90P/Gehrels 1

 

This comet was discovered by Neil Gehrels on January 24^th 1974. It was found to have an orbital period of 14.5 years and perihelion at 2.94AU. The 2002 return was its third observed apparition. The orbit is unusually stable, with no significant planetary encounters (i.e. none to less than 1AU) over the period 1795-2107, although between 1810 and 1958 the perihelion distance did reduce very slowly from 3.09AU to 2.9AU and the period from 15.4 to 14.5 years. After an increase to 3.00AU in 2047, the perihelion distance will again reduce slowly to 2.93AU by 2107. Overall though the brightness of the comet will be virtually unchanged over 300 years by such small variation in the orbit.

 

90P/Gehrels is a rather faint object and, in 2002, the circumstances were poor with opposition coming some months after perihelion. Observations are only available on four nights with a single night by Pepe Manteca (MPC 170) in September 2002 and three by Ramón Naves & Montse Campàs (MPC 213) over New Year 2002/03.

 

They show an apparently flat light curve. This though is misleading as 90P/Gehrels 1 shows an unusually large perihelion asymmetry with the maximum of the light curve being delayed 130 days. The comet also shows a large power law dependence of brightness, with its light curve varying as 22 log r.

 

The maximum of the light curve and activity falls almost exactly between the two clusters of points. As a consequence, the values of Afrho show no variation at all, with a constant value of 20-cm.