The Mars Science Laboratory (MSL 1) mission to land a large rover on Mars is being planned for Launch in 2009. As currently conceived, the rover would use a Multimission Radioisotope Thermoelectric Generator (MMRTG) to generate about 110 W of electrical power for use in the rover and the science payload. Usage of an MMRTG allows for a large amount of nearly constant electrical power to be generated day and night for all seasons (year around) and latitudes. This offers a large advantage over solar arrays. The MMRTG by its nature dissipates about 2000 W of waste heat to produce 110 W of electrical power. The basic architecture of the thermal system utilizes this waste heat on the surface of Mars to maintain the rover's temperatures within their limits under all conditions. In addition, during cruise, this waste heat needs to be dissipated safely to protect sensitive components in the spacecraft and the rover. Mechanically pumped fluid loops 2 are used to both harness the MMRTG heat during surface operations as well as reject it to space during cruise. This paper will describe the basic architecture of the thermal control system, the challenges and the methods used to overcome them by the use of an innovative architecture to maximize the use of heritage from past projects while meeting the requirements for the design. MISSION OVERVIEW CRUISE CONFIGURATION-While the MSL mission is still in the earliest stages of its design cycle, the mission will follow the general design paradigm of the previous JPL rover missions to Mars (Mars Pathfinder, MPF 3 and Mars Exploration Rovers, MER 4). MSL will feature a rover enclosed in an aero-shell for protection during entry and descent onto the planet's surface. A cruise stage will carry the lander and aero-shell enclosure from Earth to Mars and will separate from the lander just prior to entry, descent and landing (EDL). Figure 1 shows a rendering of the rover packed into the aero-shell enclosure with the cruise stage attached at the top.
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.
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