The TOPEX/Poseidon (T/P) altimetry mission operated for 13 years before the satellite was decommissioned in January 2006, becoming a large space debris object at an altitude of 1,340 km. Since the end of the mission, the interaction of T/P with the space environment has driven the satellite's spin dynamics. Satellite Laser Ranging (SLR) measurements collected from June 2014 until October 2016 allow for the satellite spin axis orientation to be determined with an accuracy of 1.7°. The spin axis coincides with the platform yaw axis (formerly pointing in the nadir direction) about which the body rotates in a counterclockwise direction.
The combined photometric and SLR data collected over the 11-year time span indicates that T/P has continuously gained rotational energy at an average rate of 2.87 J/day and spins with a period of 10.73 s as of Oct. 19, 2016. The satellite attitude model shows a variation of the cross sectional area in the sun direction between 8.2 m2 and 34 m2. The direct solar radiation pressure is the main factor responsible for the spin-up of the body and the exerted photon force varies from 65 μN to 228 μN around the mean value of 138.6 μN.
Including realistic surface force modeling in orbit propagation algorithms will improve the prediction accuracy, giving better conjunction warnings for scenarios like the recent close approach reported by the ILRS Space Debris Study Group - an approximate 400 m flyby between T/P and Jason-2 on June 20, 2017 (ILRS, 2017).