Ed Barker (NASA/Johnson Space Center), M. J. Matney (National Aeronautics and Space Administration), T. Yanagisawa (Japan Aerospace Exploration Agency), J.-C. Liou (ESCG), K. J. Abercromby (ESCG), H. M. Rodriguez (ESCG), M. F. Horstman (ESCG), P. Seitzer (University of Michigan)
Keywords: Orbital Debris
Abstract:
In February 2007 dedicated observations were made of the orbital space predicted to contain debris from the breakup of the Titan 3C-4 transtage back on February 21, 1992. These observations were carried out on the Michigan Orbital Debris Survey Telescope (MODEST) in Chile with its 1.3? field of view. The search region or orbital space (inclination and right ascension of the ascending node (RAAN) was predicted using NASAs LEGEND (LEO-to-GEO Environment Debris) code to generate a Titan debris cloud. Breakup fragments are created based on the NASA Standard Breakup Model (including fragment size, area-to-mass (A/M), and delta-V distributions). Once fragments are created, they are propagated forward in time with a subroutine GEOPROP. Perturbations included in GEOPROP are those due to solar/lunar gravity, radiation pressure, and major geopotential terms. Barker, et. al, (Proceedings of AMOS 2006 Technical Conference, pp. 596-604) used similar LEGEND predictions to correlate survey observations made by MODEST in February 2002 and found several possible night-to-night correlations in the limited survey dataset.
One conclusion of the February 2002 survey search was to dedicate a MODEST run to observing a GEO region predicted to contain debris fragments and actual Titan debris objects (SSN 25000, 25001 and 30000). Such a dedicated run was undertaken with MODEST between February 17 and 23, 2007 (UT dates). MODESTs limiting magnitude of 18.0 (SN~10) corresponds to a size of 22cm assuming a diffuse Lambertian albedo of 0.2. However, based on observed break-up data, we expect most debris fragments to be smaller than 22cm which implies a need to increase the effective sensitivity of MODEST for smaller objects. MODESTs limiting size could not be lowered by increasing the exposure time from 5 to 20 seconds due to trailing of the image. However, special image processing did allow the detection of smaller debris. Special processing combined several individual CCD images to detect faint objects that were invisible on a single CCD image. Sub-images are cropped from six consecutive CCD images with pixel shifts between images being consistent with the predicted movement of a Titan object. A median image of all the sub-images is then created leaving only those objects with the proper Titan motion. Limiting the median image in this manner brings the needed computer time to process all images taken on one night down to about 50 hours of CPU time.
Successful observations were carried out over 6 consecutive nights. Positions for each of the 62 detected targets on individual nights were fit under the assumption of circular orbits (ACO). Those targets that were observed on other nights and that had similar ACO orbital parameters will be combined and their observed positions fit to a full 6 parameter orbit. Combinations of targets having RMS fits less than ~10 arcseconds were considered to be the same target. Six combinations were correlated to cataloged targets (CTs). Cataloged Titan debris (SSNs 25000, 25001, 30000) were not detected because they were not observable during nighttime hours. Nine combinations could not be correlated to cataloged targets, hence they were defined as UCTs. These UCTs have orbital elements very similar to those predicted by LEGEND and thus are strong candidates for Titan debris.
Date of Conference: September 12-15, 2007
Track: Orbital Debris