U.S. Government Efforts to Deal With Orbiting Space Junk by Michael Erbschloe - HTML preview

NASA Orbital Debris Program Office

The NASA Orbital Debris Program Office (ODPO) has taken the international lead in conducting measurements of the orbital environment and in developing the technical consensus for adopting mitigation measures to protect the users within it. Located at the Johnson Space Center, the Office continues to develop an improved understanding of the orbital debris environment and measures that can be taken to control debris growth.

Measurements of near-Earth orbital debris are accomplished by conducting ground-based and space-based observations of the orbital debris environment. Data is acquired using ground-based radars and optical telescopes, space-based sensors, analysis of spacecraft surfaces returned from space, and ground-based laboratory experiments, such as DebriSat. Some important data sources have been the U.S. Space Surveillance Network, the Haystack X-Band Radar, and returned surfaces from the Solar Max, the Long Duration Exposure Facility (LDEF), the Hubble Space Telescope (HST), and the Space Shuttle. The data provides validation for the environment models and identifies the presence of new sources.

NASA's main source of data for debris in the size range of approximately 5 mm to 30 cm is the Haystack Ultrawideband Satellite Imaging Radar (HUSIR). The HUSIR radar, operated by the Massachusetts Institute of Technology’s Lincoln Laboratory, has been collecting orbital debris data for the ODPO since 1990 under an agreement with the U.S. Department of Defense. HUSIR statistically samples the debris population by "staring" at selected pointing angles and detecting debris that fly through its field-of-view.

The data are used to characterize the debris population by size, altitude, and inclination. From these measurements, scientists have concluded that there are approximately 500,000 debris fragments in orbit with sizes down to one centimeter. The NASA ODPO also collects data from the Haystack Auxiliary Radar (HAX) located next to the main HUSIR antenna. Although HAX is less sensitive than HUSIR, it operates at a different wavelength (1.8 cm for HAX versus 3 cm for HUSIR) and has a wider field-of-view.

Since 1990, the Goldstone Orbital Debris Radar has collected orbital debris data for debris as small as about 2 mm in LEO for the NASA ODPO. It is located in the Goldstone Deep Space Communications Complex in the Mojave Desert near Barstow, California and is operated by NASA’s Jet Propulsion Laboratory. The Goldstone Orbital Debris Radar is an extremely sensitive sensor capable of detecting a 3-mm metallic sphere at 1000 km, which makes it an incredibly useful tool in the characterization of the sub-centimeter-sized debris population.

Optical telescopes passively measure sunlight reflected by debris and allow us to routinely observe objects in low Earth orbit out to the more distant geosynchronous (GEO) orbital regimes. Combining optical and radar measurements into the ORDEM model allows NASA to obtain a more complete picture of the orbital debris environment at all altitudes.

Currently optical measurement research- and GEO-survey observations of the orbital debris environment are accomplished with the 1.3-m Eugene Stansbery Meter-Class Autonomous Telescope (ES-MCAT) and the Optical Measurement Center (OMC). Predecessors to ES-MCAT include: a charge-coupled device (CCD) equipped 0.3 m Schmidt camera, referred to as the CCD Debris Telescope (CDT); a 3-m-diameter liquid mirror telescope, the Liquid Mirror Telescope (LMT); and the 0.6-m Michigan Orbital Debris Survey Telescope (MODEST). MODEST data was used to build ORDEM 3.0.

The Eugene Stansbery–Meter Class Autonomous Telescope was deployed on Ascension Island and achieved first-light in June 2015. The ES-MCAT is a 1.3-m, f/4, DFM Engineering optical telescope with an ObservaDome, both fast-tracking to easily accommodate tracking debris at all orbital altitudes. The custom Euclid Research Corp. control software queries and controls all functions of the observatory, including autonomously monitoring weather and system health, conducting all observations, and processing the data. Ultimately, the data will be incorporated into the ORDEM model, an engineering model designed to describe the risk from orbital debris in Earth orbits, ranging from low Earth orbit (LEO) to geosynchronous orbits (GEO).

ES-MCAT, a joint NASA-Air Force Research Labs (AFRL) project, is located on the U.S. Air Force base (45th Space Wing, Detachment 2) near the Ascension Auxiliary Air Field. Ascension was chosen because: (1) its near equatorial latitude ensures that low-inclination LEO, GEO, and GEO transfer orbit (GTO) target orbits pass overhead; (2) it fills a gap in longitudinal coverage of the GEO belt that is not covered by other U.S. ground-based telescopes; (3) its remote location provides very dark skies for astronomical observations, particularly important for small, faint debris; and (4) the infrastructure and personnel on the island allow for long term logistical support and maintenance.

The primary observing goal for MCAT is to statistically characterize under-sampled orbital regimes, with emphasis on monitoring the GEO debris belt to determine the distribution function of debris. The prime objective is to monitor and assess the orbital debris environment by surveying orbiting objects at all orbital altitudes: LEO, medium Earth orbit, GTO, and GEO. As a dedicated NASA asset, ES-MCAT can be tasked for rapid response to break-up events after they have occurred, and is intended to become a contributing sensor for the Space Surveillance Network for the purposes of Space Situational Awareness. ES-MCAT is expected to make a valuable contribution to our understanding of the orbital debris environment around Earth for years to come.

