2. It’s a Matter of Time
The Sun is the lifeblood of humanity. It warms us and gives us
light. Although our everyday existence depends upon the Sun,
the evolution of life on Earth was only possible because the Sun
and the Solar System are partly composed of the embers cast into
space by a long line of stellar ancestors. Indeed, the intricate and
multitudinous steps that have paved the way to our existence
started as soon as the Milky Way galaxy formed some 12 to 13
billion years ago. Step by tiny step our journey has been traced out.
We are both ancient and modern, with our bodies being composed
of archaic matter forged in the unimaginably intense fires of the
primordial moment of creation (the so-called Big Bang) and from
matter transformed and fused in the dense and blistering hot cores
of massive stars.
The calcium that is contained within the bones that animate
our all-too-temporary bodies was formed in the violence of star
explosions. As pioneer astrophysicist Sir Arthur Eddington once so
aptly put it, “We are the journey work of the stars.” Every atom in
the universe, other than those atoms of hydrogen and helium,1 was
made by the Sun’s distant ancestors, generated piece by piece from
the time of the first stars onward. Earth and the life that teems
on, above, and through it; the other planets that orbit the Sun;
their attendant satellites, and the swarm of comets that surrounds
the Solar System, are all made of material produced by long-dead
stars.
The Sun provides Earth with energy. If it were not for the Sun,
we would not be here—it’s as simple as that. It is also because of
the Sun, however, that we may not be here forever. Deep within
the Sun there is a demon, a lurking monster that will eventually
see release and, if nothing is done to tame it, it will destroy all
life on Earth. We are in the midst of a horror movie. The twist,
however, is that we know the cause from the very outset. Indeed,
the only part of the movie that we don’t know is exactly when
23
24 Rejuvenating the Sun and Avoiding Other Global Catastrophes
the horror will be perpetrated. We can be fairly sure the demon
won’t be released in our lifetime; even our descendants a million
generations from now will not necessarily be its victims.
Although we live in the midst of this movie, we will never
know the final outcome of the saga. It is our distant descendants
(perhaps 50 million generations from now) that will face the demon
in the Sun—unless, that is, they do something about it, and for
the present we can only guess at how well they might fare. Not
only will we not know the outcome of this story, there is the
open-ended possibility that no destruction will actually be perpetuated.
Perhaps our distant descendants will actually survive the
onslaught.
Time and Transformation
What is this demon that lies in the Sun? It is, in fact, the very
two-headed demon that led to our existence in the first place:
time and transformation. The Sun is converting a staggering (that
is, by human standards) 4 billion kilograms of matter (actually
hydrogen; see below) into energy every second of the day, day after
day after day. Since it formed some 4.56 billion years ago, the
Sun has converted about 6 x 1026 kg of matter into energy. That’s
equivalent to the mass of the planet Saturn and five Earths. It is a
ceaseless process; the Sun is hungry, and it feeds upon itself. And
yet, the total amount of matter already transformed into energy is
a mere 0.03 percent of the Sun’s current mass—a minuscule diet
and one that it can easily afford to consume.
In order for the Sun to replenish the energy radiated into space
at its surface, and for that matter for the Sun to remain inflated,
it has tapped into a seemingly inexhaustible energy source—the
hydrogen out of which it is predominantly made. The energy is
derived via the conversion of hydrogen into helium through the
proton-proton chain (described in Chapter 3) of fusion reactions—
a beautiful sequence of chance encounters, interactions, and
explosive transformations, described once again in prosaic form
by Sir Arthur Eddington as “a jolly crockery-smashing turn of a
music-hall.”2 Out of the chaos deep within the Sun’s core, energy
is generated, the Sun remains hot and stable, and in consequence
It’s a Matter of Time 25
we can live on a gently warmed Earth. Not too hot, not too cold, we
occupy the Goldilocks planet of the Solar System. But the demon
lurks and grows within the Sun.
The Sun contains within its massive girth something like
1.4 x 1030 kg of hydrogen (and about 0.589 x 1030 kg of material
other than hydrogen)—a seemingly endless supply of fuel, but for
an ever energy hungry Sun it is not enough. It can never be enough.
One day the hydrogen within its core will all be gone, transformed
into energy and an ‘ash’ of helium. This is deep time, some 5
billion years from now, when the demon will be born: a roaring
cuckoo-child that will grow into a bloated red giant.
The time that it takes a star to use up the hydrogen supply in
its core is called the main-sequence lifetime, or TMS. The details
relating to stellar structure, internal energy generation, and stellar
evolution will be presented in detail in the next chapter. For
the moment let us simply take it as a fact that a fraction q of
a star’s total mass M* is available in the form of hydrogen to
produce energy. The main-sequence lifetime can now be calculated
according to the total amount of energy available to the star
divided by the rate at which that energy supply is used up. The
total fusion energy available to a star can be determined from
an experimental result first established by chemist Francis Aston
in 1919.
Specifically, Aston, who was working at the Cavendish
Laboratory in Cambridge, found that the mass of four protons was
smaller than the mass of a helium atom nucleus (composed of
two protons and two neutrons) by 0.007 percent of the mass of
the protons. From this observation, Eddington realized in 1920
that if nature could coax (somehow) four hydrogen (H) atoms into
forming a helium (He) atom, then energy could be made available
to a star. Symbolically, 4H ⇒ He + E, where E is the energy
liberated per conversion. The energy term, Eddington reasoned,
would be related to the mass difference found by Aston—that is,
using Einstein’s famous equation linking mass and energy, E = m
c2, where c is the speed of light and m = 0.007 MH, and where
MH is the mass of the hydrogen atom. In this fashion, the total
amount of energy available to a star is Etotal = 0.007 c2 q M* X,
where X is the mass fraction of a star that is hydrogen (X = 0.75,
for our purposes). Now, the rate at which a star consumes its
26 Rejuvenating the Sun and Avoiding Other Global Catastrophes
energy supply is given by its luminosity L*, which accounts for
the electromagnetic energy radiated into space per unit time at its
surface. Accordingly, we have:
TMS ≈ Etotal/L∗ = 0007c2
qM∗
X /L∗ (2.1)
For the Sun, detailed numerical models indicate that q ≈ 0.13
and consequently TMS(SUN) ≈ 3 x 1017 seconds ≈ 1010 years.
On this basis the Sun is middle-aged with respect to its mainsequence
lifetime, and has some 5 to 6 billion years of hydrogen
fuel supply left.
Time and solar transformation—these are humanity’s deeptime
enemies. Time we cannot control, but the transformation
and mixing of matter in the Sun we might just be able to influence,
and this proactive engineering option – literally the rejuvenation
of the Sun – is perhaps the last best hope for providing our distant
offspring with a long-term future, and our best hope for saving
Planet Earth from total destruction.
The Deadly Earth
Life is a continuous struggle against the elements and, apparently,
there are many more ways of dying than there are of staying alive. It
is a narrow path that we have to tread to literally keep life and limb
together. Over the timescales of concern in this book, however, it is
not the survival of any one individual that is especially important.
Rather, it is the long-term survival of humanity and Earth that
matters, along with all the incredible diverse species of animals
that live upon and in it.
The scale of any disaster can be measured in numerous ways—
some more impersonal than others. The scale might be measured
in lives lost, or in financial infrastructure undermined. It can be
measured in irreplaceable historical artifacts destroyed or future
growth delayed. But in a typical human lifetime (a measure of,
say, 70 years) the sum total of natural disasters that might be
witnessed – be they hurricanes, tsunamis, earthquakes, or volcanic
eruptions – will be small fry compared to the disasters caused
It’s a Matter of Time 27
Figure 2.1. A satellite image of Hurricane Katrina. This one event devastated
the city of New Orleans in August 2005. It caused the untimely death
of 1,836 people and produced an estimated $81.2 billion in damage and
destruction. (Image courtesy of NASA).
by extraterrestrial influences that operate on timescales of many
thousands of years and longer. Indeed, on the timescale of tens of
thousands of years countless numbers of hurricanes (Figure 2.1)
will have whipped across the face of Earth, buzzing over the
landscape, in accordance with our accelerated time frame, like
a carefree fly on a hot summer’s day. Whole segments of continents
will have been shaken and shifted by earthquakes, or become
covered in ash and blistering lava by volcanic eruptions. But all of
28 Rejuvenating the Sun and Avoiding Other Global Catastrophes
these disasters combined will melt into insignificance on the day
when a 2-km-diameter asteroid ploughs into Earth’s atmosphere.
And here the correct adjective is when, not if. This event will
happen—it’s a certainty, unless humanity does something about it.
The Deadly Solar System
The main belt asteroid region, located between the planetsMars and
Jupiter, is a reservoir for planetesimals that failed to be accreted into
planets. They are primordial objects formed at the very beginning
of Solar System history, and they are the scourge of the inner
Solar System. Countless collisions, grazing sideswipes, and smallparticle
strafing have ground the initial population of asteroids into
a vast swarm of blasted and fragmented shards. There are literally
hundreds of thousands of asteroids now orbiting our Sun. The
meteorites that occasionally careen through Earth’s atmosphere,
in a blaze of light and concussion of sonic booms, are asteroid
fragments, and it is by collecting these small fragments that we have
learned about the processes by which our Solar System formed.
The chances of an individual being struck by a meteorite
are (luckily) very slim. Certainly, a few people have been bruised
by meteorites throughout recorded history, but as far as reliable
sources go, no one has ever been killed by a meteorite strike.3 The
main belt asteroids are not a direct threat to Earth. The simple
reason for this is because they move along circular orbits located
between Mars and Jupiter. There is, however, a small subset of
asteroids (admittedly derived from the main belt region) that do,
however, pose a collisional threat to Earth. These are the nearEarth
asteroids (NEAs). The NEAs have more elliptical orbits than
their main belt relatives, and this property can bring them into
the region of Earth’s orbit4 where collisions (and close encounters)
will eventually occur.
