Galileo uses two Radioisotope Thermoelectric Generators (or RTGs for short) to generate electrical power. The RTGs power the spacecraft through the radioactive decay of plutonium-238. The decay emits heat, which is converted into electricity for the spacecraft to "see, sense, hear, and speak." Each RTG is mounted on a 5 meter long boom.
The spacecraft was able to generate 570 watts of electric power at launch; we expect that it will be able to produce 480 watts when the spacecraft actually arrives at Jupiter in December of 1995.
What type of fuel do Galileo's engines use?
Galileo uses monomethylhydrazine for fuel. The fuel has to be oxidized in order to ignite, so nitrogen tetroxide is mixed into the fuel right before a burn. There are two separate tanks of helium pressurant. In all, Galileo carried 932 kg (2,050 lbs) of propellant at launch.
Why do we have to use plutonium? Why not use solar panels?
Spacecraft that travel at or within Earth's orbit can use solar energy to power their instruments. However, at the great distance of Jupiter, the only feasible power source means using Galileo's Radioisotope Thermal Generators (or RTGs for short). Galileo would need a minimum of 700 to 1,600 square feet of solar panels--a solar panel about the size of a house!
Unlike other power sources, the RTGs are insensitive to the freezing cold of space, and are virtually invulnerable to high radiation fields, such as Earth's Van Allen belts and Jupiter's magnetosphere.
For additional details, and a discussion of the safety review conducted on the RTGs, see What's in an RTG?
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