Patrick Kelly, Millennium Space Systems, A Boeing Company; Ellen Glad, Millennium Space Systems, A Boeing Company; Tyler Ritz, Millennium Space Systems
Keywords: Space Debris, Tether, Telescope, Detection, Orbital Debris Mitigation, Scientific Method
Abstract:
DRAGRACER was a 2020 flight demonstration mission to characterize the orbital debris mitigation properties of deployable tethers as well as to validate ground observability of commercial-off-the-shelf (COTS) LEDs on-orbit. The scientific-method-based deorbit study was executed using two nearly identical modules that were deployed simultaneously into a 500 km, sun-synchronous orbit. One module was equipped with a 70-meter-long tether while the other served as a control unit to provide a one-to-one basis for deorbit comparison. The tethered module deorbited within 8 months while the other will remain on orbit for over a decade. Both satellites were observed using COTS ground equipment, confirming observability of the LEDs. This paper summarizes the findings of the DRAGRACER flight experiment, including insights into orbital decay rates, drag estimation parameters, and a radiometric study analyzing the observability of COTS LEDs on-orbit.
The primary motivation for the DRAGRACER program was to validate the deorbit performance of space tethers in low-earth orbit. Two modules were used to test the deorbit performance: ALCHEMY and AUGURY. When stowed for launch, ALCHEMY and AUGURY formed a single 12U stack. ALCHEMY was equipped with a deployable, 70 m x 15.4 cm long aluminized Kapton tape while AUGURY maintained identical mass and volume characteristics as the stowed ALCHEMY configuration. Upon orbit insertion, the two modules would separate naturally. ALCHEMY’s tape was deployed 32 minutes after insertion and unfurled naturally due to induced tip-off rates, gravity gradient effects between a tip mass affixed to the free end of the tether, and the local atmospheric drag interacting with the exposed surfaces on the terminator tape. The tape was not electrically conductive and was not designed to impart any electromotive forces to assist with deorbit. The significant increase in surface area introduced by the tether was predicted to result in an accelerated deorbit trajectory for ALCHEMY as compared to AUGURY.
Deorbit data was collected primarily using TLEs and ELSET data from space-track.org. Using information on the mean elements, decay rates and drag properties were extracted for each satellite. Flexible body dynamics and uncertainty in atmospheric drag intensities make modeling the reentry trajectory a difficult challenge for a tethered satellite system. The data collected for each satellite was used to help refine existing models and inform on areas for model improvements. Initial decay estimates were baselined assuming a 50% effective surface area efficiency for the terminator tape. Actual data supports a 20% effective surface area efficiency, resulting in a total orbital lifetime of 8 months for ALCHEMY.
The secondary motivation for the DRAGRACER program was a radiometric study to understand the observability of COTS LEDs affixed to the outer surfaces of the satellites. The LEDs were powered using a combination of hobbyist battery packs and solar cells on the outer surfaces of each DRAGRACER module. During the early months of the study, the team was able to collect imagery of the satellites as they passed over select ground sites, all using COTS observation equipment. The team utilized a monochromatic camera sensor, an 11-inch optical telescope, and a tracking mount designed to open-loop track satellites based on TLE inputs.
Data collected from the optical ground station was collected and post-processed to generate light curves for each pass. Images were filtered and normalized for each pass and ultimately confirmed the observability of each DRAGRACER module using LEDs. A machine learning algorithm was used to identify the DRAGRACER satellites from hundreds of collected images. Optical signatures from the LEDs were compared against measurements of the DRAGRACER modules from sun-illuminated passes.
The DRAGRACER flight experiment proved successful on all fronts. Space tether performance was effectively quantified using reported orbit tracking information. Existing and developed deorbit model accuracies were tested against real life data sets and used to help predict the reentry of the ALCHEMY flight module. Tether dynamics were investigated using two-separate multi-link tether simulations. The observability of the COTS LEDs, reflective tether, and satellite surface were each observed using ground-based electro-optical tracking equipment. Space tethers have been validated as a viable, low-complexity means of orbital debris mitigation and LEDs have been proven to be useful ground-observable light sources for LEO satellites. This paper will expand on the outcomes and conclusions drawn from the DRAGRACER flight experiment.
Date of Conference: September 19-22, 2023
Track: Space Debris