Plutonium-powered spacecraft to zoom by Earth on way to Saturn : LUSENET : TimeBomb 2000 (Y2000) : One Thread

A two-story, $3.4 billion spacecraft carrying a load of deadly plutonium will zoom within 725 miles of Earth this week to gain momentum for the final leg of its meandering, seven-year voyage to Saturn.

Cassini's return, two years after NASA launched the largest and most expensive unmanned spacecraft ever, poses virtually no risk, mission officials say.

But anti-nuclear activists, concerned over the 72 pounds of carcinogenic cargo, aren't so sure.

"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."

Closest point over South Pacific

The flyby at 8:28 p.m. PDT Tuesday will use Earth's gravity to change the probe's direction and speed relative to the sun. Without the "gravity assist" and two previous close encounters with Venus and a future flyby of Jupiter, the probe would never reach its destination in 2004 to study Saturn's rings and moons.

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 might 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 chances of an accidental re-entry of Cassini are about 1 in 1.2 million, according to a NASA estimate.

Fear of error, and plutionium contamination

"It's just not a credible event," said Bob Mitchell, Cassini's program manager at NASA's Jet Propulsion Laboratory in Pasadena. "I'm not telling you it's impossible, but it's just not credible."

Activists fear that some sort of navigation or human error could cause the craft to burn up in the Earth's atmosphere, showering the planet with deadly plutonium dioxide.

Gagnon's group organized protests in the United States, England, Germany and elsewhere in June, but he admits there is little that can be done to change the spacecraft's course. 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 spacecraft requires plutonium not for propulsion but to power its dozen scientific instruments. The probe's three radioisotopic thermoelectric generators, or RTGs, convert heat from the naturally decaying plutonium 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.

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 flyby. 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.

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 point out 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.

********** I plan on living forever:So far so good

-- Drken (Drken@bubble.gone), August 15, 1999


Beyond a really really hazy possible parallel between the doomies and the anti-nuke nutballs, I can't find anything on-topic here at all. But I do hope I live to see the Cassini results. Saturn is a fascinating planetary system.

-- Flint (, August 15, 1999.

If those who protest the use of plutonium-fueled RTGs in spacecraft want to do something about it other than just gripe several years too late, then get busy lobbying U.S. Senators and Representatives to put more money into the NASA budget so that it is financially feasible to use solar power instead of RTGs on missions past Mars.

In case you weren't watching, Congress recently slashed the NASA budget again. (One good place to monitor NASA-related news is NASA Watch at

It takes money to build spacecraft. For missions beyond Mars, the size of solar panels needed to provide sufficient electrical power means that they will be more expensive than RTGs of the same electrical power output. (The farther one gets from the Sun, the less sunlight falls on each square meter of solar panel and the less electricity that square meter can generate. OTOH, RTG power output is unaffected by distance from the Sun. RTG output does decline slowly with time, but this factor is far less than the distance-to-Sun factor, and solar panels degrade slowly over time, too.)

It takes hundreds of millions of dollars more to use solar panels rather than RTGs on a mission to Jupiter or Saturn. (Remember, every single extra kilogram of mass in the spacecraft needs many extra kilograms of fuel and rocket to boost it into orbit.) If it's important to you to make this tradeoff of dollars for extra safety, let your Representatives and Senators know that's what you want! Then don't gripe about the extra cost.

-- No Spam Please (, August 15, 1999.

It's nice to see that the right wing has not grabbed a total monopoly on finding silly things to worry about. Accidents do happen, but if we let that slow us down, who would ever drive a car?

-- Bokonon (, August 16, 1999.

For all this hype and conjecture, I see the AP and CNN still haven't answered the only question that matters:

Is the Cassini ACTUALLY on track or not? If it is within course parameters, THERE ARE NO PROBLEMS. If the spacecraft is off-course, when must they make the burn to adjust it?

If it is "off-course" and so won't get to the outer planets correctly - but it can't hit the atmosphere - then NASA can simply wait until the spacecraft is past earth's orbit to make the burn. Then an error in the rocket circuits won't affect the trajectory - it's already "past" the closest point of approach.

-- Robert A. Cook, PE (Kennesaw, GA) (, August 16, 1999.

Jonathan, I like your signature line. You will soon have your hoards of panicking people, trust me.

-- Fed Up (, August 16, 1999.

I don't care how Cassini gets its power. I'm extremely curious about what Titan is going to look like below those clouds.

-- coproilith (, August 16, 1999.

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