96P/Machholz

Comet 96P/Machholz, also known as Machholz 1 is a periodic sunskirting comet discovered on 12 May 1986, by amateur astronomer Donald Machholz on Loma Prieta peak, in central California using binoculars. On 6 June 1986, 96P/Machholz passed from the Earth. 96P/Machholz last came to perihelion on January 31, 2023. The comet has an estimated diameter of around.
This comet is the parent body of both the Kracht and Marsden sungrazer families.
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.

Orbit

The orbit of 96P/Machholz corresponds to the Arietids and the Marsden and Kracht comet groups. Its Tisserand parameter with respect to Jupiter, T_J, is 1.94 and comets are generally classified as Jupiter family if T_J > 2. Orbital integrations indicate that T_J 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. It may eventually be ejected from the Solar System.

Date & time of closest approach	Earth distance	Sun distance	Velocity wrt Earth	Velocity wrt Sun	Uncertainty region	Reference
2139-Jun-13 23:28 ± 13 hours			41.4	37.1	± 500 thousand km

Perihelion

96P/Machholz has a perihelion of for 2028. At perihelion Comet Machholz passes the Sun at. It comes closer to the Sun than any numbered comet less than 321P/SOHO. Between 1897 and 2102 perihelion gradually drops from to. 2081 will be the first perihelion below 0.1 AU.

Observations

Machholz 1 entered the field of view of the orbiting Solar and Heliospheric Observatory in 1996, 2002, 2007, 2012, and 2017, where it was seen by the corona-observing LASCO instrument in its C2 and C3 coronagraphs.

2001/02 perihelion

During the 2001/2002 passage the comet brightened to magnitude −2, and was very impressive as seen by SOHO.

2007 perihelion

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 comet SOHO 2333 is believed to be a fragment of Machholz that came off during the 2007 perihelion. It was discovered by Indian amateur astronomer Prafull Sharma in August 2012 by analyzing data from the Solar and Heliospheric Observatory, specifically the Large Angle and Spectrometric Coronagraph. Data analysis of this sort has become commonplace based on public availability of SOHO images. Sharma became the third Indian to have discovered a comet in this manner.

2012 perihelion

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.

2017 perihelion

The 2017 perihelion was on October 27, 2017. At closest approach, it passed 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.

2023 perihelion

The January 31, 2023 perihelion passage was the sixth passage observed by SOHO. On February 4, 2023, the comet was recovered in the morning sky 2 degrees above the horizon at around magnitude 7.
Using observations through October 2022, which is three months before the 2023 perihelion passage, the comet will next come to perihelion around 12 May 2028.

Unusual composition

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 two comets previously seen with similar depletion both in carbon-chain molecules and cyanogens were C/1988 Y1 (Yanaka) and C/2025 K1 (ATLAS), but they both have substantially different orbits.
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. Given how close it approaches the Sun at perihelion, repeated baking by the Sun may have stripped most of its cyanogen.
The following table represents future orbital elements for 96P keeping in mind that results hundreds of years in the future are highly speculative given the uncertain behavior of nongravitational forces over long time intervals and divergent solutions. By the year 2235, the uncertainty in the comets position is more than.

Year	1600	1700	1800	1900	2000	2100	2200	2300	2400	2500
Periapsis	0.34	0.27	0.23	0.17	0.12	0.09	0.07	0.04	0.03	0.03
Apoapsis	5.73	5.77	5.85	5.88	5.90	5.97	6.01	6.01	6.01	6.02
Inclination	70.87	69.65	66.35	63.62	60.11	53.20	44.00	32.36	15.08	16.82
Eccentricity	0.89	0.91	0.93	0.94	0.96	0.97	0.98	0.99	0.99	0.99