74P/Smirnova-Chernykh

 

This is one of a small number of comets that have practically asteroidal objects. 74P/Smirnova-Chernykh has a low eccentricity (e=0.147) orbit in the outer part of the asteroid belt, ranging from 3.57AU at perihelion, to 4.60AU at aphelion, always within the orbit of Jupiter. In fact, after discovery, it was later identified with the supposed Main Belt asteroid, 1967 EU, observed between March 9^th and April 4^th 1967. In fact, it is now known that the comet was actually co-orbital with Jupiter for a brief period between 1957 and 1963, between Jupiter encounters and major changes in the orbit.

 

The comet was identified by Tarnara Smirnova on plates taken at the Crimean Astrophysical Observatory on March 4^th and 16^th 1975. At the time, its nature was unclear and so on March 30^th 1975 Nikolaj Chernykh re-observed it and confirmed that it was indeed a comet rather than an asteroid. It was around magnitude 15. Due to the very low orbital eccentricity the magnitude changes little between perihelion and aphelion and the comet can be followed all around its orbit.

 

74P/Smirnova-Chernykh is subject to frequent encounters with Jupiter and important changes in its orbit. From 1900-1955 perihelion was at 5.68AU and the comet had a moderately elliptical orbit (eccentricity 0.35) with a period of 26 years. A close approach to Jupiter (0.244AU on October 29^th 1955) circularized the orbit (eccentricity 0.040) and made the comet almost co-orbital with Jupiter, setting it up for a second encounter on September 20^th 1963 that, although more distant (0.467AU), reduced the perihelion distance considerably from 5.25AU to 3.55AU and making the comet much brighter and detectable from Earth, although slightly increasing the eccentricity.

 

Since then the orbit has been stable and the period 8.5 years. However, this situation is about to change, with a new double encounter with Jupiter in 2021 and 2030 that will again briefly circularize the orbit, this time to a co-orbital situation with a period almost exactly the same as Jupiter’s (11.75 years) before returning to an orbit similar to the present one, although with a slightly greater perihelion distance.

Due to the small range of heliocentric distance the brightness of the comet changes by only about 3 magnitudes between perihelion and aphelion. This is evident from the light curve (left) based on eight nights of observations by Pepe Manteca (MPC 170), Ramón Naves & Montse Campàs (MPC 213) and José Antonio Reyes & Sensi Pastor (MPC J76) which, although poorly sampled shows only a 3 magnitude decline in 4.5 years, one magnitude of which is due to the changing geocentric distance. Despite this, it shows activity only close to the Sun. A 360s exposure by Pepe Manteca on March 3^rd 2000 (T-318 days) shows only a faint, slightly diffuse image. In contrast, a 5x300s integration by the same observer on March 15^th 2001 shows a faint but obvious gas tail some 400 000km long.

 

Despite the rather large perihelion distance the comet shows a quite high level of activity. Values of Afrho slightly greater than 100-cm were registered 2-3 months after perihelion, indicating that the comet is quite active, despite its large perihelion distance. However, the dust production falls as r^-6.8, a somewhat more rapid decline than usual in Jupiter Family comets.