An optical search for small comets

Abstract

We have conducted an extensive optical search for small comets with the characteristics proposed by Frank et al. [1986] and Frank and Sigwarth [1993, 1997]. The observations were made using the 0.5‐m reflector of the Iowa Robotic Observatory between September 1998 and June 1999. The search technique consisted of tracking a fixed point in the ecliptic plane at ±9° geocentric solar phase angle. The telescope scan rate was chosen to track objects moving prograde at 10 km s −1 relative to the Earth at a distance of 55,000 km. The camera was multiply shuttered to discriminate against trails caused by cosmic rays and sensor imperfections. Of 6143 total images, we selected 2713 which were suitable for detection of objects with a magnitude 16.5 or brighter with 120 pixel trails. The sensitivity and reliability of the visual detection scheme were determined by extensive double‐blind tests using synthetic trails added to over 500 search images. After careful visual inspection of all images, we found no trails consistent with small comets. This result strongly disagrees with previous optical searches of Yeates [1989] and Frank et al. [1990], whose detection rates and magnitudes, when converted to the present search, predict 65 ± 22 detections. We conclude that at 99% confidence, the number density of any prograde objects in the ecliptic plane brighter than magnitude 16.5 with speeds near 10 km s −1 have a number density less than 5% of the small‐comet density derived by Frank et al. [1990]. Any object fainter than this magnitude limit with a mass corresponding to the small‐comet hypothesis ( M > 20,000 kg) must have either an implausibly low geometric albedo ( p < 0.01) or a density larger than that of water.