We investigate the possibility that the anomalous acceleration of the Pioneer 10 and 11 spacecraft is due to the recoil force associated with an anisotropic emission of thermal radiation off the vehicles. To this end, relying on the project and spacecraft design documentation, we constructed a comprehensive finite-element thermal model of the two spacecraft. Then, we numerically solve thermal conduction and radiation equations using the actual flight telemetry as boundary conditions. We use the results of this model to evaluate the effect of the thermal recoil force on the Pioneer 10 spacecraft at various heliocentric distances. We found that the magnitude, temporal behavior, and direction of the resulting thermal acceleration are all similar to the properties of the observed anomaly. As a novel element of our investigation, we develop a parameterized model for the thermal recoil force and estimate the coefficients of this model independently from navigational Doppler data. We find no statistically significant difference between the two estimates and conclude that once the thermal recoil force is properly accounted for, no anomalous acceleration remains.

NASA launched two rovers in June and July of 2003 as a part of the Mars Exploration Rover (MER) project. MER-A (Spirit) landed on Mars in Gusev Crater at 15 degrees South latitude and 175 degree East longitude on January 4, 2004 (Squyres, et al., Dec. 2004)). MER-B (Opportunity) landed on Mars in Terra Meridiani at 2 degrees South latitude and 354 degrees East longitude on January 25, 2004 (Squyres, et al., August 2004) Both rovers have well exceeded their design lifetime (90 Sols) by more than a factor of 4. Spirit and Opportunity are still healthy and continue to execute their roving science missions at the time of this writing. This paper discusses rover flight thermal performance during the surface missions of both vehicles, covering roughly the time from the MER-A landing in late Southern Summer (Ls = 328, Sol 1A) through the Southern Winter solstice (Ls = 90, Sol 255A) to nearly Southern Vernal equinox (Ls = 160 , Sol 398A). This paper describes the MER rover thermal design, its implementation and performance on Mars. The rover surface thermal design performance was better than pre-landing predictions. The very successful thermal design allowed a high level of communications immediately after landing without overheating and required a minimal amount of survival heating in the dead of winter. An analytical thermal model developed for the rover was used to predict surface operations performance. A reduced-node version of this model was integrated into the mission planning tool to achieve the proper balance between: 1) desired science and communications operating profile, 2) available energy from the power system and 3) temperature limits prescribed for the hardware. One of the more challenging thermal problems during surface operations, predicting the performance of actuator and camera electronics warmup heaters, was automated by using heater lookup tables that were periodically updated based on flight telemetry. Specific MER rover thermal flight experiences are discussed in this paper. Lessons learned and suggestions for improvement of future Mars surface vehicle designs are presented.

In January 2004, two Mars Exploration Rovers (MER) landed on the surface of Mars to begin their mission as robotic geologists. A year prior to these historic landings, both rovers and the spacecraft that delivered them to Mars, were completing a series of environmental tests in facilities at the Jet Propulsion Laboratory. This paper describes the test program undertaken to validate the thermal design and verify the workmanship integrity of both rovers and the spacecraft. The spacecraft, which contained the rover within the aeroshell, were tested in a 7.5 m diameter thermal vacuum chamber. Thermal balance was performed for the near earth (hot case) condition and for the near Mars (cold case) condition. A solar simulator was used to provide the solar boundary condition on the solar array. IR lamps were used to simulate the solar heat load on the aeroshell for the off-sun attitudes experienced by the spacecraft during its cruise to Mars. Each rover was tested separately in a 3.0 m diameter thermal vacuum chamber over conditions simulating the warmest and coldest expected Mars diurnal temperature cycles. The environmental tests were conducted in a quiescent nitrogen atmosphere at a pressure of 8 to 10 Torr. In addition to thermal balance testing, the science instruments on board the rovers were tested successfully in the extreme environmental conditions anticipated for the mission. A solar simulator was not used in these tests.

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