31st Annual Meeting of the DPS, October 1999
Session 40. Dust
Contributed Oral Parallel Session, Thursday, October 14, 1999, 8:30-9:40am, Sala Kursaal

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[40.01] Mars Dust Counter on Board NOZOMI: Initial Operation

S. Sasaki (Geol. Inst., Univ. Tokyo), E. Igenbergs (LRT, TU-Muenchen), H. Ohashi (Tokyo Univ. Fisheries), R. Muenzenmayer, W. Naumann, G. Hofschuster, G. Farber, F. Fischer (TU-Muenchen), A. Fujiwara (ISAS), A. Glasmachers (Univ. Wuppertal), E. Gruen (MPI-Kernphysik), Y. Hamabe (Univ. Tokyo), H. Iglseder (STMS), H. Miyamoto (Univ. Tokyo), K. Morishige (ISAS), T. Mukai (Kobe Univ.), K. Nogami (Dokkyo Univ. School Med.), G. Schwehm, H. Svedhem (ESA/ESTEC), K. Yamakoshi (Univ. Tokyo)

Mars Dust Counter (MDC) is an impact-ionization type dust detector on board Japanese mission NOZOMI (PLANET-B), which was launched on July 4th, 1998. MDC will discover dust particles around Mars and reveal distribution of the proposed Martian ring or torus of dust from Phobos and Deimos. Since NOZOMI will take elongated retrograde orbits whose apoapsis is beyond the Dei-mos' orbit, it can encompass the expected distribution (ring/torus) of the dust ring/torus. Although relative velocity between ring particles and NOZOMI is 1-3 km/s, expected dust mass is large (a few tens micron in size) enough to be detected by MDC. If the self-sustaining mechanism, where collisons between the satellite and ring dust should supply additional dust, is being operated, we can expect dust abundance detectable by MDC.

On July 13th, MDC detected the first impact signal. This was the first scientific data obtained by NOZOMI. During parking orbits around the Earth for five months, NOZOMI detected about 20 clear impact signals. On November 18th 1999, NOZOMI encountered the Leonids meteor tube, which was formed by Comet 55P/Tempel-Tuttle. Timing of NOZOMI-Leonids encounter was predicted to be about one day later than that of the Earth-Leonids encounter. Relative velocity between MDC with Leonids dust was 70km/s. MDC detected two questionless dust impacts whose impact velocities may be higher than a few 10km/s. NOZOMI is a spin-stabilized satellite. For those two impacts, phase angles from solar direction were 75 and 154 degree that were involved in the anti-Leonids direction (10-190degree).

MDC will also measure other particles such as interplanetary dust and interstellar dust during the cruising phase, which will continue by the end of 2003. From 2004, NOZOMI will investigate dust particles around Mars.

The author(s) of this abstract have provided an email address for comments about the abstract: sho@geol.s.u-tokyo.ac.jp

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