On March 5 the Japan Aerospace Exploration Agency (JAXA) released the extraordinary video shown above. The sequence of 233 images shows a spacecraft descending to collect material from the surface of an asteroid, before rising amidst fragments of ejected debris. It is an event that has never been captured on camera before.
The images were taken by a camera onboard the Hayabusa2 spacecraft, a mission to explore a C-type asteroid known as “Ryugu” and bring a sample back to Earth.
C-type asteroids are a class of space rock that is thought to contain carbonaceous material and undergone little evolution since the early days of the Solar System. These asteroids may have rained down on the early Earth and delivered our oceans and possibly our first organics. Examination of the structure of Ryugu and its composition compared to Earth will help us understand how planets can become habitable.
Hayabusa2 arrived at asteroid Ryugu on June 27, 2018. The spacecraft spent the summer examining the asteroid with a suite of onboard instruments. Despite being a tiny world at only 1km across, Hayabusa2 spotted different seasons on Ryugu. Like the Earth, the asteroid’s rotation axis is inclined so that different levels of sunlight reach the northern and southern hemispheres.
It also rotated upside down, spinning in the opposite sense to the Earth and its own path around the Sun. This is likely indicative of a violent past, a view supported by the heavily bouldered and cratered surface. This rugged terrain presented the Hayabusa2 team with a problem: where could they land?
After a summer of observations, Hayabusa2 had been planning three different operations on the asteroid surface. The first was the deployment of two little rovers known as the MINERVA-II1. The second was the release of a shoebox-sized laboratory known as MASCOT, designed by the German and French space agencies. Finally, the spacecraft was planning to land and collect a sample.
After careful consideration by the mission’s international science team, three landing sites were chosen. Ryugu was devoid of flat, open regions, but team selected sites based on gradient (not too steep), temperature (not too hot for the instruments) and boulder density (as low as possible).
The two MINERVA-II1 rovers were the first to be deployed on September 21, 2018. These little 1kg vehicles were designed to test mobility in a low gravity environment. As there is not enough friction for wheels, the two rovers “hopped” by rotating a weight that sprung back to bounce the rover across the surface. The pair began sending images back straight away, including a beautiful sequence showing the Sun passing over the asteroid surface.
Around two weeks later on October 3, MASCOT was successfully deployed to the surface. To guarantee power for the instruments, the lander was equipped with a lithium battery with an anticipated lifetime of up to 16 hours. MASCOT surpassed this expectation, operating for 17 hours and sending back data that is currently being analysed by the European teams.
The next step was the sample collection, originally scheduled for the end of October. But now the team paused to consider the situation. Images from low sweeps over the surface and from the rovers and lander highlighted exactly how difficult it would be to land. With the safety of the spacecraft at stake, the team decided to examine the landing site in more detail and postpone until the new year.
One of the main issues was that the original landing plan needed a 100m wide space for touchdown, but such a flat site simply did not exist on Ryugu. Close inspection of the best location —labelled L08— turned out to still be littered with boulders that posed a threat to the spacecraft.
In response to this, the team changed to a different navigation technique known as “pinpoint touchdown”. In the original plan, Hayabusa2 would descend and drop a shiny object known as a target marker onto the surface. By reflecting light from the target marker’s surface, Hayabusa2 could track the target marker to the surface of the asteroid and land safely. However, the precision of this technique depended on how accurately the target marker could be dropped, which was estimated at about ±50m. In the pinpoint touchdown technique, the target marker lands on the surface before the spacecraft’s descent, and Hayabusa2 descends relative to its position. The accuracy of this method depends on the distance from the target marker. For the region finally selected this permitted a landing accuracy of just ±2.7m, allowing Hayabusa2 to touchdown in a region just 6m wide.
Touchdown was scheduled for about 8am on February 22. On February 21 just after 1pm, Hayabusa2 began its descent. As with previous descent operations by the mission, navigation images were posted to the web, allowing everyone to follow the progress of the spacecraft as the asteroid loomed larger on camera.
When the spacecraft reached 500m above the surface, Hayabusa2 switched to autonomous descent mode. At 8.5m, the spacecraft turned so that it was aligned parallel to the surface for touchdown. As the sample horn reached the asteroid, 5g bullet was fired down the sampler horn to break up material and stir up the surface. This allowed material to rise through the sample horn and into one of the chambers of the sample container. When the spacecraft began to ascend again, the speed briefly slowed to allow any material caught in the inverted teeth of the sampler horn to rise into the container.
With Hayabusa2 moving away from the asteroid and back to its home position at a distance of 20km, data from the touchdown could be downloaded. This included images from a small monitor camera known as CAM-H. CAM-H had been developed and installed by donations from the general public and provided a view straight down the sampler horn. With a perfect view of the looming asteroid surface, CAM-H captured a series of frames during touchdown, creating the spectacular video of the event.
But this does not conclude the mission and one of the most exciting operations is still to come. Hayabusa2 is equipped with a small carry-on impactor (SCI) that is designed to strike the surface of Ryugu and generate an artificial crater. This will expose sub-surface material that has been protected from any weathering by the Sun’s radiation. A second touchdown will then be considered, possibly in or around the site of the fresh crater.
Hayabusa2 is now performing low altitude operations to examine Ryugu’s unforgiving surface for the best impact site. The plan is to deploy the SCI in early April, with a possible second touchdown after the start of May. Hayabusa2 will leave Ryugu at the end of the year and return to Earth at the end of 2020.
The mission is a breath-taking exploration of a new world, and one that may give us all information about where we come from.
Elizabeth Tasker is an astrophysicist and science communicator at the Japan Aerospace Exploration Agency (JAXA). Her research explores the formation of stars and planets, while her science articles have covered topics from Egyptian coffins to deep sea drilling (but mainly focus on exoplanets and space missions!). She also keeps her own website and personal blog.