Comet 96P/Machholz or 96P/Machholz 1 is a short-period 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, comet 96P/Machholz passed 0.40373 AU (60,397,000 km; 37,529,000 mi) from the Earth. Comet 96P/Machholz last came to perihelion on July 14, 2012, and will next come to perihelion on October 27, 2017. 96P/Machholz has an estimated radius of around 3.2 km (2.0 mi).
Machholz 1 is unusual among comets in several respects. Its highly eccentric 5.2 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, Machholz 1 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 Machholz 1 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. Machholz 1 is currently in a 9:4 orbital resonance with Jupiter. 96P 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). Observations
Machholz 1 entered the field of view of the orbiting Solar and Heliospheric Observatory (SOHO) in 1996, 2002, and 2007, where it was seen by the corona-observing LASCO instrument in its C2 and C3 coronagraphs. During the 2002 passage the comet brightened to magnitude −2, and was very impressive as seen by SOHO. During the comet's last perihelion passage 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.
The most recent opportunity to observe Machholz 1 was when it returned to perihelion in 2012. Between July 12–17, 2012, comet Machholz was visible in the SOHO LASCO/C3 field of view and expected to brighten to about magnitude +2. Two small faint fragments of Comet Machholz were detected in the SOHO C2 images. The fragments were 5 hours ahead of Comet Machholz, and probably fragmented from the comet during the 2007 perihelion passage.
Spectrographic analysis of the coma of Machholz 1 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 Machholz 1 to have far fewer carbon molecules than the 150 other comets. These other comets had on average 72 times as much cyanogen as Machholz 1.
The only comet previously seen with similar depletion both in carbon-chain molecules and cyanogens is Yanaka (1988r; 1988 Y1), but it has a substantially different orbit.
Possible cause of the unusual chemical composition
There are currently three hypotheses to explain the chemical composition of Machholz 1.
One hypothesis for the difference is that Machholz 1 was an interstellar comet from outside the Solar System and was captured by the Sun.
Oort cloud origin
Other possibilities are that it formed in an extremely cold region of the Solar System (such that most carbon gets trapped in other molecules).
Extreme thermal alteration
Given how close it approaches the Sun at perihelion, repeated baking by the Sun has stripped most of its cyanogen.