103P/Hartley
Small periodic comet visited by Deep Impact in 2010.
103P/Hartley, also known as Comet Hartley 2, is a small periodic comet that orbits the Sun once every 6.48 years. Malcolm Hartley spotted it in 1986 using the Schmidt Telescope Unit at Siding Spring Observatory in Australia. Its nucleus measures between 1.2 and 1.6 kilometers across.
This comet holds the record, as of November 2010, for being the smallest comet ever visited by a spacecraft. It was the fifth comet to receive a spacecraft visit overall, and the second for the Deep Impact probe, which had previously flown by Comet Tempel 1 in July 2005. The EPOXI mission sent Deep Impact within 700 kilometers of Hartley 2 on 4 November 2010.
The comet belongs to the Jupiter family and has a perihelion of 1.05 AU from the Sun, close to Earth’s orbit. On 20 October 2010, it passed 0.12 AU from Earth, just eight days before reaching perihelion on 28 October. That November, from northern latitudes, it was visible around midnight without moonlight interference, reaching an apparent magnitude of about 5 and becoming faintly visible to the naked eye. On 26 September 2023, it came within about 57.3 million kilometers of Earth, reaching perihelion on 12 October and brightening to roughly magnitude 8. Although it is not currently a known meteor shower source, dust trails from its recent returns could change that; the trail from 1979 is expected to intersect Earth in 2062 and 2068.
Spitzer Space Telescope observations in August 2008 gave the nucleus a radius of 0.57 ± 0.08 kilometers and a very low albedo of 0.028. Its mass is estimated at about 300 megatonnes, and at its current rate of mass loss, it should survive roughly 100 more apparitions (about 700 years) unless it breaks apart. Radar data from Arecibo during the 2010 approach showed the nucleus is highly elongated, rotating every 18 hours. The EPOXI mission manager described its shape as “a cross between a bowling pin and a pickle.” In 2011, the Herschel Space Observatory detected vaporized water in the coma, and Hartley 2’s heavy-water-to-regular-water ratio matched that of Earth’s oceans—half the heavy water content seen in other comets. For years, the comet produced more water vapor than expected from direct ice sublimation; the flyby revealed that icy grains in the coma are driven out by outgassing carbon dioxide, which is the main source of the water.
Quick Facts
- Pronounced
- ' · h · ɑ:r · t · l · i
- Discovery Ref
- IAUC_4197
- Discoverer
- Malcolm Hartley
- Discovery Site
- Siding Spring, Australia
- Discovery Date
- 15 March 1986
- Mpc Name
- P/1986 E2, P/1991 N1
- Designations
- 1985 V, 1991 XV · 1986c, 1991t
- Orbit Ref
- MPC1 · jpl1
- Epoch
- 5 May 2025 (JD 2460800.5)
- Observation Arc
- 20.44 years
- Obs
- 8,857
- Perihelion
- 1.06 AU
Facts from the source article.
Lore & Background
Comet Hartley 2 is a Jupiter-family comet with an orbital period of 6.46 years. It was discovered by Malcolm Hartley in 1986 at the Schmidt Telescope Unit, Siding Spring Observatory, Australia. Its perihelion is near Earth's orbit at 1.05 AU from the Sun. The comet passed within 0.12 AU of Earth on 20 October 2010, eight days before perihelion on 28 October 2010, reaching an apparent magnitude of about 5 and becoming dimly visible by naked eye. On 26 September 2023 it passed about 57 million km from Earth, brightening to about magnitude 8.
Observations by the Spitzer Space Telescope in August 2008 gave a nucleus radius of 0.57 ± 0.08 km and a low albedo of 0.028. Radar observations by the Arecibo Observatory during the 2010 apparition revealed a highly elongated nucleus rotating over an 18-hour period, described as 'a cross between a bowling pin and a pickle'. The Herschel Space Observatory in 2011 detected vaporized water in the coma, with a heavy water ratio matching Earth's oceans. The flyby showed that many icy grains are driven out by outgassing of carbon dioxide, and that carbon-monoxide and carbon-dioxide abundances indicate short-period comets formed under warmer conditions closer to the Sun.
Reader's Guide
The Deep Impact spacecraft, as part of the EPOXI mission, flew by Hartley 2 on 4 November 2010 at a distance of 700 km and a speed of 44,300 km/h. This was the second comet visited by Deep Impact, which first visited Tempel 1 on 4 July 2005. The flyby revealed the comet to be 2.25 km long and 'peanut shaped', with jets of material ejected from the dark side. Scientists described it as 'hyperactive, small and feisty'. The material ejected was primarily CO2 gas, marking the first time comet activity powered by sublimation of frozen carbon dioxide was observed near the Sun. Key findings included a smooth, inactive waist probably re-deposited; a tumbling rotation; and glittering blocky objects up to 50 m high on the rougher ends, two to three times more reflective than the surface average. The comet spews more water than other comets its size, with dry ice deep in the body turning to gas and dragging water ice with it. Some material from the ends falls back into the middle due to the comet's weak gravity.
Did You Know?
- Hartley 2 is the smallest comet visited by a spacecraft as of November 2010.
- It was the fifth comet visited by spacecraft and the second visited by Deep Impact.
- The comet's heavy water ratio matches that of Earth's oceans.
- Its nucleus rotates over an 18-hour period and is shaped like a cross between a bowling pin and a pickle.
More in Periodic Comets 1-24
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