NASA is currently working on one of the most difficult parts of its asteroid-retrieval mission: finding an appropriate space rock to shrink-wrap in space.

Dozens of asteroids have been identified by scientists as "promising targets" for NASA's asteroid-capture mission. The space agency will attempt to drag a small asteroid, or a piece of a larger one, to a stable orbit around the moon, where it can be visited by astronauts by 2025, according to SPACE.com.

"For either concept that's being looked at right now, either the capture of a small asteroid less than 10 meters in size, or going after a boulder, large boulder, on a larger asteroid, we have a list of about six or so candidates each," Lindley Johnson, program executive for NASA's Near-Earth Object (NEO) Observations Program, said, according to SPACE.com.

The NEO program's main goal is to identify and track potentially dangerous asteroids, but good candidates are usually a subset of a larger group, so assessment of their suitability for the mission doesn't prevent NEO scientists from doing their job.

NASA wants the first manned visit to an asteroid to take place around 2025, which would go well with an exploration timeline set by the White House, according to SPACE.com.

Back in 2010, President Barack Obama told NASA he wants to get astronauts to a near-Earth asteroid by 2025, and the vicinity of Mars by mid-2030s.

This mission will essentially be a stepping stone, according to NASA officials, by demonstrating technologies that could allow humans to reach Mars and other deep space destinations in the future.

It will also help scientists develop asteroid-mining technology, and advance their understanding of the early solar system.

"We continue to look for additional candidates," Johnson said, confirming that NEO program scientists "will continue to do that over the next two to three years, until the time comes to actually determine which will be the best object for the mission."

Last week NASA announced that it is looking for ideas that could help make the asteroid-capture mission a reality. It will select 25 proposals and give away a total of $6 million, according to a company release. Awards will be announced July 1.

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