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Session 33 - Comets & Asteroids.
Oral session, Wednesday, January 07

[33.03] The Lost Tribe of Small Comets (SCs)

J. Brandt, C. Randall, I. Stewart (U. Colorado), M. A'Hearn, Y. Fernandez (U. Maryland), D. Schleicher (Lowell Observatory)

Evidence for an extensive population of comets with nuclear radii, r, less than 1000 meters continues to accumulate. Because they are faint, we see them when special circumstances produce intrinsic or apparent brightening and when improved observational techniques become available.

Intrinsic brightening occurs near the sun where SCs are detected by coronagraphs on the SOLWIND, SMM, and SOHO spacecraft. These comets are too faint to be detected when passing the earth’s orbit, but brighten considerably at small heliocentric distances. A large number of such comets are known. Apparent brightening occurs when comets pass close to earth; several are known. Finally, observations using the WF/PC 2 on the Hubble Space Telescope have isolated the light from the nucleus and determined radii for comets Honda-Mrkos-Pajdusakova (340 meters) and Wirtanen (600 meters). Lamy et al. conclude that comet Wirtanen “...belongs to a class of small, highly active nuclei...”

The distribution function f(r) \propto r^-3 (differential) probably holds down to 100 meters and almost certainly does not turn over at 1000 meters where our traditional detection methods begin to fail. We estimate that 16 to 800 250-meter SCs are in a shell between 1 and 2 AU heliocentric distance. But, at present, the detection of SCs is a haphazard process. We advocate a dedicated search program that is optimized by using all the unique properties of comets: extended source (most light coming from the coma rather than the nucleus); motion with respect to the background stars; and characteristic spectral signature. We report results on algorithm development using ground-based data.

Program listing for Wednesday