NASA has released new details on how it plans to boldly go to an asteroid and come back with a bit of it. The Asteroid Redirect Mission (ARM) is part of the space agency's Asteroid Initiative announced in 2013, which envisions the capture and return of an asteroid to lunar orbit for study by astronauts as a rehearsal for a later mission to Mars.
Scheduled to begin in 2020, the purpose of the ARM is to test new technologies and techniques that would be needed for later manned deep space missions while learning more about asteroids and how to defend the Earth against them. This involves sending a robotic spacecraft to collect a boulder from a near-Earth asteroid, then return it to lunar orbit, where a later manned mission will rendezvous to retrieve samples.
The ARM will begin with an unmanned Asteroid Redirect Vehicle being sent to a target asteroid, which has yet to be selected. The collector spacecraft will travel to the asteroid on a multi-year trajectory using Solar Electric Propulsion (SEP). This uses solar panels to power ion thrusters that provide a very low, constant thrust for years on end by charging xenon atoms and accelerating them. The system is currently being used on the Dawn mission and NASA hopes that it could one day be used to preposition supply craft for a Mars mission.
Once at the asteroid, the spacecraft will select a likely boulder and collect it using its robotic arms equipped with microspine grippers, then lift it off by hopping and using thrusters. NASA says it will take about six years to bring the boulder back to Earth. On return, the Asteroid Redirect Vehicle will go into a distant retrograde orbit around the Moon at a distance of 61,500 km (38,200 mi), which is very stable and requires relatively little energy to reach. This will not only place the returned boulder within the reach of astronauts, it will also test the orbit's suitability for parking future interplanetary spacecraft, such as a Mars mission habitat.
Another objective of the asteroid return mission will be to test methods for defending the Earth against asteroid strikes. One of these will be to use the mass of the Asteroid Redirect Vehicle and its boulder cargo as an interplanetary tug called a gravity tractor. By going into a halo orbit around the target asteroid and orienting in a particular direction, this will pull the asteroid minutely and while this fractional change might not seem like much, over hundreds and millions of miles that deflection can add up, making the difference between a hit and a miss.
After the boulder has been placed into lunar orbit in the mid-2020s, NASA will launch an Orion spacecraft with two astronauts aboard on a 25-day mission to rendezvous with the asteroid fragment for study and collecting samples. While there, the astronauts will test new sensors and a new docking system to link the Orion and the Asteroid Redirect Vehicle, after which the astronauts will spacewalk to the boulder using a new generation of spacesuits.
NASA says it will select a target asteroid by 2019, which will be about a year before the unmanned collector spacecraft is launched. These will be assessed based on size, shape, rotation, and orbit. The current candidates include the asteroids Itokawa, Bennu, and 2008 EV5, but up to two more candidates will be added each year until the mission starts.
The collection of asteroid samples will also help train astronauts and mission managers in how to collect and secure samples for return on future Mars missions.
"The Asteroid Redirect Mission will provide an initial demonstration of several spaceflight capabilities we will need to send astronauts deeper into space, and eventually, to Mars," says NASA Associate Administrator Robert Lightfoot. "The option to retrieve a boulder from an asteroid will have a direct impact on planning for future human missions to deep space and begin a new era of spaceflight."
The agency is asking the US Congress for US$50 million towards the mission in the 2016 budget.
The animation below shows one of the latest scenarios envisioned for the asteroid return mission.