NASA's next marquee mission might be described as the great asteroid boulder pluck.
At a news conference Wednesday, agency officials said they had revised their original plan to capture an asteroid and drag it into deep lunar orbit.
The new plan calls for a spacecraft with two robotic arms to remove a boulder of up to 12 feet in length from the surface of an asteroid and bring that into orbit around the moon instead.
The agency still plans to send two astronauts to collect a sample of the boulder once it is in a stable orbit around the moon.
The new plan may seem less dramatic than the original concept for the Asteroid Redirect Mission (ARM) first announced in 2013, but NASA associate administrator Robert Lightfoot said the boulder-plucking plan ultimately utilizes more of the technologies needed for humans to eventually get to Mars.
"ARM is an important part of the overall mission of us taking humans further into space," he said. "The systems we are going to bring into play are the kinds of things we know we are going to need when we go to another planetary body."
He added that the new plan also allows for more flexibility within the mission itself.
"There will be a sensor suite on the spacecraft that will let us look at the boulders and make an educated choice about which one we pull up," he said. "We'll also have three to five opportunities to pull up the boulders, lowering the mission risk."
The timeline of the mission, for now, is to launch the spacecraft in 2020 and have it arrive at the asteroid about two years later.
After capturing the boulder, the spacecraft is also to test a new technique, called a gravity tractor, that could be used to alter the orbits of asteroids headed for a collision with Earth. Once the spacecraft has procured the boulder, it will fly in a halo orbit around the asteroid. Lightfoot said that the mass of the boulder combined with the mass of the spacecraft should be able to exert enough gravitational pull to tug the asteroid into a new orbit.
Lightfoot said the asteroid will likely be moved just a tiny bit off its orbit -- and even that will take many months.
"It takes a while," he said. "You are talking about a very large mass you are trying to influence with a very small mass."
Once telescopes on the ground determine that the asteroid's orbit has been altered, the spacecraft would fly the boulder to lunar orbit and deposit it there. Ideally, officials said, astronauts would be able to visit it sometime in the mid-2020s. They expect the manned part of the mission to last about 25 days.
The boulder-plucking plan is shaping up to cost about $100 million more than the original plan. As of now, Lightfoot said he expects the entire mission to cost $1.25 billion, not including the cost of the launch vehicle.
A few target asteroids have been identified, but NASA won't know until 2019 exactly which one will get the visit.
Now that the new plan has been decided, Lightfoot said, NASA is moving on to the next phase of the mission, which is figuring out the costs, deciding who will build what and managing risk.
"It's going to be a tight schedule," Lightfoot said. "The teams are glad to be moving."