The most recent impact of a 10-m-sized asteroid with Earth
occurred on September 3, 2004. Military reconnaissance spacecraft
recorded this particular event, and the material that survived
atmospheric passage (fortunately) ploughed into the Antarctic ice
shelf. Unfortunately (for science), the fall location of this particular
impact means that no ground investigation took place, and the
It’s a Matter of Time 29
Figure 2.2. The Tunguska fireball devastation. At 07:17 local time on
June 30, 1908, the sky over central Siberia filled with the light of a massive,
detonating fireball. In an instant thousands of trees were felled and scorched
by a searing blast wave generated by the catastrophic breakup of the
meteoroid just 10 km above the ground. It is still a matter of contention
between planetary scientists whether the Tunguska object was a comet
(predominantly composed of water ice) or an asteroid (predominantly stony
in composition). The photograph reproduced here was taken during the
1927 expedition led by Leonid Kulik to study the impact area.
crater and possible fragments are now lost. Historically speaking,
the best-studied large impact event occurred in Siberia on June
30,1908. This – the so-called Tunguska event5 devastated some
2,000 square kilometers of forest and produced a series of longwavelength
acoustic waves that propagated around the world
multiple times (Figure 2.2). The last major event involving a 10- to
15-km-sized impactor occurred some 65 million years ago. It has
been argued that it was as a result of this particular impact that
the dinosaurs (and indeed, most of the world’s large land animals)
became extinct (Figure 2.3), although many researchers also argue
that the impact was more of a coup de grace than the principal
agent behind the extinction. Many other abrupt extinction events
are recorded in the fossil record and only a few of these have been
linked to large impact structures.6
The extensive and, indeed, ongoing collisional alteration of
the main-belt asteroid population dictates that there are now many
more small asteroids than large ones. Earth impact probability
increases, therefore, with decreasing asteroid size (or mass). So,
for example, Earth experiences a collision with a 50-m-diameter
30 Rejuvenating the Sun and Avoiding Other Global Catastrophes
Figure 2.3. The Chicxulub impact crater located on the Yucatan peninsula,
Mexico. The actual crater is no longer a distinctive surface structure, and
the image shown here is a reconstruction of the crater through surface
magnetic and gravitational anomaly measurements. The crater is 170 km
across and is dated to the time of the great dinosaur extinction 65 million
years ago. (Image courtesy of V. L. Sharpton, LPI).
asteroid once every few hundred years; an encounter with a
1-km-sized asteroid occurs every few hundred thousand years, and
an impact from a 10- to 15-km-sized asteroid takes place every
few tens of millions of years (Figure 2.4). An approximate formula
Figure 2.4. The total number of objects N(M) with masses larger than
mass M, falling to Earth per year. Extinction event (EE) impacts occur
on timescales of several tens of millions of years, whereas Tunguska-like
events occur on timescales of a few hundred years.
It’s a Matter of Time 31
for the time interval between impacts from asteroids of diameter
D-km is: TNEA (yrs) = 10 5+08ln D−km .
Dealing with potential Earth-impacting comets and asteroids
will presumably be one of the first priorities of future space
engineers. The nature of the danger posed by such objects,
however, must first be understood. Indeed, we still need to know
which objects might impact Earth. To this end a small number of
astronomers, working from observatories dotted around the world,
are scanning the sky in the search for rogue asteroids, annotating
their numbers and determining their orbits. The orbits and nearterm
evolution of all asteroids, both main-belt and Earth crossing,
with sizes larger than a few kilometers will soon be known,
and in principle the times of potential future impacts from these
objects will be predictable. Our present knowledge of the asteroid
population with sizes less than about 1 km is limited, and the
lead-time we might currently expect before the next Tunguskalike
impact is perhaps a few seconds. In other words, the first we
will know about the event is when it has happened. Although this
is far from comforting news, within the next half-century (perhaps)
most of the potential Tunguska-like impactors, with diameters of
just a few tens of meters, will also be catalogued.
In 1999 MIT researcher Richard Binzel developed a 10-point
scheme to measure the impact threat posed by an asteroid. Based
upon the estimated size, potential impact velocity and impact
probability, the so-called Torino scale developed by Binzel assigns a
color and a number to any Earth-approaching asteroid. The Torino
scheme is described in Table 2.1. At the time of this writing
only one object – an asteroid called 2004 VD17 – has a Torino
scale designation greater than white 0. This particular asteroid is
believed to be some 600 m across, with a mass of about 2.5 x 1011
kg. Having an estimated impact velocity of 21 km/s this asteroid
could impart some 14,000 megatons of TNT-equivalent energy
upon impacting Earth. The Torino scale designation for 2004
VD17 is Green 1, and while an Earth impact is highly unlikely, a
relatively close approach on May 4, 2102, is predicted. It is highly
likely that follow-up observations of this particular asteroid, during
the next several months and years, will see it downgraded to a
Torino scale white 0 object.
32 Rejuvenating the Sun and Avoiding Other Global Catastrophes
Table 2.1. The Torino impact scale. Further details can be found at
http://neo.jpl.nasa.gov/torino_scale.html.
Color Number Comments
White 0 No direct impact threat. Or a small object (size less
than 50 m) that might possibly produce a
meteorite shower.
Green 1 Close Earth passage, but very low probability of
impact.
Yellow 2 An object warranting further attention, but no great
impact concern.
Yellow 3 An object with a > 1% chance of hitting Earth.
Impact would produce local destruction. Object
warrants further study.
Yellow 4 An object with a > 1% chance of hitting Earth.
Impact would produce regional destruction.
Object warrants further study.
Orange 5 Close encounter posing a serious threat of impact
resulting in regional destruction. Impact
lead-time less than 10 years.
Orange 6 Threat of a global catastrophe. Impact lead-time
less than 30 years.
Orange 7 Very close encounter by a large object that might
hit Earth during the next 100 years.
Red 8 A certain impact, resulting in localized devastation.
Red 9 A certain impact, resulting in regional devastation.
Red 10 A certain collision, resulting in catastrophic global
devastation.
Knowing when an impact might occur, however, is only part
of the story; what to do about the impending impact is something
else altogether. A number of proposals concerning asteroid defense
tactics have been put forward in recent years, and they range from
the “blast ‘em to pieces” type of proposal to the “change their
orbit slowly and gently” type. Of course, the action that might
be taken is dictated by the size of the asteroid and the potential
lead-time before Earth impact is going to occur. If the asteroid is a
small one (perhaps a few hundreds of meters across; see Figure 2.5)
and the lead-time to an impact is very short (a few years) then
blasting it with nuclear weapons is probably the best last-choice
action.7 If the asteroid is large (in excess of, say, 500 m or so) and
the lead time is many tens to hundreds of years, then attempts to
just slightly alter the asteroid’s orbit (for example, by attaching a
set of large space sails) would seem to make sense.
It’s a Matter of Time 33
Figure 2.5. Asteroid (25143) Itokawa is some 0.535 by 0.294 by 0.209 km in
size and weighs in at about 3.5 x 1010 kg. Visited by the Japanese Hayabusa
spacecraft in late 2005, it is presently believed that the complex structures
observed on its surface are the result of a collisional breakup followed by
a reassembly phase. That many (possibly most) asteroids are loosely structured
rubble piles has important consequences for any potential collisional
avoidance strategies. (Image courtesy of JAXA).
The potential impact threat posed by near-Earth asteroids and
methods by which such impacts might be avoided are relatively
well understood (at least in principle) at the present time. Indeed,
the stage is set (literally ready and waiting) for the first attempts to
alter the orbit of a ‘safe’ non-impacting asteroid.8 The first (terrestrial)
asteroid impact avoidance engineers have already been born.
The Deadly Stars
The Sun is situated at a distance of 8,000 pc from the galactic
center. It travels through space at a speed of 230 km/s and
completes one galactic orbit every 200 million years. On its
journey around the galactic center, the Sun is accompanied by an
ever-moving host of stellar companions. The nearest star to the
Sun is currently the faint dwarf star Proxima Centauri. Located
just under 1.3 pc away, however, our nearest stellar neighbor is
not actually visible to the naked eye. Within the volume encompassed
by a sphere of radius 5 pc centered on the Sun, there are 49
star systems. Indeed, the number of stars per unit volume of space
in the solar neighborhood is determined to be * ≈ 0.1 stars/pc3.
This is a useful number, since it enables us to determine the
34 Rejuvenating the Sun and Avoiding Other Global Catastrophes
typical time interval (TCE) for close encounters between the Sun
and another star.
To determine TCE we need to first set up an impact cross
section area:
= d2, where d is the encounter distance as
measured from the Sun. Given that the Sun has a relative velocity
V* (km/s) with respect to its nearest neighbors, then in time T the
volume swept out by the Sun will be V =
V* T. In the volume
of space swept out, however, there will be N* = V * actual stars.
Hence, in time T, the number of stars encountered by the Sun
within distance d is N* = *
V* T. The time interval between
individual encounters is, therefore, about TCE =1/ *
V*.
The question now is, what do we mean by a close encounter?
As the approach distance d is increased so the cross sectional area
increases and TCE correspondingly decreases, so distant encounters
become common. The time interval between stars being as close
to the Sun as Proxima Centauri (d ≈ 1.3 pc) is now 74,000 years,
given a typical relative velocity of V* = 25 km/s. What about an
even closer encounter, say at a distance of 5,000 AU, well inside
of the Solar System’s cometary reservoir, the Oort Cloud?9 In this
case TCE ≈ 200 million years. How about an approach as close as
5 AU? At this encounter distance the Solar System will almost
certainly be destroyed (depending on the mass of the interloping
star), with the planets being scattered out of their current orbits
and most probably ejected to a cold, dark death in interstellar space.
For this situation, TCE = 2 x 1014 years, which is well in excess of
the present age of the galaxy (Tgal ≈ 13 x 109 years). Within the
main-sequence lifetime of the Sun [TMS(Sun) ≈ 1010 years], no star
is likely to pass closer than d ≈ 730 AU.
Although a direct collision – or even a very close
encounter – between the Sun and another star is a highly unlikely
event, encounters at the distance of the Oort Cloud and of a few
thousand astronomical units will happen reasonably often, and
these encounters can still be deadly to life on Earth. Cometary
nuclei in the outer part of the Oort Cloud will have their orbits
gravitationally perturbed by passing nearby stars. The smaller the
closest approach distance, the more massive and the slower the
speed of the perturbing star, the greater the number of cometary
nuclei that will have their orbits changed. Many of the cometary
nuclei will be ejected from the Solar System, doomed thereafter to
It’s a Matter of Time 35
Figure 2.6. Close-up of a cometary nucleus. In this image the surface of
periodic comet Wild 2 is revealed by a camera carried aboard the Stardust
spacecraft (http://stardust.jpl.nasa.gov/home/index.html). The nucleus is
about 5 km across. (Image courtesy of NASA)
rove the depths of interstellar space. Other cometary nuclei will
have their orbits perturbed in such a fashion that they will swing
into the inner Solar System, and while on their journey around the
Sun they may potentially hit one of the planets (Figure 2.6).
Although Proxima Centauri is currently the closest star to the
Sun, we can ask which stars in the solar neighborhood are going
to make even closer approaches to the Sun in the future. This
question, in fact, was recently answered by Joan Garcia-Sanchez of
the University of Barcelona and co-workers.10 Using data gathered
by the Hipparcos astrometric satellite, the team found a total of
87 stars that will pass within 5 pc of the Sun during the next 10
million years. Of these, the star Gliese 710 will make the closest
approach of all, skimming the edge of the Oort Cloud at a distance
of about 0.3 pc (or 70,000 AU) from the Sun (Figure 2.7). The closest
approach will occur some 1.4 million years from now. Estimates
of the effect of Glies 710 on the Oort Cloud suggest that around 2
million long-period comets will be perturbed into potential Earthcrossing
orbits over a time interval lasting perhaps 5 million years.
36 Rejuvenating the Sun and Avoiding Other Global Catastrophes
0
0.5
1
1.5
2
1 1.2 1.4 1.6 1.8 2
Time (millions of years)
Distance (pc)
Gliese 710
HD33487
HD179939
Oort cloud boundary
Figure 2.7. Closest approach distances of some selected nearby stars. The
curved solid line shows the distance variation with time of Gliese 710.
The stars HD 33487 and HD 158576 will pass as close as 2 and 0.9 pc
respectively to the Solar System. After Gliese 710, the star HD 158576
makes the second closest approach to the Solar System during the next 5
million years.
