(Guest post Summarized by Hirokuni Oda)
The special issue “Science of solar system materials examined from Hayabusa and future missions” was published as one of the thematic series of SpringerOpen journal Earth, Planets and Space (EPS). Hayabusa was the Japan Aerospace Exploration Agency (JAXA) engineering mission to explore and return a sample from a near-Earth asteroid. It was launched in 2003 by 5th M-V launch vehicle, visited and explored asteroid 25143 Itokawa in 2005, and finally returned surface materials from there to Earth in 2010. The purpose of this mission was an engineering experiment of a round-trip journey to the asteroid, as well as scientific experiments for understanding the origin and evolution of the early solar system by exploring a primitive body. The S-type, sub-kilometer-sized asteroid Itokawa was found to be chondritic in composition and low-density rubble-pile in structure. Its surface was basically rough and covered with boulders, but smooth terrains such as MUSES-C Regio also existed where the first and second touchdown operations were conducted for sample collection (see Fig. 1).
The returned samples have been curated in JAXA’s Extraterrestrial Sample Curation Center. These samples are of small amount and very fine grained, because the impact-sampling method failed to shoot projectiles for sample acquisition and only floating dust grains entered into the sample catcher (see Fig. 2). Thousands of micron-sized silicate-dominated grains other than artificial contaminants were found by scraping them from the inside wall of the catcher with a Teflon spatula and tapping on the chamber. These samples are consistent with chondritic composition as shown by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). The removal of tens of micron-sized particles (up to 300 μm) in the catcher was started using the newly developed electrostatic control manipulator. So far, about 500 particles have been catalogued for initial description about size, shape, SEM images, and representative elemental and mineral composition from EDS spectra, and part of those catalogued particles have been delivered for more advanced research including preliminary examinations and through an international announcement of opportunity.
The preliminary examination of Hayabusa-returned samples has successfully unveiled the mineralogical, petrographic, chemical, and isotopic relationships between an S(IV)-type asteroid and ordinary LL chondrite meteorites as predicted by ground-based observation and provided the first direct evidence that meteorites originate from asteroids. The particle-size distribution and existence of rounded grains as well as the noble gas isotopic compositions have also recorded asteroid surface processes such as meteoroid impacts, possible granular flow, and solar wind irradiation on the asteroid surface, which were not observed in meteorites. Organic analyses have been also performed, but indigenous organic compounds have not been identified from the samples to date.
Investigation of Hayabusa-returned samples has expanded to any interested researchers by applying to the International Announcement of Opportunity, and the successful winners started their advanced and sophisticated research. Hayabusa 2013: 1st Symposium of Solar System Materials was held for these researchers to come together and discuss their latest results, giving new prospects to understanding the asteroid-meteorite connection and the early solar system origin and evolution. All the papers presented in the symposium were invited for submission to this EPS special issue, and any papers related to the topics were also welcomed. In the end, 14 manuscripts were successfully completed for publication in this special issue.