NASA has decided to pluck a small boulder off an asteroid and bring it back to the vicinity of Earth, rather than bag up an entire asteroid, agency officials in charge of the Asteroid Redirect Mission (ARM) announced today.
The $1.25 billion mission, which is planned to launch in December 2020, would send a robotic spacecraft for a rendezvous with an asteroid in 2022. After touching down on the asteroid’s surface, the spacecraft would snatch a boulder several meters across. The spacecraft would then orbit the asteroid for up to 400 days, testing out an idea for defending Earth from a catastrophic asteroid impact: using the spacecraft’s own gravitational field to subtly alter the asteroid’s orbit. Next, the spacecraft would bring the snatched rock back to Earth’s vicinity in 2025. Finally, as part of preparations for a possible mission to Mars, astronauts would visit and examine the rock for some 25 days, using the planned Orion spacecraft to make the trip.
The boulder-snatch concept is expected to cost $100 million more than the bagging concept, but it would be better for developing technologies that would have greater value for exploring Mars, explained Robert Lightfoot, NASA’s associate administrator, during a teleconference today. Moreover, he says, whereas a bagging mission might get only one chance to snare its target, a boulder-snatching spacecraft will have a chance to survey the asteroid ahead of time before picking a target, and it could make several attempts at grabbing a boulder. “I’m going to have multiple targets when I get there, is what it boils down to,” he says. “That was the better value, in my opinion, for what we’re trying to do.”
The leading target for ARM now is a 0.45-kilometer-wide carbonaceous C-type asteroid called 2008 EV5, Lightfoot says. The two other candidates are asteroids called Bennu and Itokawa, and ongoing searches are expected to yield one or two more candidates each year leading up to mission launch.
Scientists say that there is intrinsic interest in C-type asteroids, which have never been visited by a spacecraft. They are darker than many asteroids because of all the primitive carbonaceous material they hold. Some may contain hydrated minerals, or even water ice, says Tim Swindle, director of the University of Arizona’s Lunar and Planetary Laboratory in Tucson. “They definitely have the potential of being a dirtier version of a comet.”
But scientists have been skeptical about the mission, mostly because of concerns that its costs could end up threatening science missions, even though ARM is primarily designed to demonstrate capabilities for NASA’s human spaceflight program. Already, two science missions are planning on visiting a C-type asteroid. In December 2014, Japan’s space agency launched Hayabusa 2, which aims to return a few grams of asteroidal material to Earth in 2020. And in 2016, NASA plans to launch OSIRIS-REx, which aims to return at least 60 grams of material by 2023. “A lot of the wariness was that [ARM] would be funded out of science, and that the science return after going to other carbonaceous asteroids would not be that great,” Swindle says. “Everyone is going to remain wary until the mission has flown and the cost hasn’t come out of science one way or another.”
ARM has also drawn skepticism from lawmakers in Congress, who will make the ultimate decision on whether to fund it.