An optical search for small comets

Abstract

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 ø 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 Yeares [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.

Reply to comment on “Detection of small comets with a ground‐based telescope” by L. A. Frank and J. B. Sigwarth

“…Instead Mutel and Fix [2003] executed a qualitative ''visual inspection'' of three small comet trails and declared them faulty by their appearance in the images. This inspection was similar to that published in their original paper on this topic [Mutel and Fix, 2000].…”

Comment on “An optical search for small comets” by R. L. Mutel and J. D. Fix

“…The range for the previous searches with the Spacewatch Telescope is also identified inFigure 3. The value given byMutel and Fix [2000] from their equation (6) for the volume at a distance of 55,000 km is 7.9 x 1017 m 3, whereas the correct volume is only 4.1 x 10 z6 m 3 at this distance as shown in our The effective volume for the detection of small comets with the Iowa Robotic Observatory (IRO) as a function of distance from the telescope to the position of the small comet. This "tuning" distance is that for the telescope field-of-view at rest relative to a small comet with speed of 10 km/s perpendicular to the line-of-sight of the telescope.…”

“…Successful optical sightings with the Spacewatch Telescope have been previously published [Yeates, 1989;Sigwarth, 1989;Frank et al, 1990;Frank and Sigwarth, 1993]. Mutel and Fix [2000] claim that ff the numbers of small comets as reported from these previous Spacewatch Telescope surveys do indeed exist, then their IRO survey should have detected 65 small comets on the average, but they find none. Unfortunately, Mutel and Fix [2000] have severely overestimated the volume of space which is viewed by the IRO during their search.…”

“…Thus only a small fraction of the small comets are detected in the image. On the other hand,Mutel and Fix [2000] have assumed that all of the comets in the 50 ø cone are observed, thus yielding a large error in their expectations for detection rates for small comets.4. Confirmation by Numerical ComputationA rigorous quantitative assessment of the effective volume of the IRO for the detection of small comets is also straightforward.…”

“…For a qualitative assessment the volume in configuration space can be estimated to vary as R •. On the other hand, examination of Figure 4 finds that Mutel and Fix [2000] can enter the "window of opportunity" for the search by using their present image set at apparent brightnesses in the range of ~17.3 m. This will require more than a cursory examination of the images, but our analysis of selected images finds that the sensor noise levels are such that small comet trails with total signal-tonoise ratios of 6 to 7 should be present if the small comets exist. We encourage Mutel and Fix to enter the window of opportunity with their future analyses.…”