In an ambitious five-year mission, NASA hopes to rendezvous with a near-Earth asteroid, pluck a boulder from its surface and practice a technique called a “gravity tractor” in an attempt to alter an asteroid’s orbit.
It will then return the boulder to orbit around the moon where astronauts will collect samples and practice space techniques that may someday take us to Mars.
The Asteroid Redirect Mission (ARM) is currently in the planning stages with an anticipated launch date of December 2021. Its goals range from science to technology development to planetary defence, says John Holdren, science advisor to US president Barack Obama.
Scientifically, studying asteroids is important because they contain remnants of the early solar system – remnants that could provide clues to the formation of the Earth, and ultimately us.
But the mission will also rehearse technologies valuable for a manned mission to Mars, including having astronauts work in high lunar orbit, which Holdren calls a much more sensible staging ground for missions to Mars than the surface of the moon itself.
“You don’t have to lift stuff off the moon,” he says. “You do it from a jumping-off point in stable orbit around the moon.”
Ultimately, he adds, asteroid materials such as those ARM hopes to bring back to lunar orbit may also supply construction materials for space missions, refuelling stations for spacecraft and other valuable materials.
“President Obama said we’re going to space not just to visit but to stay,” Holdren says, “and the asteroid mission is the next step on that trajectory.”
Rendezvous with the moon-orbiting boulder will also give astronauts practice working at distances farther from Earth than anyone has ever been, even in the Apollo days.
For 16 years, says NASA administrator Charles Bolden, astronauts have worked on the International Space Station.
But that’s only two hours from Earth. High lunar orbit is far enough away that the boulder-rendezvous mission will take close to a month.
“We’re going to have to figure out new ways to supply ourselves to test the technologies that will be necessary for when we go to Mars,” Bolden says.
The plan begins with an advanced solar-electric propulsion system that uses solar panels to provide energy to shoot out propellant at high velocity.
Because it doesn’t require burning propellant, it’s far more efficient than conventional rockets. “It takes six to 10 times less propellant,” says Jim Reuter, of NASA’s Space Technology Mission Directorate.
Its only downside is that it doesn’t provide a lot of acceleration – so little, in fact, that it’s been compared to a car that could go from zero to 60 on a thimbleful of fuel … but might take two days to get up to speed.
Once it reaches the target asteroid, a rock roughly 400 metres in diameter called 2008 EV5, the ARM spacecraft will pick a likely boulder, approach and pluck the 20-tonne rock from the surface.
Then, it will hover above the asteroid, using its solar-electric propulsion engines to stay in place. By doing so, it will use the gravitational attraction of the boulder-plus-spacecraft to gently tug on the asteroid, slowly pulling it off course – a method NASA calls a gravitational tractor.
The effect is small but adds up. “It’s quite elegant in that it doesn’t even require us to touch the asteroid,” says Lindley Johnson, planetary defence officer at NASA headquarters.
“Nature’s tow rope, gravity, can be used to tug this asteroid, even though it’s a half-kilometre across.”
Over time, he adds, such a method should be usable to do “a very controlled and gradual adjustment” of a dangerous asteroid’s trajectory, so that when it comes by Earth, “it would be a miss, instead of a hit”.
And that’s a critically important piece of the mission because asteroids do occasionally strike.
The most recent exploded above Chelyabinsk, Siberia, in 2013, with a force about 30 times larger than the atom bomb that levelled Hiroshima, Holdren says, and injuring more than 1,000 people.
In 1908, an even larger asteroid exploded over an uninhabited region of Siberia with a force about 10 times larger yet – and the object that caused that explosion was puny compared to what might hit us.
“This is a hazard that 65 million years ago the dinosaurs succumbed to,” Holdren says.
“We have to be smarter than the dinosaurs.”