73P/Schwassmann–Wachmann

Observational history

Discovery

Comet Schwassmann–Wachmann 3 was one of the comets discovered by astronomers Arnold Schwassmann and Arno Arthur Wachmann, working at the Hamburg Observatory in Bergedorf, Germany. The comet was discovered as the astronomers were exposing photographic plates in search of new asteroids for a minor planet survey, on 2 May 1930. On 31 May 1930 the comet passed about 0.062 AU (9.3 million km; 5.8 million mi) from Earth. The comet was lost after its 1930 apparition as the 1935 apparition had poor viewing geometry, but was recovered in 1979. During perihelion in 1985, the comet was unobserved as it was on the far side of the Sun 1.9 AU from Earth. In 1990 the comet reached apparent magnitude 9 and was the best appearance since 1930.

Breakup

In September 1995, the comet began disintegrating on its re-entry to the inner Solar System, in a reaction triggered by the Sun's heating of the comet as it emerged from the colder regions of the outer Solar System. It was seen to break into four large pieces labeled 73P-A, B, C and D. As of March 2006, at least eight fragments were known: B, C, G, H, J, L, M and N. On April 18, 2006, the Hubble Space Telescope recorded dozens of pieces of fragments B and G. It appears that the comet may eventually disintegrate completely and cease to be observable (as did 3D/Biela in the 19th century), in which case its designation would change from 73P to 73D. In May 2006, it was known to have split into at least 66 separate objects. In April 2006, fragment C was the largest and the presumed principal remnant of the original nucleus.

The fragments passed Earth in May 2006, with the comet coming nearest to Earth on May 12 at a distance of 0.078 AU (11.7 million km; 7.3 million mi), a close pass in astronomical terms though with no significant threat of debris–Earth collision. With a 34-day observation arc fragment 73P-T was known to pass Earth on May 16 at roughly a distance of 0.059 AU (8.8 million km; 5.5 million mi). In 1930 when the comet passed Earth that close, there was a meteor shower on June 9 with as many as 100 meteors per hour. Analysis by P. A. Wiegert et al. suggested that a recurrence of that spectacle was unlikely.

Over many decades the fragments of 73P from 1995 and 2006 will disperse over the orbital path of 73P as they are all moving at a slightly different speed. Known fragments of 73P have orbital periods of 4.7 years (73P-AJ) to 6.1 years (73P-Y). While the main fragment of 73P came to perihelion (closest approach to the Sun) on 25 August 2022 when it was 1 AU from Earth, fragment 73P-Y (with a short 34-day observation arc) had a best-fit of being near the orbit of Jupiter about 6.0 ± 0.5 AU (898 ± 75 million km) from Earth. During the 2011 perihelion passage the primary component 73P-C was recovered on 28 November 2010 near apparent magnitude 21.3; it came to perihelion on 16 October 2011.

The non-primary fragment 73P-BT which has an observation arc of 250 days from February 2017 to October 2017 and (if it had survived) was expected to come to perihelion on 26 August 2022. On 23 July 2022 fragments JD001 (73P-BU) and JD002 (73P-BV) were detected and came to perihelion on 24 August 2022. Three additional fragments "BW, BX, and BY" that were discovered in mid-August were announced on 2 September 2022. 73P-BV had a 22-day observation arc giving it the longest observation arc of the five fragments discovered in 2022.