A search for Vulcanoids with the STEREO Heliospheric Imager

Abstract

a b s t r a c tInterior to the orbit of Mercury, between 0.07 and 0.21 AU, is a dynamically stable region where a population of asteroids, known as Vulcanoids, may reside. We present the results from our search for Vulcanoids using archival data from the Heliospheric Imager-1 (HI-1) instrument on NASA's two STEREO spacecraft. Four separate observers independently searched through images obtained from 2008-12-10 to 2009-02-28. Roughly, all Vulcanoids with e 6 0.15 and i 6 15°will pass through the HI-1 field of view at least twice during this period. No Vulcanoids were detected. Based on the number of synthetic Vulcanoids added to the data that were detected, we derive a 3r upper limit (i.e. a confidence level >0.997) that there are presently no Vulcanoids larger than 5.7 km in diameter, assuming an R-band albedo of p R = 0.05 and a Mercury-like phase function. The present-day Vulcanoid population, if it exists at all, is likely a small remnant of the hypothetical primordial Vulcanoid population due to the combined effects of collisional evolution and subsequent radiative transport of collisional fragments. If we assume an extant Vulcanoid population with a collisional equilibrium differential size distribution with a power law index of À3.5, our limit implies that there are no more than 76 Vulcanoids larger than 1 km.

The Wide-Field Imager for Solar Probe Plus (WISPR)

“…Searches for the existence of Vulcanoids have not been successful. Durda et al (2000), Merline (2008), and Steffl et al (2013) have used LASCO, Messenger and SECCHI observations to search for Vulcanoid objects and have put upper limits on the number of objects above certain sizes. While asteroids have been detected within the Vulcanoid region (0.08-0.2 AU), none were Vulcanoids.…”

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

“…A principle of mediocrity approach therefore suggests that it is the detection threshold condition that likely applies, and that a Vulcanoid population genuinely does exist but that it contains no large objects at the present epoch. Indeed, a search for Vulcanoids using the data archive generated by the STEREO Heliospheric Imager by Steffl et al [11] reveals that there are no objects larger than about 6 km across within the present Vulcanoid population (for an assumed R-band albedo of 0.05 -a value characteristics of that derived for Mercury and the C-type asteroids). Steffl et al [11] additionally estimate, assuming a steady-state collisional fragmentation history, that there are no more than 76 Vulcanoids larger than 1 km across at the present epoch.…”

“…The complex volcanic history and surface evolution of Mercury, however, conspire to make such an analysis difficult, at best, and at present there is no clear-cut data to suggest that an excess of craters (or an extended cratering history) due to Vulcanoid impacts exists. A number of searches for Vulcanoids, with the SOHO and STEREO Sun-monitoring space-platforms, have been made during the past 15 years [9,10,11] but, again, no Vulcanoid objects have been positively identified. These collective nullobservations, of course, cut two ways; either there are no such objects to be found, or, the size distribution and surface albedos of the Vulcanoids are such that their reflected-light values continue to fall below the threshold for detection with the available instrumentation.…”

“…Based upon both dynamical and thermodynamic lifetime against sublimation arguments Campins et al [21] suggest that the search area for Vulcanoids might best be confined to the ecliptic at distances between 0.1 and 0.25 au from the Sun. Steffl et al [11] and Durda et al [9] place the Vulcanoid inner and outer boundaries at 0.07 and 0.21 au from the Sun and allow for inclinations as high as 15 o away from the ecliptic. Stern and Durda [22] set a thermodynamic limit of 0.06 au for the inner boundary of the Vulcanoid region and allow orbits with inclinations up to 16 o , while Vokrouhlicky et al [23] adopt the radial limits set by Evans and Tabachnik [19].…”

“…For Jupiter family comets, therefore, collisions are most likely going to take place when they are close to aphelion. Alternatively, however, collisions close to perihelion might occur for both long period and Jupiter family comets, sungrazers, and Halley type comets when they pass through the inter-Mercurial region where the long-suspected population of Vulcanoid asteroids is reasoned to reside [7,8,9,10,11]. Although no member of the Vulcanoid asteroid family has (so far) been detected (see section 2 below), the rational in this review is to investigate the history of outburst behavior associated with those comets known to have passed through the putative Vulcanoid zone.…”