Astronomer Malcolm Hartley working with the UK Schmidt Telescope Unit of the Siding Spring Observatory, New South Wales, Australia discovered this comet. The comet was exposed on photographic plates. The photographic plates were exposed on 15th, 17th and 20th of March, 1986. Astronomer Hartley noticed a diffused tail on one of the plates. Hence, a new comet was discovered. Malcolm Hartley announced his discovery to the Central Bureau of Astronomical Telegrams. As astronomer Hartley had discovered it, the comet was named as Hartley.
Observations made about this comet 103P/Hartley 2 include:
- Astronomer D. W. E Green (Central Bureau of Astronomical Telegrams) made an initial observation and computed an uncertain parabolic orbit for the comet.
- B. G Marsden calculated that the comet had passed perihelion on 4th June, 1985.
- He also noted that the distance of the comet from the sun was too small to make observations in 1985.
- The composition of gases in comet Hartley includes a large amount of carbon dioxide or monoxide.
- The diameter of its nucleus was small in size, which is only 2.2 km.
- It takes about 18.1 hours for one complete rotation.
At its discovery apparition, the magnitude of comet 103P/Hartley was noted at about 17–18. It reached a magnitude of 18 around 5th April. In the 1991 apparition, comet Hartley was bright at a magnitude of 11 in early July and by the end of July, it reached a magnitude of 10. The 2010 apparition was the most favourable as the comet reached a magnitude of 7.5 –4.5 and became visible to the naked eye in the dark skies.
Perihelion distance (closest to the sun) of comet 103P/Hartley is noted at 1.05 AU. The aphelion distance (far from the sun) of this comet is noted at 5.87 AU. The perihelion and aphelion distances change due to the gravitational effects of the giant planet Jupiter.
The orbital period of comet 103P/ Hartley is noted at 6.46 years.
Comet Hartley was accidently rediscovered on 9th July, 1991. Astronomer G. R Kastel (Institute for Theoretical Astronomy) reported and confirmed this rediscovery made by astronomer T. V Kryachko (Majdanak). Astronomer Nakano, who had predicted comet Hartley’s return for 1992, confirmed that this comet was 103P/Hartley 2. The comet was recovered on 2nd May, 1997 at Whipple Observatory, Mt. Hopkins.
Comet 103P/Hartley had close encounters with Jupiter in August 1947, 28th April, 1971 and 2nd November, 1982. It had close approaches to the earth in 1991, 1997 and 2010.
Comet 103P/Hartley is expected to reappear or pass perihelion on 20th April, 2017. Its last perihelion was on 28th October, 2010.