Comets and Asteroids

Comet 38P/Stephan-Oterma.

Comet 38P/StephanOterma has been gracing the night sky for a while now, but has been overshadowed by the prospect of 46P/Wirtanen reaching naked eye status. Ironically, both comets ended up in the indistinct constellation of Lynx at the end of December and that’s where we pick up the path of 38P during January.

At 00:00 UT on New Year’s Day the 10th magnitude comet can be found 4° southeast of mag. +4.2 31 Lynxis. It then travels north, veering slightly northwest towards the end of the month when it will have dimmed to mag. +11.6. Thanks to its late course correction, 38P remains within the boundaries of Lynx all month long. Travelling a relatively small distance through the month, 38P is well suited to astrophotography, its relatively slow motion amongst the stars helping to avoid motion blur during a 1-2 minute exposure.

Comet 38P/Stephan-Oterma was first sighted by Jérôme Eugène Coggia in Marseilles, France on 22 January 1867. However, he quickly lost sight of his ‘uncatalogued nebula’ when clouds drifted in. It wasn’t until the night of 24 January that EJM Stephan, also in Marseilles, was able to see the object again. He determined that it had moved and recognised it as a comet. Its second name was provided by Liisi Oterma from Turku, Finland who recovered the comet in November 1942.

38P/Stephan-Oterma is a Halley-type comet with an orbital period of 37.7 years and an aphelion distance of 20.9 AU. Halley-type comets have eccentric orbits, with periods ranging from 20 to 200 years. Over the years 38P has had numerous close passes of main planets. At the end of November 1866, weeks before its discovery, it passed Earth by 0.92 AU. In December 1942 it came as close as 0.63 AU, then on 9 December 1980 just 0.59 AU. Last month, on 17 December, it passed Earth by 0.77 AU.