In April 2010, amid mounting criticism that his space plan lacked direction, President Barack Obama gave a speech in Florida to lay out a few ambitious goals he had in mind for NASA. The details of how those targets would be met remain somewhat sketchy even today, but the goals themselves were clear—sometime around 2025, the U.S. would perform an unprecedented feat. "We'll start by sending astronauts to an asteroid for the first time in history," Obama said.

NASA already sent a robotic spacecraft to land on the near-Earth asteroid Eros in 2001, and the Japan Aerospace Exploration Agency (JAXA) even managed to return a sample from the near-Earth asteroid Itokawa in 2010. So where will the next robotic mission go, and where will astronauts first touch down in the decades to come?

A group of astronomers working through a census of nearby asteroids has a few suggestions. Michael Mueller of the Côte d'Azur Observatory in Nice, France, and his colleagues have since 2009 been using NASA's Spitzer Space Telescope to gauge the properties of hundreds of near-Earth objects (NEOs), a campaign known as ExploreNEOs. In the April issue of The Astronomical Journal, Mueller and his colleagues report that three asteroids look especially promising as spacecraft destinations.

The motivations for visiting an asteroid are myriad. Asteroids have much weaker gravity than the moon or the planets and so can be approached without a complicated, expensive landing module. From a scientific perspective, asteroids are leftovers from the epoch of planet formation and preserve a critical record of the primitive solar system. And a better understanding of the composition and properties of near-Earth asteroids would be invaluable in the event that a deflection or detonation became necessary to avert a catastrophic impact with Earth.

The ExploreNEOs group used a series of criteria to filter the NEOs already examined by Spitzer into good destinations for future missions, manned or unmanned. First, the orbit of the asteroid should allow for a relatively easy rendezvous; that is, the amount of fuel required to boost a spacecraft from Earth orbit to the asteroid's vicinity should be minimized. Second, the asteroid should be fairly dark—asteroids that reflect very little light are thought to be among the most primitive. And lastly, the asteroid should have undergone minimal solar heating, which might taint its primitive composition. So the study's authors modeled each asteroid's orbital history to determine how close to the sun it may have been in the past.

Of the 293 NEOs Spitzer had observed by mid-2010, 65 of them had orbits that a spacecraft could reach with a boost of less than seven kilometers per second, meeting the easy-rendezvous criterion. Of those 65, seven asteroids were especially nonreflective, satisfying the primitive-makeup criterion. And of those seven, three asteroids—known as 1992 UY4, 2001 SK162, 2001 PM9—are likely to have remained relatively far from the sun, and hence relatively cool, since their formation. Those three asteroids, then, are at least worth a look as possible destinations.

Absent from the list are the asteroids 1999 RQ36 and 1999 JU3, which have already been nominated as targets for proposed unmanned missions from NASA, JAXA and the European Space Agency. Those asteroids simply do not appear in Spitzer's somewhat narrow visibility window during the ExploreNEOs campaign, Mueller says. But the survey is ongoing, and the researchers expect to have characterized some 700 NEOs by the end of 2011. If the statistics hold up, Spitzer should by that time have turned up six or seven desirable target asteroids in the entire sample. It remains to be seen what direction the world's space agencies will take with their missions, but learning more about the asteroids in our neighborhood certainly can't hurt their planning.

Photo of near-Earth asteroid Eros, where NASA's NEAR-Shoemaker mission landed in 2001: NASA/NEAR (JHU/APL)