Article: "Finding the Right Asteroid for Astronauts to Land on"

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Already, researchers have begun culling the list of potential candidates. Martin Elvis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, proposed criteria for identifying “potentially visitable objects” on April 28 in Brookline, Massachusetts, at a meeting of the American Astronomical Society’s Division on Dynamical Astronomy.

Asteroids come in a menagerie of sizes, shapes and trajectories. Some are little more than giant loose rubble piles, while others are densely packed. Though Obama’s proposal didn’t point to any specific destinations, Elvis says that a worthy asteroid ought to have a few key features, including a slow spin rate, no problematic satellites and a solar orbit that allows for a long and recurring launch window.

“Are they spinning rapidly? Are they elongated? Is there strange, irregular gravity?” Elvis asks. If the asteroid is “lumpy and nasty, that’s not good.”

The most important consideration, though, is that the asteroid is easy to get to. While the majority of asteroids reside in a belt between the orbits of Mars and Jupiter, some come close to Earth. A relatively nearby asteroid that circles the sun at a speed similar to the Earth’s would be ideal, Elvis reported. So far, six of 6,699 known near-Earth asteroids seem to have amenable orbits.

To find their Mount Rainier, astronomers first need to map all the asteroids. Scientists have pinpointed many of those big enough to destroy the Earth, but a lot of the rocks smaller than a kilometer in diameter haven’t been identified, says planetary scientist Bill Bottke of the Southwest Research Institute in Boulder, Colorado. Bottke recently coauthored a National Research Council report outlining possible approaches to cataloging all asteroids near Earth.

Once the asteroids are tallied, selection criteria such as those proposed by Elvis can be considered. (Regardless of choice, it is unlikely that the asteroid will have enough gravity to allow a landing. Rather, astronauts would probably tether their spacecraft to the asteroid and move as it moves, possibly zipping to the rock in a smaller vehicle.)

Planetary scientist Paul Abell of NASA’s Johnson Space Center in Houston says an asteroid’s composition might also affect its desirability. Visiting an asteroid that holds water-ice, for example, might help astronauts figure out how to extract water for drinking and for fuel, a technique that could come in handy during pit stops on a long trip to Mars.

More than 6,500 asteroids are known to enter Earth’s neighborhood. Of these, 1,100-plus are classified as “potentially hazardous” — meaning they can approach Earth relatively closely and have diameters larger than 150 meters. The orbits of a few of these asteroids are shown below.

25143 Itokawa grabbed public attention when it became the target of the Japanese Hayabusa mission, which launched in 2003, imaged the asteroid and attempted to collect soil samples. (The recovery capsule is expected to land in Australia in June.) The asteroid’s next close approach will be in March 2030, when it will pass within 56.3 million kilometers of Earth.

Recent observations suggest asteroid 2005 YU55 is 400 meters long, twice as large as previously thought. The measurements were taken in April as the Arecibo telescope in Puerto Rico tracked the asteroid passing within 2.3 million kilometers of Earth. On its next approach, in November 2011, the body is expected to get much closer — a mere 325,000 kilometers away.

6344 P-L was first discovered in 1960, but then researchers lost track of it. The asteroid was rediscovered in 2007 and given the name 2007 RR9 before it was recognized. The asteroid has a highly elongated orbit that takes 4.7 years to traverse, and its next close approach to Earth will be in November 2040, when it will pass within 11 million kilometers.

Link: Article ("Finding the Right Asteroid for Astronauts to Land on")