4. Dissipating comets and comet pairs
I introduced the term dissipating comets (Sekanina 1984, referred to hereafter as Paper 2) to describe a group of comets observed to undergo rapid physical changes. A fading sets in suddenly, without warning, and the central condensation disappears usually in a matter of days, terminating astrometry. The coma expands gradually and becomes progressively elongated. The surface brightness drops at an alarmingly fast rate until, in a few weeks, the head essentially vanishes before the eyes of the surprised observers. Interestingly, the comet is sometimes survived by a dust tail, the signature of a flare-up that had preceded the fading but for whatever reasons remained unobserved.
The terminal changes experienced by the dissipating comets were shown in Paper 2 to bear a strong resemblance to the physical behavior of secondary nuclei of the split comets. This similarity is illustrated by 1996 Q1 (Tabur), the most recent dissipating comet (Green 1996), which confirms that the dissipating comets are secondary nuclei of split comets: the orbits of 1996 Q1 and 1988 A1 (Liller) indeed are practically identical (Jahn 1996). The two objects make a comet pair (Table 3) and were unquestionably a single object in the past, probably as recently as one revolution, or 2900 years, ago. An estimate for the deceleration in the relative motion of two comets of the common parentage, based on the assumption that their breakup occurred exactly at previous perihelion, is given by , where is the revolution period of the original orbit of the principal comet (in this case 1988 A1) and is the time difference between the perihelion passages of the secondary (in this case 1996 Q1) and the principal comets; is again in units of the solar attraction. If the breakup occurred n revolutions in the past, the value of from the formula must be divided by a factor of . For the 1988 A1/1996 Q1 pair, yr and units (for ).
Table 3. Known comet pairs.
Two other comet pairs are also listed in Table 3. The orbits of Neujmin 3 and Van Biesbroeck were found to have virtually coincided before a close approach to Jupiter in 1850. Although the numbers are somewhat uncertain, this pair is likely to be of tidal origin. The remaining pair (Bardwell 1988) includes comets 1988 F1 (Levy) and 1988 J1 (Shoemaker-Holt), whose yr, yr, and for which therefore units (), or a factor of 200 smaller than the value for the Liller/Tabur pair. The low may explain why 1988 J1 was not observed to disintegrate. The splittings of 1988 F1/1988 J1 and 1988 A1/1996 Q1 are nontidal and in both cases the comet that appeared first was intrinsically the brighter one. Finally, of course, there is the sungrazer comet group, which has 24+ known members (cf. Sec. 3). For more on this group's history and orbital evolution, the reader is referred to Marsden (1967, 1989). Other proposed comet groups (e.g., Porter 1963) can be dismissed as products of chance orbital coincidences.
An outstanding issue is why most dissipating comets do not pair with other objects. The answer may be observational selection: the missing principal comets should have appeared at earlier times, when the discovery probability was lower. For comets of longer orbital periods ( yr), the time between the perihelion passages of the components could reach decades or even centuries. Perhaps the most difficult case to explain is 20D/Westphal. Will another comet be eventually discovered in its orbit?
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998