Conor Benson, University of Colorado Boulder; Charles Naudet, Jet Propulsion Laboratory; Daniel Scheeres, University of Colorado Boulder; Joseph Jao, Jet Propulsion Laboratory; Lawrence Snedeker, Goldstone Deep Space Communications Complex; William Ryan, New Mexico Tech/MRO; Eileen Ryan, New Mexico Tech/MRO; Marc Silva, Goldstone Deep Space Communication Complex; Jeffrey Lagrange, Goldstone Deep Space Communication Complex
Keywords: radar, light curves, GEO debris, spin states, YORP effect
Abstract:
The debris population near geosynchronous earth orbit (GEO) continues to grow with ongoing launches and no natural deorbit mechanisms. Understanding the long-term dynamical evolution of these debris objects is necessary to protect active assets and preserve GEO for future use. This includes both the long-term orbital and rotational evolution. The former has been studied comprehensively over the past fifty years, while research on the latter has been much more limited. Nevertheless, the spin states of GEO debris, particularly retired and otherwise defunct satellites, are known to be diverse and evolve significantly over time [1-7]. A better understanding of long-term defunct satellite spin state evolution would aid GEO space situational awareness, predictions for attitude-dependent debris perturbations (e.g. solar radiation pressure), active debris removal, and satellite servicing. Recent observations and dynamical simulations have shown that the spin states of some defunct GEO satellites are predominantly driven by the Yarkovsky-OKeefe-Radzievskii-Paddack (YORP) effect [5-8]. The YORP effect is spin state evolution caused by solar radiation and thermal re-emission torques [9].
Observations are valuable to gain direct insight and validate theories about defunct GEO satellite spin state evolution. Unfortunately, extracting GEO satellite spin states (i.e. spin periods and attitude) from ubiquitous photometric light curve observations can be challenging, even for known satellites. Fitting satellite spin states often results in numerous local minima, even with high fidelity (e.g. ray-traced) photometric models. A number of defunct GEO satellites are also in non-principal axis rotation (i.e. tumbling), and recent work indicates that the YORP effect can cause defunct satellites to transition from uniform rotation to tumbling where their dynamical evolution continues [5-8]. Tumbling light curve analysis is more challenging than the uniform rotation case because there are two fundamental spin periods: one corresponding to precession of the satellites long axis around the rotational angular momentum vector (pole) and the second to rotation of the long axis about itself [5]. Dominant light curve frequencies for tumblers consist of several or more (a priori unknown) low-order harmonics of the two fundamental frequencies [5]. The common analysis approach requires testing a handful of candidate tumbling period pairs over the sphere of possible attitude phasing and angular momentum vector directions [5], often resulting in a number of similarly well-fitting period pairs and complete solutions.
The aim of this work is to reduce light curve spin state ambiguity by incorporating ground-based radar measurements. For this work, we observed both uniformly rotating and tumbling defunct GEO satellites with Deep Space Network (DSN) antennas at NASAs Goldstone Deep Space Communications Complex in California. Two 34 meter DSN antennas were used in a bi-static configuration with one antenna transmitting and the second receiving. The transmitted signal consisted of continuous wave (cw) carrier at X-band. Reflection of the signal off of the rotating satellite and back to the receiving antenna yielded time-varying Doppler spectra. Each satellite was observed periodically over the course of several hours to sample different viewing geometries. The satellites were observed in this manner on a number of days from late 2019 through early 2020.
The selected targets were NOAAs retired GOES 8-12 weather satellites. These satellites are nearly ideal targets given their well-documented geometry. Their known shapes and significant asymmetry aid in interpreting radar echoes. This shape asymmetry also makes GOES 8-12 particularly susceptible to the YORP effect and therefore dynamically interesting. The strong Doppler echoes obtained during this study allowed for clear identification of GOES satellite features including their conical solar sail, bus, and solar array. Analysis of the echoes yielded unambiguous long axis precession period estimates for all satellites, both the uniform rotators and tumblers. Leveraging the varied viewing geometries, pole estimates were also obtained. For the tumbling satellites, spectral analysis of the Doppler echo time histories also yielded long axis rotation period estimates. These collective radar-derived estimates were consistent with near simultaneous photometry obtained at Magdalena Ridge Observatory.
The radar study provided a number of interesting dynamical findings. First, it showed significant spin state diversity among the nearly identical GOES satellites with: GOES 10 in fast uniform rotation with a 30 s spin period, GOES 12 in slow uniform rotation, and GOES 8 tumbling. The pole estimates differed significantly among the satellites with GOES 8s pole near the sun/anti-sun direction and GOES 12s approximately normal to ecliptic plane. Clear changes in long axis precession periods were also observed for GOES 8 and GOES 12 from December 2019 through February 2020. GOES 8s long axis precession period decreased from 353 s to 216 s while GOES 12s decreased from 882 s to 454 s. These findings are all consistent with YORP-driven spin state evolution [7-8].
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Date of Conference: September 15-18, 2020
Track: Non-Resolved Object Characterization