Japanese contribution to in-situ meteoroid and debris measurement in the near Earth space

This paper reviews major results of present studies and recent developments for future missions in the Japanese space program regarding in-situ measur ement and collection of micrometeoroids and orbital debris in the near Eart h space. Japan's contribution in this area began with the post flight impac t analysis of the Space Flyer Unit (SFU) satellite which was returned to Ea rth in 1996 after 10-month exposure in space. Despite a decade later than s imilar efforts first conducted in the USA and Europe, it resulted in a reco rd of over 700 hypervelocity impact signatures, which now forms the nation' s first database of real space impacts being open to public in the Internet . Together with laboratory impact tests, both morphological and elemental a nalyses of the impact craters yielded new insights of the meteoroid to debr is ratio as well as flux variation compared with the previous spacecraft. T he next step was a passive aerogel exposure in the STS-85 shuttle mission i n 1997. No hypervelocity impact was found there but its experience has been incorporated for designing a microparticle collector to be on-board the Ja pan Experiment Module-Exposed Facility of the International Space Station. All of such "passive" collection of micro-impact features, however, still l eave the significant uncertainty in the quest of their origins. Therefore a n aerogel-based "hybrid" dust collector and detector (HD-CAD) is currently under the development. It measures time of impact and deduces impactors' or bital and physical parameters by detecting impact flash while still capturi ng them intact. The system is suitable for both (1) sample return missions in LEO as well as to parent bodies of meteoroids, i.e., comets and asteroid s, and (2) one-way mission to where the thermal and plasma environment is s uch that impact induced plasma detectors may suffer from significant noise, e.g., a Mercury orbiter and a solar probe. Together with unambiguous dust samples from a comet by STARDUST and an asteroid by MUSES-C as references, the HD-CAD in the LEO will be able to deduce the accretion rates of the com etary and asteroidal dust grains on the Earth.