Fireball created by the Hayabusa2 re-entry capsule as it passes through the Earth’s atmosphere towards the ground (JAXA).

In the mission control room in Japan, all eyes were fixed on one of the large screens that ran along the far wall. The display showed the night sky, with stars twinkling in the blackness. We were waiting for a delivery from space.

Japan’s Hayabusa2 mission launched from the Tanegashima Space Center on December 3, 2014. The spacecraft was headed to asteroid Ryugu, with the intention of studying the tiny world and collecting a sample to return to Earth.

The mission would prove to be an incredible success. Not only did the spacecraft gather two samples from the asteroid, but it was the first mission to deploy autonomous rovers to explore an asteroid’s surface, generate an artificial crater in order to study the asteroid’s structure and collect a sample of the interior, and additionally, deploy a lander to make scientific measurements from the surface itself. The mission finale was to return the samples safely back to Earth on December 6, 2020. The grains in that sample container may hold clues as to how the Earth became habitable.

Ryugu is an example of a C-type or “carbonaceous” asteroid. These asteroids have undergone relatively little change since the start of the solar system, and are thought to contain hydrated minerals (minerals containing water in their structure) and possible organics. It is this class of asteroid that may have crashed into the early Earth and delivered the necessary tools for life to begin. Analysis of the Ryugu sample could therefore tell us about our own beginnings and how terrestrial planets develop habitable conditions.

Images before and after the first touchdown of Hayabusa2 on asteroid Ryugu, taken with CAM-H on February 21, 2019 (animation plays at 5x speed) (JAXA).

As the Hayabusa2 spacecraft drew near the Earth, five “trajectory control manoeuvres” (TCMs) were planned. The first four of these were designed to put the spacecraft onto a collision course with the Earth, aimed at the Woomera desert in Australia. The re-entry capsule would then be released, and the spacecraft would make a final manoeuvre to divert onto an orbit that swept past the Earth and back into deep space.

Despite the smooth progress so far, there were concerns. The capsule release mechanism had not been tested since launch six years previously and it was always possible that separation would fail.

On Saturday December 5, 2020 at 14:30 JST, data from the spacecraft confirm that the separation operation had worked as planned and the capsule was now falling towards Earth. However, an even better confirmation arrived later that evening, when telescopes in Japan successfully imaged both the spacecraft and the capsule from the ground. The asteroid sample really was heading home.

Hayabusa2 (left) and the separated re-entry capsule (right) images from Kyoto University’s Seimei telescope (Kyoto University Okayama Observatory). Link to animation.

Landing on Earth is not an easy process. Upon entering the atmosphere, the surface of the re-entry capsule would have reached temperatures of about 3000°C. To avoid frying the sample, the capsule had a heat shield that kept the sample container to temperatures of about 80°C; cool enough to prevent damage. As the capsule approached the ground, the heat shield was jettisoned and a parachute deployed. The expected landing area was about 100 kilometres squared, which is an amazingly precise region given that the capsule was released at an altitude of 220,000 km; more than half the distance from the Earth to the moon.

Despite this precision, it was a big search area for the capsule recovery team on the ground in Woomera. The capsule therefore began to emit a beacon as the parachute deployed. The recovery team had set up five antennas to listen for the beacon and report the direction of the signal. The intersection of the reported direction from each station then gave the rough landing area of the capsule.

Like the capsule separation, the beacon’s functionality was another uncertainty after six years in space. The team therefore had a heap of contingency plans for finding the capsule. One of these was to observe the capsule’s trajectory when the high temperatures experienced by the heat shield would cause the capsule to glow like a fireball in the night sky.

Re-entry capsule of Hayabusa2, captured near Coober Pedy, Australia on December 6, 2020 at around 02:29 JST (JAXA).

It was the appearance of the fireball that everyone was waiting for in the mission control room. The display screen showed the view from near Coober Pedy, close to Woomera in Australia. Above the screen, two clocks counted down the minutes in JST and UTC time zones. It was just before 02:30 am on December 6 in Japan.

As time ticked on, a feeling of diappointment arose in the control room. The capsule had clearly separated from the spacecraft, but perhaps the fireball had been missed.

Then, a bright star appeared on the dark screen and started moving rapidly, shooting across the screen from right to left. A new view through a telescopic lens shortly afterwards would show the tail of the artificial shooting star, due to the extreme heat surrounding the capsule.

A loud cheer erupted around the control room. Our sample capsule was home. Now, we just had to find it.

The re-entry capsule and parachute spotted from the air in Woomera, Australia (JAXA).

News came at 03:07 am JST that the capsule beacon had also operated as planned and an approximate site for the capsule was now known. At 03:17 am JST, a helicopter took off in the direction that the beacon signals had indicated, and at 04:47 am JST, confirmation arrived from Woomera: the capsule had been found.

While the capsule location had been sought immediately after re-entry, approach to collect the capsule from the ground was more cautious. Pyrotechnics had been used to release the parachute, and it was possible not all the explosives had been detonated. Among the first images we received of the capsule was therefore team members in protective gear to confirm the capsule’s safety.

Team members wear protective gear as they confirm the capsule is safe, since explosives were used to deploy the parachute (JAXA).

However, no problems were found and the capsule was transported to a “Quick Look Facility” (QFL) that had been assembled in Woomera. The purpose of the QFL was to test for gases that may have been emitted from the Ryugu sample. Although the capsule seal was undamaged, volatiles should be tested as quickly as possible to prevent any chance of escape. Without opening the sample container, the gases were extracted and analyzed. Results from this have the hallmarks of scientific caution, with the team reporting that gases were detected but waiting for further analysis before confirming that they originate from the asteroid sample.

To ensure no chemical reactions would alter the sample, the capsule had to be stored in the specialised curation chamber back in Japan within 100 hours of collection. The aeroplane therefore left Woomera at 22:30 JST on December 7, touching down in Tokyo’s Haneda airport at 07:20 JST on December 8.

The Hayabusa2 Extended Mission to 1998 KY26 (JAXA).

The sample is now in the curation chamber, but patience is still required before we can expect the first results. The re-entry capsule will be deconstructed and the sample container opened in a vacuum environment in the next week. However, Astromaterials Science Research Group Manager Tomohiro Usui noted at the JAXA press conference that it would be about a month before a definite weight of the sample would be known, and about six months for the initial analysis and catalogue of the sample would be complete. This will include optical and infrared examinations of the sample and should provide the first clues to the piece of our past we are holding.

Meanwhile, Hayabusa2 has turned away from the Earth and is heading to a new destination: a tiny, rapidly rotating asteroid known as 1998 KY 26. It is a journey that will last 11 years with a team that never gives up a challenge.