As this is a chapter concerned with timescales, we can ask
the following: What is the likely time interval TLPC between longperiod
comet hits on Earth? The target area of Earth (ignoring
gravitational focusing) is
E =
RE
2, where RE = 6371 km is Earth’s
radius. The probability that a single Earth-orbit crossing comet
might actually strike Earth is, therefore, Phit = 2
E/(4
DE)
2), where
DE = 1 AU = 149.6 million km is Earth’s orbital radius. The factor
of 2 accounts for the fact that the cometary orbit cuts through the
sphere of radius 1 AU twice, and it is assumed that long-period
comets can approach the Sun from any direction (an assumption
that is not strictly true for cometary showers). If the number of
comets heading into the inner Solar System is fcom comets per year,
then the time interval between possible Earth impacts will be of
order TLPC = 1/[(RE/DE)
2 fcom/2] ≈ 109 (yr)/fcom.
The typical flux of long-period comets approaching to within 1
AU of the Sun is estimated to be fcom ∼100 per year. (This may rise
to ∼200 per year during a cometary shower such as that induced
by Gliese 710.) These numbers indicate that long-period comet
impacts upon Earth are going to be rare, even under cometary
shower conditions, with TLPC ∼5 – 10 x 106 years.
The situation with long-period comet impacts is actually
more complicated than indicated above. Oscillations of the Solar
System above and below the galactic plane (with a period of
It’s a Matter of Time 37
about 35 million years), gravitational tides resulting from galactic
spiral arm encounters, and the close passage of giant molecular
clouds can also perturb the orbits of cometary nuclei in the Oort
Cloud, causing cometary showers. Mass extinction events deduced
from the fossil record (Figure 2.8) and the estimates of terrestrial
crater ages suggest that cometary impacts occur in discrete bursts,
lasting for perhaps a few million years, separated by intervals corresponding
to 25 to 30 million years.11
Long-period comet impacts may well be rare, but they pose
a tremendous threat to life on Earth. Their large, kilometric size
and high encounter velocities indicate that they can deliver devastatingly
large amounts of impact energy. Not only this, the lead
time between the detection of a long-period comet and its possible
impact with Earth might be just a few months. This is a very
different situation to the NEA threat discussed earlier, and there
is no obvious engineering option that can easily save Earth from a
Chicxulub Wilkes Land
% Extinction
100
End
Late Ordovician
Devonian
End
Triassic
End Permian
End
Cretaceous
500
50
0
0 100 200 300 400
Time before present (Myr)
Figure 2.8. Schematic diagram of marine species extinctions as recorded
in the fossil record over the last 550 million years. The so-called ‘big five’
events are labeled. The dinosaurs and the Chicxulub impact crater are dated
to the end-Cretacious extinction some 65 million years ago. The greatest
mass extinction occurred at the end of the Permian, about 250 million years
ago. It has been suggested that the end-Permian extinction coincided with
the formation of the 480-km-diameter Wilkes Land impact crater recently
found in eastern Antarctica. The ripple effect, with a period of about 25 to
30 million years, is clearly visible in the extinction curve.
38 Rejuvenating the Sun and Avoiding Other Global Catastrophes
Figure 2.9. On July 4, 2005, the Deep Impact mission produced a new
impact crater on the surface of Comet Tempel 1. The comet is some
14 km x 5 km x 5 km in size and predominantly composed of water
ice. The impact was produced through the direct hit of a 1-m-long, 1-mdiameter
copper cylinder striking the icy surface at about 10 km/s. The
impact crater produced is thought to be about 100 m across. (Image courtesy
of NASA)
direct hit by a long-period comet. Brute explosive force is perhaps
one solution (Figure 2.9), but the multiple fragments produced by
such actions may well exacerbate rather than fix the problem.
Indeed, one of the most pressing issues related to the long-term
survival of humanity is how to quickly (on a time scale of months)
alter, albeit very slightly, the orbital track of a directly impacting
long-period comet without breaking it apart.
Deadly Novae
The end phases of some stars are very violent. They literally
blow themselves to pieces (Figure 2.10), spraying processed nuclear
material into interstellar space at immense speeds and irradiating
vast volumes of space with lethal X- and -ray radiation.12 There
It’s a Matter of Time 39
Figure 2.10. The Crab Nebula, the icon of a supernova remnant. The ‘New
Star’ associated with the production of the Crab Nebula was recorded in
numerous Chinese and Korean chronicles, and was first seen in July 1054.
The object was visible for many months, and was reportedly visible in broad
daylight. At a distance of 2,000 pc, the maximum brightness of the nova
would have been about magnitude -6, brighter than the planet Venus but
not as bright as a half-illuminated Moon. (Hubble Space Telescope image,
courtesy of NASA)
is some debate concerning the minimum distance beyond which
Earth’s atmosphere might be safe from a nearby supernova event,
but it is typically thought to be between 50 and 100 pc.
An approximate timescale for nova irradiation can be made as
follows. Here we will work in two dimensions only and consider
the galactic disc to be a circular band with an inner radius of 1 Kpc
and an outer radius of 15 Kpc. The area of this galactic disk is then
Agal ≈ 7 x108 pc2. If the critical distance from the Sun for a nova
detonation is taken to be Rcrt, then on average the number of nova
NNVA that must occur before one is at least within a distance Rcrt
from the Sun is NNVA = Agal/( R2
crt) . If we take Rcrt = 60 pc, then
40 Rejuvenating the Sun and Avoiding Other Global Catastrophes
NNVA ≈ 6,000. Taking a disk nova rate of SNVA = 0.04 per year, then
the typical time interval TNVA between critical nova explosions is
going to be TNVA = [Agal/( Rcrt
2
)]/SNVA ≈ 1.5 x 106 years.
The next closest Type I supernova-producing system13 to us
may have already been identified, and it is the binary star known as
IK Pegasus (also designated HR 1820). In this particular case one of
the stars in the binary has already formed into a white dwarf, while
its companion is presently in a subgiant phase. The two stars are
sufficiently close (just 42 R apart, in fact) that the subgiant star is
actively adding mass to the white dwarf star. This latter accretion
is the critical point. In 1930 Nobel Prize-winning physicist and
mathematician Subrahmanyan Chandrasekhar showed that there
is a maximum mass limit for a white dwarf to remain stable.
The critical mass – the Chandrasekhar limit – beyond which a
white dwarf will catastrophically collapse is MCLM ≈1.4 M Once
MWD > MCLM then gravitational collapse will ensue, and a Type I
supernova will result (Figure 2.11).
It is estimated that the white dwarf component in IK Pegasus
has a mass of about 1.2 M and this means that once it has
accreted an additional 0.2 M of material from its companion
a dramatic collapse will occur. If the accretion rate is as high
as 10−6 solar masses of material per year, then IK Pegasus will
undergo supernova disruption in about 200,000 years from now,
at a distance of some 44 pc—well inside the ‘danger zone’ for
perturbing Earth’s atmosphere (Figure 2.12). For an accretion rate
of 10−7 solar masses per year, supernova disruption will occur
about 2 million years from now, when the system is at a distance
of 43 pc from us. The rate at which the white dwarf star in IK
Pegasus accretes matter from its companion must be less than
5 x 10−8 solar masses of material per year if supernova disruption
is to take place at a distance when the system is beyond the safe
distance boundary of 60 pc. At the present time astronomers do
not know what the accretion rate for the white dwarf in the IK
Pegasus system is.
The speed with which material is blasted into space by
a supernova is typically several thousands of kilometers per
second. Clearly if a planet is within a few tens of parsecs of a
supernova explosion and chances to encounter such a disruptive
blast wave, then its atmosphere would be severally denuded.
It’s a Matter of Time 41
Star of mass M2 Star of mass M1
MS star Giant star
MS star WD star
Giant star WD star
Binary star
Secondary Primary
(1)
(2)
Ejection (3) SN
Figure 2.11. Stages in the production of a Type I supernova. The process
begins with the formation of a binary star system in which one star – the
primary – has a mass M1 greater than that of its companion – the secondary –
of mass M2. Since the primary star is the more massive of the pair it will
evolve into a giant star long before the secondary does (Stellar evolution
will be discussed in the next chapter). At stage (1) the giant primary star
begins to lose mass to the secondary, and the two begin to spiral together
within a common envelope. Eventually, the primary giant evolves into a
white dwarf star in close orbit about its companion (the now-enhanced
mass secondary). Eventually, at stage (2), the secondary star evolves away
from the main sequence to become a giant star and mass exchange begins
again. At this stage, however, the white dwarf can only accrete so much
material before the Chandrasekhar limit is reached. Finally, at stage (3),
the white dwarf collapses to produce the Type I supernova and the giant
secondary companion is slung outwards at high speed into interstellar
space.
Not only this, the atmosphere of such a planet would probably
have been badly disrupted long before the material blast wave
actually arrived. At distances of less than about 50 to 100 pc,
the flux of energetic cosmic rays, combined with that of the
X-ray and -ray radiation produced by the supernova explosion
will probably destroy or at least badly perturb the atmosphere
surrounding an Earth-like planet. An intriguing book – The Cycle
of Cosmic Catastrophes [Bear and Company, Rochester, Vermont
(2006)] by Richard Firestone, Allen West, and Simon WarwickSmith
– presents the argument that Earth suffered a triple blow
from the supernova responsible for producing the Geminga pulsar
and gamma-ray source.14 Based upon radiocarbon data (and a host
42 Rejuvenating the Sun and Avoiding Other Global Catastrophes
35
45
55
65
0 0.5 1 1.5 2 2.5 3
Time (millions of years)
)cp(ecnatsiD
-6 -7
Minimum safety zone
t(min)
Figure 2.12. Variation in the distance to IK Pegasus. The system will be
at its closest to the Solar System at a time t(min) = 1.14 million years
from now. The safe distance beyond which the supernova will have little
effect on Earth’s atmosphere is estimated to fall between 50 to 60 pc. The
points labeled -6 and -7 correspond to the distances and times at which
a supernova will occur when the accretion rate is 10−6 and 10−7 M/yr
respectively.
of other environmental markers), Firestone and co-authors argue
that 41,000 years ago the atmosphere encountered the X- and
gamma-ray burst from the Geminga supernova event, then 23,000
years later(18,0000 years ago) Earth encountered the supernova
shockwave, followed 5,000 years later (13,0000 years ago) by a
slower-moving debris wave. Firestone and his co-authors link the
various events to mega-fauna extinctions, massive flooding, and
dramatic climate change.
The peak energy output per unit time (the luminosity) of a
typical Type I supernova is LSN ∼1036 (watts). This is a tremendous
amount of energy being radiated into space—indeed, 10 billion
times greater than the Sun’s present luminosity—and yet it is by
no means the most energetic of celestial cataclysms.
