ONE day there will be no more white Christmases. Ever. Not even in Lapland.
The reason has nothing to do with the greenhouse gases we are pumping into the
atmosphere—it’s just that all stars grow steadily brighter as they age, so
as our Sun heats up, it gradually turns up the Earth’s thermostat. In about a
billion years, it will cook any complex life on Earth, in three billion it will
evaporate the oceans, and eventually the Sun will bloat into a red giant and
melt the Earth’s surface.
But don’t panic. Earlier this year, three American astronomers worked out a
way to keep our planet cool. Fred Adams of the University of Michigan in Ann
Arbor and Greg Laughlin of the University of California at Santa Cruz had been
studying how the orbits of planets change over extremely long time frames. They
found that over billions of years there’s a remote chance that the gravity of a
passing star could dislodge the Earth from its orbit. That led them to wonder if
they could use the gravity of other passing bodies to move the Earth away from
the warming Sun.
NASA has already tweaked the Earth’s orbit a few times. For example, in 1999
the Cassini spacecraft used the Earth’s gravity to help send it on its journey
to Saturn. In the process Cassini gave the Earth a minuscule pull. But moving
our planet by a significant amount would require close encounters with much
bigger objects.
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So Don Korycansky, another astronomer at Santa Cruz, suggested raiding the
Kuiper Belt. This is a cloud of up to 100,000 giant icy bodies—100
kilometres across or larger—that circle the Sun beyond the orbit of
Neptune. These bodies are thought to occasionally plunge into the inner Solar
System, becoming comets that light up the night sky on Earth.
The idea would be to engineer some special comets: to somehow slow down some
of these Kuiper Belt objects, so that the Sun’s gravity would pull them inwards
in a long, elliptical orbit towards the Earth, skimming by in just the right way
to enlarge our own orbit. Of course, each one would need quite a kick to start
the process, so a pretty advanced space industry would be needed. One option
would be to attach nuclear fusion engines to the comet, fuelled by hydrogen
extracted from its ice.
The three astronomers made a series of rough calculations. They set their
goal as very gradually moving the Earth, so that after 6.3 billion years it
would be 50 per cent further from the Sun. By that time, the Sun would be 2.2
times brighter than today, and so the Earth would receive the same flux of solar
energy as it does now.
A single close encounter with a 100-kilometre object could transfer about a
millionth of the total energy needed, so the whole task would take about a
million close encounters, say one every 6000 years. Fortunately, you don’t have
to send in a million fresh comets. With careful orbital engineering, each one
could loop around again and again, via the two most massive planets, Jupiter and
Saturn. Each time, the body would steal some orbital energy from the gas giants
and give it to the Earth. Jupiter and Saturn would move inwards a little,
although not nearly as far as the Earth would move out. It would be like a vast
and stately game of snowballs, with Jupiter and Saturn on one side and the Earth
on the other—only the aim of this game would be to just miss.
As sunlight gradually evaporates a comet’s ice, each snowball’s orbit would
have to be adjusted from time to time; and the ice wouldn’t last forever, so
fresh snowballs would have to be supplied. But there are plenty out there.
Of course, a disaster that won’t happen for another billion years is too far
off to worry about, and the solution is in the realms of fantasy—albeit
highly educated fantasy. Adams, Laughlin and Korycansky were merely playing with
ideas. “We weren’t advocating policy,” says Korycansky.
So mere amazement doesn’t adequately describe the astronomers’ reaction when
an article in The Observer took them seriously, and suggested that
moving the Earth could combat global warming caused by human production of
greenhouse gases. Laughlin caught a lot of flak because the article quoted him.
Then dismay took hold when a few commentators implied that the idea was a real
NASA plan. “Fortunately, the whole thing died down after that,” says
Korycansky.
The three astronomers list many reasons why their original scheme should not
be taken too seriously. Their calculations were based on a greatly simplified
version of the Solar System. That’s fine for a theoretical study, but real
planetary engineering would be a far more complex—and vastly
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Complicated interactions among the planets and asteroids produce
hard-to-predict side effects in the Solar System. Isaac Newton himself realised
that the planets’ gravitational effects on each other’s orbits are too complex
to calculate. “He thought that their evenly spaced orbits were evidence of a
benign Creator,” says Laughlin. Recent research suggests that moving the Earth
might destabilise the orbits of Venus and Mercury—they could end up
anywhere. Asteroid orbits are already chaotic on much shorter timescales, and
moving the planets would add to the disruption. The Moon would have to be moved
separately to keep up with the Earth. Mars is another problem, because it
already occupies the orbit proposed for the Earth six billion years in the
future. Maybe it would be simpler to up sticks and move to Mars.
More obviously, hurling huge comets within several thousand kilometres of the
Earth leaves a very thin margin of error, says Korycansky. A mere 10-kilometre
asteroid is blamed for wiping out the dinosaurs. A 100-kilometre object might
pack a big enough wallop to sterilise the Earth.
And the billion-year project raises problems that go beyond planetary orbits.
“You’d have to have automatic machinery, or a civilisation capable of making
plans on a geological timescale,” says Korycansky. “Imagining a civilisation
that lives for 100 million years is the most far-out part of the whole thing.”