In a paper published today on the Los Alamos astro-ph preprint website, two Chinese scientists have proposed using a solar sail for deflecting any asteroid that happens to be aimed at the earth. The diagram to the right is their simulated mission to impact the asteroid Apophis, which will pass close to the earth in 2029 and — depending on whether that flyby puts it through a very small 600 meter-wide mathematical “keyhole” — could then return in 2036 on a collision course.
The idea is to use the sail to slow the spacecraft down enough so that it starts to fall towards the sun. The sail is then used to maneuver it into a retrograde orbit. When it impacts the asteroid the impact will therefore be similar to a head-on collision, thereby imputing the most energy in the least amount of time with the least amount of rocket fuel. In their Apophis simulation, a mission, weighing only 10 kilograms (about 22 pounds), launched around 2025, and hitting the asteroid in this manner in 2026, would deflect its flyby in 2029 enough to guarantee it will not fly through the “keyhole” and therefore eliminate any chance of it hitting the earth in 2036.
Obviously many questions must be answered before such a mission should fly.
For example, we need to know more about Apophis, or any target asteroid for that matter, in order to determine the best way to deflect it. If the target asteroid is a rubble pile rather than a solid object, an impact like this might simply disperse the material instead of deflecting it, thereby making things worse.
In addition, a lot more engineering work and testing of solar sails is required. At this time, only one solar sail, the Japanese sail Ikaros, has ever been successfully launched, flown, and maneuvered in space. The authors of the above paper themselves admit that such a mission requires “a high performance solar sail,” with a very large area as well as a very sophisticated “navigation, guidance, and control system.” Otherwise, even the smallest error could “make the sail miss the asteroid.”
What I find most fascinating about this proposal, however, is how it illustrates the versatility and maneuverability of solar sails. Properly designed, they could easily make travel throughout the inner solar system fast and easy.