The star Eta Carina is the next most likely candidate, among
the known stars within a few thousand parsecs of us, to produce
a Type II supernova—the result of the catastrophic collapse of
a single massive star. Eta Carina is some 5 million times more
luminous than the Sun, and it is estimated to be about 120 times
larger in size. There is some evidence that Eta Carina is actually
a binary system, but it is reasonably clear (whether singular or
a binary) that the system contains at least one 50 to 100 M
star. For at least the last century and a half it has undergone
It’s a Matter of Time 43
Figure 2.13. Eta Carina as photographed by the Hubble Space Telescope
in the mid-1990s. The twin lobes of the expanding envelope are clearly
visible, and the surrounding trelliswork of twisted veins indicates those
locations where carbon grains have formed. The envelope is expanding into
space at a speed of about 700 km/s. An equatorial disk can also be seen in
this image, which has been reproduced in a negative format to enhance the
visible detail. (Image courtesy of NASA)
quasi-periodic outbursts in brightness, and since the early 1840s
it has lost an estimated2–3 M of material into space. Much of
this material resides in a surrounding double-lobed nebula called
the Homunculus (Figure 2.13). Various model calculations suggest
that Eta Carina will undergo supernova collapse within the next
100,000 years, but since it is situated at a distance some 2,500 pc
away from us, the effects of Eta Carina’s destruction will be of
little consequence to Earth. Indeed, as seen from Earth, the peak
brightness of the Eta Carina supernova will be about magnitude
−4, comparable in brightness to the planet Venus.15 In contrast,
if IK Pegasus undergoes supernova disruption when it is at its
closest point to Earth, it will attain a peak brightness of about
magnitude −16, brighter than the full Moon.
44 Rejuvenating the Sun and Avoiding Other Global Catastrophes
GRBs and Hypernovae
By far the most energetic of explosions within the entire universe
are those associated with the hypernovae that are responsible for
producing gamma ray burster (GRB) events. The GRB production
mechanism is believed to result from the gravitational collapse
of a rapidly spinning, massive, magnetic star (sometimes called a
collapsar; see Figure 2.14). The rotation and the strong magnetic
field combine to produce two exceptionally strong and twisted
magnetic columns protruding along the star’s spin axis. Electromagnetic
radiation, such as gamma rays, are then directed along
these magnetic columns and literally ‘squirted’ into space as two
oppositely directed jets.16
Although the details of the GRB production mechanism are
still being worked out, it is clear that they can release a staggering
1054 joules worth of energy into space in just a few tenths of
seconds. At these energy-release levels a GRB occurring within a
few kilo parsecs of Earth could potentially cause biologically significant
irradiation. The time interval between GRB events occurring
in our galaxy has been estimated to be of order TGRB ∼108 years (but
see later), and although the exposure time to the burster radiation
is just a few seconds (affecting just one hemisphere of Earth,
Figure 2.14. Artist’s image of a collapsar producing a GBR event. The
gamma rays are channeled along the oppositely directed, magnetically
confined jets. (Image courtesy of NASA)
It’s a Matter of Time 45
therefore), the energy flux might nonetheless seriously damage
Earth’s upper atmospheric structure. One set of calculations by
Brian Thomas and co-workers at the University of Kansas17 finds
that even a 10-second exposure to the gamma rays from a GRB
2,000 pc away can severely damage Earth’s ozone layer. Indeed, the
effect of the gamma rays is to break apart the molecular nitrogen
(N2) in the atmosphere, which then reacts with molecular oxygen
(O2) to produce nitric acid (NO). It is the nitric acid that destroys
the ozone (O3), further producing nitrogen oxide (NO2). Then, in
a feedback loop, the NO2 reacts with O2 to produce more NO,
which then destroys more O3. The computer model indicates that
it takes at least five years for the atmosphere to recover the ozone
lost due to the gamma ray exposure. It is not so much the loss of
atmospheric ozone that is the problem for life on Earth, but rather
the problem that the Sun’s UV radiation, normally absorbed by the
ozone, will be able to penetrate to the ground. Extended exposure
to solar UV rays will, for example, result in a dramatic increase
in animal and human skin cancers; surface dwelling plankton (the
bottom of the aquatic food chain) would also be killed off, probably
resulting in numerous ecosystem failures.
It has been suggested that the Ordovician mass extinction
(see Figure 2.10), in which 60 percent of marine species became
extinct some 450 million years ago, was the result of atmospheric
disruption caused by a nearby GRB event. The evidence for this,
however, is not fully convincing,18 and recent studies suggest that
GRBs must occur well within 1,000 pc of Earth before any serious
atmospheric damage is likely to occur. In addition, Krzysztof
Stanek and co-workers19 have recently argued that GRBs tend
to occur mostly in small, irregularly shaped galaxies having low
heavy element abundances. (These elements will be discussed in
the next chapter.) What this means for us is that the odds of
any GRBs occurring in our Milky Way galaxy now and in the
future are actually very low indeed, and the interval between
events is probably even longer than the 108 years indicated
earlier.
Can anything be done to protect Earth from GRBs? In
principle, perhaps surprisingly, yes. A hypernova precursor star
will of necessity be highly luminous, and if situated within the
range at which damage to Earth’s atmosphere might result (i.e., less
46 Rejuvenating the Sun and Avoiding Other Global Catastrophes
than 1,000 pc), it will be a very obvious astronomical object. In
principle, future inhabitants of Earth will at least know which
stars to guard against. Further, detailed spectral observations could
also determine the direction of the progenitor star’s spin axis, and
this would determine if Earth were situated in the direct line of
fire. If Earth were actually exposed to a burst of gamma rays from
a GRB, then in principle large UV sunscreens could be placed in
near-Earth orbit to temporarily protect Earth’s surface-dwelling life
forms from direct solar UV exposure (Figure 2.15).
The potential importance of hypernovae and GRBs in
controlling the emergence of intelligent life has recently been
discussed by James Annis.20 Indeed, Annis argues that our Milky
Way galaxy has been sterilized of life on numerous occasions in
the past. It is generally believed that the GRB rate decreases with
time—that is, GRB events were much more common when our
galaxy was younger. Annis argues that because of this effect the
Sunshield
located at L1
1 AU
Sun
Dia. ~ 20,000 km
L1
~ 0.01 AU
Earth
Figure 2.15. It has been suggested that Earth’s climate might be manipulated
by placing a sunshield at the Sun-Earth L1 point, where gravitational
interactions are equal and opposite. Technically, to completely
cover the Sun at L1 the sunshield would need to have a diameter of
20,000 km. Clearly a much smaller diameter sunshield is required in
practice, since it is not the intention to block out all of Earth-incident
sunlight.
It’s a Matter of Time 47
galaxy has only recently undergone a phase-transition allowing
for the long-term survival of complex life forms (such as us).
His argument runs like this: In the early galaxy GRB events were
so common that even if life did manage to evolve on some specific
planet it would soon thereafter be destroyed. As the GRB rate
decreased with time, however, various strongholds of life could
develop and undergo advanced evolution, eventually producing
(in some cases) intelligent species capable of space exploration.
Annis, in fact, argues that this phase transition requirement offers
a solution to Fermi’s Paradox. His point is that since intelligent
life will not evolve in the galaxy until the GRB rate drops to
a level allowing for the phase-transition to occur, the time for
the emergence of extraterrestrial civilizations capable of galactic
colonization cannot be much different from TUS ∼4.5 billion years.
Hence, extraterrestrial civilizations are only just on the verge of
beginning galactic colonization, and perhaps they will be here
very soon.
The Embrace of Andromeda
Of order 20 million Sun diameters separate our Sun from
Proxima Centauri. In contrast, only about 20 galaxy diameters
separate the Milky Way galaxy from its nearest comparable-sized
companion M31—the Andromeda Galaxy. It is because of this
basic size compared to separation conditions that star collisions
are rare, but galaxy interactions are relatively common. It is
certain that the Milky Way galaxy has ‘cannibalized’ numerous
smaller Local Group21 galaxies in the past with little effect
upon its overall structure; the forthcoming interaction with M31,
however, will be much more extensive than any of its previous
encounters.
The Andromeda Galaxy is presently some 730 Kpc away, but
it is moving towards us at a speed22 of about 120 km/s. We do
not know if a direct collision and interaction with our Milky
Way galaxy is actually going to occur, but if it does, then at its
current speed M31 will merge with our galaxy in something like
TM31∼6 x 109 years (Figure 2.16).
48 Rejuvenating the Sun and Avoiding Other Global Catastrophes
Figure 2.16. A snapshot from a computer model simulation of the collision
between the Milky Way galaxy (to the upper right) and M31. (Image
courtesy of John Dubinski, University of Toronto).
The possible consequences for our Solar System as the merger
between M31 and our Milky Way galaxy proceeds are difficult to
predict. It is clear, however, that star collisions are still highly
unlikely to happen; the average separation between stars will
continue to remain at many millions of Sun-diameters. This being
said, a close stellar encounter is certainly possible, and our Solar
System may yet be disrupted in TM31 years from now—intriguingly,
a time not far removed from the canonical end of the Sun’s mainsequence
phase; our distant descendants may be in for double
trouble.
Although the star-Sun close-encounter timescale will not
be greatly changed by the merger of our galaxy with M31, the
time interval TNVA between Type II supernova12 encounters will
decrease. This result follows in the wake of the enhanced star
formation that will occur during the M31 collision.23 The star
formation rate will inevitably increase during the M31 merger
because of the compression and interaction of the large gas and dust
clouds known to reside in the interstellar media of both galaxies.
Our distant descendants will certainly experience a wonderful
view as the Andromeda galaxy approaches over the next several
billion years. Two Milky Way bands may eventually grace the
night sky, and myriad glowing gas clouds, star formation regions
and supernovae will sparkle in the heavens. Indeed, a beautiful but
potentially deadly vista lies ahead for humanity.
It’s a Matter of Time 49
Deep Time
Even if Earth physically survives the inevitable future close
encounters with wayward stars, nova explosions, and the ravages
of an aging Sun, it still has a ‘natural’ winding-down mechanism in
the form of inescapable gravity. Indeed, any object orbiting around
a fixed center will lose energy due to gravitational radiation. It
is certainly a small amount of energy that is lost per orbit, but
how long it takes is not the issue at this stage. As Earth loses
orbital energy through the generation of gravitational waves it will
spiral in towards the Sun. The decay time, TGRV will be of order
TGRV ≈ (c/Vorb 5 P, where Vorb is the orbital velocity, c is the speed
of light and P is the orbital period. Earth currently has Vorb = 30
km/s and P = 1 year, which dictates a decay time of TGRV ≈ 1020
years. For the planet Mercury the decay time is TGRV ≈ 1016
years. Mercury, however, will be consumed by the expanding Sun
in about 6 billion years, long before its TGRV destruction time.
Earth, on the other hand, may physically survive the Sun’s red
giant phase, and correspondingly, some 8 billion years from now
find itself in orbit around a white dwarf star. If it continues to
survive against disruption through close stellar encounters, then
its eventual demise (in the deep, deep future) will be to spiral into
the surface of a zero temperature, zero luminosity black dwarf
sphere24 supported by degenerate electron pressure—the cold and
dark relic of the Sun.
