81P/Wild 2


81P/Wild 2 was the fifth comet to be studied by a space probe (after 21P/Giacobini-Zinner, ICE (1985), 1P/Halley, Vega 1, Vega 2, Sakigake, Suisei, Giotto (1986), 26P/Grigg-Skjellerup, Giotto (1988) and 19P/Borrelly, Deep Space 1 (2001)), but only the third to have its nucleus imaged. On January 2nd 2004 the Stardust probe passed just 236km from the nucleus taking samples of the cometary dust that were successfully returned to Earth on January 15th 2006.


It is hard to believe that a comet which has become so well known is a comparatively recent discovery for reasons that will be explained below. Paul Wild (pronounced “Vilt”) of the Berne Astronomical Institute discovered the comet on photographic plates exposed on January 6th and 8th 1978. A third observation by Wild on January 25th allowed a provisional orbit to be calculated that showed the comet to be a short period object with a period of 6.17 years. What was also obvious was that the comet was relatively bright and that at a favorable opposition such as that of 1997 or 2010 its magnitude could reach approximately 9; how could it have been missed?


The answer was found when the history of the orbit was studied. From the start of the 19th Century though to 1973 81P/Wild 2 had a perihelion distance around 4.95AU and a period close to 40 years. At this distance the comet was unobservable. However, late on September 9th 1973 the comet passed just 0.0061AU from Jupiter, well inside Jupiter’s outer moons. The result was to reduce the perihelion distance from 4.95AU to 1.49AU and the period from 43.2 years to 6.17, making it an easy object to detect.


In its new orbit 81P/Wild 2 is able to have very close encounters (less than 0.1AU) with Mars and Jupiter and more distant ones with the Earth. As a result the orbit is subject to continuous modifications that will slowly increase its perihelion distance and period until, at the end of the 21st Century, they will reach 1.80AU and 6.73 years respectively. Undoubtedly, at some time in the future 81P/Wild 2 will be, once again, thrown into a completely different orbit.



The current period of 81P/Wild 2 is 6.4 years, making returns alternate between very good (1997 and 2010) and very bad (2003). At a good return the comet is at perihelion close to opposition and may reach magnitude 9. In a poor one, such as 2003, perihelion occurs close to conjunction and observations are difficult or impossible for as much as a year around perihelion.


As we can see, there is a large gap in observations around perihelion. The light curve shows that the brightness of the comet was rather stable from T-300 days to T+300 days. Before T-300 days (r=3.0AU) there is a rapid brightening as the nucleus shows a sudden turn-on (this is beautifully shown in Seichii Yoshida’s light curve), with the brightness increasing as 45 log r. Similarly, after T+300 days there is a rapid fade as the nucleus before a steady state is reached.


















Total visual magnitude estimates are available from early December 2002 to early March 2003 (r=2.95AU to r=2.45AU).


The light curve fit that is obtained is:


m1 = 6.6 + 5 log Delta + 15.1 log r


Seichii Yoshida finds a strong perihelion asymmetry in the light curve of the 1997 apparition, with the maximum displaced to T+20 days. The absolute magnitude is somewhat brighter in 1997 (5.3), although as “n” are larger than the fit found here (21.6 log r) the intrinsic brightness of the comet was probably no different in 2003 to 1997. The implied rate of increase of water production with heliocentric distance in 2002-3 is a rather modest r-1.8.


The Stardust encounter occurred at T+98 days (the black vertical line in the plot, left). Although some visual observers could get data at this time, the comet was not observable for northern hemisphere CCD observers at this time. In fact, very few observations were obtained anywhere in the world before T+200 days, 100 days after the Stardust encounter, so knowledge of the comet’s activity at the time of the encounter is extremely limited. We do not know if the comet showed a perihelion asymmetry similar to that of 1997, although the limited data that does exist suggests that the activity was highly symmetrical about perihelion and flat at maximum.


The data for Afrho shows that in a 10” aperture Afrho was essentially constant at 70-cm from T-300 days to T+300 days. There is essentially a zero slope (r-0.2) in the dust production rate against heliocentric distance. However, from the plot left it is also evident that the value of Afrho calculated in large apertures is significantly greater than that for a small aperture. 81P/Wild 2 shows a coma light distribution that is significantly flatter than 1/r.