Here is Apophis!

Details Here is Apophis! Here is Apophis!

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.4305t&link_type=abstract

American Astronomical Society, DPS meeting #41, #43.05

Astronomy and Astrophysics

Astronomy

Scientific paper

Previously we had called attention to a positive declination bias of 0.1 to 0.2 arcsec in over a hundred observations of (99942) Apophis made at Mauna Kea Observatory since the last published observations in 2006. After investigating a variety of possible causes, we have determined that the vast majority of the bias is due to the reference catalog used for the observations (the USNO-B1.0), with differential refraction being the second largest effect, but usually smaller than the catalog error by over an order of magnitude. To correct for this problem, we have begun systematically remeasuring all of our Apophis images against the 2MASS point source catalog. Although the brighter magnitude limit of this catalog provides typically about half the number of reference sources compared to the USNO-B1.0, the number is still adequate to perform good astrometric solutions. The best of our data has astrometric uncertainty as small as 0.05 arcsec, thus offering several times higher fractional precision than the five available Doppler radar observations of Apophis, and approaching within a factor of two the fractional precision of the two available delay radar observations. However, on one of the nights already remeasured, the bias was not completely eliminated. Comparison to the UCAC2 catalog revealed a bias in the 2MASS catalog for the corresponding position on the sky. Our complete data set now encompasses 40 nights spanning from the discovery observations on 2004 June 19 to 2008 January 9, thereby extending the observational arc by about 50 percent more than the currently published observations do. These data will be used to refine the details of the 2029 close approach to the Earth, which will indicate whether an impact risk for the Earth remains in the decade of the 2030s.

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