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Nuclear-powered NASA craft to zoom by Earth on Tuesday

Cassini's Earth flyby


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The Cassini Mission


August 16, 1999
Web posted at: 2:30 p.m. EDT (1830 GMT)

In this story:

Closest point over South Pacific

Fear of error and plutonium contamination

Anti-nuclear activists unhappy


(CNN) -- NASA's mammoth Saturn-bound spacecraft will skirt within 725 miles of Earth Tuesday, picking up a boost for its seven-year voyage and refueling anxiety among Cassini's anti-nuke opponents.

The two-story, $3.4 billion spacecraft and the 72 pounds of on-board plutonium that keeps it operational sparked an intense pre-launch war of words two years ago.

Activists feared an accident during launch or the Earth fly-by could rain down Cassini's carcinogenic cargo. NASA managers said the risks were extremely low and the scientific benefits very high.

The launch went off without a hitch in October 1997 from Cape Canaveral. Now everyone wants a safe fly-by, and NASA expects just that.

"There is just no prospect of having an accident," said Bob Mitchell, Cassini's program manager at NASA's Jet Propulsion Laboratory. "We've been doing this kind of thing now for nearly 40 years and have lots of fly-bys of planetary bodies."

Anti-nuclear activists disagree, and they made their objections known with demonstrations at the launch and at other protests earlier this year.

"The fact is, space technology can and does fail," said Bruce Gagnon of the Global Network Against Weapons and Nuclear Power in Space. "And when you start using nuclear materials in increasing numbers, the odds of an accident increase."

NASA puts the risk of a fly-by disaster at one in a million.

Michio Kaku, a City University of New York theoretical physicist, calls that a flimsy estimate.

"They basically ask their engineers to volunteer some probability figures, then they take the average. This is not science. This is voodoo," Kaku says.

The spacecraft requires plutonium not for propulsion but to power its dozen scientific instruments. The probe's three radioisotopic thermoelectric generators, or RTGs, generate heat from the naturally decaying plutonium, which circuitry converts into electricity.

The units were built especially strong in case of an accident during launch or flyby. Each pellet is boxed in layers of heat- and corrosion-resistant iridium and graphite. Cassini is the largest and most expensive unmanned spacecraft ever.

Closest point over South Pacific

The flyby at 11:28 p.m. EDT Tuesday will use Earth's gravity to change the probe's direction and speed relative to the sun. The "gravity assist" and two previous close encounters with Venus and a future flyby of Jupiter are the only practical way for the probe to make the long journey to study Saturn's rings and moons, starting in 2004, NASA says.

The probe will approach Earth at about 35,000 mph. Its speed will increase by about 11,000 mph after the swingby. At its closest point over the South Pacific, the probe may be visible from Pitcairn or the Easter islands.

NASA has used planets' gravity to fling its probes through space since 1973. The plutonium-powered Galileo probe to Jupiter twice swung by Earth in the early 1990s at altitudes much lower than Cassini's closest point. The nuclear opposition was less organized then.

Fear of error and plutonium contamination

Activists fear navigation or communications mishaps could cause the craft to burn up in the Earth's atmosphere, showering the planet with carcinogenic plutonium dioxide.

Cassini roared into space on October 15, 1997  

Gagnon's group organized protests in the United States, England, Germany and elsewhere in June. A handful of anti-Cassini Web sites also have been set up.

The protests pale in comparison to events leading up to Cassini's October 1997 launch, when demonstrators threatened to chain themselves to the pad and filed lawsuits to stop the mission. The disaster risk at launch is estimated to be far greater than the upcoming risk during the fly-by.

Anti-Cassini news abounds in Bangladesh, says F. R. Sarker, the general secretary of the astronomical society there.

A June 12 demonstration in Dhaka drew several government officials and hundreds of people who stopped traffic.

"If you ask any person in Bangladesh who reads news papers, 'What is Cassini?' he will tell you that 'It is a spacecraft coming near to Earth in August with radioactive materials which might cause deaths of lots of people,'" Sarker said.

The situation is reversed in India, he said, where astronomers are unaware of the mission, let alone the fly-by.

Anti-nuclear activists unhappy

Mitchell said for re-entry to occur, a failure aboard the probe would have to cause an exact change in its speed before the fly-by. A hit by a meteor larger than a pea is the biggest risk, he said. Cassini is wrapped with layers of mylar blankets to prevent at least smaller meteors from posing a problem, Mitchell said.

And then something would have to happen to prevent NASA from transmitting corrective orders.

"We've been flying this thing for two years now and we got a lot of practice," he said.

Kaku says debris from past missions and satellites also is a risk.

"Cassini will probably execute a flawless mission around the Earth if it can make the last hurdle through 8,000 pieces of 'space junk' surrounding the Earth," Kaku said.

The maximum danger comes 30 minutes into the actual fly-by, he said, when Cassini enters a belt of dense debris that starts about 20,000 miles from Earth.

Saturn flyby
Cassini passing over the rings of Saturn  

Even if the capsules were to vaporize during an accidental re-entry, the effects on Earth's population over 50 years would be less than the amount of radiation from dental X-rays or a round-trip flight across the United States, according to NASA.

Anti-nuclear activists, who dispute the numbers, say the space agency should be using safe solar energy to power all its probes.

But scientists say that Saturn is 10 times as far away from the sun as Earth and its solar panels would have to be the size of two tennis courts to harness enough energy. Others say we should wait until engineers develop more efficient solar panels.

"Saturn is not going away," Kaku says. "Neither are the planets. What's the rush? Why not delay our space probes a bit, make them smaller and more sophisticated and use solar power?"

The Associated Press contributed to this story.

In-Depth Special:
Journey to Saturn - The Cassini Mission

Telescope may have found liquid seas on Titan
July 30, 1999
Cassini picks up speed around Venus again
June 25, 1999
NASA expects to get Cassini back on track soon
January 15, 1999
Cassini spacecraft swings by Venus
April 26, 1998
Cassini roars into space
October 15, 1997

NASA Jet Propulsion Laboratory
    •Cassini: Mission to Saturn
Global Network Against Weapons and Nuclear Power in Space
The City University of New York
Astronomical Society of India
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