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Letters to Nature

Nature 354, 287-289 (28 November 1991) | doi:10.1038/354287a0

Near miss of the Earth by a small asteroid

J. V. Scotti*, D. L. Rabinowitz* & B. G. Marsden†

  1. *Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA
  2. †Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA
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THE 0.91-m Spacewatch Telescope, on Kitt Peak in Arizona, is being used to search for new Earth-approaching asteroids. On the night of 18 January 1991 a streaked image extending over more than 161 arcseconds was recorded. Further observations over the next 4.6 hours showed that the images were of an object travelling in an independent orbit around the Sun; it was given the asteroidal designation 1991 BA. Orbital computations, presented here, indicate that 1991 BA was only 0.0053 AU from the Earth at the time of discovery, closing to 0.0033 AU at the last detection. Extrapolation of the calculated orbit shows that the object passed only 0.0011 AU (170,000km) from the Earth 12 hours after it was found. The observed brightness translates into an absolute visual magnitude 28.9, corresponding to a diameter of only 5–10 m. 1991 BA is the closest and smallest asteroid yet observed outside the Earth's atmosphere.