Sen—NASA says it will learn more about what it takes to send astronauts to Mars by practicing with a piece of an asteroid relocated into lunar orbit rather than by snaring and transporting an entire small body.

After detailed studies of both options, NASA on Wednesday unveiled its preference for Plan B -- plucking a 2- to 4-meter diameter boulder off the surface of a larger asteroid and nudging it into a high lunar orbit. Despite a price tag of roughly $100 million more than Plan A, NASA associate administrator Robert Lightfoot told reporters it's worth the cost.

"It really gives us an opportunity to demonstrate capabilities we're going to need for future human missions beyond low-Earth orbit and then ultimately, to Mars," Lightfoot said.

“The option to retrieve a boulder from an asteroid will have a direct impact on planning for future human missions to deep space and begin a new era of spaceflight,” he added in statement.

Congress has been lukewarm and occasionally hostile to the initiative, which is estimated to cost about $1.25 billion, plus launch costs.

The mission grew out of President Obama’s April 2010 call to send astronauts to an asteroid by 2025 as a steppingstone to a human mission to Mars. The previous plan to follow up the International Space Station program was to develop a base on the Moon before heading to Mars, but the initiative was canceled due to budget shortfalls.

NASA is now aiming to launch the robotic precursor mission in December 2020. It would arrive at a still-to-be selected asteroid about two years later and begin a survey that would last about a year. Once a suitable boulder was identified, the spacecraft would descend to the surface and deploy a pair of robot arms to grapple the selected target.

NASA also wants to conduct a practice deflection maneuver, technology that one day might be needed to divert an asteroid on a collision course with Earth. Ultimately the boulder, still attached to the spacecraft, would be captured by the moon’s gravity, opening the door for a follow-on visit by astronauts around 2025.

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