The body of a rover is an enclosed box called the Warm Electronics Box (WEB). This box is essential because temperatures at night can fall to -100 degrees C (-150 F). The batteries would stop functioning, as would many of the electronic components, if some warmth were not provided to get the temperature up toward 0 degrees C (32 F).
The WEB is an insulated box that contains:
• The rover's computer brain • The lithium-ion batteries • The radios and amplifiers for the radios • Various pieces of control electronics for the different spectrometers, etc.
Basically, anything that cannot survive at -100 degrees C is inside the box. The box keeps warm through three different mechanisms:
• When they are on, the different electronics modules produce their own heat. The computer, for example, can consume 7 watts and therefore produces heat like a 7-watt night-light bulb would. • There are small, 1-watt resistance heaters that the computer can turn on to raise the temperature. • Eight radioactive pellets (plutonium dioxide) give off heat as the plutonium atoms decay. The pellets are tiny -- the size of a pea. They are wrapped in a protective alloy and then in a carbon-fiber case. If the Delta launch rocket had blown up on the pad, or the spacecraft had re-entered earth atmosphere, these cases would protect the pellets.
The Onboard Computer
The rovers use a RAD6000 computer produced by BAE systems. This processor is nearly identical in architecture to an old PowerPC processor used in early Macintosh computers. By today's standards, these processors are slow. They run at 20 megahertz, about 1/100th the speed of a typical desktop computer today. They have 128 kilobytes (KB) of RAM, 256 KB of flash memory and some ROM to hold the boot code and operating system. There are no disk drives. Although they are slow and incredibly expensive ($200K to $300K per computer), they have two big advantages:
1. They are radiation-hardened so they are immune to the cosmic radiation falling on Mars. 2. They run the ultra-reliable VxWorks (PDF) real-time operating system from Wind River Systems.
This computer makes the rover that much more reliable than a typical desktop computer because it is never crashing or corrupting data. The computer helps with power management, image processing, motor control, and instrument management. It also handles navigation. The rover has six navigation cameras arranged in three pairs. The computer processes stereo images from the camera pairs. It uses binocular vision algorithms, and it can identify the distance to and size of the different rocks in the field of view. Using this information, the computer can build a map of all the nearby obstacles and then maneuver the rover to avoid them when it is moving.
The rovers have 1.3 square meters (14 square feet) of high-efficiency solar cells to provide power. When the panels are first unfolded by the rover, they are clean, and at noon the Sun is "strong" by Martian standards because of the season. The panels produce about 140 watts peak, or about 900 watt-hours total per day (with this much power, you could run a single 100-watt light bulb for nine hours). In other words, the Sun is bright enough to activate the solar panels for only about six hours per Martian day.
The power from the solar panels goes to the devices that need it (computer, motors, RAT, instruments, radios, etc.). Any excess power is stored in two 28-volt, 10-amp-hour, lithium-ion batteries.