The Doomsday Event
The great philosopher and biologist J. B. S. Haldane once remarked
that “The universe is not only as queer as we suppose, but queerer
than we can suppose.” Haldane is indeed right, and it is entirely
possible that there are physical entities within our Milky Way
galaxy that might destroy Earth (and the Solar System) in its
entirety if encountered just the once. Such entities might include
massive black holes, putative strange matter, and of course, the
phenomena that Haldane tells us we know nothing about. More
will be said about black holes in Chapter 5, but since there is
currently no consensus on what the mass distribution of black
50 Rejuvenating the Sun and Avoiding Other Global Catastrophes
holes is, we can presently say very little about the chances of
Earth or the Solar System encountering one (other than it hasn’t
happened in 4.5 billion years). Strange matter is certainly odd stuff,
but entirely possible within the context of allowed modern atomic
physics. The suggestion is that such matter, made of strange quarks
(the fractionally-charged, basic building blocks of matter), might
form within the interiors of high-density neutron stars, and it may
also be stable (in the form of so-called stranglets) outside of such
objects. It has been further speculated that should ordinary matter
(such as the stuff Earth is made of) meet a stranglet, then it will
become transformed into strange matter. There is currently no
proof that strange matter exists, but Haldane’s comment surely
resonates on such issues.
The fact that Earth formed some 8 or 9 billon years after the
universe came into existence (the moment of the Big Bang) and has
survived for at least 4.5 billion years tells us that doomsday catastrophes
must be rare in our galaxy, but the question is, how rare?
If a randomly occurring event (i.e., a stranglet or massive black
hole encounter) destroys a planetary system at some constant rate
tau, , then the probability that a specific planetary system will
survive a time t decreases as e−t/ . That is, the survival probability
decreases exponentially with time. If the event causing destruction
occurs relatively often, then will be small and the survival probability
soon becomes very small. If destruction events are rare, then
will be large and the probability of a planetary system surviving
for a long time is high. It is not, unfortunately, possible to constrain
with any certainty at the present time, other than by making
the statement that the continued existence of the Solar System
suggests that it is unlikely that tau is less than several billion
years.
The Long and the Short of It
The timescales upon which astronomical disasters are likely to
occur are compared in the inequality below.
TNEA 1−km << TNVA ≈ TLPC ≈ TGRB < TM31 < TMS
<< TCE d = 5AU << TGRV
It’s a Matter of Time 51
At the far right hand end of the time sequence, if nothing else
destroys Earth first, it will meet its ultimate doom by being
accreted onto the surface of the Sun, which will have evolved into
a black dwarf star. In the near term, however, the most likely astronomical
disaster would be that due to an impact by a relatively
small (perhaps 100 to 1,000 meters across) asteroid or comet. These
are the smaller, more common near-Earth objects (NEOs) that feed
into the inner Solar System from the main belt asteroid region and
the Jupiter family of comets. The most serious impacts, however,
will be those from the long-period comets. On about the same
timescale that a cometary shower impacts the Solar System, it is
liable to be irradiated by a near-by supernova outburst and possibly
by a GRB event.
Although Annis20 has argued that GRB events are capable of
sterilizing the galaxy, not all researchers agree with this claim.
Indeed, it has been pointed out that just a few centimeters of
rock, soil, or water would completely protect any life forms on the
Earth’s surface from GRB irradiation.25 Perhaps the most important
issue with respect to surviving a GRB event is the time required to
regenerate atmospheric ozone (destroyed by the incoming -rays).
It is the ozone shield that protects life on Earth from the harmful
UV solar-background radiation.21
Making the Best of It
The first steps toward securing humanity’s long-term survival
will probably focus on the means of detecting and then either
destroying or diverting potential Earth-impacting asteroids and
comets. Methods will also have to be developed for repairing
Earth’s atmosphere following the ravages imposed by supernovae
and GRB irradiation events. Although each of these catastropheavoiding
steps will require great ingenuity, enormous engineering
skill, tremendous daring, and total cooperation between the
world’s nations, there is in principle no specific or fundamental
reason why they cannot be achieved. Time, of course, will tell
how well humanity lives up to the task. What is perhaps most
important to note at this moment, however, is that there is a
52 Rejuvenating the Sun and Avoiding Other Global Catastrophes
straightforward progression in project size and required skill development
that takes us from impact avoidance to terraforming, Solar
System colonization, and ultimately to solar rejuvenation. Once
again, the deep future is not isolated from our present.
Before moving on to consider the detailed properties of the
Sun and how it might be rejuvenated, let’s briefly review a few of
the larger-scale, stepping-stone projects that our descendants may
eventually want to undertake.
Dyson Spheres
The Sun radiates a total of 3.85 x 1026 joules of electromagnetic
energy into space per second. At a distance of 1 astronomical
unit from the Sun, Earth receives a flux of 1,369 watts/m2 of
solar energy at the top of its atmosphere. Multiplied by Earth’s
cross sectional area of 1.3x108 km2, this translates to an energy
budget of about 1.8 x 1011 watts. Earth, therefore, taps only a
minuscule amount of the Sun’s energy output, something like
3.4 x 10−17 percent of the available energy, in fact. Indeed, small
fry. In what has become a classic paper, Freeman Dyson in
1960 proposed that the existence of extraterrestrial civilizations
might be revealed through the infrared emission from artificially
constructed worlds.26 Specifically, Dyson argued that a sufficiently
advanced civilization might utilize the material extracted from a
Jovian-type planet to engineer a sphere around its parent star. The
engineered shell in our Solar System might have a radius of about
1 AU and be several meters thick. In principle this shell could
support extended human colonies and large industrial complexes
capable of tapping a substantial fraction of the Sun’s energy output.
The heat signature of the shell (now commonly called a Dyson
sphere) would be that of a blackbody radiator with a temperature
of 200 to 300 K. Such structures would likely be bright
infrared sources in the 10-micron wavelength region of the electromagnetic
spectrum. Interestingly, and unlike the radio surveys
that search for intentional beacons or leaked transmissions from
extraterrestrial civilizations, Dyson-like spheres would be potentially
observable without the specific intent of the originating
civilization.
It’s a Matter of Time 53
Dyson spheres have indeed been searched for, but no finds
have been reported to date.27 The problem, however, with serendipitous
detection surveys (just as with the radio search for extraterrestrial
signals) is to know when a null result actually indicates
that there are no sources. At present we cannot say, one way or the
other, whether any galactic civilization has built Dyson sphere-like
structures. Clearly, engineering ability aside, there is a cost versus
potential payback tradeoff that any civilization has to consider
before embarking on the construction of a Dyson sphere. It may,
in fact, make more sense to engineer the smaller real estate within
a planetary system first. For example, within our Solar System, the
terraforming of Mars and Venus might represent the first steps in
this direction.
Terraforming
Vast amounts of literature have been published on terraforming—
literally, the engineering of an otherwise barren world into one
compatible with biological life. As a basic minimum requirement,
however, the process of terraforming should result in the presence
of liquid water on the surface of the planet being engineered.
Liquid water will only exist on the surface of a planet if the
average surface temperature (Figure 2.17) is above 273 K and if
the atmosphere provides a surface pressure in excess of 610 Pa
(which defines the so-called triple-point condition). In the case of
Mars and Venus, the would-be engineer faces a range of problems—
problems that, in fact, fall at the two extremes of the terraforming
spectrum. Mars is a frozen, geologically inactive world with a
tenuous atmosphere, while Venus is (possibly) geologically active,
has a massive atmosphere, and a surface temperature in excess of
700 K. For Mars, the atmospheric pressures and temperatures are
too low to allow liquid water to remain very long on its surface,
while on Venus the surface temperature is far too hot and the
atmospheric pressure is 90 times greater than that on Earth.
Mars is a relatively small planet, about half the size of Earth,
and its interior consequently cooled off rapidly, resulting in the
permanent shutdown of the majority of its volcanic networks.28
This latter effect is particularly important with respect to the
54 Rejuvenating the Sun and Avoiding Other Global Catastrophes
Orbital
eccentricity
Greenhouse
effect
Sun’s
luminosity
Axial
inclination
Orbital semimajor
axis
Atmosphere
Geological
activity
Surface Temperature
Stabilization
Figure 2.17. Factors determining the surface temperature of a planet. The
irradiance by the Sun will depend upon the planet’s orbital characteristics,
the planet’s geological activity, and the structure and composition of its
atmosphere. The variation in the orbital and axial tilt parameters (dashed
line box) are governed by the Croll-Milankovitch cycle in the case of Earth.6
heating of the planet’s surface, since it is the geochemical carbon
cycle that normally controls the greenhouse gas content of a
planet’s atmosphere. On Earth, for example, chemical weathering
and plate tectonic activity control the cycling of carbon
between gaseous and mineral phases. The volcanoes formed at
the plate subduction zones allow for the release of carbon dioxide
(CO2) into the atmosphere, thus increasing the greenhouse heating
effect. At the same time the CO2 in the atmosphere is chemically
incorporated into rocks such as limestone by the weathering
and erosion of silicate rocks, once again trapping the carbon in
a solid phase. It is the feedback loop between the temperaturesensitive
chemical weathering process (it runs more rapidly at
warmer temperatures) and the release of CO2 by volcanic activity
that has allowed Earth to maintain a near constant temperature
for the past several billions of years (Figure 2.18). On Mars, the
CO2 release mechanism via volcanic outgassing no longer works
and, consequently, its ancient atmosphere has literally frozen out.
The CO2 is now mostly trapped within its constituent rocks.
For the surface temperature of Mars to once again rise above
273 K the CO2 content of its present atmosphere would have to be
increased by a factor of about 200. This would raise the greenhouse
It’s a Matter of Time 55
Atmospheric CO2
Biotic up-take Oceans
(T)
Silicate rocks
Weathering
(T)
Sedimentation
Volcanic activity
Human activity Solar heating
Figure 2.18. The terrestrial carbon-dioxide cycle. The (T) term in the
Oceans and Weathering boxes indicate specific temperature sensitivity.
If, for example, the oceans are warm, then carbonate minerals are more
rapidly formed, and the rate at which the ocean takes up CO2 increases
concomitantly, reducing the atmospheric CO2 concentration. The reduced
atmospheric CO2 content then results in global cooling, which eventually
checks the ocean-warming effect. Likewise, an increase in atmospheric
temperature increases the rate at which silicate rocks can dissolve CO2 to
produce carbonate rocks. The hotter the temperature, the faster the ‘weathering’
effect on silicate rocks, and the greater the rate at which carbonates
are deposited into the ocean. The CO2 contained in the sedimentary
carbonate rocks is eventually returned to the atmosphere through volcanic
outgassing. Human activity is presently causing an increase in atmospheric
CO2 through, for example, the burning of fossil fuels and the clear-cutting
of ancient forests. The atmosphere is directly heated by the Sun, and
this heating effect will increase with time, eventually forcing – if left
unchecked – the loss of Earth’s oceans and the death of the biosphere.
gas content to a level that it could offset the low solar irradiance
at the orbit of Mars. Such an increase in the CO2 abundance would
also raise the surface pressure well above 610 Pa, allowing liquid
water to exist in a stable form at its surface. In principle such a
process could be initiated by manipulating (that is, increasing) the
brightness of Mars with the aid of space reflectors.29
Although the atmosphere of Mars needs to be warmed up
by the terraforming engineer, the atmosphere of Venus needs to
be cooled down. This process could in principle be achieved by
reducing the amount of CO2 in the Venusian atmosphere, either
by physically removing it or by increasing the CO2 uptake by the
planet’s crust. Radiation shields or reflectors would also need to be
installed in orbit around the planet to reduce the solar irradiance.30
56 Rejuvenating the Sun and Avoiding Other Global Catastrophes
Indeed, even if there were no greenhouse heating factor for Venus,
its surface temperature would be about 313 K (that is, an uncomfortably
high 40°C) based purely upon its orbital proximity to
the Sun.
