“In the red corner, weighing in at 1048 Earth masses, King of the Planets, give it up for Juuuupiteeeer! And in the blue corner, weighing in at one ten-billionth of an Earth mass, the new kid on the block, Comet 2011 P1!”
A recent comet discovery by prolific comet discoverer Rob McNaught has turned out to be an interesting example of large short-term orbit changes. Even before we put out the initial discovery announcement on MPEC 2011-P19 it was apparent that the comet was physically close to Jupiter. This caused some problems with determining the initial orbit and deciding whether to call it a C/ or a P/ comet. The difference between the C/ and P/ classifications is that P/ comets have orbital periods under 30 years and C/ comets have longer periods (or won’t return, because their future barycentric elements are hyperbolic). I decided that since the orbit that appeared on the initial MPEC had a 6.3-year period, it had to be P/. This initial orbit was noted as being very uncertain. As more observations were received, such a short orbital period was no longer tenable. Some solutions had periods greater than 30 years and I decided that the safest course of action was to switch the comet to a C/ designation. The orbit that appeared on the August 10 comet observation and orbit MPEC was parabolic and based on an observation arc of 10 days. A further orbit, this time a perturbed elliptical solution with a period (P) of 43.7 years, and based on an observation arc of 11 days appeared on MPEC 2011-Q05. The latest orbit, with the comet reverting back to P/ status and P = 25.3 years, appears on MPEC 2011-Q34.
The publication of the latest orbit was accompanied by a short note explaining that the epoch of the orbit was the current standard epoch (2011 August 27.0 TT), rather than the traditional 40-day date closest to the time of perihelion passage, T. The published orbit had T = 2010 June 29.6, but when the orbit was integrated back to that date, the time of perihelion passage moved to 2013! The reason for this unusual behavior was clear: an approach to within 0.025 AU (3.74 million km, or 53.5 Jupiter radii) of Jupiter in 2010 December had caused radical changes in the orbit. The post-encounter orbit is given in full on MPEC 2011-Q34. Here I will just mention that the perihelion distance, q, is 4.86 AU, the orbital inclination, i, is 6.3 deg and P is 25.3 years. Prior to the 2010 encounter, q was 4.24 AU, i was 4.7 deg and P was 12.2 years.
A diagram showing the motion of P/2011 P1 from 1980 to 2040 is shown above. From 1980 to 1999 the comet made 1.3 orbits (q = 4.48 AU, i = 4.6 deg and P = 14.0 years) before an approach to 0.73 AU (109 million km) of Jupiter in May changed the orbit to the 12.2-year orbit mentioned above. One orbit later, the comet made the aforementioned 0.025 AU approach and entered the larger 25.3-year (actually, when the comet is well away from Jupiter it becomes 23.7 years) orbit. After only one more orbit, another close approach in 2033 September, this time to within 0.11 AU (16.4 million km) of Jupiter, reduces the size of the orbit, making P = 13.1 years and raising i to more than 9°.
The case of 39P/Oterma is a similar story. Prior to a 0.16 AU (23.9 million km) Jovian approach in 1937 October, the comet was in a 18 year orbit with q = 5.8 AU. This encounter changed the orbit radically, such that the period dropped to 8 years and q to 3.4 AU. This allowed the comet to be discovered in 1943 by Liisi Oterma (“Miss Oterma” in old publications) at Turku in Finland. After completing three orbits, the comet encountered Jupiter in 1963 April. The approach, to within 0.095 AU (14.2 million km), moved q back out to 5.5 AU and raised P to almost 20 years. This put the comet out of reach of the telescopes of the day. Near perihelion (which occurred in 1983 June and 2002 December), the comet was expected to be around magnitude 22. This is “easy” with modern detectors and the comet was recovered in 2001 August, only some two arc-minutes from a prediction by Brian Marsden. I am listed on that MPEC as a measurer and I also did some uncertainty calculations for one of the observing teams, but I can’t remember which and I no longer seem to have the relevant e-mail (or I just can’t find it)!
A number of comets, other than the Jupiter-impactor D/1993 F2 (Shoemaker-Levy), almost certainly spent time captured, albeit temporarily, in orbit around Jupiter. But that is material for a future article…
Given the closeness of the 2010 encounter of P/2011 P1 and Jupiter, it is clear that a much longer observational arc will need to be obtained before we can make definitive statements about the long-term past and future motion of this comet. However, the scenario described above is one that is fully supported by the current available observational data.