Information about Cassini's RTG's
(Radioisotope Thermoelectric Generators)


Mirror Sites

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Information for the MEDIA and general public. Comments, Contact information and more info about RTG safety, alternatives, etc.

THIS SITE HAS THE MOST USEFUL COMPREHENSIVE INFORMATION ABOUT THE RTG, THEIR NEED, SAFETY, ETC

Cassini : Mission to Saturn Nuclear Safety Issues and Answers

Explaining The Risks of The Cassini Mission

Here is Florida Todays (newspaper) article about the RTG's and their safety (sept 14, 97)

SEDS: Space Views - CAST your vote about RTG's

Send an EMAIL to President Clinton supporting Cassini and stress NASA's safety and success with past spacecrafts using RTG's

The Dept of Energy (DOE) was responsible for producing the RTG (Radioisotope Thermoelectric Generators) for NASA which are necessary to power the spacecraft especially since no battery can last that long, nor can it recharge since it will be so far from our sun. Such a power supply source much be devised.
RTG's have been used in countless spacecrafts ( Pioneer 10 & 11 , Ulysses, Voyager 1 and 2), and even Apollo missions, and numerous military Earth orbiting spacecraft and most recently Galileo .

Send your Emails !!!!!

Additional Web Sites

Rallies

SUPPORT Cassini and inform the public about NASA and the safety with RTG' s in Washington DC in front of the White House on Sept 28th, 1997 around 12 Noon. Members of the National Space Society are organizing this event. Email me with your name, phone, email, organization for more info. Email me

Other Missions using RTG's

Other Sites

Radioisotope Thermoelectric Generators (RTGs) are the only sensible source of power for spacecraft to the outer solar system at the present time. To date they have been used on Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses and Cassini all of which travelled beyond 5 AU from the sun. They have also been used on the two Viking landers as well as by all the Apollo ALSEP experiment packages and numerous military Earth orbiting spacecraft. They all rely on the basic principle that if you take a piece of metal and make one end hot and one end cold then current will flow from the hot end to the cold end. This fact can be used to generate electricity.

Cassini's electrical power source - Radioisotope Thermoelectric Generators (RTGs) - have provided electrical power for some of the U.S. space program's greatest successes, including the Apollo lunar landings and the Viking Landers that searched for life on Mars. RTGs made possible NASA's celebrated Voyager explorations of Jupiter, Saturn, Uranus and Neptune, as well as the Pioneer missions to Jupiter and Saturn. RTG power sources are enabling the Galileo mission to Jupiter and the international Ulysses mission studying the Sun's polar regions.

Extensive studies conducted by NASA's Jet Propulsion Laboratory (JPL) have shown that NASA's Cassini mission, given its science objectives, available launch systems, travel time to its destination and Saturn's extreme distance from the Sun, requires 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.

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. 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. The United States has an outstanding record of safety in using RTGs on 23 missions over the past three decades. While RTGs have never caused a spacecraft failure on any of these missions, they have been on-board three missions which experienced malfunctions for other reasons. In all cases, the RTGs performed as designed. More than 30 years have been invested in the engineering, safety analysis and testing of RTGs. Safety features are incorporated into the RTG's design, and extensive testing has demonstrated that they can withstand physical conditions more severe than those expected from most accidents. First, the fuel is in the heat-resistant, ceramic form of plutonium dioxide, which reduces its chance of vaporizing in fire or reentry environments. This ceramic-form fuel is also highly insoluble, has a low chemical reactivity, and primarily fractures into large, non-respirable particles and chunks. These characteristics help to mitigate the potential health effects from accidents involving the release of this fuel. Second, the fuel is divided among 18 small, independent modular units, each with its own heat shield and impact shell. This design reduces the chances of fuel release in an accident because all modules would not be equally impacted in an accident. Third, multiple layers of protective materials, including iridium capsules and high-strength graphite blocks, are used to protect the fuel and prevent its accidental release. Iridium is a metal that has a very high melting point and is strong, corrosion resistant and chemically compatible with plutonium dioxide. These characteristics make iridium useful for protecting and containing each fuel pellet. Graphite is used because it is lightweight and highly heat-resistant.

Potential RTG accidents are sometimes mistakenly equated with accidents at nuclear power plants. It is completely inaccurate to associate an RTG accident with Chernobyl or any other past radiation accident involving fission. RTGs do not use either a fusion or fission process and could never explode like a nuclear bomb under any accident scenario. Neither could an accident involving an RTG create the acute radiation sickness similar to that associated with nuclear explosions.

NASA places the highest priority on assuring the safe use of plutonium in space. Thorough and detailed safety analyses are conducted prior to launching NASA spacecraft with RTGs, and many prudent steps are taken to reduce the risks involved in NASA missions using RTGs. In addition to NASA's internal safety requirements and reviews, missions that carry nuclear material also undergo an extensive safety review involving detailed verification testing and analysis. Further, an independent safety evaluation of the Cassini mission will be performed as part of the nuclear launch safety approval process by an Interagency Nuclear Safety Review Panel (INSRP), which is supported by experts from government, industry and academia.

More information from the Dept of Energy (DOE)

Information about radioisotope thermoelectric generators can be located via the DOE Reports Bibliographic Database at
http://apollo.osti.gov/html/dra/dra.html

Select the link "Search The DOE Reports Bibliographic Database."

perform a search using the keywords radioisotope thermoelectri generators in the "Enter Search Terms Here" area. By selecting a report link you will reach information about that report. For each report, availability information can be found by selecting the link "Report Ordering and Retrieval

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