Transforming Mars and Venus into habitats suitable for
nurturing biological life will only be achieved through the
investment of considerable money and through long-term cooperation
between all nations on Earth. It would also require the
development of the engineering skills needed to perform such
large-scale operations. Some of these skills will, with little doubt,
first be honed in the engineering of Earth’s atmosphere. Global
warming resulting from the generation and release of greenhouse
gases, wide-scale industrial pollution, along with poor land use
and agricultural management, are all conspiring to destroy the
natural feedback systems that have maintained Earth’s relatively
steady global temperature over the eons. Professor James Lovelock
has introduced the profoundly important Gaia concept to describe
the whole-Earth feedback system and, in his recent book The
Revenge of Gaia: Why Earth Is Fighting Back—and How We Can
Still Save Humanity (Penguin Books, London, 2006), he paints
a disturbing picture for our near-term future. Human activity is
changing Earth’s atmosphere and there are clear signs that traditional
weather patterns have changed and will continue to change
for the worse. Stronger and more numerous hurricanes (Figure 2.1)
and tropical storms, the melting of Arctic and Antarctic ice fields,
the retreat of glaciers around the world—all are being observed.31
Humanity’s short-term (over the next few hundred years) survival
may well depend upon the engineering skills that our children
develop in order to revitalize Gaia and Earth’s atmosphere.
One suggestion that has been made for solving the global
warming problem calls for the injection of sulphur dioxide (SO2)
into Earth’s atmosphere to increase its albedo, thereby causing
more solar energy to be reflected back into space.32 This, in fact,
mimics the cooling effects associated with the sulfur plumes
emitted by volcanoes.33 Another possible solution is to increase
the absorption of atmospheric CO2 in the oceans by seeding
surface waters with iron particles.34 Physicist Edward Teller and
co-workers35 have further suggested that the global warming
effects due to enhanced atmospheric CO2 concentrations could
It’s a Matter of Time 57
be counteracted by the deployment of an electrically conducting
mesh in Earth’s stratosphere (or in near-Earth orbit) with the aim of
reducing the direct solar heating of the atmosphere. Alternatively,
Jerome Pearson and co-workers36 have proposed that atmospheric
insolation could be increased by engineering an equatorial ring of
particles around Earth (similar to Saturn’s rings). Indeed, by making
the ring particles out of dismantled near-Earth asteroids (NEAs),
this proposal also solves some of the potential impact problems
faced by our descendants.37
In an ideal world the drastic manipulation of Earth’s
atmosphere would not be required. Gaia currently does it for free.
However, given the apparent lack of political leadership on industrial
emissions issues, such as those recommended in the Kyoto
Accord, it is not at all likely that the basic engineering skills
needed to terraform Mars and Venus will be developed on Earth
during the next 100 years. Although we already have the ability
(by default) to alter planetary atmospheres (that is, terraform) by
the careless exploitation of Earth’s resources, what is needed in the
future is the ability and skill to control the alterations.38 Lovelock
poignantly writes in his latest book, however, “The idea that
humans are yet intelligent enough to serve as stewards of Earth is
among the most hubristic ever.” Humanity has much to learn and
there is much work that needs to be done.
Space Structures
That our descendants will eventually exploit all of the useful and
available real estate within our Solar System seems inevitable.
Terraformed Mars and Venus, enclosed Moon and Mercury bases,
industrial processing plants located in the main asteroid belt and
on the Jovian Moons—all may come about. Part and parcel with
this development will be humans adaptating to living in space.
Skylab, Mir, and the International Space Station have already
supported small (non-reproducing) human colonies, but in the
future kilometric-scale space structures, such as those envisioned
by physicist Gerard O’Neill, will presumably be engineered.39
Indeed, the space structures considered by O’Neill are truly
grand in conception. Composed of 30-km long x 6-km diameter
58 Rejuvenating the Sun and Avoiding Other Global Catastrophes
rotating cylinders, the space platforms would have an Earth-like
atmosphere (and weather system) and potentially house, feed, and
employ 10,000 people. The challenge, as noted by O’Neill,39 “is to
bring the goal of space colonization into economic feasibility now,
and the key is to treat the region beyond Earth not as a void but
as a culture medium, rich in matter and energy.”
In addition to building space colonies and terraforming,
Paul Birch has suggested that rigid shells might eventually be
engineered around all of the planets (and some of their moons)
within the Solar System.40 Such supramundane planets would have
habitable atmospheres, natural gravity, and be made of material
extracted from asteroids, smaller planetary moons, and even the
Sun. Perhaps somewhat overly optimistic, Birch suggests that the
first supramundane habitats will be built around the planet Venus
by 2040!
Thinking Long-Term
The solution to Fermi’s Paradox that is being followed in this
book, as outlined in Chapter 1, is that advanced civilizations do
not engage in galactic colonization because they have found no
need to. This is not to suggest that stellar exploration won’t ever
take place. But a clear distinction between local exploration and
galactic colonization is drawn. Perhaps the first stellar voyages will
be initiated in order to divert nearby rogue stars that are heading
for an uncomfortably close encounter with the Solar System
[i.e., on a timescale of TCE (d = 5000 AU) ≈ 200 million years,
or TM31 ∼6 billion years]. Indeed, the possibility of controlling
the space motion of a star through the induction of mass loss
has been discussed by David Criswell41 (an operation described
in more detail in Chapter 5). Long-time advocate and researcher
into terraforming and space colonization Martyn Fogg42 has further
investigated the possibility of star exchange to ensure the longterm
habitability of the Solar System.
It does not seem unreasonable to assume (or at least adopt as a
working hypothesis) that all intelligent life forms that are capable
of molding and adapting their environments will expand to live
upon and commercially exploit all of the accessible components
It’s a Matter of Time 59
of their parent planetary system.43 In addition, it also does not
seem unreasonable to assume that all advanced civilization will
find some way of avoiding a Malthusian catastrophe44 in which
the demands of a population overreach the available food and raw
materials supply. Indeed, our descendants will have to solve this
problem if they are to have a long-term future. In addition, the longterm
survival of a civilization (in, say, a planetary system similar
to our own) will require the development of planetary defense
programs to shield against asteroid and comet impacts. Such
programs will, in fact, probably constitute the first truly global
space initiatives, literally (certainly hopefully) uniting humanity
through a common goal for survival against otherwise devastating
odds. By tackling the NEO impact problem, as a space-based
initiative, humanity will begin to develop the skills necessary to
tackle even larger-scale projects.
Again, as a working hypothesis, it does not seem unreasonable
to assume that all advanced extraterrestrial civilizations will
undertake terraforming projects, construct colonies and outposts
on planetary moons, and engineer O’Neill-style spacecraft to
produce their food. All such projects will anchor a civilization to
its planetary system. Its financial investments, its sense of self, and
indeed all that a civilization might physically require to exist will
be infused within its home planetary system. Such an investment
of time, lives, and money is hardly something that any civilization
will willingly abandon and move away from. Indeed, such a holistic
system is worth saving by any and all means that fall within the
realms of possible physics and engineering.
In terms of timescale the first astronomical danger that
our descendants will need to guard against is that from NEO
impacts (TNEO ∼105−6 years), then long-period comet impacts and
supernova irradiation (TLPC ∼TNVA ∼106−7 years). The next significant
timescale – indeed the ultimate timescale of concern in this
book – will be the exhaustion of hydrogen in the Sun’s core about 5
billion years from now. If Earth and our descendants are to survive
beyond the canonical TMS then two options present themselves:
either tame the natural gigantism of the aging Sun by rejuvenating
it, or leave the Solar System for good. Our descendants may be the
first to reach for the stars in a quest for a new Earth, leaving a
devastated and blistered Solar System (as described in Chapter 4)
60 Rejuvenating the Sun and Avoiding Other Global Catastrophes
far behind them. But, as shall be explained later in Chapter 5, it is
not unreasonable to think of extending the lifetime of the Sun, and
thereby the potential life-supporting viability of the Solar System,
by a factor of between 10 and 15, to TMS(engineered) ∼1011 years.
This engineered lifetime is still three orders of magnitude smaller
than the planet scattering encounter timescale by a nearby rogue
star [i.e., TCE (d = 5 AU) ∼2 x 1014 years], but it is surely better
than doing nothing. Either the Sun becomes a giant some 5 billion
years from now and in the process destroys all life within the Solar
System, or through the act of rejuvenation our descendants create
an additional 95-billion-year time span over which to enjoy the
nurturing light and warmth of a tamed Sun.
Notes and References
1. Strictly speaking there are small amounts of deuterium and lithium
produced during the time of primordial nucleosynthesis. Interestingly,
the fusion of deuterium to produce 3He (via the reaction: D + P
⇒ 3He + gamma ray) is of great importance during the early (pre-main
sequence) stage of star formation. Some of the consequences of stars
undergoing an early deuterium burning phase are discussed in M.
Beech and R. Mitalas, The formation of massive stars, Astrophysical
Journal Supplement, 95, 517–534 (1994).
2. A. S. Eddington, The Internal Constitution of the Stars, Cambridge
University Press, Cambridge (1926). Although the physical arguments
presented in this book are now somewhat dated, they remain a
brilliant example of lucid science writing.
3. K. Yau, P. Weissmann, and D. Yeomans, Meteorite falls in China
and some related human casualty events, Meteoritics, 29, 864–871
(1994) provide a summary of meteorite falls recorded in Chinese
chronicles between 700 B.C. to A.D. 1920. They report on one event
that occurred in Ch’ing-yang in 1490, where it is claimed that stones
fell like rain and many tens of thousands of people were killed. The
reported death toll seems very high, but this may possibly describe
a Tunguska-like impact (see Note 5 below) event where meteorites
actually survived to hit the ground (and people). As shown in the
author’s book Meteors and Meteorites: Origins and Observations
(Crowood Press, 2006, p. 26), the probability of a person who lives to
the ripe old age of 99 years being struck by a 1-gm meteorite is one
It’s a Matter of Time 61
chance in 1.5 billion. The other way of saying this is that one person
should be hit (not necessarily killed) worldwide every 33 years (or so).
4. Three NEA groups are generally recognized: the so-called Apollo,
Amor, and Aten asteroid groups. Both the Apollo and Amor groups
have their aphelia (greatest distance from the Sun) within the main
belt asteroid region. The Apollo NEA group has perihelia (closest
point to the Sun) inside of Earth’s orbit, while the Amors have
perihelia just outside of Earth’s orbit. The Aten NEA group have
perihelia well inside and aphelia just outside of Earth’s orbit.
5. It has been estimated that the equivalent of some 5 megatons of
TNT energy was liberated in the Tunguska explosion. In the comprehensively
titled research paper, Earth impact effects program: a
web-based computer program for calculating the regional environmental
consequences of a meteoroid impact, by Gareth Collins,
Jay Melosh and Robert Marcus [Meteoritics and Planetary Sciences,
40 (6), 817–840 (2005)], the equations that describe the environmental
effects of an impact (such as crater size, overpressure, thermal
radiation, blast wave propagation, and seismic effects, as well as
casualty levels) have been gathered together. The web site is accessible
at http://www.lpl.arizon.edu/impacteffects.
