Hayabusa2: From Exploring Ryugu to Returning Samples

Hayabusa2 was organized by the Japan Aerospace Exploration Agency (JAXA) to return material from the C-type asteroid Ryugu and investigate relationships among minerals, water and organic matter. Its predecessor, Hayabusa, had visited the S-type asteroid Itokawa. The change of target type allowed the two sets of samples to be used to compare early Solar System materials preserved in different small bodies.

Studies for a successor mission began in 2006. A preparatory team was established in 2007, became a project team in May 2011, and entered critical design review in 2012. The spacecraft carried cameras, spectrometers, sampling equipment and an impactor, as well as the MASCOT lander developed by the German Aerospace Center and France's National Centre for Space Studies. It launched from Tanegashima on H-IIA Launch Vehicle No. 26 on 3 December 2014; the original 30 November launch had been postponed because of weather.

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Hayabusa2 arrived at Ryugu on 27 June 2018 and began observations from its home position about 20 kilometers from the asteroid's center. Engineers including Yuichi Tsuda worked with the science team to combine data on shape, gravity, temperature and boulder distribution into maps of candidate landing sites. Boulders covered the surface, leaving less safe space than expected and making engineering constraints crucial to site selection.

The first touchdown rehearsal in September 2018 was aborted when the dark surface interfered with laser ranging, prompting the spacecraft to ascend automatically. The team added rehearsals and used deployed reflective target markers for autonomous tracking to reduce touchdown error. Two MINERVA-II1 rovers and MASCOT carried out surface investigations in succession. On 22 February 2019, Japan time, the main spacecraft made its first sampling touchdown. Subsequent image analysis indicated a landing accuracy of about 1 meter.

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On 5 April 2019, the Small Carry-on Impactor created an artificial crater on Ryugu. The deployable camera DCAM3 recorded the ejecta, and the spacecraft later imaged the terrain again. Masahiko Arakawa and colleagues compared photographs taken before and after the impact and used elevation models to measure the crater's shape, testing crater-formation mechanisms under the asteroid's microgravity conditions.

The crater was asymmetrical and semicircular, with a rim spanning more than 10 meters. Boulders and subsurface structures affected the direction in which it expanded. The observation that the ejecta curtain did not detach from the ground as a whole supported the interpretation that crater growth was primarily constrained by gravity. This experiment, conducted at a known time and location, provided observational evidence for interpreting Ryugu's natural craters and exposing subsurface material.

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In July 2019, Hayabusa2 completed its second touchdown at a sampling site near the artificial crater. Material from the first sampling was stored in chamber A and material from the second in chamber C, facilitating comparisons between the collection sites. Researchers considered that some particles in chamber C might have come from subsurface layers excavated by the impact, rather than identifying every sample in that chamber as material unaffected by the space environment.

The return capsule landed in southern Australia on 6 December 2020, and the sealed samples were transported to the Extraterrestrial Sample Curation Center in Sagamihara. That date marked the completion of the Ryugu sample-return phase. After separating from the capsule, the mother spacecraft departed Earth's vicinity and entered an extended-mission trajectory; its flight did not end with the capsule's return.

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Research after recovery began with gas analysis and contamination control. JAXA performed successive mass-spectrometric analyses at a temporary facility in Woomera, Australia, and at Sagamihara, comparing gas from the container. A report issued on 15 December 2020 concluded that the extracted gas came from Ryugu on the basis of consistent composition, seal checks and subsequent outgassing observations.

The sample container was opened in a clean vacuum system. Chamber A was examined first and a small number of particles removed; subsequent operations took place in purified nitrogen. Different clean chambers handled opening, vacuum sampling, environmental transfer, sorting and weighing. The report confirmed the presence of abundant black particles in chamber A while leaving further analytical steps to follow. This phased handling created a traceable chain of curation between capsule recovery and sample research.

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Toru Yada and colleagues reported a total returned mass of 5.424 ± 0.217 grams, consisting mainly of millimeter- and submillimeter-sized particles. The team began with nondestructive characterization before allocating samples for further research. The measured mean bulk density of the particles was about 1.282 ± 0.231 grams per cubic centimeter, indicating substantial internal porosity. This was a measurement of particle density, not the mean density of the entire asteroid.

The samples' low reflectance and infrared absorption characteristics supported the interpretation that they were carbon-rich and had undergone aqueous alteration. Compared with CI carbonaceous chondrites, the Ryugu samples were darker, more porous and more fragile. Cross-checking physical specimens kept under controlled conditions against close-range remote sensing connected asteroid surface topography and spectra with tiny particles in the laboratory.

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