Asteroid-bound: Scientists look for worthy rock

Proximity and slow spin rate are desirable for exploration

The Little Prince, who stood tall on his fictional house-sized asteroid B612, may soon have company. Since President Obama announced last month that NASA plans to send people to an asteroid by 2025 (SN: 5/8/10, p. 10), scientists have been scrambling to fill in the details. Before astronauts can embark on such a journey, they need to choose a destination.

Already, researchers have begun culling the list of potential candidates. Martin Elvis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., proposed criteria for identifying “potentially visitable objects” on April 28 in Brookline, Mass., 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.

For many researchers, the visit will be a mini–Mars-mission — a chance to test strategies and equipment before traveling to the Red Planet. A round-trip journey to a nearby asteroid might take about half a year. A mission to Mars would take more than twice as long.

“If you want to climb Mount Everest, you don’t climb K2 first,” says astronaut and astronomer John Grunsfeld of the Space Telescope Science Institute in Baltimore. Practicing deep space maneuvers on a nearby asteroid would be like climbing Washington’s Mount Rainier before tackling the Himalayas.

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, Colo. 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.

Of course, a crewed mission to an asteroid would garner rich scientific rewards in its own right. Visiting an asteroid “tells you about what existed back when planets were forming,” Bottke says. Asteroids may host carbon-containing molecules, which could hold clues to the beginning of life on Earth. So far, scientists have gleaned much of their information about the early solar system from meteorites that have landed on Earth, but these samples lose a lot of material as they flame through the atmosphere, he says.

Though robots have successfully landed on two asteroids so far — Eros and Itokawa — people could accomplish experiments that robots couldn’t. “Having humans in the mix gives you a lot of flexibility,” Abell says. A human with a hammer could pick up a rock and then choose to discard it in favor of a more intriguing rock somewhere else.

But having “non-artificial intelligence,” as planetary astronomer Andrew Rivkin of Johns Hopkins University’s Applied Physics Laboratory in Laurel, Md., puts it, doesn’t mean a thing unless the astronauts survive the trip. Keeping them safe on a long flight to an asteroid, as well as to Mars, will pose new challenges.

“Going to an asteroid is a new idea, but I don’t think all of the complications have been thought through,” Bottke says. “I think everyone’s being a little cavalier about jumping on the bandwagon.”

For instance, researchers will need to quantify the doses of radiation that astronauts will experience on the journey. An inopportune solar flare could be deadly, and the requisite protective shielding could be too heavy to carry.

If researchers can identify a flight plan that will keep astronauts healthy and safe, Earth’s homebodies may be protected as well. Another hope — and another reason to probe a nearby asteroid — is that such a mission could uncover new ways to deflect or destroy a life-threatening rock careening toward Earth.

“A lot of the objects that we’ll be able to get to as human beings are the ones that represent the greatest threat,” Abell says.

The difficulties of the proposed visit are great, but the morale boost from accomplishing the mission is “powerful, if not tangible,” Rivkin says.

Adds Grunsfeld, who has been on five space flights: “This is about the bigger picture. It’s the start of humans going out and exploring the solar system.”

DESTINATION ASTEROID Scientists have begun working out what makes an asteroid, Eros shown here, suitable for a crewed mission. Size, shape and composition are a few considerations. Nasa-JPL, APL/JHU

The NEAR spacecraft (artist’s illustration shown) landed on Eros in 2001. Scientists are now evaluating other asteroids for manned missions. SPL/Photo Researchers, Inc.


Back Story | Asteroids to watch

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

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.

2005 YU55

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

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.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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