Note: The original text of the resolution is printed below in italics and is also available in its entirety on a separate page. The rebuttal is in plain text.
RESOLUTIONS: MEMORIALIZING THE PRESIDENT AND THE CONGRESS OF THE UNITED STATES TO STOP THE LAUNCH OF THE CASSINI SPACE PROBE UNTIL A SAFER POWER SOURCE CAN BE SUPPLIED.
WHEREAS, NASA has scheduled the launch of the Cassini space probe to Venus and Saturn for October 6, 1997, from Cape Canaveral, Florida; and
This is not entirely accurate. The Cassini mission's sole destination is the planet Saturn, where it will arrive in July 2004. To reduce the travel time to a manageable length, the spacecraft uses "flybys" of the planets Venus (twice, in 1998 and 1999), Earth (in 1999), and Jupiter (in 2000), using a technique known as "gravity assist" to build up enough velocity to reach Saturn by 2004.
Launch of Cassini is now scheduled for October 13; there was a one-week delay while minor repairs were made to some insulation on part of the spacecraft.
WHEREAS, three out of 24 United States space missions involving nuclear materials have met with accidents and a Titan IV rocket similar to the one which is to launch the Cassini probe exploded on its launch pad in 1993; and
It is correct that there have been three accidents involving radioisotope thermoelectric generators (RTGs) since the mid-1960s. In 1964 a SNAP-9A RTG burned up in the atmosphere when a Navy Transit 5BN-3 satellite burned up in the atmosphere. The RTG carried about 2 pounds of plutonium-238, which was presumably scattered in the atmosphere. Slight levels of Pu-238 that could be traced to the accident were detected in soil samples taken 6 years later, but were small compared to the amount of plutonium in soil samples from nuclear weapons testing. Nevertheless, RTG containers were strengthened considerably to prevent any future release of plutonium.
In 1968 a Nimbus B-1 weather satellite, launched from Vandenberg Air Force Base in California, crashed into the Pacific Ocean just off the coast when its launch vehicle failed. The RTG it carried was retrieved from the ocean floor intact a few months later, and was successfully reused on another satellite. None of the 4 pounds of uranium-238 it carried was released.
In 1970, the Apollo 13 lunar module reentered the Earth's atmosphere over the Pacific Ocean at the end of that tumultuous mission. The lunar module, which was intended not to return to Earth, had a SNAP-27 RTG, carrying 8 pounds of plutonium-238, in its base intended to power experiments on the lunar surface. The spacecraft was targeted to reenter over the Tonga Trench, one of the deepest spots in the Pacific Ocean. Surveys of the area after reentry showed no evidence of increased radiation, presumably meaning the RTG remained intact with no ill effects on the delicate ecosystem there.
These accidents affirm the safety of RTGs: in no case was there a significant release of radiation, and in one case the RTG was salvaged intact and used again!
Regarding the Titan IV launch vehicle: there was one, and only one, accident, involving that vehicle in twenty launches. The 1993 mission failure discussed in the resolution was traced to problems with a solid rocket booster, which were corrected before the rocket was allowed to fly again. The 95% success rate for Titan IV launches is among the best of any launch vehicle currently in service.
WHEREAS, NASA estimates that cleanup of large areas of contaminated land in the event of a launch pad accident would cost $247 million per square mile; and
However, not only is a launch pad accident unlikely, any sort of accident which could rupture the RTGs and release their plutonium is extremely unlikely. RTGs have multiple layers of protection to keep the plutonium inside secure. The plutonium is stored in a ceramic form that is highly resistant to heat, is insoluble to water, and has a low chemical reactivity. The plutonium is protected by multiple layers of materials such as iridium, a strong, dense, highly-heat resistant metal; and graphite, which is also highly-resistant to heat from an explosion or reentry. The plutonium is stored in 18 individual containers, each protected separately to reduce the chances of a large release of plutonium in an accident.
WHEREAS, safer means, including highly efficient solar cells, exist for providing the power required by the Cassini probe; and
Unfortunately, this is not the case. Solar cells work very well in the inner solar system, and are less expensive than RTGs, which is why the recent successful Mars Pathfinder and Mars Global Surveyor missions both used solar cells.
At the orbit of Jupiter and beyond, though, solar cells are not an efficient way to generate electricity. At Saturn sunlight is only about 1% as strong as it is at the Earth, which means solar panels would need to have 100 times the area to generate the same amount of electricity as panels on the Earth. Given the high electricity requirements for the Cassini spacecraft and its numerous scientific instruments, the solar panels required would have to span area about one-fourth the size of a football field! Such panels would be extremely heavy, difficult to deploy, expensive, and make it difficult to control the spacecraft.
Some anti-nuclear activists have claimed that more efficient solar cells are available from the European Space Agency. It is true that some initial laboratory tests created solar cells with a higher efficiency than the standard cells used in current spacecraft. However, these cells have not entered into production yet or have even been extensively tested. These cells are also much heavier than standard solar cells, such that if used on Cassini they would make the spacecraft too heavy to launch. The European scientists who developed the advanced solar cells themselves agreed that they were not suitable for use on Cassini.
WHEREAS, the Cassini space probe will carry 72 pounds of plutonium-238, which is 280 times more radioactive than the plutonium which is used in nuclear weapons; and
The radiation released by Cassini's power supply is in the form of "alpha particles", slow, heavy particles which can be stopped with something as insignificant as a piece of paper. The danger is when these alpha particles are inhaled or ingested; inside the body they can do cell damage. To enter the body, though, the plutonium must be ground down into particles the size of dust. The plutonium on Cassini is in the form of plutonium dioxide, a ceramic material which does not easily turn to dust but instead breaks into large chunks. Thus even if the RTG container was somehow breached, very little of the plutonium would be ground into particles dangerous enough to cause damage.
Even if dust-sized particles of plutonium were released in an accident, they would have little impact on the radiation everyone is exposed to on an everyday basis. NASA studies have estimated the 50-year dose for a person exposed to plutonium dust released in a launch accident to be 1 millirem. Radiation from natural sources -- rocks, cosmic rays, radon in homes, and even the decay of naturally-occurring radioactive elements in the human body -- would amount to 15,000 millirem in the same time period.
WHEREAS, NASA's own environmental impact statement says that if the Cassini probe makes an inadvertent reentry into the Earth's atmosphere during a planned flyby, five billion of the seven to eight billion people living on the Earth could be exposed to the radiation from the plutonium on board; and
The Earth flyby during the Cassini mission is scheduled to occur at an altitude of more than 500 miles. To reenter the Earth's atmosphere the spacecraft would have to pass much less than 100 miles from the surface. No NASA spacecraft has ever had such a large error during a flyby. The Galileo mission, which used RTGs and made two flybys of the Earth in 1990 and 1992 en route to Jupiter, was accurate to within 5 miles on its first flyby and 1 mile on its second. The spacecraft has been ever more accurate while passing by the moons of Jupiter, hundreds of millions of miles from the Earth in orbits not known as well as the Earth's. NASA estimates that the odds of a flyby accident are about one in a million; given past successes, this may be an overstatement of risk!
Written by Jeff Foust. Thanks to Bruce Mackenzie, Mike Tabaczynski, and Richard Wagner for their comments while drafting this rebuttal. More information on the Cassini mission and its use of RTGs is available at http://www.seds.org/spaceviews/cassini/.
Updated 1997 September 29.