Between June 2018 and November 2019, the Japanese spacecraft Hayabusa2 examined asteroid Ryugu, which is just under one kilometre in size, at close range. The spacecraft came into contact with Ryugu twice and collected samples from the asteroid's surface.
"Hayabusa2 was and is one of the most complex missions in the history of research into the bodies of the Solar System," emphasises Anke Pagels-Kerp, DLR Divisional Board Member for Space. "In addition to collecting samples, several unusual experiments were carried out on and around asteroid Ryugu for the first time. Studying the samples from Ryugu is also a recognition of DLR's expertise in the field of planetary research."
Hayabusa2 then made its way back to Earth. The sample capsule separated from the spacecraft at a distance of 220,000 kilometres from Earth on 5 December 2020. The capsule entered Earth's atmosphere, slowed down significantly and floated down to the surface with the valuable cosmic cargo attached to a parachute at the end of the manoeuvre. A search team soon found the capsule intact in the target area in central Australia. About five grams of Ryugu have since been catalogued and curated in Japanese laboratories. Now the Japan Aerospace Exploration Agency (JAXA) has approved an application from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) to study a small part of the samples.
"With the analyses in our planetary laboratory, we want to answer a fundamental methodological question regarding the interpretation of sample material and remote sensing data," explains Alessandro Maturilli from the DLR Institute of Planetary Research in Berlin-Adlershof." Typically, the reflection light curves of planetary analogue material studied in the laboratory always show greater contrasts than 'real' extraterrestrial material. We need to find out if this is due to differences in composition or other material properties, or due to the different measurement methods used in space and on Earth." Answering this question is important for the evaluation of remote sensing measurements of the various bodies in the Solar System, such as observations made from a distance using spacecraft.
Comparison of remote sensing data with laboratory measurements
The laboratory team will compare hyperspectral data and spectral image data acquired by Hayabusa2 from different heights above ground – and directly on the surface – with the two DLR experiments in the MASCOT landing module, MASCam and MARA, using the new laboratory measurements. The laboratory analyses are carried out using 'bidirectional reflectance spectroscopy' under simulated conditions similar to those on the asteroid surface in the wavelengths of UV light, visible light and near and far infrared. "We are curious to find out whether we can see a difference again in the samples from Ryugu, or if the differences between remote sensing measurements and the studied sample material no longer appear."
In total, the Hayabusa2 mission returned just over five grams of material – about a teaspoon of dust. Over the past year, these samples have been analysed and catalogued in Japan by the 40 members of the Hayabusa2 Preliminary Analysis Team. DLR's Enrica Bonato and Gabriele Arnold were also involved in this preliminary analysis in Japan. At the same time, the global scientific community was able to apply for laboratory analyses of Ryugu material. The team at the DLR Institute of Planetary Research has been assigned the sample A0112. "This is a three-millimetre grain of Ryugu weighing 5.1 milligrams," says Enrica Bonato. "It will arrive in Berlin before the end of 2022. We will examine it here in the Planetary Spectroscopy Laboratory (PSL). Afterwards, the sample will be analysed by a functional camera model like the one used in the MASCOT lander on the asteroid. Then we will compare the spectral characteristics obtained in the laboratory with the analysis made using MASCam." The Berlin Museum für Naturkunde, which closely cooperates with the DLR Institute of Planetary Research, will also take a 'close look' at the sample and examine it with a scanning electron microscope, a microprobe and X-ray spectroscopy.
Results will improve spectral analysis of other celestial bodies
The measurements of the Ryugu samples will make it possible to better link laboratory and remote sensing observations. In a next step, the findings could lead to better results not only in the evaluation of the Hayabusa2 observations, but also in all remote sensing observations in which planetary surfaces are analysed using spectroscopy. The study of the mineralogical composition and organic matter of the samples collected by Hayabusa2 also builds on this. "We hope to use this to better understand how asteroids, these primordial building blocks of the Solar System, formed. On Ryugu, we have obtained valuable samples from the earliest period of the Solar System," says Alessandro Maturilli, who also hopes to gain insights beyond asteroid research. "It should expand our knowledge of the mineralogy of the dust and carbonaceous organic matter in the protoplanetary disk from which the planets formed 4.5 billion years ago."
It may also be possible to draw conclusions about how water and water ice in the parent body of asteroid Ryugu led to changes in the mineralogical composition by comparing the sample with measurements of unaltered carbonaceous analogue meteorites from the class of chondrites. Spectral data will be supplemented with data measured using Raman spectroscopy under simulated asteroid surface conditions. X-ray diffraction studies of the crystal lattices of the various minerals in the Sample Analysis Laboratory (SAL) at DLR's Institute of Planetary Research would also make it possible to determine the overall mineralogy of the samples and to investigate the presence and nature of organic material in the samples.
Hayabusa2 was, or is, one of the most complex missions in the history of Solar System exploration. In addition to sampling, several extraordinary, first-time experiments were conducted on and near asteroid Ryugu. Among other things, JAXA deployed several small rovers on the surface, equipped with cameras. An explosive device hovering above the surface was detonated to create a crater to investigate Ryugu beneath the surface. Finally, on 3 October 2018, the MASCOT lander developed by DLR and the French space agency (CNES) was deployed, landed on Ryugu and carried out a day of measurements using its four instruments. Finally, the two mission highlight sampling sessions took place on 22 February and 11 July 2019, before the spacecraft then returned to Earth. Hayabusa2 is still on a heliocentric orbit following its delivery of the asteroid samples to Earth and is scheduled to study asteroid 2001 CC 21 in July 2027 and the near-Earth asteroid 1998 KY 26 in July 2031 during flybys.