My 8-year-old son is very worried about Earth being destroyed by the sun when it becomes a red giant billions of years from now. I have tried to comfort him by suggesting we could shift Earth’s orbit by hurling asteroids towards the planet on trajectories that will gradually move it away from the sun. Is this feasible?
• Maybe your best hope is that your son forgets he asked the question, because humans may well be extinct by the time the sun swells into a red giant. Although some mammalian species have been known to last as long as 10 million years, their average length of existence is about 1 million years.
But setting this aside, using “gravity assists” would be a plausible strategy to help shift Earth. Rockets on asteroids or comets could push those objects along trajectories that would pull the planet into a higher orbit. Yet given that Earth might need to increase the distance of its orbit by 15 per cent to avoid spiralling into the red giant that our sun will become, many tens of thousands of these assists would be required. That would increase the possibility of an asteroid impact and a mass extinction, if not the end of life on Earth.
An alternative is to construct a “gravity tug”. This could take the form of a solar sail attached to a huge mass. Sunlight would push away the solar sail, with Earth then gravitationally tugged by the mass.
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However, either solution would simply replace one problem with another, because as the sun evolved into a red giant and then a white dwarf, the habitable zone would first expand but then contract. Perhaps our efforts would be better invested in developing the technology to move to a different planet.
Mike Follows, Sutton Coldfield, West Midlands, UK
• Sorry, it’s not feasible. Most asteroids orbit between Mars and Jupiter, and it would require huge changes in their velocity or momentum to get them on an orbit that would make them hit Earth. And if the sun became a red giant, Earth would have to be moved a long way beyond the orbit of Mars to get it far enough away from the sun’s heat. The angular momentum imparted to Earth would need to be much greater than that of all the asteroids in the solar system put together, which have a total mass about a two-thousandth that of Earth.
Eric Kvaalen, Les Essarts-le-Roi, France
• It depends what you mean by feasible. From the laws of conservation of momentum and energy, if you hit Earth with enough asteroids at the correct angle, then yes, in theory you could shift its orbit further out. However, the logistical challenge of achieving that goal would be impossible with current technologies. You would have to send rockets to many millions of asteroids, with enough fuel to alter those asteroids’ orbits such that they would eventually impact on Earth from the correct side. It might be better to spend all those resources to colonise a planet further out.
A second problem to consider is the effect of all these asteroid impacts on Earth. It is estimated that the dinosaurs were driven extinct by the impact of a single asteroid roughly 10 kilometres in diameter about 65 million years ago, yet the mass of that asteroid was only a tiny fraction of the mass of Earth and made negligible difference to its orbit. Imagine what might happen to life on Earth if it were bombarded with enough asteroid impacts to actually shift our orbit.
Simon Iveson, University of Newcastle, Callaghan, New South Wales, Australia
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