133P/(7968) Elst-Pizarro = P/1996 N2 (Elst-Pizarro)


This is one of a small group of apparently normal main-belt asteroids that show occasional cometary activity. Originally discovered in 1979 as asteroid 1979 OW7, it has a low excentricity (e=0.167) orbit with a period of 5.607 years and a=3.156AU, putting it slightly outside the centre of the asteroid belt. The orbit has a very low inclination (1.4 degrees) and the distance from the Sun varies from 2.63AU at perihelion to 3.69AU at aphelion, thus there is nothing in the orbit to make it look in any way unusual.


However, on July 14th 1996, three months after perihelion, Eric Elst and Guido Pizarro, detected a faint tail and also noted that the “asteroid” was 2-3 magnitudes brighter than usual, although no coma was present. The comet passed perihelion again at the end of November 2001 and again, several months after perihelion a tail was detected, although it was much fainter and any photometric outburst had an amplitude of less than one magnitude.


An excellent summary of the photometric history of the comet is given in Seichii Yoshida’s superb web pages.



Observations are only available for a single night in 2005. Taken by Josep Lluis Salto at MPC A02 from his dark site in the Spanish Pyrenees, the magnitude is slightly brighter than expected, but within the errors.


At the time of observation the comet was, in theory, on the inbound leg from aphelion towards the Sun, although the very low excentricity makes the difference in insolation between perihelion and aphelion so small that it is difficult to see why the perihelion pass should triggered delayed activity. The comet will reach perihelion in mid-2007, although minimum geocentric distance several months after perihelion will provide an excellent opportunity to see any new activity.



Although a single point is little guide to an object’s behaviour, the observation from August 2005 gives an equivalent value of Afrho of just 2-cm. If there is any cometary activity present, it is at a very low level indeed.