A sobering thought experiment is to contemplate a world without electricity. Not only is electricity exploited as a convenient means of delivering mechanical or thermal energy to remote locations, but electricity is vital in information transmission and in sensing and control. Although the first planetary missions contemplated involved launching to the Moon a vehicle containing flash powder with which it would optically announce its arrival, and some early spacecraft used clockwork timers to sequence operations, every mission actually flown has been electrically powered.

In this chapter we first consider the overall requirements on the probe's power system, and how these requirements favour the various means adopted to meet them. The power supply and storage possibilities are then discussed, with particular reference to planetary probes. A general reference for power considerations is the book by Angrist (1982).

It is instructive to consider the electrical power requirements of various household devices to place spacecraft requirements in context. A modern PC may consume perhaps 200 W; a laptop perhaps an order of magnitude less. The Viking lander ran on 90 W. The Huygens probe's batteries supplied around 300 W for about 5 hours. The Sojourner rover had a solar array that delivered a mere 15 W.

System requirements

The total energy requirement of a mission (i.e. its integrated power requirement) is the most fundamental parameter for designing the power system.