6. Not every mass-extinction event observable in the fossil record
can be associated with an impact. Some are, without a doubt,
geological in nature and relate, for example, to variations in sea-level,
anoxic ocean events, extensive volcanism, tectonic activity as well
as climate change [see the excellent book by Tony Hallam, Catastrophes
and Lesser Calamities—The Causes of Mass Extinctions,
Oxford University Press, Oxford, 2005]. Impact extinction events can
be identified by a significant increase in the iridium abundance in the
terrestrial rocks that indicate the extinction boundary. Comets and
asteroids are relatively rich in iridium (compared to Earth’s crustal
rocks) and, following a large impact event, the constituent iridium is
deposited over much of Earth’s surface, forming a thin iridium-rich
layer. Perhaps the most famous such iridium rich boundary layer is
that which was produced by the impact that resulted in the 180-km
diameter Chicxulub crater in the Yucatan peninsula at the end of
the Cretaceous Period (65 million years ago). On a smaller scale Jan
van Dam of Utrecht University in the Netherlands, and co-workers
[Long-period astronomical forcing of mammal turnover. Nature, 443,
687–691 (2006)] have found that there are ‘spikes’ in rodent extinction
record spaced at intervals of 2.4 million and 1.2 million years. These
intervals, van Dam argues, correspond to variations in Earth’s orbit
and shifts in the tilt of Earth’s rotation axis to its orbital plane. The
62 Rejuvenating the Sun and Avoiding Other Global Catastrophes
cycles of extinction have apparently been active for at least the past 22
million years. The Croll-Milankovitch cycle, which links long-term
climate change to variations in Earth’s orbit, is generally believed to
control the timing of Earth’s ice ages; events that invariably lead to
many species becoming extinct [for a description of the C-M cycle
see, Doug Macdougal, Frozen Earth: The Once and Future Story of
Ice Ages. University of California Press, Berkeley (2006), pp. 65–88].
7. Simply blasting an asteroid with a volley of, say, nuclear missiles may
actually increase the impact devastation. Instead of one object hitting
Earth, a series of explosions might result in multiple fragments
striking Earth over a much larger area than that corresponding to a
single-body impact. This is especially a problem given the current
understanding that many asteroids are ‘rubble piles’ of loosely bound
components. A good general review of the observational status and
future spacecraft missions to comets and asteroids can be found in
the article by A. C. Levasseur-Regourd, E. Hadamcik, and J. Lasue,
Interior structure and surface properties of NEOs: what is known and
what should be understood to mitigate potential impacts. Advances
in Space Science, 37, 161–168 (2006).
8. A space tractor concept is discussed in some detail by E. Lu and
S. Love in their article, Gravitational tractor for towing asteroids,
Nature, 438, 177–178 (2005). The European Space Agency (ESA) is
currently considering a spacecraft mission (the Don Quijote mission)
to study the internal structure of an asteroid (presently unspecified).
The mission will also attempt to change the orbit and rotation
state of the target asteroid through high-speed surface impacts. An
overview of the ESA mission can be found in a 2006 paper presented
by Ian Carnelli, Andres Galvez and Dario Izzo, Don Quijote: a NEO
deflection precursor mission, to a NASA workshop dedicated to
near-Earth object detection, characterization, and threat mitigation—
available at: http://www.esa.int/gsp/ACT/doc/ACT-RPR-4200-ICNASANEOWS-DonQuijote.pdf.
9. The Oort Cloud, named after the Dutch astronomer Jan Oort
(1900–1992) who first suggested its existence, delineates the outer
edge of the Solar System. It is literally the three-dimensional
boundary beyond which the Sun’s gravitational influence no longer
holds sway over the rest of the galaxy. It is a dynamical boundary
that expands and contracts according to the time dependent distribution
of matter in the solar neighborhood. Of order 1012 to 1013
cometary nuclei are believed to delineate the Oort Cloud region
stretching between ∼10,000 to ∼50,000 AU from the Sun. Cometary
nuclei within the Oort Cloud region are continually ejected into
It’s a Matter of Time 63
interstellar space and into the inner Solar System as a result of gravitational
perturbations from close passing stars, massive molecular
clouds embedded within the interstellar medium, and during spiral
arm region crossings.
10. J. Garcia-Sanchez et al., Stellar encounters with the Solar System,
Astronomy and Astrophysics, 379, 634–659, (2001). In addition, R.
A. Matthews [The close approach of stars in the solar neighborhood.
Q. J. R. Astr. Soc. 35, 1–9. (1994)] finds that Proxima Centauri will
not be at its closest approach to the Sun for another 26,700 years. At
that time it will be 0.941 pc away.
11. See, for example, the recent research paper by W. M. Napier, Evidence
for cometary bombardment episodes, Monthly Notices of the Royal
Astronomical Society, 366, 977–982 (2006). A more general account
of cometary impacts is given in the very readable book by Duncan
Steel, Target Earth: The Search for Rogue Asteroids and Doomsday
Comets That Threaten Our Planet [Readers Digest Association Inc.,
New York (2000)].
12. D. H. Clark, W. H., McCrea, and F. R. Stephenson [Frequency of
nearby supernovae and climatic and biological catastrophes, Nature,
265, 318–319 (1977)] investigated the possibility of Earth encountering
the radiation burst or blast wave associated with a Type II
supernova (see Note 13 below) during galactic spiral arm crossings.
They concluded that such an ‘encounter’ might take place once every
108 years, assuming a Type II SN rate of one every 100 years. The
current, revised SN rate is actually some three to four times higher
than that adopted by Clark et al., with one SN occurring every 25 to
30 years. This brings the typical time between close SN passages to
a value of order 3 x 107 years.
13. Two main categories of supernova are generally recognized. Type I
novae are produced through the accretion-driven collapse of a white
dwarf star in a binary system. A Type II supernova is produced, on
the other hand, by the collapse of the iron-rich core of a massive
(Minitial > 8 M) star. The supernova types are generally distinguished
observationally according to their spectra, brightness variations with
time, and galactic location – Type II supernova, for example, only
occur in spiral arm regions where massive stars are actively forming.
14. A pulsar is a rapidly spinning neutron star, and it is believed that
such objects form during the supernova disruption of massive stars
(i.e., in Type II SN). Spinning at a rate of 4.2 revolutions per second,
the Geminga pulsar is believed to be about 300,000 years old. While
Firestone and co-authors have assembled an impressive quantity of
data to support their thesis, it should be pointed out that much of
64 Rejuvenating the Sun and Avoiding Other Global Catastrophes
what they claim is controversial and almost all their data has multiple
possible interpretations not necessarily requiring external (that is
astronomical) driving influences.
15. Optical astronomers measure stellar brightness in terms of apparent
magnitude. The apparent magnitude of a star is based upon the
logarithm of the energy flux received from the star at the surface of
Earth. The magnitude scale works in such a way that the brighter an
object is, the more negative its magnitude. The faintest star visible
to the human eye has an apparent magnitude of about +6, while
the Hubble Space Telescope can record stars and galaxies as faint as
magnitude +24. In contrast, the full Moon has an apparent magnitude
of –12, and the apparent magnitude of the Sun is –27.
16. Collapsars are believed to be the final stage of massive star evolution.
The progenitors are the highly distinctive Wolf-Rayet stars, first
studied by the French astronomers Charles Wolf and Georges Rayet
in the early 1900s. These stars are distinguished by showing emission
lines in their spectra (virtually all other stars show only absorption
lines) and they undergo extreme mass loss. By way of comparison,
the Sun loses mass via a stellar wind at a rate of ∼10−14 M/yr,
while a Wolf-Rayet star loses mass at a rate of 10−4 to 10−5 M/yr.
The Wolf-Rayet stars are believed to represent the end stages of the
evolution of stars with initial masses greater than ∼30 M, and the
very final hypernova collapse is believed to produce a black hole
rather than a neutron star. Working with Professor Romas Mitalas at
the University of Western Ontario, this author studied the effects of
mass loss upon massive stars [Effect of mass loss and overshooting
on the width of the main sequence of massive stars, The Astrophysical
Journal, 352, 291–299 (1990)] and found that the mass loss
strongly reduces the observed luminosity but increases the surface
temperature—a result that actually proves useful with respect to
star-engineering. An alternative model for the cause of hypernova
and GRB generation envisions the coalescence of two neutron stars
previously orbiting each other in a binary system. Such mergers will
again probably result in the formation of a black hole. Michael Shara
discusses the possible consequences of star collisions and mergers in
When stars collide, Scientific American, 287 (5), 46–51 (2002).
17. Brian Thomas et al., Terrestrial ozone depletion due to a Milky Way
Gamma-Ray burst. Astrophysical Journal, 622, L153–L156 (2006).
The authors conclude that a 10-second burst of gamma rays results
in a globally averaged ozone depletion of order 35 percent (with some
latitudes seeing a 55 percent reduction). A 50 percent decrease in
It’s a Matter of Time 65
the ozone column translates to a three-fold increase in the UVB flux
received at Earth’s surface.
18. J. Scalo and J. C. Wheeler [Astrophysical and astrobiological implications
of gamma-ray burst properties. Astrophysical Journal, 566,
723–737 (2002)] have discussed the potential biological consequences
of Earth encountering a strong burst of gamma rays. They point out
that very little gamma-ray radiation actually reaches Earth’s surface
and, indeed, a column of water just a few tens of centimeters thick
would fully protect any marine organisms from a dangerous dose of
radiation. Shielding by small amounts of rock and soil would protect
other organisms.
19. Krzysztof Stanek et al, Protecting the Milky Way: metals keep the
GRBs away. Paper submitted to the Astrophysical Journal in April
2006.
20. J. Annis. An astrophysical explanation for the Great Silence. Journal
of the British Interplanetary Society. 52, 19–22 (1999).
21. Our Milky Way Galaxy is part of the so-called Local Group (LG) of
galaxies. The LG contains at least 36 members within a region that
stretches about 1 Mpc across its longest dimension. Most of the LG
members are dwarf elliptical and irregular galaxies with masses of
between one thousandth to one millionth that of the Milky Way.
The Andromeda Galaxy (M31) is comparable in size and mass to our
galaxy. Detailed studies of the central region of M31 reveal a double
nucleus indicative of a past ‘collision’ and ‘disruption’ of a smaller
LG member by Andromeda.
22. In order to determine the true spatial velocity and direction of motion
of M31 a measure of both its line-of-sight (radial) velocity as well as
its velocity tangential to the line-of-sight is required. Only the radial
component is known and, consequently, it is not clear under what
circumstances M31 will actually ‘collide’ with the Milky Way.
23. The burst of star formation triggered by the (potential) merger of
M31 with the Milky Way will certainly produce large numbers of
massive stars (stars with initial masses greater than 8 M). These
massive stars have lifetimes of just a few million years (as indicated
by Equation 2.1), and they eventually undergo core-collapse as Type
II supernovae. Type II supernovae are typically less energetic than
Type I supernovae (see Note 13 above), but the starburst conditions
should result in a far greater number of such events occurring in our
galaxy than observed at the present.
