WASHINGTON — NASA has identified about a dozen potential targets for its proposed Asteroid Redirect Mission, but will make no decision until at least 2017 about which space rock to bring back to the Earth-Moon system to be probed by astronauts, an agency official said.

That means the new robotic spacecraft that will redirect the chosen asteroid into a distant lunar retrograde orbit — where NASA engineers believe it could be stored for close to a century — will launch no earlier than 2018, according to Lindley Johnson, program executive for the Near-Earth Object Program Office at NASA headquarters here.

The proposed NASA mission uses as its blueprint a concept developed at the California Institute of Technology in Pasadena, Calif. Unveiled last April as part of the White House’s 2014 budget request for NASA, early concepts called for launching a robotic spacecraft in 2017 to capture an asteroid roughly 10 meters in diameter, which astronauts could visit later.

However, “a launch as soon as 2017 is probably not feasible, particularly given the budget situation,” Johnson told SpaceNews in a Dec. 19 phone interview. “We’d have to be ramping up, starting to build, be into the preliminary design phase already to be able to launch in 2017. And of course that’s not going to happen.”

Once corralled into the distant lunar orbit, the captured asteroid would, according to NASA’s current plans, be visited by human explorers using the Orion crew capsule and Space Launch System (SLS) heavy-lift rocket the agency is working on. That mission would notionally take place around 2025 — a date that has more political than practical significance, being the year by which U.S. President Barack Obama, back in 2010, challenged NASA to send astronauts to an asteroid. 

The mission, which in Congress has received reactions ranging from lukewarm to hostile, was among the subjects discussed here Jan. 8-9 at the NASA-chartered Small Bodies Assessment Group. 

At the moment, NASA is looking at seven small, free-flying asteroids and six larger space rocks from which boulder-size samples could be pried. 

However, the list of potential targets is bound to grow between now and 2017, and some — maybe all — of the asteroids discovered so far could fall off NASA’s list, depending on what follow-up observations reveal about their spin rates and composition.

At the moment, NASA knows only that the orbits of the potential targets are such that a robotic craft should be able to nudge them into the desired capture orbit around the Moon — provided that the asteroids are not spinning too fast, or composed of materials that make capture an impossible ordeal, Johnson said.

The seven free-floating asteroids that could be retrieved using the Caltech concept are: 2007 UN12, 2008 EA9, 2010 UE51, 2013 LE7, 2009 BD, 2013 PZ6 and 2011 MD. These asteroids could be put into NASA’s desired lunar retrograde orbit between 2020 and 2024, Johnson said.

Larger asteroids from which boulder-size samples could be collected and redirected are: Itokawa, a sample of which was returned to Earth in 2010 under Japan’s Hayabusa mission; Bennu, from which NASA’s Osiris-Rex mission, launching in 2016, plans to return a small sample to Earth in 2023; 1999 JU3; 2008 EV5; 2011 UW158; and 2009 DL46. 

NASA has yet to put a price on its proposed asteroid redirect mission, which would require the construction and launch of a new solar-propelled robotic spacecraft to retrieve the asteroid. NASA Administrator Charles Bolden has said the endeavor would cost less than the $2.2 billion estimate that came attached to the Caltech concept. 

Bolden has said the Caltech concept did not figure in work NASA has already done on SLS and Orion, which Congress ordered the agency to build in 2010 using hardware and contracts left over from the space shuttle program, and the Constellation Moon exploration program canceled by the Obama administration that year. These programs are among the most expensive in NASA’s portfolio.

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Dan Leone is the NASA reporter for SpaceNews, where he also covers other civilian-run U.S. government space programs and a growing number of entrepreneurial space companies. He joined SpaceNews in 2011.Dan earned a bachelor's degree in public communications...