The rovers

Each rover has a mass of 174 kg, and when fully deployed from their cruise configuration, each stands around 1.5 m tall with a width of 2.3 m and a length of 1.6 m. Once free of its carrier, each rover communicates with terrestrial ground-stations of the Deep Space Network (DSN) directly via their low-gain and high-gain antenna at X-band frequencies of 7.2 GHz for the uplink, and 8.4 GHz for the downlink. The omnidirectional UHF antenna can communicate with orbiting spacecraft (Mars Odyssey, MGS, MEx) at rates up to 128 kbps, while the 28 cm diameter HGA allows data rates of 2 kbps with the much more distant DSN.

A notable feature of the MER project is the presence of an arm on the forward face of each rover. This arm, having five degrees of freedom, is able to position the instrument platform at its tip up to 0.8m in front of the rover within a volume of around 0.1m3. Clearly, exceptional science can be performed by locating payload sensors, and not necessarily whole instruments, on the end of an articulated arm. The inevitable tradeoff between mass and actuator size for the arm versus the reach and carried mass has been solved for the MER project by having the arm of each rover carry a four-way turret which allows one of four devices to be offered up to a surface presented to the rover. This arrangement is shown in Figure 27.2 along with the positions of the other notable rover items.

In Table 27.1 the data products from the payloads of Spirit and Opportunity are summarized and the payload instruments are described in detail thereafter.

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