24. A black dwarf is the name given to the cold (near zero degrees Kelvin)
remnant of a fully cooled-off white dwarf. Such objects remain stable
against collapse due to the electron degeneracy of their interior, the
66 Rejuvenating the Sun and Avoiding Other Global Catastrophes
pressure support for such gases being independent of temperature.
The cooling time required to produce a black dwarf, however, is
longer than the present age of the universe.
25. J. Scalo and J. C. Wheeler (see Note 18) point out that any life
forms that might have evolved on Mars, even with its higher
density atmosphere of the past, would not have fared so well as
terrestrial organisms. They estimate that since the formation of
the Solar System some 1,000 biologically significant GRB-related
events have possibly occurred, and that surface life on Mars (i.e.,
eukaryotic bacteria) has probably been ‘sterilized’ many times
over. This being said, Professor Alexander Pavlov (University of
Arizona) and co-workers have recently argued [Was Earth ever
infected by Martian Biota? Clues from radioresistant bacteria. Astrobiology,
6 (6), 911–918 (2006)] that the radiation tolerance of
some terrestrial bacteria (e.i., Deinococcus radiodurans) is more
likely to have evolved on Mars than Earth. The bacteria, having
evolved on early Mars, were later transported to Earth via Martian
meteorites.
26. Freeman Dyson, Search for artificial stellar sources of infrared
radiation, Science, 131, 1667 (1960). A shell of radius 1 AU has a
surface area of order 2 x (4
) x (1.496 x 108)
2 = 5.6 x 1017 square
kilometers. The factor of 2 accounts for both the inner and outer
surfaces of the shell. Compared to Earth’s present land area, this
corresponds to a 4 billion-fold increase in potential lebensraum. This
being said, the shell of a Dyson sphere cannot, for dynamical stability
reasons, be solid, but must consist of a ‘swarm’ of moonlets. Richard
Carrigan, Jr. [Searching for Dyson spheres with Planck spectrum fits
to IRAS, International Astronomical Congress paper: IAC-04-IAA-
1.1.1.06] provides an extensive review of observational searches. In
a Phase I study of some 4,400 infrared point sources detected with
IRAS (the Infra-Red Astronomical Satellite flown in 1983), Carrigan
finds that about 1 in 600 have flux characteristics consistent with the
radiation expected from a pure blackbody radiator with a temperature
in the range 150 < T(K) < 500 [see, http://home.fnal.gov/∼carrigan/
Infrared_Astronomy/Fermilab_search.htm].
27. It is not absolutely clear that Mars is volcanically dead. The crystallization
ages of a number of Martian basaltic meteorites are as young
as 165 million years. This corresponds to about 4 percent of the
planet’s age. As pointed out by William Hartmann [A Traveler’s
Guide to Mars, Workman Pub. NY (2003)], it is unlikely that Mars
has been volcanically active for the past 96 percent of its history and
then suddenly, in recent times, all eruptions stopped.
It’s a Matter of Time 67
28. Solar reflectors can be used to either enhance the heating of a planet
by reflecting additional sunlight into its atmosphere, or they can act
as cooling shades by directly blocking the incoming sunlight. It has
been proposed that the effects of CO2 global warming on Earth could
be offset by placing a 2,000-km diameter parasol between Earth and
the Sun [see, for example, Martin Hoffert and co-workers, Advanced
technology paths to global climate stability: energy for a greenhouse
planet, Science, 298, 981–987 (2002)].
29. The U.S. National Academy of Sciences published a policy report in
1992 in which it was suggested that the effects of CO2 warming could
be counteracted by installing 55,000 mirrors – each of surface area
100 km2 – in random inclination, near-Earth orbits. The effect of the
mirrors would be to reduce the amount of solar energy intercepting
Earth’s atmosphere. A similar concept could be used to reduce the
direct solar heating of Venus.
30. The U.S. Climate Change Science Program (www.climatescience.
gov/) released a report in April 2006 concluding that over the past
50 years there is clear and uncontroversial evidence for human
activity-induced climate change. Likewise, Gabrielle Walker [The
tipping point of the iceberg, Nature, 441, 802–805 (2006)] describes in
sobering detail the distinct climate changes being recorded in Earth’s
polar regions. It is perhaps a little ironic that some of the effects of
human-induced global warming have been masked by anthropogenic
aerosol production, which tends to actually cool the planet [see, for
example, Jim Coakley’s commentary article, Reflections on aerosol
cooling. Nature, 438, 1091–1092, 2005]. The irony of this situation
is that as some (indeed a sad, small few) nations begin to curb and
set new limits on the industrial emission of aerosols, the cooling
effect that the aerosols cause will be lost—a process that will actually
enhance global warming.
31. F. J. Dyson and G. Marland, Technical fixes for the climatic effects
of CO2. Workshop on the global effects of carbon dioxide from fossil
fuels. DOE report CONF-770385 (1979).
32. The 1991 eruption of Mt. Pinatubo induced a global mean temperature
cooling of ∼0.5 K as a result of the release of sunlight-scattering
SO2-based particles. The dispersal of SO2 into the atmosphere would,
in principle, increase Earth’s insolation by increasing the amount of
solar radiation that is reflected back into space [see, for example, the
atmospheric model described by K. Taylor and J. Pinner, Response
of the climate system to atmospheric aerosols and greenhouse gases,
Nature, 369, 734–737 (1994)].
68 Rejuvenating the Sun and Avoiding Other Global Catastrophes
33. J. H. Martin and co-workers [Testing the iron hypothesis in
ecosystems of the equatorial Pacific Ocean, Nature, 371, 123–129
(1994)] seeded a 64-square-km area of the open Pacific with 15,600
liters of iron solution, and found a dramatic increase in the biomass
of phytoplankton. It is the increased photosynthetic activity of the
enhanced biomass that absorbs the atmospheric CO2.
34. E. Teller, L. Wood, and R. Hyde [Global warming and ice ages: I
Prospects for physics-based modulation of global change. 22nd International
Seminar on planetary emergencies, Erice (Sicily), Italy (1997)]
argue that the predicted increase in global warming due to the
enhanced CO2 abundance can be offset by a 1 percent reduction in
the amount of solar radiation reaching Earth. They also argue that the
onset of ice ages could be canceled by the appropriate manipulation
of atmospheric CO2 concentrations and the amount of solar energy
reaching Earth.
35. James Peason and co-workers [Earth rings for planetary environment
control, 53rd International Astronautical Congress, 2002 (IAF-02-
U.1.01)] modeled various ring systems extending out to distances
between 1.2 and 1.8 Earth radii. The ring system casts a variable
width and location shadow upon Earth’s surface, and it is this shading
effect that results in atmospheric cooling.
36. O’Leary and co-workers [Retrieval of Asteroidal Materials, Space
Resources and Space Settlements, NASA SP-428, 173–189 (1979)]
studied the methods by which material from the Moon and Earthapproaching
asteroids might be mined for chemical processing and
the fabrication of large space structures. Indeed, O’Leary et al.,
conclude in their report that “immediate studies on asteroidalretrieval
mission opportunities” be initiated, with “technology
readiness for asteroid retrieval by the mid-1980s”. Twenty years late
perhaps, in 2005, the first direct sampling of an asteroid surface was
completed (hopefully) by the Japanese Space Agency’s Hyabusa spacecraft.
The term ‘hopefully’ is employed since it is currently unclear
if material was actually collected. We will only know for sure when
the spacecraft returns to Earth in June 2010.
37. David Keith [Geoengineering, Nature, 409, 420 (2001)] discusses how
the damaging side effects of pollution, derived from human industrial
activity, might be redressed by planetary scale environmental
engineering. He concludes: “I judge it likely that this century will
see serious debate about—and perhaps implementation of—deliberate
planetary-scale engineering.”
38. O’Neill outlines his plans in The Colonization of Space [Physics
Today, September (1974)]. His study concludes that, given the
It’s a Matter of Time 69
appropriate funding and political commitment, space colonies could
be constructed “without robbing or harming anyone and without
polluting anything.” He also argues that virtually all industry could
be moved away from Earth and into space within 100 years. Writing
these notes 32 years from the time of O’Neill’s original publication,
it is clear that his ideas have not, as of yet, found the required imagination,
funding and political will for action from either industry or
national governments.
39. P. Birch, Supramundane planets, J. Brit. Interplan. Soc. 44, 169–182
(1991). The name supramundane is derived from the Latin supra
(above) and mundus (world). The shell of a supramundane planet
is not physically anchored to the surface of its central body and,
consequently, the Jovian planets could be utilized as regions of
colonization. A suprajupiter shell would have a surface area some
316 times that of Earth and potentially house, feed, and support up
to 300 billion people.
40. D. R. Criswell [Solar system industrialization: implications for interstellar
migrations, Interstellar Migration and the Human Experience,
R. Finney and E. Jones (eds.) University of California Press, Berkeley
(1985). pp. 50–87] has described human industrial development in
terms of a triangular pyramid. The base of the pyramid is labeled
according to ‘skill,’ ‘matter,’ and ‘energy.’ As human civilization has
learned to control and use these three ‘bases,’ so it has produced
the tip of the pyramid that Criswell calls “cumulative controlled
connectives,” or CCCs. It is the future growth of CCCs that will
determine human destiny and, increasingly, the ‘matter’ and ‘energy’
bases markers of the industrial growth pyramid that will ‘demand’
the extraction and exploitation of the raw materials residing within
the greater Solar System beyond Earth.
41. M. J. Fogg [Solar exchange as a measure of ensuring the long term
habitability of Earth, Speculations in Science and Technology. 12
(2), 153–157 (1988)] builds upon the idea of star lifting described by
Criswell (see Note 40 above and the detailed discussion in Chapter 5).
42. Philosopher Nick Boston [Astronomical waste: The opportunity cost
of delayed technological development. Utilitas, 15, 308–318 (2003)]
takes this argument even further and argues that we (indeed, any
galactic civilization) should strive to embrace and utilize the entire
gamut of resources available within the Milky Way galaxy (and the
Virgo super cluster beyond). That is, he argues humanity should
strive to become a Kardashev Type III civilization (see Chapter 1,
and especially Note 18 in that chapter). Indeed, he opens his paper
in the following way: “As I write these words, suns are illuminating
70 Rejuvenating the Sun and Avoiding Other Global Catastrophes
and heating empty rooms, unused energy is being flushed down
black holes, and our great common endowment of negentropy is
being irreversibly degraded into entropy on a cosmic scale. These
are resources that an advanced civilization could have used to create
value structures, such as sentient beings living worthwhile lives.”
43. The idea of a dynamic relationship between human population levels
and the land carrying capacity was first articulated by Thomas
Malthus (1766–1834) in 1798. The Malthusian catastrophe is often
presented in the form that it is only such factors as epidemics and
wars that hold human population levels in check. That is, if left
unchecked the population level would simply increase to a point
whereby the demand for food would outstrip all possible supply.
Presumably (or at least hopefully) our descendants will solve the
problem of epidemics and become more peaceful. If one follows the
arguments presented by Criswell (Reference 40), then a Malthusian
meltdown can be avoided by taking agriculture into space, indeed by
adopting, for example, the space colonization strategies suggested by
O’Neill (see Note 38).
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