To Photo of the RTGOnce at Jupiter, Galileo will begin an exhaustive study of the Jovian environment. This furious activity will test the limits of Galileo's Flight Team, as well as its hardware. In spacecraft terms, a great deal of energy will be required to run the instruments with which Galileo will probe the Jovian depths. In the past, spacecraft which travel at or within Earth's orbit have utilized solar energy to power their instruments. However, at the great distance of Jupiter, the only feasible power source involves the use of Radioisotope Thermoelectric Generators (RTGs).
The RTGs power the spacecraft through the radioactive decay of plutonium-238. This decay emits heat, which is converted into electricity for the spacecraft to "see, sense, hear, and speak." This power supplies a reliable, long-lasting source of electricity which is insensitive to the chilling cold of space and virtually invulnerable to high radiation fields, such as Earth's Van Allen belts and Jupiter's magnetosphere.
An RTG consists of two parts: a source of heat and a system for converting the heat to electricity. The heat source contains a radioisotope, such as plutonium-238, that becomes physically hot from its own radioactive decay. This heat is converted to electricity by a thermoelectric converter which uses the Seebeck effect, a basic principle of thermoelectricity discovered in 1822. An electromotive force, or voltage, is produced from the diffusion of electrons across the joining of two different materials (like metals or semiconductors) that then form a circuit when the ends of the converter are at different temperatures.
Each RTG contains 18 separate heat source modules, and each module encases four plutonium-238 pellets. The modules are designed to survive a range of postulated accidents: launch vehicle explosion or fire, reentry into the atmosphere followed by land or water impact, and post-impact situations. An outer covering of graphite provides protection against the structural, thermal, and eroding environments of a potential reentry. Additional graphite components provide impact protection, while iridium cladding of the actual fuel cells provides post-impact containment. The fuel is in the form of plutonium-238 dioxide, a ceramic material which is resistant to fracturing.
As the launch of Galileo neared, anti-nuclear groups, concerned over what they perceived as risks to the public safety from Galileo's RTGs, sought a court injunction prohibiting Galileo's launch.
From the outset, the Project understood the need to safely operate an RTG-powered spacecraft, and had developed appropriate safety-related data. The RTGs themselves were carefully designed and extensively tested. And, in fact, RTGs have been safely used for years in planetary exploration. The Lincoln Experimental Satellites 8/9, launched by the Department of Defense, had 7% more plutonium onboard than does Galileo, and the two Voyager spacecraft each carried 80% of the plutonium Galileo does.
After the Challenger accident, a study considered additional shielding. Additional shielding was not adopted, even though it would offer some protection near the launch area, because the great complexity of such a design significantly increased the risk of mission failure. If a failure on orbit occurred, additional shielding would significantly increase the consequences of a ground impact. The two close flybys of Earth had raised questions about the possible inadvertent reentry of Galileo into Earth's atmosphere.
Exhaustive studies were made by JPL and were independently reviewed regarding the safety of the Venus-Earth-Earth Gravity Assist (VEEGA) mission. These studies showed that the Galileo Team had designed the spacecraft and its trajectory to keep the probability of an inadvertent reentry at Earth to less than one in two million. Over the course of the Project, millions of dollars were spent to improve the safety of shuttle flights and the Galileo mission. The Department of Energy, as required by law, completed a mission risk analysis, with full disclosure of those results to State and local governments. The Interagency Nuclear Safety Review Panel completed an independent Safety Evaluation Report and the Office of Science and Technology Policy approved the mission. As a result of the Project's testing and rationale for the Galileo mission, the Court found in the Project's favor and the launch was a splendid success.
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