An Optical Search for Small Comets
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An Optical Search for Small Comets R. L. Mutel & J.D. Fix University of Iowa. Small Comet Detection Papers. DE-1 (April 1986). Polar (May 1997). Small Comet Scenario (From L. Frank Website). Small Comet Parameters (from Frank and Sigwarth 1993, Small comet Web site)

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An Optical Search for Small Comets

R. L. Mutel & J.D. Fix

University of Iowa


Small Comet Detection Papers

DE-1 (April 1986)

Polar (May 1997)


Small comet scenario from l frank website
Small Comet Scenario(From L. Frank Website)


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



  • 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.





Visual magnitude calibration using standard stars adu counts vs v fwhm

FWHM = 2.8"

FWHM = 4.2"

Visual Magnitude Calibration using Standard Stars:ADU counts vs. V, FWHM

16.7

15.6

17.1


Visual magnitude detection vs trail length 20 april 1999 60 s fixed 30 pixel trailed

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


Example of trails caused by cosmic rays geostationary satellite

Cosmic Ray

Example of Trails Caused by Cosmic Rays, Geostationary Satellite


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 .


V = 16.6

103 pixels

V = 16.4

39 pixels

V = 15.1

417 pixels

Sample Synthetic Comet Calibration Images


Synthetic comet trail nearing limiting magnitude v 17 0

V = 17.0

124 pixels

Synthetic Comet Trail Nearing Limiting Magnitude (V=17.0)


Calculation of sampled volume

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 ):



Detection probability per image assumes n sc 3x10 11 km 3
Detection Probability Per Image(assumes nsc = 3x10-11 km-3)


Upper limit to small comet number density 99 confidence level
Upper Limit to Small Comet Number Density (99% confidence level)

Rejected density region

0.05 n0

Allowed density region


Probability of non detection vs number density n 2 713 no detections n 0 3 10 11 km 3
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



Comparison with previous searches detection magnitude comparison
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:


Multiple scattering factor q versus albedo for solar system objects from lumme bowell aj 86 1705
Multiple Scattering Factor Q versus Albedo for Solar System Objects(from Lumme & Bowell AJ 86, 1705)

Asteroids

Planets, Satellites

Small comet albedo range


Phase angle versus local time for IRO search

8 average solar phase angle

Midnight

6 am/pm


Q = 0.3

Q = 0.6

Q = 0.0

Magnitude difference between IRO search and previous searches at fixed  = 20

0.54


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


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.


  • 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).


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