Optical and radar telescopes are tools used in order to obtain a more complete picture of the orbital debris environment. Each of these tools sees a somewhat different debris environment. Some debris objects will reflect radar well, but sunlight poorly; while some will reflect sunlight well, but radar poorly. An advantage to using an optical telescope over radar is that telescopes can more easily detect debris objects in higher altitudes, such as geosynchronous orbit. NASA has previously used two optical telescopes for measuring orbital debris: a 3 m diameter liquid mirror telescope, which is referred to as the LMT, and a charge-coupled device (CCD) equipped 0.3 m Schmidt camera, which is referred to as the CCD Debris Telescope (CDT). Currently optical measurement research of orbital debris continues with the MODEST, MCAT and NASS projects that are explained below.

Liquid Mirror Telescope (LMT)

The LMT was developed at NASA JSC and was moved to Cloudcroft, New Mexico for the purpose of measuring the population of small orbital debris particles. The LMT consisted of a 3 m diameter parabolic dish that held four gallons of liquid mercury. The dish was spun up to a rate of 10 rpm. Centrifugal force and gravity caused the mercury to spread out in a thin layer over the dish creating a reflective parabolic surface that was as good as many polished glass mirrors. To provide the required stability, the mirror was mounted on a precision air-bearing. By "staring" straight up, the telescope was able to observe the orbital debris that passed overhead through its 0.26° field-of-view. The LMT was "housed" inside a large six story observatory with a 50 ft diameter dome which was originally built by the U.S. Air Force for satellite observations and studies of missile launches from nearby White Sands.

This observatory, located just outside of Cloudcroft, New Mexico, provided excellent viewing conditions. The elevation above sea level is 9,061 ft. The skies are among the darkest in the United States, and the atmosphere contains relatively little water vapor, dust, smoke, and/or aerosols that affect the transmission of light through the air. Cloudcroft is located at 32.9° N, which means that debris having orbital inclinations less than 32.9° cannot be seen. However, the major fraction of debris in low Earth orbit (LEO) has inclinations larger than this value. During its data collection period, 1997-2001, the LMT collected approximately 1,322 hours of observations. The LMT was shut down and disassembled in December 2001.

CCD Debris Telescope (CDT)

CCD Debris Telescope (CDT)

The CDT is a 12.5 in. aperture Schmidt portable telescope with point capability that was used in 1990 and 1991 at Rattlesnake Mountain Observatory in Washington for measuring the optical properties of known particles of orbital debris. The results were published in the Journal of Spacecraft and Rockets , Volume 31, Number 4, p. 671-677. It was also used for several weeks at a time in 1992, 1993, and 1994 at Maui, Hawaii for geosynchronous debris observations. The CDT was moved to the same facility as the LMT in Cloudcroft, New Mexico. With the existing sensor (27 m, 384x576 pixel CCD), the CDT could see objects as faint as 17.1 magnitude for a 30 sec exposure. It could see approximately 0.8 m diameter objects (assuming an albedo of 0.1) at geosynchronous altitude. During its data collection period, 1997-2001, the CDT collected approximately 1954 hours of observation. The CDT was shut down and moved to storage in December 2001. Future plans for the CDT include deployment to another location for site surveys in association with the next generation of optical telescope observations.

MODEST

MODEST, Cerro Tololo Inter-American Observatory in Chile

Using the University of Michigan's Curtis Schmidt telescope located at the Cerro Tololo Inter-American Observatory in Chile (30.2° S, 70.8° W, 2200 m altitude), observations of geosynchronous orbit (GEO) debris are taken throughout the year. The system is given the acronym MODEST, for Michigan Orbital DEbris Survey Telescope. From this location one can observe orbital longitudes ranging from 25° W to 135° W, which covers most of the orbital slots assigned to the continental U.S. The telescope is shown in the figure at the right. The telescope is a 0.61/0.91 m f/3.5 Schmidt of classical design. A 2048x2048 SITe thinned, backside illuminated CCD is mounted at the Newtonian focus. The field-of-view is 1.3x1.3°, with 2.318 arc-second pixels. All images are obtained through a broad R filter (200 nm FWHM) selected to maximize detected counts from objects with a reflected solar spectrum, and at the same time to minimize signals from night sky emission lines and the effects of atmospheric refraction. Standard exposure time is 5 sec, which reaches a signal-to-noise (S/N) of 10 for an 18^th magnitude object. All magnitudes are referred to as the Cousins R system of astronomical magnitudes, and are calibrated by nightly observations of standard stars. Observations with this telescope are ongoing.

MCAT

NASA and the Air Force Maui Optical and Supercomputing (AMOS) site are cooperating to place a wide field-of-view, one-meter-aperture telescope on Kwajalein Atoll for space debris research. The telescope system, designated the Meter-Class Autonomous Telescope ( MCAT ), will be deployed as part of the High Accuracy Network Orbit Determination System (HANDS) and will use the Oceanit, Inc. K-Star design. The telescope will operate in two different modes. During twilight hours it will sample low inclination orbits in a "track before detect" mode. In the middle of the night it will perform a more standard geosynchronous orbit (GEO) search. Kwajalein Atoll was chosen as the location for MCAT because: 1) its low latitude location is necessary for sampling low inclination orbits, 2) its location allows it to measure a part of the GEO region not covered by other optical sensors, and 3) it has a technically skilled workforce which can act in a caretaker capacity. Three islands in the Atoll are under consideration for location of the MCAT: Kwajalein Island (8.7° N), Roi-Namur (9.4° N), and Legan (9.0° N).

The future for the MCAT project is to build the 0.5 m prototype wide field-of-view (5°) system. The deployment of a standard HANDS telescope and the prototype MCAT will occur by the end of 2005. This will allow for establishment of infrastructure and refinement of the autonomous operation and data reduction. In addition, this interim step will refine the corrosion control procedures, which is a problem on the Atoll due to the sea salt spray. The deployment of MCAT is expected during 2005-2006.

NASS

In order to characterize the space environment, the physical characteristics of orbiting objects are taken into consideration. These properties are used in current space environment models and the building of shields for spacecraft, as well as in providing base work for future environment studies. Some of these characteristics, including material type, are currently assumed. Each material type shows a different spectrum based on its composition. Using low-resolution reflectance spectroscopy and comparing absorption features and overall shape of spectra, it is possible to determine material types of man-made orbiting objects in both low Earth orbits (LEO) and geosynchronous orbits (GEO).

NASS (NASA AMOS Spectral Study) began observations in May 2001 collecting data for 23 nights. Currently, data on more than 60 rocket bodies (R/Bs) and spacecraft (S/C) spectra have been collected using the 1.6 m telescope at the Air Force Maui Optical Supercomputing (AMOS) site. The remote spectra were compared to the database of spacecraft material spectra kept at JSC. Shown at the right is the reflectance spectrum of a LEO R/B (shown in black with more noise) overlaid with a laboratory sample (in red and smoother) in an attempt to characterize the material. This rocket body is identified as aluminum coated with white paint.