From 2001 to 2014, MODEST served as NASA’s primary optical detector for statistically surveying the GEO region to provide flux estimates of the number of detected objects at GEO, as input for the ORDEM model. MODEST is a 0.6-m aperture, f/3.5 Schmidt of classical design and is located at the Cerro Tololo Inter-American Observatory (CTIO) in Chile (30.2° S, 70.8° W, 2200 m elevation).

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. MODEST detected 17.5-magnitude objects routinely and occasionally as faint as 18.3 magnitudes by using 5-second exposures with the R filter. These magnitudes correspond to objects that are 25 and 17-cm diameter, respectively, assuming a 0.175 albedo object at 36,000 km altitude (i.e., at GEO) that follows a Lambertian phase function.

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. Unlike radio/radar waves, visible light waves are much smaller than typical debris sizes, and as such, we are able to probe physical characteristics that are complimentary to radar-based properties.

For example, time-dependent photometric observations acquired in multiple wavelength bandpasses yield data that aid in both material identification and assessment of possible periodicities in object orientation. This data can also be used to help identify shapes and optical properties at multiple solar 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 simulates 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-µm pixel CCD camera (350 nm to 1100 nm bandwidth) with conventional astronomical colored filters (Johnson/Bessel), and a robotic arm to orient/rotate objects to mimic an object’s orbit/rotational period. A high-resolution, high bandwidth (350 nm to 2500 nm) Analytical Spectral Devices (ASD) spectrometer also is employed to baseline the spectral signature of various material types as a fiduciary reference to filter photometric assessment of these materials.

CTIO 0.9-m SMARTS and 0.6-m MODEST telescopes to better understand the GEO orbital debris environment and will be used for MCAT data in the future. Thus far, materials that are known to be in the space environment (i.e., materials from spacecraft), debris fragments generated in laboratory-based hyper- and low-velocity 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 (MLI), solar panels, solar cells, plastics, carbon-fiber reinforced plastics (CFRP), glass-fiber reinforced plastics (GFRP), and different types of metal in a range of shapes and sizes.

After approximately 16 years in low Earth orbit aboard the Hubble Space Telescope (HST), the Wide Field Planetary Camera 2 (WFPC-2) was returned to Earth in 2009 by Servicing Mission 4 (STS 125). The WFPC-2 radiator was exposed to the micrometeoroid (MM) and orbital debris (OD) environments and provides a unique record of the MMOD environment due to the duration on orbit as well as its relatively large 1.76 m2 surface area.

The radiator’s outermost layer is a 4-mm-thick aluminum, curved plate coated with YB-71 Zinc Ortho-Titanate white thermal paint. Immediately apparent was the presence of large impact features featuring spallation of the surrounding paint as well as craters resident only in the thermal paint layer; similar phenomenology was observed during a prior survey of the WFPC-1 radiator.

Craters were extracted by coring the radiator in the NASA Johnson Space Center’s Space Exposed Hardware clean room and were subsequently examined using scanning electron microscopy / energy dispersive X-ray spectroscopy to determine the likely origin, e.g., micrometeoritic or orbital debris, of the impacting projectile.

Recently, a selection of large cores was re-examined using a new technique developed to overcome some limitations of traditional crater imaging and analysis. This technique, motivated by thin section analysis, examines a polished, lateral surface area revealed by cross-sectioning the core sample.

Like other returned surfaces, the MLI display both impact craters and penetrations. A relatively complex 'petaling' phenomenon along with multiple-layer penetration features have been observed in both hypervelocity testing and in the Bay 5 MLI. This indicates a more complicated structure for impact features than earlier assumed. A new methodology of characterization techniques was developed and employed during data collection and analysis of the HST MLI and will provide greater understanding and more accurate estimation of impacting particle parameters.

The NASA Orbital Debris Program Office (ODPO) is leading the effort, with full support from the NASA Hypervelocity Impact Technology, the NASA Meteoroid Environment Office, and the NASA Astromaterials Acquisition and Curation Office, to conduct an MMOD impact survey of the WFPC-2 radiator. The goal is to use the data to validate the near-Earth MMOD environment definition and the Orbital Debris Engineering Model (ORDEM). This effort is also very well supported by the HST Development Project Office located at the NASA Goddard Space Flight Center.

The NASA ODPO Space Debris Sensor (SDS) is the first flight demonstration of the Debris Resistive / Acoustic Grid Orbital NASA-Navy Sensor (DRAGONS) technology developed and matured over 10 years by the NASA ODPO, in concert with the DRAGONS consortium, to provide information on the sub-millimeter orbital debris environment.

