Optical telescopes and radar are tools used 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 rather than 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 Liquid Mirror Telescope (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, and the OMC Laboratory, which are explained below.
NASA and the Air Force Research Laboratory (AFRL) Maui Optical and Supercomputing (AMOS) site are collaborating to place a wide field-of-view (FOV), 1.3-m aperture telescope on Ascension Island for space debris research. The telescope system, designated the Meter-Class Autonomous Telescope (MCAT), is being designed and constructed by DFM Engineering and will be deployed in 2014.
Ascension Island is a British overseas territory located in the Atlantic Ocean, midway between Brazil and Africa (7° 58’ S, 14° 4’ W, 350’ elevation). MCAT will be located on the US Air Force base (45th Space Wing, Detachment 2, Ascension Auxiliary Air Field (AAF)). Ascension was chosen as the location for MCAT because: (1) its low geographic latitude ensures that low-inclination LEO, GEO and GTO target orbits pass overhead; (2) it provides coverage of the GEO belt that is not well covered by other existing ground sensors within the Ground Based-Electro-Optical Deep Space Surveillance (GEODSS) network of sites; (3) its remote location provides dark skies for astronomical observations, particularly important for small, faint debris; and (4) it has outstanding infrastructure and personnel on the island for long term logistical support and maintenance.
Utilizing a 4K, 15 µm pixel, back-illuminated Spectral Instruments CCD, the MCAT telescope will have a diagonal FOV of nearly one degree and operate in several different modes, all run autonomously. During twilight hours it will sample low inclination, low-Earth orbit (LEO) space debris in either a “stare and chase” or "rate track" mode. During dark hours, it will perform a conventional geosynchronous orbit (GEO) search. Targeted searches are also possible. With detection sensitivity potentially reaching the 10-15cm size regime in GEO and the 1cm regime in LEO, MCAT is expected to make a valuable contribution to our understanding of the orbital debris environment around Earth.
Observations of geosynchronous orbit (GEO) debris are taken throughout the year using the University of Michigan's Curtis Schmidt telescope located at the Cerro Tololo Inter-American Observatory (CTIO) in Chile (30.2° S, 70.8° W, 2200 m elevation). The system is given the acronym MODEST, for Michigan Orbital DEbris Survey Telescope. From this location, orbital longitudes ranging from 25° W to 135° W are observed, which covers most of the orbital slots assigned to the continental United States. The telescope, shown in the figure at the right, is a 0.61-m aperture, f/3.5 Schmidt of classical design. A 2K, SITe thinned, back-illuminated CCD is mounted at the Newtonian focus. The field-of-view is 1.3 x 1.3°, with 2.3 arc-second pixels. Standard exposure time is 5 sec, which reaches a signal-to-noise (S/N) of 10 for an 18th magnitude object. To perform follow-up observations of interesting debris targets, MODEST is frequently used in conjunction with the 0.9 m SMARTS telescope (also at CTIO). After initial detection with MODEST, extended observations are made possible using SMARTS, often resulting in repeated reacquisition of uncatalogued debris over several nights.
In characterizing the space environment, the physical characteristics of orbiting objects must be considered. These properties are used both in current space environment models and in building shields for spacecraft, as well as in providing a baseline for future environment studies. Some of these characteristics, including material type, currently are 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. Figure 1 shows the reflectance spectrum of a LEO R/B (in black with more noise) overlaid with a laboratory sample (in red and smoother) in an attempt to characterize the material. This rocket body was identified as aluminum coated with white paint.
A project that originated from the astronomy community highlighted the range of possible uses for the spacecraft materials spectral database. An object thought to be an asteroid, J002E3, was observed in September 2002. Its erratic orbit made astronomers question whether or not it was actually an asteroid. A spectrum, taken at the Infrared Telescope Facility (IRTF) on Mauna Kea, was sent to JSC to see if it matched man-made materials. For the test, a materials model was created from the known dimensions and paint scheme of a Saturn V upper stage. Both the models and the remote data results are seen in Figure 2. A variation between the two models occurs because a different type of white paint was used for each model.
This object was concluded to be 60% white paint that has turned gold in color due to space environment exposure, with the remaining materials consisting of a mix of 10% black and 10% yellow paint, and the remaining 20% consisting of exposed metals.
