|Discovered by||Donald Machholz|
|Discovery date||May 12, 1986|
|96P, Machholz, Machholz 1, 1986 J2, 1991 XII, 1986e, 1986 VII|
|Orbital characteristics A|
|Epoch||October 14, 2017|
|Semi-major axis||3.03527 AU|
|Orbital period||5.29 yr|
|Max. orbital speed||118 km/s (420,000 km/h)|
|Last perihelion||October 27, 2017, 23:03 UTC|
|Next perihelion||January 31, 2023, 02:02 UTC|
Comet 96P/Machholz or 96P/Machholz 1 is a short-period sungrazing comet discovered on May 12, 1986, by amateur astronomer Donald Machholz on Loma Prieta peak, in central California using 130 millimetres (5.1 in) binoculars. On June 6, 1986, 96P/Machholz passed 0.40373 AU (60,397,000 km; 37,529,000 mi) from the Earth. 96P/Machholz last came to perihelion on October 27, 2017, and will next come to perihelion on January 31, 2023. The comet has an estimated diameter of around 6.4 km (4.0 mi).
96P/Machholz is unusual among comets in several respects. Other than small SOHO comets, its highly eccentric 5.29 year orbit has the smallest perihelion distance known among numbered/regular short-period comets, bringing it considerably closer to the Sun than the orbit of Mercury. It is also the only known short-period comet with both high orbital inclination and high eccentricity. In 2007, 96P/Machholz was found to be both carbon-depleted and cyanogen-depleted, a chemical composition nearly unique among comets with known compositions. The chemical composition implies a different and possible extrasolar origin.
The orbit of 96P/Machholz corresponds to the Arietids and the Marsden and Kracht comet groups. Its Tisserand parameter with respect to Jupiter, TJ, is 1.94 and comets are generally classified as Jupiter family if TJ > 2. Orbital integrations indicate that TJ was greater than 2 about 2500 years ago. 96P/Machholz is currently in a 9:4 orbital resonance with Jupiter. It will not make another close approach to the Earth until 2028, when it will pass at a distance of 0.31972 AU (47,829,000 km; 29,720,000 mi). It may eventually be ejected from the Solar System.
96P/Machholz has a perihelion (closest approach to the Sun) of 0.124 AU (18,600,000 km; 11,500,000 mi). At perihelion Comet Machholz passes the Sun at 118 kilometers per second (420,000 kilometers per hour). It comes closer to the Sun than any numbered comet less than 321P/SOHO.
Machholz 1 entered the field of view of the orbiting Solar and Heliospheric Observatory (SOHO) in 1996, 2002, 2007, 2012, and 2017, where it was seen by the corona-observing LASCO instrument in its C2 and C3 coronagraphs.
In 2007, it appeared in SOHO's LASCO C3's field of view from April 2 to 6, peaking in brightness on April 4, 2007, around magnitude +2. In these observations, its coma was substantially smaller than the Sun in volume, but the forward scattering of light made the comet appear significantly brighter.
Between July 12–17, 2012, 96P/Machholz was visible in the SOHO LASCO/C3 field of view and expected to brighten to about magnitude +2. Two small faint fragments of 96P/Machholz were detected in the SOHO C2 images. The fragments were five hours ahead of 96P/Machholz, and probably fragmented from the comet during the 2007 perihelion passage.
The 2017 perihelion was on October 27, 2017. At closest approach, it passed 0.12395 AU (18,543,000 km; 11,522,000 mi) from the Sun. Coronagraphs on SOHO were monitoring the flyby for a fifth time. Its peak brightness was expected to be about 2.0, when it was closest to the Sun.
Spectrographic analysis of the coma of 96P/Machholz was made during its 2007 apparition, as part of the Lowell Observatory comet composition long-term observing program. When compared with the measured abundances of five molecular species in the comae of the other 150 comets in their database, these measurements showed 96P/Machholz to have far fewer carbon molecules. These other comets had on average 72 times as much cyanogen as 96P/Machholz.
The only comet previously seen with similar depletion both in carbon-chain molecules and cyanogens is C/1988 Y1 (Yanaka), but it has a substantially different orbit.
There are currently three hypotheses to explain the chemical composition of 96P/Machholz. One hypothesis for the difference is that 96P/Machholz was an interstellar comet from outside the Solar System and was captured by the Sun. Other possibilities are that it formed in an extremely cold region of the Solar System (such that most carbon gets trapped in other molecules). Given how close it approaches the Sun at perihelion, repeated baking by the Sun may have stripped most of its cyanogen.
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