An Optical Search for Small Comets R. L. Mutel & J.D. Fix University of Iowa

1. An Optical Search for Small Comets R. L. Mutel & J.D. Fix University of Iowa

2. Small Comet Detection Papers DE-1 (April 1986) Polar (May 1997)

3. Small Comet Scenario(From L. Frank Website)

4. Small Comet Parameters (from Frank and Sigwarth 1993, Small comet Web site) Mass: 20,000 – 40,000 kg Size: 4 – 10 m Density: ~ 0.1 x H20 Number density: (3 ± 1) · 10-11 km-3 Flux at Earth: 1 every 3 seconds (107 per yr.) Composition: Water ice with very dark mantle (albedo 0.01-0.02) Orbit: Confined to ecliptic, prograde Speed: ~10 km-sec-1 at 1 a.u. Origin: Hypothesized comet belt beyond Neptune

5. Tests of the Small Comet Hypothesis

6. Observations • The observations were made using the 0.5 m f/8 reflector of the Iowa Robotic Observatory between 24 September 1998 and 11 June 1999. • Observations were scheduled every month within one week of new moon. A total of 6,148 images were obtained, of which 2,718 were classified as category A (visual detection magnitude 16.5 or brighter in a 100 pixel trail). • Seeing conditions varied from 2 - 5 arcsec (see histogram). For quality A images, seeing was < 3.5 arcsec. • All images were has thermal and bias corrections applied. • Images were recorded on CDROM and sent to the University of Iowa for analysis. • All images are available for independent analysis via anonymous ftp at node atf.physics.uiowa.edu.

7. Search Geometry

8. Iowa Robotic Observatory

11. Star Visual Magnitude Calibration Faint Galaxy V=14.9

12. FWHM = 2.8" FWHM = 4.2" Visual Magnitude Calibration using Standard Stars:ADU counts vs. V, FWHM 16.7 15.6 17.1

13. 16.7 17.1 17.1 15.6 16.7 Visual Magnitude Detection vs. Trail Length(20 April 1999, 60 s: fixed & 30 pixel trailed

14. Cosmic Ray Example of Trails Caused by Cosmic Rays, Geostationary Satellite

15. V = 16.4 39 pixels Visual Detection Calibration Using Synthetic Trails • Synthetic comet trails were added to 520 search images with randomly chosen magnitudes and trail lengths. • Three observers independently inspected all images • Result: Visual detection threshold is ~0.9  per pixel, with a suggestion that longer trails can be detected slightly fainter, perhaps 0.7 - 0.8 .

16. V = 16.6 103 pixels V = 16.4 39 pixels V = 15.1 417 pixels Sample Synthetic Comet Calibration Images

17. V = 17.0 124 pixels Synthetic Comet Trail Nearing Limiting Magnitude (V=17.0)

19. Observer r- r+  Calculation of Sampled Volume 1.Sampled volume as function of trail length L, field of view  : 2.Use faintest visual magnitude vs. trail length from synthetic comet test (60 s,  = 17 ADU  = 3.5"): 3. Detection volume as a function of visual magnitude (mv), speed (vobj ):

20. Trail Length versus Range

21. Detection Probability Per Image(assumes nsc = 3x10-11 km-3)

22. Upper Limit to Small Comet Number Density (99% confidence level) Rejected density region 0.05 n0 Allowed density region

23. Probability of Non-detection vs. Number Density(N=2,713, no detections, n0 = 310-11 km-3) n = 0.05  n0 n = 0.25  n0

24. Small Comet Optical Search Comparison

25. Comparison with Previous Searches:Detection magnitude comparison 1. Visual magnitude m correction for distance (55,000 km vs. 137,000 km) is 2.0 magnitudes. 2a. Visual magnitude m as function of solar phase angle , scattering parameter Q, phase function () [Lumme & Bowell 1981]: 2b. Best fit phase function for solar system objects is: 2c. For Q ~ 0, magnitude difference between previous searches (fixed phase angle 20) and present search (4<  < 9 ) is:

26. Multiple Scattering Factor Q versus Albedo for Solar System Objects(from Lumme & Bowell AJ 86, 1705) Asteroids Planets, Satellites Small comet albedo range

27. Phase angle versus local time for IRO search 8 average solar phase angle Midnight 6 am/pm

28. Q = 0.3 Q = 0.6 Q = 0.0 Magnitude difference between IRO search and previous searches at fixed  = 20 0.54 8°

29. V=16.5 Implications for Physical Characteristics of Small Comets The magnitude limit can be converted to limits on the physical properties of small comets. Assuming a single scattering function Q = 0 and and a mean solar phase angle of 8°, the allowed range of geometrical albedo and density for a mass of 20,000 kg (Frank et al. 1990) is shown below. V=16.0 Permitted Region Forbidden Region Darkest solar system objects (Iapetus) Darkest part of Halley nucleus

30. Density 0.1 x H2O Frank et al. (1990) estimated mass range V = 16.5 V = 17.0 Physical Conditions of Small Comets Alternatively, assuming a mass density of 0.1 gm-cm-3 (e.g. Frank and Sigwarth 1993), the mass-albedo allowed range is shown below.

31. Summary • We have conducted an extensive optical search for small comets proposed by Frank et al. (1986; Frank & Sigwarth 1997,1999). • After careful visual inspection of more than 2,700 images, we found no objects consistent with small comets. The detection limit depends on magnitude and trail length: e.g. for V = 16.5, trail lengths up to 120 pixels are robustly detected. • These results strongly disagree with previous optical searches of Yeates (1989) and Frank et al. (1990). Extrapolation of their detections to our search predicts more than 60 detections brighter than V = 16.5. • The null detections place an upper limit to the number density n < 0.05 (99% confidence) of the value claimed by Frank and Sigwarth (1990). • Any object with mass M = 20,000 kg and fainter than the magnitude-trail length limit must have either: • An implausibly low geometric albedo (p<0.01) or • Density greater than ice ( > 1 gm/cm3).