Optical observations of orbital debris offer insights that compliment radar measurements by yielding a more comprehensive description of individual debris pieces and the space environment as a whole. Since optical observations operate in a different portion of the electromagnetic spectrum, with wavelengths much smaller than accessible debris size, they are able to probe physical characteristics normally unavailable to radar.
For example, time-dependent photometric observations acquired in multiple bandpasses yield data that aid in both material identification and assessment of possible periodicity in object orientation. This data can also be used to help identify shapes and optical properties at multiple phase angles. Capitalizing on optical data products and applying them to generate a more complete understanding of orbital space objects is a key objective of NASA’s Optical Measurement Program, and a primary driver for creation of the Optical Measurements Center (OMC). The OMC attempts to emulate space-based illumination conditions using equipment and techniques that parallel telescopic observations and source-target-sensor orientations. The OMC uses a 75 watt, Xenon arc, collimated lamp as a solar simulator, a 1024 x 1536, 9 um pixel CCD camera (350 – 1100 nm bandwidth) with conventional astronomical colored filters (Johnson/Bessel), and a robotic arm to orient/rotate objects to simulate an object’s orbit/rotational period. A high-resolution, high bandwidth (350 - 2500 nm) Analytical Spectral Devices (ASD) spectrometer is also employed to baseline the spectral signature of various material types as a fiduciary reference to filter photometric assessment of these materials.
The laboratory data is compared directly with the photometric data collected from the Cerro Tololo Inter-American Observatory 0.9 m SMARTS and 0.6 m MODEST telescopes to better understand the GEO orbital debris environment. Thus far, materials that are known to be in the space environment (i.e., materials from spacecraft), materials from hyper- and low-velocity ground impact tests of mock satellites, and debris from a mock Arian 4 tank composed of aluminum alloy have been investigated. These materials consist of multi-layered insulation, solar panels, solar cells, plastics, carbon-fiber reinforced plastics, glass-fiber reinforced plastics, and different types of metal in a range of shapes and sizes.
The goal for the OMC is to produce light curves representative of the orbital debris environment to better understand their properties and evaluate potential risks.
The Liquid Mirror Telescope (LMT) was developed at NASA JSC and then 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 4 gallons of liquid mercury spinning at 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 f/1.5-parabolic surface whose optical quality matched conventional, 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 and celestial objects that passed overhead. An image-intensified video camera system (with a 0.26 deg FOV and limiting magnitude of 18th) was used for debris observations and yielded LEO detections to 1 cm diameter a 2K, 24 um pixel, back-illuminated CCD was used during non-twilight hours for astronomical observations with a 24th magnitude (white-light) detection limit.
The LMT facility became the NASA Orbital Debris Observatory (NODO) at the commencement of routine operations in 1995. Ironically, this six-story observatory with a 50-m-diameter dome, was built by the U.S. Air Force for satellite observations and studies of missile launches from nearby White Sands. It had housed a 1.2m. Cassegrain telescope that was operated from 1965 until 1982, then abandoned until becoming home to NASA's LMT. Located near Cloudcroft, New Mexico, at a 2770m elevation, the NODO site provided excellent viewing conditions. Operations continued every clear night until project termination in 2001. Over 1300 hours of twilight debris observations were acquired, making the LMT dataset one of the most comprehensive optical debris datasets to date. Larger LMTs have since been constructed by other groups for astronomical and debris observations, including the 6-m-diameter Large Zenith Telescope (LZT) at the University of British Colombia.
The CCD Debris Telescope (CDT) is a portable 32cm aperture, f/1.3 Schmidt telescope that was developed for measuring the optical properties of known orbital debris particles. It was used in many observing campaigns, including New Zealand in 1989, two summer runs at Rattlesnake Mountain Observatory in Washington State in 1990 and 1991, as well as a semi-permanent installation in Maui, Hawaii from 1992-1995, where it performed GEO observations. In 1997, the CDT was moved to the NASA Orbital Debris Observatory (NODO) in Cloudcroft, New Mexico to join the Liquid Mirror Telescope (LMT). With the existing sensor (a 27 m, 384 x 576 pixel CCD), the CDT could see objects as faint as 17.1 magnitude with a 30-sec exposure. It could detect approximately 0.8-m-diameter objects (assuming an albedo of 0.1) at geosynchronous altitude. During its tenure at NODO from 1997-2001, the CDT collected approximately 1954 hours of observations. With the closure of NODO in December 2001, the CDT was moved to storage. It has since been donated to Embry-Riddle University in Prescott, Arizona.