Thinking ahead ... 6.3 billion years ... and moving the planet Earth

... 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. ... 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. ...

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." ...  - newsci