[tt] Moving the Earth: a planetary survival guide

[Eugen Leitl]

http://space.newscientist.com/article/dn14983-moving-the-earth-a-planetary-survival-guide.html


Moving the Earth: a planetary survival guide

* 22:10 20 October 2008 * NewScientist.com news service * Jeff Hecht

When the Sun expands into a red giant several billion years from now, the
Earth will be dragged into its atmosphere (Illustration: Mark Garlick/HELAS)

Evacuating all 6.7 billion Earthlings would take the equivalent of a billion
space shuttle launches (Image: NASA)

Humans cannot live on Mars without life support. So if large numbers of
people were to take refuge there, the Red Planet would have to be made more
like Earth, or 'terraformed' (Illustration: NASA)

The Kuiper belt is a ring of icy objects beyond Neptune (Illustration: NASA)

The clock is ticking inexorably toward doomsday even if we don't kill
ourselves by poisoning the environment or overheating the planet. You see,
there's a little problem with the Sun.

The Sun is slowly getting warmer as it burns the hydrogen in its core. In
about 5 billion years, the Sun will begin evolving into a bloated red giant.
Its outer gas shell will swell up, engulfing the Earth by the time it reaches
its peak size and brightness 7 billion years from now.

But long before that, in 1.1 billion years, the Sun will grow 11% brighter,
raising average terrestrial temperatures to around 50 °C (120 °F). That will
warm the oceans so much that they evaporate without boiling, like a pan of
water left on a sunny kitchen counter.

Plants and animals will have a very tough time adapting to that hothouse,
although some single-celled organisms called Archaea might survive. But only
for a while. Once the water vapour is in the atmosphere, ultraviolet light
from the Sun will split the water molecules, and the hydrogen needed to build
living cells will slowly leak into space. If our descendants – or other
intelligent life-forms that follow us – want to survive, they'll have to
migrate elsewhere. But where and how?

One approach would be to fire up rockets and move to another planet. Back in
1930, British science-fiction author Olaf Stapledon wrote about a future
where our descendants fled to Venus, and later Neptune, when the Earth became
uninhabitable. Eminent scientists such as Stephen Hawking have endorsed the
idea of establishing colonies on the Moon or other planets so humanity would
survive any disaster that wiped out life on Earth.

Yet evacuating all 6.7 billion Earthlings would take the equivalent of a
billion space shuttle launches. Even if we could launch 1000 shuttles a day,
it would still take 2700 years to move the whole planet's population.

Then there's the matter of taking care of people once they reached their new
home. Moving to any other planet would require "terraforming" it to provide
food, water and oxygen to support colonists. Why not bring our own planet
along with the resources we would need?

Tiny change

Elementary physics tells us that we actually can move the planets. Launching
a rocket into space pushes the Earth a bit in the opposite direction, like
the recoil from a gun.

Science-fiction author and trained physicist Stanley Schmidt exploited this
fact in his novel The Sins of the Fathers, in which aliens built giant rocket
engines at the South Pole to move the Earth. (Read about other sci-fi novels
and films that have tackled the problem of moving worlds.)

In real life, however, the Earth is so massive that a rocket would have
little effect on its motion. Launching a billion 10-tonne rockets in exactly
the same direction would change the Earth's velocity by just 20 nanometres
per second – peanuts compared to the planet's current speed of 30 kilometres
per second.

A few astronomers have tackled the problem of moving planets, but not for
dealing with emergencies on human time scales. They're actually devising
thought experiments to understand the dynamics of planetary systems, says
Greg Laughlin of the University of California, Santa Cruz. So processes that
occur on geologic time scales work perfectly well.  Moving out

Planetary dynamics seemed simple and orderly when we knew only our own solar
system, but that changed with the discovery of "hot Jupiters" on tight orbits
around other stars. The planets couldn't have formed in the scorching regions
where they orbit – there was not enough gas and dust there to amass such
giant worlds. Instead, they must have migrated there from more distant
birthplaces.

To understand how planetary systems might rearrange themselves, Laughlin, his
Santa Cruz colleague Don Korycansky, and University of Michigan astronomer
Fred Adams posed themselves the problem of how to move the Earth so the
warming Sun didn't cook the planet.

