Understanding and predicting the evolving spin states of defunct geosynchronous (GEO) satellites and rocket bodies is important for space situational awareness, active debris removal, satellite servicing, and anomaly resolution. There is clear evidence that many defunct GEO satellite spin states are predominantly driven by the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. The YORP effect is spin state evolution due to solar radiation and thermal re-emission torques. Observations are crucial to understand how YORP drives spin states and to validate dynamical models. Unfortunately, GEO satellites are non-resolved from ground-based telescopes and extracting spin states (spin periods, rotational angular momentum vector, instantaneous attitude) from ubiquitous photometric light curve data is challenging. Even for well-known objects, light curve inversion often yields several or more well-fitting spin state solutions within the measurement noise and modeling uncertainty (i.e. detailed satellite geometry, reflective properties, etc.). Also, there is strong evidence that the YORP effect drives satellites from uniform rotation to non-principal axis tumbling. Such tumbling states further complicate the light curve inversion process because the motion is driven by two independent periods rather than one. To aid complete spin state analysis, particularly for the tumbling case, Doppler radar observations collected at NASA Goldstone Deep Space Communications Complex are incorporated. Observing the well-documented retired GOES 8–12 weather satellites, the radar data yielded unambiguous spin period estimates for all targets and greatly narrowed pole solutions, independent of light curve data. Significant changes in spin rates and pole directions were observed over a two month span. These findings are consistent with YORP-driven evolution.

Estimating and predicting the spin states of defunct satellites and rocket bodies is important for space situational awareness, active debris removal, and satellite servicing. Observations show that the spin states of defunct geosynchronous (GEO) satellites are diverse and can change significantly over time. Spin state evolution for many defunct GEO satellites is primarily driven by solar radiation torques via the Yarkovsky-O’Keefe-RadzievskiiPaddack (YORP) effect. To better understand the YORP-driven evolution of these objects we obtain spin state estimates from a combination of Deep Space Network (DSN) Doppler radar echoes and optical light curves. The resolved nature of Doppler radar echoes allows for clear identification of satellite spin periods and can greatly constrain possible spin pole directions. Observations of the defunct GOES 8-12 GEO weather satellites demonstrate ongoing spin period evolution and pole motion consistent with YORP theory. Observations of two spent upper stage rocket bodies yield spin periods, center of mass offsets, and constraints on spin pole directions.