The Original IRO Search

From September 1998 to June 1999 we 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. 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-sec-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 6,143 total images, we selected 2,713 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 was 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/sec 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.

Mutel, R.L. and J. D. Fix 2000, An Optical Search for Small Comets, J.G.R., 105, 24907-24915.

A MS-Powerpoint presentation of this paper (6.1MB) can be downloaded here

The Search Volume Controversy

Frank and Sigwarth (2001, Comment, JGR, 106, 24857) have argued that the sample volume calculation used in the above paper was incorrect. We have written a Reply (JGR, 106, 24863,2001) with detailed calculation of sample volume as a function of orbital parameters. We extended the sample volume calculation for the IRO search to consider objects with a range of perihelia and inclinations (the original paper only considered objects with orbits confined to the ecliptic).

We find that for objects with absolute magnitude V > 32.8 (corresponding to maximally stealth parameters of albedo p = 0.01, and density 0.3 gm-cm-3, and mass M > 8,000 kg) the expected number of detections averaged over a uniform range of perihelia (0.8 < q < 1.0 AU) is 235. For non-zero inclinations, the expected number depends on the distribution function; for a uniform distribution from 0 to 17 degrees, the expected number is 14. While smaller than the expected number in the original paper, the null detection results still implies rejection of the claimed small comet number density at 99.999% confidence.

The Frank and Sigwarth Small Comet Search and our Analysis

After our original search was published, Frank and Sigwarth (Detection of small comets with a ground-based telescope, 2001, JGR, 106, 3665) reported detection of nine faint trail using a selection of 1,500 images from the original survey. All the claimed detections were in the magnitude range 16.8 - 17.7, fainter than our limiting magnitude of 16.5, so this result does not contradict the our null result. We have conducted an independent analysis of all nine images and cannot find any evidence for faint trails with the requisite properties (correct shutter modulation, point-spread function equal to the stellar trails). In one case, we find that the candidate trail lies on a star trail (figure at right). Furthermore, the integral number density of detections versus apparent magnitude is inconsistent both in scale and form with the alleged distribution in several small comet papers (e.g. Fig. 3, Frank & Sigwarth 2001). We have submitted a Comment to JGR summarizing these results.

The image database

Interested parties can access FITS and GIF format images of all nine images in which there is a claimed detection in the Frank and Sigwarth paper here. All other images from the original search can be made available on request.