Mathematics – Probability
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.2702t&link_type=abstract
American Astronomical Society, DPS meeting #40, #27.02; Bulletin of the American Astronomical Society, Vol. 40, p.434
Mathematics
Probability
1
Scientific paper
Between 2006 March, when the last observations of (99942) Apophis were obtained by others, and 2008 January, we have acquired over 200 additional observations on 27 different nights, with one of the goals being to improve the orbit sufficiently to eliminate the 2036 impact possibility. Rather surprisingly, the astrometry shows a systematic offset of about 0.2 arcsec to the north of the ephemeris position. During the time spanned by these observations, Apophis made nearly two complete trips around the sky, thus sampling a wide variety of background reference fields. We have been studying this problem, which has implications for the impact probability calculations. Because of the small solar elongations at which these observations were made, we investigated relativistic deflection of the light from background reference stars. The displacement is primarily in ecliptic longitude for a source located near the ecliptic, with the declination component being insufficient to explain the observed offset. Because the observations were also made at large zenith distances and often unfiltered, we investigated the effect of differential refraction. Although considerably larger than the relativistic effects, differential refraction also appears to be insufficient to explain the observed offset. Our attention is currently focused on potential systematic errors in declination in our astrometric reference catalog of choice, namely the USNO-B1.0 catalog. A comparison of positions for 64 ICRF sources that appear in the USNO-B1.0 catalog by da Silva Neto (Astron. Astrophys. 429, 739-745, 2005) has shown an average declination offset of +0.116 arcsec, about half of what we see in the Apophis data, with 45 of the 64 sources showing positive differences. Systematically positive declination offsets are also seen for other asteroids, suggesting that the problem is not unique to Apophis.
Bernardi Fabrizio
Micheli Mario
Tholen David J.
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