After years of weighing the options, NASA has decided to launch a robotic probe to a near-Earth asteroid in 2020, have it pull off a boulder from the space rock and bring it to a spot near the moon where astronauts can study it.
The decision to go with "Option B" marks a big milestone for a mission concept that was born in the early days of the Obama administration and is expected to cost $1.25 billion, not including launch costs. But there's still the possibility that a future administration will change NASA's course.
The other option for what's most recently been called the Asteroid Redirect Mission, or ARM, would have involved identifying an asteroid small enough to be moved wholesale to a parking orbit in the Earth-moon system.
This "Option A" would have cost about $100 million less than Option B, NASA Associate Administrator Robert Lightfoot told reporters during a Wednesday teleconference. He said Option B was chosen instead because it would demonstrate more of the capabilities that NASA would need for more ambitious trips to Mars.
The top target for the mission is currently a quarter-mile-wide (400-meter-wide), walnut-shaped asteroid known as 2008 EV5. Two other prospects are asteroid Itokawa, which was visited by Japan's Hayabusa probe in 2005; and Benno, which NASA's OSIRIS-REx spacecraft will sample in 2018. Lightfoot said NASA doesn't have to decide on a target until 2019.
Launch in 2020
Lightfoot said the ARM mission would go on to the next phase of preparation. The scenario currently under review calls for sending a robotic spacecraft to the target asteroid in December 2020. It'd take two years for the craft to cruise to the space rock, using a solar electric propulsion system.
Once it arrived, the probe would spend 215 to 400 days hovering around the asteroid, to select the boulder to be plucked and also test techniques for influencing the asteroid's trajectory — for example, by using subtle gravitational force over a long period of time. Such techniques might come into play if space agencies had to divert a potentially threatening asteroid in the future.
The probe would reach out with its robotic arms, grab onto a boulder that could be up to 13 feet (4 meters) wide, and eventually carry it away to a stable spot near the moon — perhaps in what's known as a distant retrograde orbit.
Meanwhile, NASA would get astronauts ready to head for the boulder's parking spot in the 2025 time frame, aboard an Orion capsule that would be launched by the yet-to-be-built, heavy-lifting Space Launch System.
Lightfoot said the two-person crew would take 24 or 25 days to rendezvous with the boulder, go on spacewalks and collect samples of the rock for return to Earth.
Science vs. exploration
Lindley Johnson, program executive for NASA's Near-Earth Object Program, said 2008 EV5 was a carbonaceous C-type asteroid — a stony type of space rock that's of particular interest to astrobiologists because they're thought to contain large amounts of organic compounds.
However, Lightfoot acknowledged that science isn't the driving factor behind the ARM mission. Rather, the effort has a "human exploration emphasis," he said. "The technologies used to grab onto the asteroid ... are the kinds of things we know we're going to need when we go to another planetary body," Lightfoot said.
For example, the solar electric propulsion system could pre-position cargo or vehicles for future crewed missions into deep space. The docking system, sensor technologies and spacesuits being developed for the asteroid mission could also be used on Mars missions.
Lightfoot said that although Option B was more expensive, the technologies developed for Option A would not be as "extensible" to future human missions. He also said Option B received more positive feedback from potential commercial partners.
"We had a lot of interest from traditional folks we work with, and non-traditional folks, " he said. Those "non-traditional" companies might well include Planetary Resources and Deep Space Industries, which plan to mine asteroids for water and other potentially valuable space materials.
Although NASA officials are strongly committed to the asteroid mission, the concept hasn't exactly caught fire with the general public or members of Congress, particularly on the Republican side. The mission came in for sharp criticism in a 2012 report from the National Academy of Sciences, and some GOP leaders have said NASA's long-range focus should be trained on the moon and Mars rather than an asteroid.
NASA's timetable means the details of the boulder-grabbing mission won't be set until well after President Barack Obama has left office — potentially providing an opportunity for his successor to change course.