During the launch and boost phases, only the power and pyro controllers, the data acquisition system, and the tape recorder are active. After the spacecraft separates from the launch vehicle, the orbiter commands the lander computer to separate the bioshield cap and begins the lander cruise state. During cruise, the lander is largely passive. Only the data acquisition system, ion pumps, and thermostatically controlled heaters on propulsion equipment, the biology instrument, and the inertial reference unit are powered. The heaters prevent the freezing of propellants and biology nutrients.
Heat also controls viscosity of the gyro flotation fluids.
The tape recorder is activated about every 15 days to ensure its later performance. An update to the computer requires the activation of the computer and the command detectors and decoders. The portion of the computer memory used during prelaunch checkout procedures is modified during the cruise so that it can perform other operations during the mission. About five such cycles in two groups are required, each accompanied by mass-spectrometer readings, which are analyzed to determine the performance and health of the instrument. After each activity, the lander is powered back to cruise state and, after the final bakeout of the gas chromatograph-mass spectrometer, a cruise check is made and the batteries discharged.