NASA has made a decision on the design for its Asteroid Redirect Mission, opting to retrieve a boulder from a larger asteroid.

NASA's boulder-retrieval mission
NASA has chosen to send a spacecraft to retrieve a boulder from the surface of an asteroid.

In a step forward for NASA’s Asteroid Redirect Mission (ARM), the agency announced today that it’s proceeding with plans to retrieve a boulder up to 4 meters wide from a larger asteroid. A preliminary schedule has the spacecraft set for a December 2020 launch, and by mid-decade two astronauts will have their own look-see at the asteroid boulder in lunar orbit.

NASA has been planning an asteroid mission for two years already, but until now preparations have been split between two options: bagging a small asteroid — the whole kit and caboodle — or retrieving a small boulder off a larger asteroid.

The selection of so-called “Option B” costs slightly more ($100 million) and brings the mission total, not including the launch vehicle, to $1.25 billion. But it’s also the safer choice, with more time to assess the asteroid during approach, more targets (i.e., boulders) to pick from, and multiple opportunities for the actual pick-up. More importantly, NASA associate administrator Robert Lightfoot told reporters in a press conference on March 25th, Option B will build long-term capabilities for sending humans into space.

Microspine grippers on the end of the robotics arms can be used to grasp and secure a boulder. The microspines use thousands of small spines to dig into the boulder and create a strong grip. An integrated drill will be used to provide final anchoring of the boulder to the capture mechanism.

Among the capabilities is solar electric propulsion (SEP, which has already sent the NASA’s Dawn spacecraft to two asteroids). The technology uses solar panels to produce electricity, which accelerates ions that propel the spacecraft forward. Though slower than conventional chemical propellants, SEP moves massive cargo (such as asteroid boulders) efficiently, with far less propellant required.

The robotic spacecraft will also land softly on an asteroid, a capability that future space missions may need for resupply or refueling. Boulder capture, possibly with “microspine” technology, will be another useful technique in the toolkit of future missions.

Gravity Tractor
The plan is for the boulder-retrieving spacecraft to test a planetary defense technique known as the "gravity tractor." By maintaining an orbit to one side of the asteroid, the spacecraft can over time influence the asteroid's trajectory. (Click for a larger view.)

Option B will also enable NASA to test a planetary defense technique: the gravity tractor. After retrieving the rock, the slightly more massive spacecraft will attempt to deflect the asteroid’s orbit. The spacecraft will stay on one side of the asteroid so it can exert the slightest of tugs on the asteroid’s path by maintaining a halo orbit, circling around one of the points where the asteroid’s gravitational pull cancels that of the Sun. When Earth-based telescopes have measured the asteroid’s slight deflection, probably after 215 to 400 days of halo-orbit tugging, the spacecraft will head home again, boulder in tow.

“This is a capability-demonstration mission, developing the capabilities we’ll need to take humans to space and to Mars,” Lightfoot says. “But there’s a scientific benefit to what we’re doing too.”

Three potential targets have been identified so far for this mission. The leading candidate is 2008 EV5, a carbonaceous asteroid whose surface may hearken back to the solar system’s infancy. Astronomers have studied it extensively with optical, infrared, and radar observations, garnering a good idea of its size, shape, spin, and orbit, says Lindley Johnson, the program executive of NASA’s Near-Earth Object Program. Two other candidates are Itokawa, an S-type (read: stony) asteroid visited by Japan’s Hayabusa mission in 2005, and Bennu, the planned target of NASA’s OSIRIS-REx mission, scheduled to launch in 2016 and reach the asteroid in 2019.

Next Steps

Now that the conceptual study and review are complete, ARM enters “Phase A,” the part of NASA’s mission-planning where detailed strategy and costs are laid out. This is where commercial participants will come into play — the boulder retrieval option garnered lots of interest from private industries, both traditional and non-traditional, he said. “We’ll peel back those opportunities in Phase A,” Lightfoot added, with the next milestone, an acquisition strategy meeting, coming in July.

“[This mission] is bringing together the best of NASA’s human exploration, science portfolio, technology portfolio,” Lightfoot said, “and really giving us the opportunity to demonstrate the capabilities we’re going to need to further future human missions beyond low-Earth orbit and ultimately to Mars.”

Preliminary Schedule
July 2015 Acquisition Strategy Meeting (next milestone in Phase A)
December 2020 Spacecraft launch
2022/2023 Spacecraft arrives at asteroid, retrieves boulder from surface
215 to 400 days after boulder retrieval Spacecraft will maintain “halo” orbit around asteroid
2025 Spacecraft will place boulder into retrograde lunar orbit
Mid-decade 25-day human mission to asteroid boulder


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March 26, 2015 at 10:01 pm

Hi, Monica,

The Dawn mission is NOT a European Space Agency mission. It is a JPL mission. You might be thinking of ESA's Rosetta mission.

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Monica Young

March 27, 2015 at 9:30 am

Correct, and fixed. Thanks for the catch!

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Peter Wilson

March 27, 2015 at 10:27 am

To avoid such confusion in the future, they could rename it the Ion Propulsion Lab.

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March 28, 2015 at 12:01 pm

Great article, Monica. Thanks.

Presumably, the gravity tractor's halo orbit will circle L1 -- the Lagrangian point between the asteroid and the sun. Are you (or another reader) able to enlighten this reader as to why a halo orbit about L1 is preferable to simply parking the gravity tractor at L1 and maintaining it there; that is, parking the gravity tractor at L1, and maintaining it there as the sun-asteroid Lagrangian point L1 orbits the sun?

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