Credit: JAXA, University of Tokyo & collaborators

It was a brief but historic tap on the surface of the asteroid Ryugu, at 7:29 am Japan time on 22 February. The Hayabusa2 spacecraft touched down at its target location, where it shot a projectile at the surface to collect a sample from the space rock. The probe then began ascending to its ‘parking’ altitude, the Japan Aerospace Exploration Agency (JAXA) said in a series of tweets.

The manoeuvre was considered one of the high-risk highlights of this mission — Ryugu’s surface is strewn with boulders that could damage the craft. The mission aims to return samples of asteroid material back to Earth at the end of 2020 for study.

“The touchdown has progressed very smoothly,” said Satoshi Hosoda, of JAXA, in a live webcast. “Those people who were involved were very tense, but then we saw nice smiles and they were all hugging each other.”

Hayabusa2 (illustration) temporarily touched down on asteroid Ryugu on Friday.Credit: JAXA

As the probe gently touched down, a bullet fired into the surface, kicking up sand, pebbles and fragments of rock into a collection chamber, called a sampler horn. Had this failed, the horn has teeth that can lift surface material to the probe as a back-up method.

Yuichi Tsuda, the mission’s project manager, later confirmed at a press conference that the sequence for the projectile firing to collect samples had happened as planned, but they await confirmation that there is material in the sampler horn.

If successful, it will be only the second time in history that a probe has collected a sample from an asteroid, after a predecessor mission, Hayabusa, did so in 2005.

Launch to landing

Hayabusa2 began its slow fall towards Ryugu around 26 hours before touchdown, starting from an altitude of 20 kilometres, where it had been hovering. (Ryugu’s weak gravity makes it difficult for objects to remain in orbit around the asteroid.)

The spacecraft autonomously guided its descent, ready to abort by gently pushing itself upwards, should any anomalies occur. As the asteroid’s surface slowly rotated below, Hayabusa2 locked onto a ‘target marker’ — essentially, a small, reflective beanbag it had previously deployed to the surface.

Mission scientists had dubbed the target site L08-E1. They had selected it as one of the ‘least bad’ options on the asteroid, which is covered almost entirely by rocks of varying sizes. Hitting a boulder during a touchdown manoeuvre could have disastrous consequences for the mission.

Hayabusa2 launched in late 2014 and arrived at Ryugu — an object only 1 kilometre wide in an orbit not far from Earth’s — in June 2018. JAXA then mapped the asteroid’s surface in detail, and later selected several sites for a multi-pronged assault on the space rock. The mothership craft has already released three small probes onto the surface, from where they beamed back pictures in September and October.

In the next, even more daring, phase of the mission, Hayabusa2 will shoot an explosive at the surface to create a small crater. The craft will then attempt another touchdown to collect some of the subsurface material kicked up by the impact.

The probe will head back to Earth towards the end of 2019, and its sample capsule is expected to re-enter the atmosphere in late 2020.