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?
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.
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.
That mishap 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.
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 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.