NASA redefines its asteroid redirect mission

Instead of capturing an entire asteroid and carrying it into a lunar orbit, NASA’s asteroid redirect mission will retrieve a boulder from an asteroid’s surface, agency officials announced on 25 March. The mission, which is expected to last 5–6 years, centers on the launch in December 2020 of an unmanned spacecraft that will arrive at the asteroid in 2022. After spending anywhere from 215 to 400 days at the chosen asteroid, the spacecraft will robotically grab a boulder of up to four meters in diameter and transport it to orbit around the Moon. Once the object is in place in 2025, a crewed mission lasting up to 25 days will visit and study it, NASA associate administrator Robert Lightfoot told reporters.

Retrieving a boulder instead of an entire asteroid presents less risk, Lightfoot said, and will demonstrate more of the capabilities that will be needed for a human mission to Mars. It also costs about $100 million less than moving a whole asteroid, he said. Among the technologies that will be tested are a mechanism for docking the Orion crew vehicle to the asteroid spacecraft, spacesuits designed for deep space, and advanced sensors.

The total project cost is estimated at $1.25 billion, excluding the launch vehicle, Lightfoot said. President Obama’s fiscal year 2016 budget request includes $50 million for the development program. Although the primary reason for the mission is to demonstrate capabilities needed for deep-space travel by humans, it will also benefit NASA’s science and Earth defense programs, he noted.

The spacecraft will use a solar–electric propulsion system, which is under development. The system employs electricity generated from solar arrays to ionize a xenon propellant. Although slower than conventional chemical propulsion, solar–electric propulsion requires significantly less propellant. It also is efficient for moving massive cargo, a possible consideration for an eventual human mission to Mars, for which vehicles or cargo might be positioned in advance along the way.

The asteroid redirect mission will also test the viability of a gravity tractor—a spacecraft capable of redirecting an asteroid that might some day threaten Earth. Prior to heading to the Moon, the spacecraft will be temporarily placed into an orbit around the asteroid. The combined mass of the captured boulder and spacecraft should be sufficient to deflect the asteroid’s orbit enough to be measured from Earth, Lightfoot said.

Although the target asteroid doesn’t need to be selected until 2019, NASA has been narrowing down the choices. The leading candidate is 2008 EV5, a carbonaceous C- type object that has been extensively observed, hasn’t been visited, and has a well-known orbit, said Lindley Johnson, program executive for NASA’s Near-Earth Object Program. Two other candidates are Itokawa and Bennu. NASA is expected to identify other candidates in the next several years.