Scattered Light Correction of HAYABUSA/AMICA Data and Quantitative Spectral Comparisons of Itokawa

The Hayabusa spacecraft rendezvoused with its target asteroid 25143 Itokawa in 2005 and brought an asteroidal sample back to the Earth in 2010. The onboard camera, AMICA, took more than 1400 images of Itokawa during the rendezvous phase. It was reported that the AMICA images were severely contaminated by light scatter inside the optics. The effect made it difficult to produce the color maps at longer wavelengths (>800 nm). In this paper, we demonstrate a method to subtract the scattered light by investigating the dim halos of Itokawa and the Moon taken by AMICA during the inflight operation. As the result, we found that the overall data reduction scheme including the scattered light correction enables to recognize ~3% regional differences in the relative reflectance spectra of Itokawa. We confirmed that the color variation in Itokawa was largely attributed to space weathering.Comment: 15 pages, 10 figures, 4 tables, Publications of the Astronomical Society of Japan (accepted

Large Particles in Active Asteroid P/2010 A2

Previously unknown asteroid P/2010 A2 rose to prominence in 2010 by forming a transient, comet-like tail consisting of ejected dust. The observed dust production was interpreted as either the result of a hypervelocity impact with a smaller body or of a rotational disruption. We have re-observed this object, finding that large particles remain a full orbital period after the initial outburst. In the intervening years, particles smaller than ~3 mm in radius have been dispersed by radiation pressure, leaving only larger particles in the trail. Since the total mass is dominated by the largest particles, the radiation pressure filtering allows us to obtain a more reliable estimate of the debris mass than was previously possible. We find that the mass contained in the debris is ~5x10^8 kg (assumed density 3000 kg m^-3), the ratio of the total debris mass to the nucleus mass is ~0.1 and that events like P/2010 A2 contribute <3% to the Zodiacal dust production rate. Physical properties of the nucleus and debris are also determined.Comment: 16 pages, 4 figures; ApJLett, in Pres

Estimation of space weathering timescale on (25143) Itokawa: Implications on its rejuvenation process

Context. The space weathering timescale of near-Earth S-type asteroids has been investigated by several approaches (i.e., experiments, sample analyses, and theoretical approaches), yet there are orders of magnitude differences. Aims. We aim to examine the space weathering timescale on a near-Earth S-type asteroid, Itokawa using Hayabusa-AMICA images and further investigate the evolutional process of the asteroid. Methods. We focused on bright mottles on the boulder surfaces generated via impacts with interplanetary dust particles (IDPs). We compared the bright mottle size distribution with an IDP flux model to determine the space weathering timescale. Results. As a result, we found that the space weathering timescale on Itokawa's boulder surfaces is 10$^3$ years (in the range of 10$^2$-10$^4$ years), which is consistent with the timescale of space weathering by light ions from the solar wind. Conclusions. From this result, we conclude that Itokawa's surface has been weathered shortly in 10$^3$ years but portions of the surface are exposed via seismic shaking triggered by a recent impact that created the Kamoi crater.Comment: 10 pages, 6 figures, Accepted for publication in Astronomy & Astrophysic

2006 Fragmentation of Comet 73P/Schwassmann-Wachmann 3B Observed with Subaru/Suprime-Cam

We analyzed the Subaru/Suprime-Cam images of 73P/Schwassmann-Wachmann 3B and detected no fewer than 154 mini-comets. We applied synchrone-syndyne analysis, modified for rocket effect analysis, to the mini-fragment spatial distribution. We found that most of these mini-comets were ejected from fragment B by an outburst occurring around 1 April 2006. The ratio of the rocket force to solar gravity was 7 to 23 times larger than that exerted on fragment B. No significant color variation was found. We examined the surface brightness profiles of all detected fragments and estimated the sizes of 154 fragments. We found that the radius of these mini-fragments was in the 5- to 108-m range (equivalent size of Tunguska impactor). The power-law index of the differential size distribution was q = -3.34 +/- 0.05. Based on this size distribution, we found that about 1-10% of the mass of fragment B was lost in the April 2006 outbursts. Modeling the cometary fragment dynamics revealed that it is likely that mini-fragments smaller than ~10-20 m could be depleted in water ice and become inactive, implying that decameter-sized comet fragments could survive against melting and remain as near-Earth objects. We attempted to detect the dust trail, which was clearly found in infrared wavelengths by Spitzer. No brightness enhancement brighter than 30.0 mag arcsec^-2 (3sigma) was detected in the orbit of fragment B.Comment: Total pages: 46 Figures: 12 Tables: 1 To appear ICARU