SDS was developed under the NASA Class 1E program for experimental spaceflight hardware and was robotically installed aboard the International Space Station’s Columbus module in January 2018 as an external payload. During its operational lifetime the mission was hampered by reduced operational uptime associated with loss of low data rate command/telemetry capability. The mission was effectively terminated by the loss of low and medium data rate telemetry. Efforts to recover the payload were ultimately unsuccessful.

The SDS, with a requirement to operate for a minimum of 2 years, collected 25 days of resistance/engineering data and over 1300 acoustic detection files. Despite its relatively short operational lifetime, the mission was a success in demonstrating the technology required to read and record grid resistance data at 1 Hz, grid acoustic data at 500 kHz, and store and downlink these data to the ground. Unfortunately, due to the short duration of 25 days, the mission failed to demonstrate the ability to characterize multi-grid line cuts, time of flight derived from an acknowledged impact on the second grid layer, an acknowledged impact on the third, backstop layer, and the determination of projectile mass density from impulse/impact energy delivered to the backstop.

The DebriSat project is a collaboration of the NASA Orbital Debris Program Office (ODPO), the Air Force Space and Missile Systems Center (SMC), The Aerospace Corporation (Aerospace), the University of Florida, and the Air Force Arnold Engineering Development Complex (AEDC). The project's goal is to design and fabricate a 56-kg class spacecraft mock-up (“DebriSat”) representative of modern payloads in the low Earth orbit (LEO) environment, conduct a hypervelocity impact test to catastrophically break it up, collect fragments as small as 2 millimeters in size, measure and characterize the fragment properties, and then use the data to improve space situational awareness and satellite breakup models.

Also at AEDC in 1992, a key impact test series, Satellite Orbital debris Characterization Impact Test (SOCIT), was conducted by the Department of Defense and NASA to support the development of satellite breakup models. The main target for SOCIT was a fully functional U.S. Navy Transit satellite. The DoD and NASA breakup models based on the SOCIT data have supported many applications and matched on-orbit events reasonably well over the years.

As new materials and construction techniques are developed for modern satellites, however, there is a need for new laboratory-based tests to acquire data to improve the existing DOD and NASA breakup models. The need for such tests is also supported by discrepancies between model predictions and observations of fragments generated from the breakup of modern satellites

All major design decisions, including the selection of components, subsystems, mass fractions, structure, and construction methods were reviewed and approved by Aerospace subject matter experts. Shown in Figure 1, the DebriSat body was covered with multi-layer insulation (MLI) and three solar panels were attached to one side of the main body. The table, displayed in Figure 2, shows a comparison of DebriSat and SOCIT targets and test conditions.

Figure 1: Picture of the DebriSat Satellite. Credit: University of Florida.

To reduce the project cost, a decision was made to emulate the majority of components. The emulated components were based on existing designs of flight hardware, including structure, dimensions, materials, and connection mechanisms. At the end of the assembly, DebriSat was subjected to a standard vibration test to ensure the integrity of the structure.

To increase the project’s benefits further, Aerospace designed and built a target resembling a launch vehicle upper stage (“DebrisLV”) for the pre-test shot. DebrisLV had a mass of 17.1 kg with body dimensions of 35 cm (diameter) × 88 cm (length).

Hypervelocity Impact Tests. A key element for the DebriSat impact test was the design and installation of a soft-catch system inside the target chamber that would slow down and capture fragments after the projectile impact, similar to the original SOCIT test series. Several polyurethane foam stacks, consisting of panels with different densities (0.06, 0.096, and 0.192 g/cm³) and with a total thickness of up to 25 cm, were used in the downrange and sideways directions during the SOCIT test series.

For DebriSat, the same foam material with three different densities (0.048, 0.096, and 0.192 g/cm³) was used but with an increased thickness of up to 61 cm. In addition, the interior of the target chamber was fully covered with the soft-catch foam panels to prevent any fragments from impacting the chamber walls, which would produce secondary damage (to the flying debris fragments) not associated with the breakup.

The hypervelocity impacts of DebrisLV and DebriSat were conducted at the Range G facility at AEDC. Range G operates the largest two-stage light gas gun in the United States. To maximize the projectile mass at the 7 km/sec impact speed, the AEDC team developed a special projectile design featuring a hollow aluminum cylinder embedded in a nylon cap. The nylon cap served as a bore rider for the aluminum cylinder to prevent hydrogen leakage and also to protect the barrel. Using the bore rider eliminated the need for a sabot, which holds a projectile during acceleration but is discarded after it leaves the gun barrel. This removes energy that could have been carried by the projectile, and introduces the possibility of uncharacterized secondary impacts by sabot fragments.

The DebrisLV and DebriSat impacts were successfully carried out at AEDC Range G on 1 April and 15 April 2014, respectively. Figures 3 and Figure 4 show the impact sequences of DebrisLV and DebriSat, in that order. Portions of the rear nylon cap fragmented and trailed the aluminum cylinder during flight, but this did not affect the planned catastrophic outcome of the impact.

Figure 3 Impact sequences of DebrisLV. Credit: Arnold Engineering Development Complex/Air Force.