This comet was discovered on two plates exposed with the 1.2-m UK Schmidt at Siding Springs Observatory (Australia) on June 13th 1985. The comet was magnitude 16 and had a prominent tail. It was observed for only two months, with the last observation on August 14th, but it was evident that the comet was of short period (5.66 years).
Unknown to astronomers at the time, the comet passed close to Jupiter (0.36AU on February 5th 1988), leading to a substantial modification of the orbit. This meant that the predictions for the 1991 return were considerably in error and the recovery of the comet was made by accident. On March 12th 1991 the Shoemakers and David Levy registered an apparently unknown comet on images taken with the 46-cm Schmidt at Mount Palomar. The comet was magnitude 16.5 with a significant tail.
Brian Marsden then noted that the comet was 16º from the predicted position of P/Hartley 1, but had a movement that seemed compatible with it. He then successfully linked the new comet with P/Hartley 1, discovering the close encounter with Jupiter that had increased the perihelion distance from 1.58AU to 1.82AU.
The orbit of this comet is subject to continuous perturbations with 8 encounters with Jupiter to distances ranging from 0.18-0.76AU between 1822 and 2097. As a result, the perihelion distance ranges in a regular fashion from 1.48AU to 2.02AU (see the graphic below) and the period from 5.50 to 6.58 years over this interval of time. The eccentricity of the orbit is moderate, ranging from 0.41-0.53.
The 2003 apparition of 100P/Hartley 1 was not well observed with the comet only observed between Fenruary and June 2003 and not data anywhere in the world after perihelion. The available Spanish observations that are shown here cover just two nights at the end of April 2003, with data from Ramón Naves & Montse Campàs (MPC 213) and Álbert Sánchez (MPC 442). These show a significant brightening of the comet in a week, consistent with its rapid brightening as it approaches the Sun (Seichii Yoshida finds that the light curve brightens as 40 log r).
100P/Hartley 1 is a rather low activity object with a value of Afrho below 10-cm at T-109 days, although the evidence from the small number of data points is that Afrho increased very rapidly as the comet approached the Sun. This is consistent with the comet’s known historical light curve behaviour.