An earlier mission visited the asteroid Itokawa in 2005 but suffered a string of failures. It was supposed to fire bullets into the space rock at close range and scoop up the resulting debris, but the bullets never fired. Luckily, some dust slipped into the probe's collectors and was brought back to Earth in 2010.
Hayabusa 2 will try to avoid its predecessor's mistakes when it lands on the kilometre-wide asteroid 1999 JU3. "We have learned a lot from Hayabusa," says mission leader Makoto Yoshikawa of the Japan Aerospace Exploration Agency in Sagamihara. "We modified all the parts where Hayabusa had troubles."
Hayabusa 2 will have backup software in case of failures, and instead of using bullets, the probe will drop an impactor from an altitude of 300 metres before landing and gathering the shrapnel.
"The new thing is the impactor," says Yoshikawa. "The impactor will make a small crater on the surface of the asteroid, and we will try to get the material inside the crater, which means we try to get subsurface material." That will allow scientists to compare the asteroid's radiation-scorched surface to its pristine interior.
Hayabusa 2 is not the only asteroid-grabbing mission in the works. In 2016, NASA plans to launch a spacecraft called OSIRIS-Rex to scoop up samples from a space rock.
Osiris-Rex will orbit its target asteroid for months, taking high-resolution images to determine where it should collect a sample. But rather than landing on its quarry like the Hayabusa missions, it will stick out an arm and grab what it can.
Yoshikawa says Osiris-Rex aims to collect much more rocky debris than Hayabusa 2, which is expected to bring back no more than about 1 gram. But he says Osiris-Rex will only be successful if the surface of its target asteroid is covered with soil-like particles of rock called regolith. If it's a solid body, the spacecraft might not be able to collect samples.
"I think the method of Hayabusa 2 is best, if we do not know the surface condition of the target asteroid," Yoshikawa says. "In our case, we can get the sample even if there is no regolith on the surface."
Both missions will target asteroids rich in minerals that formed in water. Isotopic studies of that water could shed light on whether Earth's water came from asteroids, comets or from the planet's own rocky building blocks.