Hayabusa2 hovering above asteroid Ryugu in October.Credit: JAXA

Over the past year, the space probe Hayabusa2 has pelted the asteroid Ryugu with bouncing probes, fired a bullet at it, and taken a bite out of it — all for science. But this week the mission performed its most daring manoeuvre yet: dropping an explosive onto the surface of the asteroid to create a small crater.

If the explosion went as planned, it will have exposed some of the asteroid’s subsurface layers, allowing the probe to gather samples during a later touchdown.

The operation took place on 5 April. First, the probe lowered itself down from an initial ‘parking’ altitude of 20 kilometres to just 500 metres above the asteroid’s surface. From there, it dropped an explosive device. Then, after making a partial ascent, it released a second device carrying a camera. The Japan Aerospace Exploration Agency (JAXA) confirmed that the probe had released the device and was safe at 11:56 am local time.

“We conducted a lot of experiments, but when we did this for real, I was still very nervous,” said Osamu Mori, an engineer from JAXA’s Institute of Space and Astronautical Science (ISAS) in Sagamihara who was involved in the impactor’s operation, during a live broadcast.

Beating a﻿ retreat

In Ryugu’s extremely weak gravity, the bomb took about 40 minutes to reach the surface. Meanwhile, the spacecraft manoeuvred itself to a safe zone behind the asteroid. That way, when the 9.5 kilograms of explosive charge went off, the debris it kicked up would not harm the probe. The single-use camera device, however, was still hovering above the target, ready to take pictures of the explosion and upload them to the mothership by way of a radio link. Images confirm that the charge detonated, but the mission team have not confirmed there is a crater.

Hayabusa2’s mission manager, Makoto Yoshikawa, also at ISAS, told Nature in June that the manoeuvre would be “a very risky operation”, but one with big potential payback. It is hoped that the experiment will give astronomers the opportunity to study material from under the asteroid’s surface, which could shed light on the early Solar System.

In the following weeks, the probe will image the crater from above. Then, at a later date, mission scientists plan to execute the final major step of the mission. They will lower the probe into the crater and collect a sample. This will be the second sample collected from Ryugu: the probe already touched down on 22 February and collected some space dirt after kicking it up with a bullet.

Impact factor

Space agencies have blown craters on bodies in the Solar System before. In 2005, NASA’s Deep Impact mission discharged a washing-machine-sized impactor at high speed onto a comet called Tempel 1. And on many occasions over the decades, researchers have sent impactors to the Moon and deliberately crashed probes on the Moon’s surface — most recently the Lunar Atmosphere and Dust Environment Explorer — and studied the effects.

Hayabusa2 left Earth in late 2014 and arrived at Ryugu in June 2018. In two separate phases in September and October, it released a total of three small probes onto the surface. The spacecraft is scheduled to begin its journey back to Earth before the end of 2019. A year later, a re-entry capsule will carry samples down with it so that scientists can study them in the lab.