WASHINGTON — At least two of the six asteroids NASA has identified as candidates for redirecting to lunar orbit for astronauts to explore by 2025 will already have been probed and sampled by robotic spacecraft by that time, an agency official told a group of scientists here July 30.

In a presentation to the NASA-chartered Small Bodies Assessment Group (SBAG), the Asteroid Redirect Mission’s so-called pre-program manager, Brian Muirhead of the Jet Propulsion Laboratory in Pasadena, California, said that all six of the asteroids on NASA’s short list could be robotically retrieved between 2023 and 2025. The candidates include both three free-flying asteroids and three boulder-sized samples that could be collected from larger asteroids, according to Muirhead.

Two of the potential boulder-retrieval targets either have been or will have been sampled by the time astronauts would get a crack at them as part of the Asteroid Redirect Mission.

Asteroid Itokawa, a boulder-sized piece of which Muirhead said would be retrievable by July 2025, was visited in 2010 by Japan’s Hayabusa mission. NASA, meanwhile, is preparing to collect a sample of Bennu as part of the robotic Osiris-Rex mission launching in 2016 to collect a 60-gram cache of surface material and bring it back to Earth by 2023. A boulder-sized chunk of Bennu could be robotically retrieved by March 2025, according to Muirhead.

The other four candidate asteroids on the list Muirhead presented July 30 are:

  • 2009 BD, a free-flyer retrievable by October 2023.
  • 2011 MD, a free-flyer retrievable by August 2025.
  • 2013 EC20, a free-flyer retrievable by October 2025.
  • 2008 EV5, a boulder-retrieval candidate retrievable by July 2025.

Until recently, the European Space Agency was considering a sample-collection mission to 2008 EV5 as part of its Cosmic Vision 2015-2025 program. The mission concept, known as MarcoPolo-R, was passed over for funding in February, when the European agency instead decided to fund an exoplanet observatory known as the Planetary Transits and Oscillations of Stars mission.

The Asteroid Redirect Mission,  NASA’s response to U.S. President Barack Obama’s challenge to send human explorers to an asteroid by 2025, notionally calls for launching a $1.25 billion robotic retrieval craft in 2019 to capture an asteroid, or a piece of an asteroid, about 10 meters in diameter.

The space rock would then be redirected to a distant retrograde lunar orbit where it would be visited by astronauts as part of a demonstration mission for the Orion deep-space capsule and Space Launch System heavy-lift rocket NASA is building.

The Small Bodies Assessment Group, in a draft report published July 30, said NASA’s human exploration and space technology divisions stand to benefit far more from the asteroid retrieval mission than the science division, and that therefore no NASA science funding should be used by the mission. The report also said scientists do not want to see the same asteroid sampled twice, a recommendation that would rule out Itokawa and Bennu, assuming NASA’s Osiris Rex mission succeeds.

SBAG member Richard Binzel of the Massachusetts Institute of Technology went even further, calling the Asteroid Redirect Mission a “one-and-done stunt” that scientists should actively oppose.

Other SBAG members, however, said the small-bodies science community needs to go along for the ride if any science is to come out of the Asteroid Redirect Mission.

“I don’t particularly like [the Asteroid Redirect Mission], but I think science needs to be involved with this,” Daniel Britt, an astronomy professor at the University of Central Florida, said during the meeting.

Mark Sykes, director of the Planetary Science Institute in Tucson, Arizona, cautioned the group that embracing the mission could end up costing the small-bodies science community.

“NASA has a history of finding ways of taxing everybody who benefits from a mission,” Sykes said.

Lindley Johnson, NASA program executive for the Near-Earth Object Program Office at the agency’s headquarters here, acknowledged that the Asteroid Redirect Mission is not the kind of mission most SBAG members would design.

“If we were to start this from a clean sheet and do it in a logical manner, I think every one of us involved with this would do it differently than how it’s being done right now,” Johnson said.

But Johnson also cautioned members “to stick to the science, as opposed to taking what some would consider political shots.”

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.