Vulcanoid Asteroids and Sun-Grazing Comets – Past Encounters and Possible Outcomes

Abstract

The region between 0.07 to 0.25 au from the Sun is regularly crossed by sungrazing and small perihelion distance periodic comets. This zone also supports stable orbits that may be occupied by Vulcanoid asteroids. In this article we review the circumstances associated with those comets known to have passed through the putative Vulcanoid region, and we review the various histories associated with a sub-group of these comets that have been observed to displayed anomalous behaviors shortly before or after perihelion passage. In all 406 known comets are found to have passed through the Vulcanoid zone; the earliest recorded comet to do so being C/400 F1, with comet C/2008 J13 (SOHO) being the last in the data set used (complete to 2014). Only two of these comets, however, are known to be short period comets, C/1917 F1 Mellish and 96P / Machholz 1, with the majority being sungrazing comets moving along parabolic orbits. We examine the case history of comet C/1917 F1 Mellish in some detail since numerical simulations suggest that over the past ~ 40 thousand years it has regularly passed through the Vulcanoid zone. Additionally, this particular comet is linked to the December Monocerotid meteor shower, which is known to have produced a series of very bright fireball displays in the 11th Century. An extremely small impact probability of order 10-19 per perihelion passage with a Vulcanoid of diameter 1 km or larger is determined for comet Mellish, and we conclude that the ancient fireball display is not likely associated with a Vulcanoid collision. Indeed, while we find no evidence to indicate that any historical collisions between a cometary nucleus and a Vulcanoid have occurred, this result, we suggest, does not automatically mean that no Vulcanoids exist at the present time, or that collisions have not taken place in the past. Likewise, these results do not rule out the possibility of collisions being observable at future times. As ever, since first being hypothesized, if they exist at all, the Vulcanoid asteroids remain elusive.