Ledyard King

Gannett Washington Bureau

WASHINGTON — NASA is reporting significant progress identifying potential asteroids where astronauts could land as part of a Mars mission, even as skeptics in Congress say the moon would make a better stepping stone.

Three candidates, none bigger than a school bus, are being monitored for a mission that would redirect a near-Earth asteroid into lunar orbit.

Sporting names like 2009 DD, 2013 EC and 2011 MD, each has the mass, shape, spin rate and orbit required for a mission that could cost as much as $1.25 billion and is still up to a decade away.

NASA also is looking at three larger asteroids (Itokawa, Bennu and 2008 EV5) as part of an "Option B." Those asteroids contain small boulders that could be broken off robotically and redirected into lunar orbit as well.

Agency officials held a news conference Thursday to discuss the status of the Asteroid Redirect Mission, or "ARM," exactly a year after NASA unveiled the unusual plan. They hope to decide on an option by early next year but don't have to choose a specific asteroid to land on until a year before the launch, expected to take place between 2021 and 2024.

The 2011 MD asteroid, with an orbit close to Earth's, has emerged as the early favorite.

But "we don't plan to nor do we want to stop actually looking for targets," said Michele Gates, program director for the asteroid mission.

Although it has support among NASA scientists, the idea has met with criticism from some lawmakers and apathy from other countries considered vital partners for any deep-space mission.

A National Research Council report issued earlier this month indicated little consensus for the asteroid mission.

"There's really no enthusiasm among any of our partners for the ARM," said Cliff Zukin, a Rutgers University political science professor and a member of the committee that issued the report. "And so if that's what the president is insisting upon, it makes it even harder for that path. He has chosen a path that makes it more difficult to find allies and co-funders."

President Obama has shown little interest in returning to the moon, viewing it as a "been-there, done-that" sort of mission.

Shortly after entering the White House, he scrapped the Constellation return-to-the-moon program — started under President George W. Bush — after an independent commission said the cost to continue it was unsustainable.

NASA says exploring an asteroid and studying its properties also will help scientists detect and divert threatening projectiles that might be headed toward Earth.

Researchers already know of some 11,000 asteroids in space and spot about 100 new ones every month, said Lindley Johnson, who heads NASA's Near Earth Object Program.

A large majority are so small they would burn up in the atmosphere. But hundreds of larger asteroids, such as the 55-foot meteoroid that crashed into Russia last year and injured more than 1,000 people, pose a threat.

"We're trying to find that population before they find us," Johnson said.