Cassini space probe poses no danger

October 01, 1997|By Stamatios M. Krimigis and Darrell F. Strobel

AS PHYSICISTS and science investigators on the Cassini Mission to Saturn, we are aware of all NASA and independent studies done on the safety issues related to the 72 pounds of non-weapons-grade plutonium 238 in the Radioisotope Thermoelectric Generator (RTG) batteries of the Cassini spacecraft.

It is our assessment that the Cassini mission does not pose any real threat to the well-being of any living organisms on the globe, let alone the human population.

But, given the amount of misinformation being circulated about RTGs, it is understandable that Mayor Kurt L. Schmoke and other protesters are concerned.

Although RTGs make use of radioactive plutonium, they are not nuclear reactors and thus pose no possible risk of nuclear explosion.

A detailed analysis of the data applicable to the probability of a Cassini accident and the subsequent potential hazard of plutonium exposure of humans reveals the following:

Any potential hazard arises from the inhalation of very tiny particles of plutonium, but the plutonium in the Cassini RTGs is in ceramic form similar to dinnerware, which minimizes the potential of vaporization and widespread distribution of fine particles, either in a local accident or globally from burn-up due to accidental re-entry.

This is similar to the fact that, for example, there are enough germs in a single sneeze to give a billion people a cold, but the distribution of the germs prevents this from happening.

Concern has been voiced of an accident during re-entry of the Cassini spacecraft. The chance of this happening is about one in million.

In the worst-case scenario of total release of the plutonium by vaporization into fine particles, the annual dose of plutonium for a person on earth's surface would be about one millirem per year (a measure of potential tissue damage), while each of us accumulates an unavoidable exposure of about 360 millirems per year from cosmic rays, radon gas, medical x-rays.

By way of comparison, a typical mammogram delivers a comparable dose to the usual annual exposure amount.

The health hazard is even smaller for a launch-pad accident, even though the probability of this happening is 1,000 times greater, because a far smaller amount of plutonium can be vaporized and exposure is limited to fewer people. The RTGs have been exhaustively tested under such explosive conditions and the carbon-iridium container of the plutonium remained intact.

Previous use

Spacecraft use of RTGs is not new. The Voyager, Galileo, and the Ulysses spacecraft are all powered by RTGs.

The Galileo spacecraft passed by the earth twice (in 1990 and 1992) for gravity assists on its way to Jupiter, performing precisely the same maneuver at much lower altitude than Cassini is scheduled to do in 1999. Detractors of Galileo raised similar highly improbable doomsday scenarios based on unsupported, unscientific assertions that never materialized.

The late Carl Sagan was an active opponent of nuclear solutions to technical problems, yet he strongly supported the launch of Galileo with its RTGs and never expressed any doubts about the safety of U.S. RTGs. Dr. Sagan was a member of science teams for the Voyager and Galileo missions and a strong supporter of Cassini.

Not a single scientist from the many countries participating in the Cassini mission would accept the risk of endangering any of our fellow human beings if he believed even for a moment that there was a realistic chance of the catastrophe feared by Cassini opponents.

We, together with members of our families and the many hundreds of Cassini scientists and engineers, will be standing only a few miles from the launch pad Oct. 13 to watch the spacecraft begin its epic journey to the ringed planet.

We believe that the overwhelming majority of Americans will eventually be as proud of the accomplishments of Cassini as they were of those of the Voyager missions, launched 20 years ago this month, that revolutionized our knowledge of the outer planets and motivated many of our young people to pursue careers in science and engineering.

Objectors to the Cassini project have yet to document their concerns by publishing the technical details underlying their claims. We hope that they will consider the scientific evidence and realize how small a possible hazard the mission poses to humans.

The authors work at the Applied Physics Laboratory of the Johns Hopkins University.

Pub Date: 10/01/97

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