Comet group

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A comet group is the term used to describe several comets that have similar orbital elements and differ essentially in the time of the perihelion passage . So far 4 groups of comets are known, all of which have small perihelion distances of q <0.1 AU .

Emergence

Comet groups presumably arise from the fragmentation of a larger original comet. This fragmentation can be triggered either by tidal forces or thermal pressure. In the case of comets with a short orbit around the sun, this process can lead to several dozen to more than 1000, mostly smaller comet fragments, after several orbits. The best-known example is the Kreutz Group .

Kreutz group

The most famous group of comets was first proposed by the German astronomer Heinrich Kreutz in 1888 and specified in 1901. The Kreutz group includes some of the brightest comets ever observed, including a. the great comets of 1106, 1680, 1843, 1880, 1882 and 1965. Today more than 1000 Kreutz comets are known. All comets show an inclination of about 144 °, perihelion distances between 0.005 and 0.015 AU, and similar perihelion lengths and widths. Their orbital period is around 750–800 years. Brian Marsden found out that the Kreutz comets can be divided into 2 subgroups. Presumably these subgroups go back to large fragments of a Kreutz comet, which 371 BC. Was observed by Aristotle and Ephorus .

Meyer Group

In Koronographendaten the space telescope SOHO , which for solar observation was used were more than 1,250 sun near comets observed (as of early 2007). More than 75% of these SOHO comets are small fragments that belong to the Kreutz group. In January 2002 the German amateur astronomer Maik Meyer found out that the orbits of comets C / 1997 L2, C / 2001 E1 and C / 2001 X8 can be represented by very similar orbital elements. Some time later he was able to identify 3 more comets that belong to the same group of comets. The Meyer Group was then officially confirmed.

The Meyer group includes more than 70 comets (as of early 2007). The orbital elements of the members of this group show a very small spread. On average they move with a 73 ° orbit inclination, an argument of the perihelion of 57 ° and a nodal line at 73 °. The perihelion distance is about 0.035 AU. So far it has not been possible to determine an orbital period for any of the comets. An origin comet is not known.

Marsden Group and Kracht Group

In the course of the discovery of the Meyer group, Brian Marsden found out that the SOHO comets C / 1999 J6, C / 1999 U2, C / 2000 C3 and C / 2000 C4 also have similar orbital elements. Other members were discovered shortly afterwards in the spacecraft's archive data. About 30 comets belong to the Marsden group (as of early 2007). They have relatively small inclinations of about 26 ° and a perihelion distance of 0.049 AU.

During a survey of the SOHO archive in 2002 , the amateur astronomer Rainer Kracht discovered other comets with a small orbital inclination that did not belong to the Marsden group. It turned out that its orbit is only inclined by 13 ° to the ecliptic , but the length and width of the perihelion and the perihelion distance of about 0.045 AU are very similar to those of the Marsden group. About 30 comets of the Kracht group are known.

The similarity in the orbital elements of the Marsden and Kracht groups suggest a common origin. Marsden noted shortly after the discovery of the Marsden group that the orbital elements of the group members show similarities with the comet 96P / Machholz 1 . This has already been associated with some meteor streams . In 2005, the American astronomers Zdenek Sekanina and Paul Chodas presented a model in which the Marsden and Kracht groups are embedded in a large interplanetary complex around 96P / Machholz, to which, in addition to 96P and the two comet groups, 7 meteor streams and the asteroid 2003 EH 1 and Comet C / 1490 Y1 are said to belong. This suggests that the comets of the Marsden and Kracht groups are short-period with an orbital period of 4 to 6 years. Closer examination of the members of both comet groups showed that some of the comets may indeed have returned between 1996 and 2006. However, due to the inaccurate position data and fragmentation that may have occurred, exact assignments or identifications are difficult to achieve.

Similar orbital elements and other possible groups of comets

There is no precise definition of what criteria must be met in order to speak of a comet group. Based on the historical models, however, there should be at least three comets with very similar orbital elements. If only two comets have similar orbital elements, they are usually referred to as a comet pair.

In 2004 Rainer Kracht suggested that the SOHO comets C / 1999 R1, C / 2002 R5 and C / 2003 R5 form the nucleus of another comet group, the Kracht II group . In 2006 the astronomer Sebastian Hönig showed that comets 1999 R1 and 2003 R5 are probably the same comet with an orbital period of only 4 years and a perihelion distance of only 0.056 AU at an orbital inclination of about 14 °. In fact, the comet's return could be observed in September 2007 at the time predicted by Hönig. The comet was given the designation P / 2007 R5 (SOHO) and the identity of 1999 R1 and 2003 R5 was confirmed, which calls into question the existence of a Kracht II group due to the lack of further members. C / 2002 R5 could be a smaller fragment.

It has been pointed out from various quarters that the SOHO comets C / 2000 Q1, C / 2000 Y6 and C / 2000 Y7 show similarities in the angle elements. However, due to the widely differing perihelion distances, a real connection is questionable.

literature

  • Green, D. (2002), New groups of near-sun comets , IAUC 7832
  • Hönig, SF (2006), Identification of a new short-period comet near the sun , A&A, 445, 759
  • Marsden, B. Kracht group , MPEC 2002-O35
  • Meyer, M. (2003), New groups of near-sun comets , International Comet Quarterly, p. 115-122
  • Sekanina, Z., Chodas, PW (2005), Origin of the Marsden and Kracht Groups of Sunskirting Comets. I. Association With Comet 96P / Machholz and Its Interplanetary Complex , ApJS, 161, 551

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