How Safe Are Nuclear-Powered Space Missions?

It was an inky black night in the Atacama desert of northern Chile on Nov. 16 when Leo Alvarado saw the blazing fire in the sky.

Stopping the car, the geologist and four colleagues watched an object change brilliant colors as it creased the heavens. It traveled "almost horizontal, not like a meteorite," Mr. Alvarado says. "We watched it break up into many pieces and burn."

Confused, Alvarado called Luis Barrera, director of the Astronomy Institute at the local Catholic University. Dr. Barrera had been following Internet accounts of the failed Russian Mars 96 probe and its encased cargo of 200 grams (.44 lbs.) of toxic plutonium. Barrera says the geologists and a second family who had called with an identical account, had seen Mars 96 burning up over the desert.

"The news is bad," Barrera said. "I think it vaporized."

Bolivian troops are now searching the desert between Chile and Bolivia for four pellets of plutonium and pieces of the Russian spacecraft. If vaporized plutonium oxide has fallen over Chile, it would be bad news for the US space program. Some scientists, physicians, and environmentalists are again raising concerns about using radioisotope thermoelectric generators (RTGs) to provide power to spacecraft.

Their attention is now fixed on NASA's launch next year of its long-awaited Cassini mission to Saturn, carrying 72.3 pounds of plutonium as an on-board power source. While Cassini promises astronomers an unparalleled four-year close up study of Saturn's atmosphere, rings, and moons, the October blast-off from the Kennedy Space Center will also place the largest-ever quantity of nuclear material in space.

"If you liked Mars 96, you'll love Cassini," says John Pike, head of space policy at the Federation of American Scientists. Dr. Pike notes that a similar type of Titan IV rocket, which will launch Cassini, blew up in 1993, destroying a billion-dollar military payload. He says the chance of a similar failure are between 1 in 10 and 1 in 20.

Even if the rocket blew up on the platform, the plutonium is housed in containers atop the rocket and are unlikely to be damaged by an explosion, NASA says.

But Pike is concerned that even if Cassini escapes Earth's orbit, there's another environmental risk. In 1999, NASA plans to have Cassini race toward Earth at 42,300 miles per hour and use Earth's gravity in a maneuver to "sling shot" the probe out to Saturn. NASA's own Environmental Impact Statement says that, in the event of a reentry disaster (the worst-case scenario), some 5 billion people could be affected by radioactivity.

NASA also says the chance of such a mishap is 1 in 1 million. "We believe it is a safe mission," says NASA spokesman Doug Isbell.

NASA successfully used a "gravity assisted" Earth flyby to send the nuclear-powered Galileo probe to Jupiter. "These devices are safe," Mr. Isbell says. He notes that the Cassini plutonium is in 18 heat-resistant ceramic modules. Each module is clad in a lightweight layer of high-melting point irridium, and further shielded with impact-resistant graphite blocks. "They have been tested to contain their plutonium," Isbell says. He notes that this is a different type of generator from the Russian Mars 96 model and "designed by us, as opposed to a foreign country."

STILL, Horst Poehler, a senior engineer for NASA contractors for 25 years, says he has studied some 30 technical reports on Cassini. His conclusion: The mission could be "the mother of all accidents." He questions the durability of the graphite blocks and whether the irridium layer surrounding the plutonium is sufficient.

Critics also point to alternative power sources, such as the European Space Agency's development of efficient solar-cell technology "suitable for deep-space missions." But to switch now, Pike allows, would require a costly multiyear redesign that might kill the $4 billion Cassini probe, already overdue and over budget.

Some NASA scientists say that solar panels wouldn't be a reliable power source so far from the sun. Says Dr. Poehler, "I support NASA when they do right things," but with Cassini, "they convict themselves with their own reports."

FALLEN WASTE

Radioactive materials from these missions have returned to Earth in recent years:

* MARS 96 (Russia)

Reentry date: Nov. 16, 1996; Location: Chile or Bolivia; .44 lbs, thought to have reached the ground.

* COSMOS 1402 (USSR)

Reentry date: Feb. 7, 1983; Location: South Atlantic; 68 lbs. of uranium 235. Unknown if any debris reached the ground.

* COSMOS 954 (USSR)

Reentry date: Jan. 24, 1978; Location: Canada; 68 lbs. of uranium 235, 75 percent was thought to have been vaporized and spread worldwide.

* RORSAT mission (USSR)

Reentry date: April 1973; Location: Pacific Ocean, north of Japan; Radiation released from the reactor was detected.

* APOLLO 13 (US)

Reentry date: April 14, 1970; Location: South of Fiji; 5.5 lbs. of plutonium 238 is believed to be intact under the ocean.

* COSMOS Lunar missions

Reentry date: 1969; Two Soviet missions failed. radiation was detected as the craft burned up in the atmosphere.

* NIMBUS B-1 (US)

Reentry date: May 18, 1968; the debris, including 4.2 pounds of plutonium 238, landed in the Santa Barbara channel off California. The fuel was recovered and used on a later mission.

* TRANSIT 5BN-3 (US)

Reentry date: April 21, 1964; Location: Indian Ocean; 2.1 lbs. of plutonium 238 vaporized in the atmosphere and spread worldwide, prompting the US to redesign its RTGs.