100P/Hartley

Comet Hartley 1 (designated 100P/Hartley) belongs to the Jupiter family of comets and completes an orbit around the Sun every 6.35 years. It is one of a dozen comets found by Australian astronomer Malcolm Hartley. On 29 April 2164, it will come within 0.487 AU (about 72.9 million kilometers) of Earth. Australian astronomer Malcolm Hartley first spotted the comet on 13 June 1985, using CCD images from the UK Schmidt Telescope. At discovery, it shone at 16th magnitude in the constellation Virgo, with a tail stretching roughly 1 arcminute toward the southeast. For three weeks after the announcement, Hartley was the only person to observe it. A follow-up observation by Hartley on 10 July 1985 allowed Brian G. Marsden to calculate the comet’s elliptical orbit for the first time, revealing it was a short-period comet with a 5.66-year orbit. James B. Gibson made the only Northern Hemisphere observations of this first apparition on 27–29 July 1985. The comet was last detected that year by Alan C. Gilmore on 14 August 1985. Because so few observations were made during its 1985 apparition, the comet’s orbit remained uncertain and it was nearly considered lost. It was successfully recovered on the night of 12 March 1991 by David H. Levy, Carolyn Shoemaker, and Eugene Shoemaker, about 16 degrees from where it had been predicted. Marsden later noted that the comet had passed close to Jupiter in February 1988, coming within 0.36 AU (54 million kilometers) of the giant planet.