10P/Tempel
A Jupiter-family comet with a prominent dust trail.
10P/Tempel, or Tempel 2, is a sizable Jupiter-family comet with a diameter of about 10 kilometers and an orbital period of 5.4 years. Wilhelm Tempel discovered it on July 4, 1873. Its most striking feature is a prominent dust trail stretching more than 48 degrees across the sky, formed by particles larger than one millimeter that the nucleus has ejected over centuries at speeds of several meters per second. Particles ahead of the comet are at least 6 millimeters in diameter. This trail was first observed by the Infrared Astronomical Satellite in 1983, when it was the most prominent such trail detected, narrow at less than 4 arcminutes thick.
The nucleus, roughly the size of Halley’s Comet at 10.6 kilometers across, has a very low albedo of 0.022. Its surface is dark because solar ultraviolet radiation has turned hydrocarbons into a tar-like substance. Even near aphelion, close to Jupiter’s orbit, the comet remains brighter than magnitude 21. Photometry from four apparitions between 1983 and 2011 shows that activity surges sharply about three months before perihelion and then declines slowly afterward. During the 2010 apparition, it reached about magnitude 8. The closest approach to Earth occurred on August 3, 2026, at 0.414 AU (61.9 million km), with perihelion the previous day at a magnitude of 8–9 and a solar elongation of 164 degrees. The most favorable apparition was in 1925, when it came within 0.35 AU of Earth and reached magnitude 6.5.
Infrared spectroscopy in July 2010 detected trace amounts of methanol (CH₃OH), ethane (C₂H₆), ammonia (NH₃), and hydrogen cyanide (HCN) in the coma, with peak intensities hinting at a distributed source from sublimed icy grains released from the nucleus. Photometry from 2010–2011 measured a rotation period of 8.950±0.002 hours, slower than the 8.941±0.002 hours recorded in 1999, which itself was slower than the 1988 value, indicating that asymmetric outgassing is gradually slowing the nucleus’s spin. The comet shows an almost linear jet in CN and red images from the Lowell Observatory, originating near one pole and changing little during rotation.
Several missions were proposed but never executed. The Jet Propulsion Laboratory planned a flyby using a spare Mariner 4 spacecraft, but it was reassigned to Venus as Mariner 5.
Quick Facts
- Discoverer
- Wilhelm Tempel
- Discovery Date
- 4 July 1873
- Mpc Name
- P/1873 N1, P/1878 O1
- Designations
- Tempel 2 · 1873 II, 1878 III, 1894 III · 1899 IV, 1904 III, 1915 I · 1920 II, 1925 IV, 1930 VII · 1946 III, 1951 VIII, 1957 II · 1962 VI, 1967 X, 1972 X · 1978 V, 1983 X, 1988 XIV · 1994 VII
- Orbit Ref
- jpldata · MPC
- Epoch
- 9 June 2026 (JD 2461200.5)
- Obs
- 7,784
- Perihelion
- 1.418 AU
- Aphelion
- 4.71 AU
- Semimajor
- 3.06 AU
- Eccentricity
- 0.5374
- Period
- 5.37 years
Facts from the source article.
Lore & Background
The nucleus of 10P/Tempel is estimated to be roughly the size of Halley's Comet at 10.6 km in diameter with a low albedo of 0.022. The nucleus is dark because hydrocarbons on the surface have been converted to a dark, tar-like substance by solar ultraviolet radiation. The nucleus is large enough that even near aphelion the comet remains brighter than about magnitude 21. Photometry during four apparitions between 1983 and 2011 shows that cometary activity increases sharply about three months before perihelion and after perihelion declines steadily and slowly. During the 2010 apparition the comet brightened to about apparent magnitude 8. The most favorable apparition was in 1925 when it came within 0.35 AU of Earth with an apparent magnitude of 6.5. Infrared spectroscopy in July 2010 revealed the presence of CH3OH, C2H6, NH3 and HCN in trace amounts within its coma, with peak intensities suggesting a possible distributed source released from the nucleus as sublimed icy grains. Observations by the Infrared Astronomical Satellite in 1983 showed the comet has a dust trail, at that time the most prominent observed, extending for over 48 degrees while less than 4 arcminutes thick. The particles have a diameter over one millimeter and were ejected over hundreds of years with velocities of several meters per second. Photometry in 2010–2011 indicated the nucleus was rotating once every 8.950±0.002 hours, higher than in 1999 (8.941±0.002 hours) and 1988, indicating the rotation rate was slowing due to asymmetric outgassing. The comet features an almost linear jet in CN and red images by the Lowell Observatory, originating from a small region near the pole.
Reader's Guide
10P/Tempel holds significance as a well-observed Jupiter-family comet with a large, dark nucleus and a distinctive dust trail that has been studied over decades. Its orbital dust trail, first noted by the Infrared Astronomical Satellite in 1983, remains one of the most prominent known, providing insight into the long-term ejection of particles from cometary nuclei. The comet's slowing rotation rate, measured across apparitions in 1988, 1999, and 2010–2011, offers evidence of asymmetric outgassing affecting its spin. The comet was considered for multiple exploration missions: the Jet Propulsion Laboratory proposed a flyby using a flight spare of Mariner 4 (later used as Mariner 5 for a Venus flyby); it was the original target of the Comet Rendezvous Asteroid Flyby (CRAF) mission before being switched to 22P/Kopff and eventually cancelled in 1992; and it was to be the target of the NASA part of the International Comet Mission, which was cancelled in November 1979. Its 2026 apparition brought it within 0.414 AU of Earth at magnitude 8–9, making it accessible to binoculars.
Did You Know?
- Its dust trail extends over 48 degrees in the sky and is less than 4 arcminutes thick.
- The comet's rotation period slowed from 8.941 hours in 1999 to 8.950 hours in 2010–2011.
More in Named Comets, Part 5 1-24
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