For the purposes of their calculation, the three chose the Earth's final
destination as an orbit 1.5 times its present distance from the Sun, at what
is now the orbit of Mars. In 6.3 billion years, when the Sun is in its
red-giant stage and is 2.2 times brighter than today, a planet at that
distance will receive about as much sunlight as the Earth receives today.

Moving the Earth to a circular orbit at that distance requires increasing its
orbital energy by about 30%. That would be possible, they say, by changing
the orbits of icy bodies in the distant solar system so they would pass close
to the Earth, transferring some of their orbital energy to the planet.

The objects lie in a ring of icy bodies beyond Neptune called the Kuiper belt
and in an even more distant shell of comets called the Oort cloud. Because
they are far from the Sun, the objects have relatively low orbital energy, so
they could be nudged using methods being developed to deflect asteroids away
from the Earth.

These range from the gentle pull of gravity tugs – spacecraft that fly near
the object and gravitationally pull them off course – to the stronger push of
mass drivers, which dig into and spew out pieces of the icy body, pushing it
in the opposite direction.

Their orbits could then be fine-tuned in the inner solar system using jets of
ices vaporised from their surfaces by equipment sent there. Nobody's thinking
about deploying a future Bruce Willis with a rocket-load of nukes to do the
job. "You need very fine-grained control, which a nuclear weapon certainly
would not produce!" says Laughlin.  Sterilised biosphere

About a million such close passes would do the trick. If we spaced them
evenly, that would mean about one close pass every 1000 to 6000 years,
depending on whether we wanted to reach the orbit of Mars by the time the Sun
started to vaporise the ocean, or when it hit its red-giant phase. Luckily,
the objects could be re-used if they looped around both Jupiter and the
Earth, taking energy from the giant planet and transferring it to Earth.

It would be a big job, and would take plenty of patience to move the Earth
consistently outwards as the Sun grew warmer. It also carries a significant
risk because the objects would have to pass just 10,000 kilometres above the
Earth's surface.

The objects would be much more massive than the asteroid that killed the
dinosaurs, so one little "oops" could be devastating. Laughlin and colleagues
take that very seriously, concluding their paper with the warning: "The
collision of a 100-km diameter object with the Earth at cosmic velocity would
sterilise the biosphere most effectively, at least to the level of bacteria.
This danger cannot be overemphasised." Push from the Sun

That danger could be avoided by using a giant solar sail, says Colin McInnes,
a mechanical engineer at the University of Strathclyde.

Solar sails are thin, mirror-like films that are propelled by the weak
pressure of the sunlight that falls on them. McInnes's idea is to put a
free-floating solar sail at a point near the Earth where the pressure of
solar radiation essentially balances the Earth's gravitational pull.

His analysis shows that the reflection of sunlight from the sail will pull
the Earth outwards along with the sail – in physical terms, increasing the
Earth's orbital energy and accelerating the centre of mass of the system
outwards, away from the Sun.

McInnes calculates that moving the Earth outwards to keep pace with the Sun's
warming would require a disc-shaped sail 19.2 times the Earth's diameter. It
would have to be tilted at an angle of 35° to the line towards the Sun, and
stationed at about five times the Moon's distance from the Earth.

He envisions building it in space by refining the raw materials in a
9-km-wide metal-rich asteroid. Nickel and iron from the asteroid would be
made into an 8-micron-thick film for the sail.

Thrown into chaos

The sail would be complex as well as large; it would need active control to
maintain the sail's proper shape, particularly in the face of perturbations
by the Moon's gravity. But McInnes says it would require moving 10,000 times
less mass than slinging objects from the Kuiper belt past Earth.

Geoffrey Landis, a science fiction author and NASA scientist, says the
concept is sound. "It looks like the physics is right, but of course there's
no technology in existence or currently proposed to make a solar sail 20
times the diameter of the Earth ¡V at the moment, that's science fiction."

McInnes admits that even he doesn't take the idea too seriously: "It's a
Friday afternoon problem."

But despite the practical difficulties of these scenarios, computer
simulations by Laughlin also point out a real danger of playing with
planetary orbits.

Planetary orbits are shaped by the gravitational pulls of their neighbours,
so moving the Earth would change the orbits of the other inner planets in
unpredictable and potentially dangerous ways.

If the move destabilised Mercury, the entire inner solar system might be
thrown into a chaotic mode "that is vastly harder and possibly impossible to
control", Laughlin says. That may be the best argument for leaving the
planets alone unless we have no alternatives.