DECIDING to send astronauts to an asteroid is all very well, but now NASA will have to find the few space rocks that are suitable to visit, and work out how to rendezvous safely.
Last month, US president Barack Obama announced the next destination for NASA astronauts would be an asteroid, as early as 2025. The goal would be to gain experience of safely sending humans far from Earth, as a stepping stone towards longer journeys to Mars. Studying the interior of an asteroid up close could also prove important if we ever need to deflect one. Yet achieving the goal will mean overcoming daunting challenges.
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Before landing on an asteroid, a spacecraft must enter its orbit, rather than simply whizzing by. That means matching the object鈥檚 speed and direction of motion, which in most cases would require burning too much rocket fuel to be practical. The only way round this would be if the asteroid鈥檚 motion happened to be very similar to Earth鈥檚 at the time of its closest approach.
Even if a space rock passes that test, few have close approaches to Earth in the right time frame, in 2025 or the following few years, points out Martin Elvis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was to speak on the subject this week at a meeting of the American Astronomical Society鈥檚 Division on Dynamical Astronomy in Boston.
A 2009 study led by Paul Abell of NASA鈥檚 Johnson Space Center in Houston, Texas, found only seven asteroids that could be visited between 2025 and 2030, from a list of more than 1200 near-Earth objects (). New discoveries since that study 鈥� which only included asteroids known in 2006 鈥� has increased that number to 42, but many of these could be rejected when further criteria are applied.
The rotation rate of most asteroids is unknown, but any fast-spinning objects will be off limits because they would be difficult for astronauts to hang onto. Combine that with potential mission delays, and it becomes clear that many more candidates are required, Elvis says. 鈥淚 think people have not appreciated how many you need,鈥� he says. 鈥淣ASA will need to survey huge numbers of asteroids to sift out the limited number of really good ones.鈥�
鈥淎ny fast-spinning objects will be off limits because they would be difficult for astronauts to hang onto鈥�
Abell is optimistic that telescopes like the Pan-STARRS observatory that recently opened in Hawaii will expand the list of candidates. 鈥淭here could be many, many targets to go to,鈥� he says.
However, ground-based telescopes are hampered because asteroids in orbits similar to Earth鈥檚 are often hidden by the glare of the sun, Elvis says. He advocates launching a space telescope to orbit the sun near Venus, from which it could look outward to see asteroids near Earth鈥檚 orbit 鈥� an idea that has long been discussed by astronomers but never funded.
Even if enough suitable targets can be found, there are more problems to overcome. Small, irregularly shaped asteroids have lumpy gravity fields, so an orbiting spacecraft would follow a chaotic trajectory, making navigation much trickier than around Earth or the moon, says Daniel Scheeres of the University of Colorado in Boulder, who has simulated such orbits (see picture).FIG-mg27584401.jpg
The surfaces of some asteroids may also be unstable, so astronauts could accidentally set off a landslide, Scheeres says, adding that it would be wise to send robots before humans. 鈥淲e don鈥檛 have the sort of data that you might want before you send an astronaut,鈥� he says.
