NASA says that poses a 1-in-1,030 possibility of a radioactive accident near the launch site and, even in the worst-case scenario, the most exposed individuals would be subjected to less radiation than a single medical X-ray.

Opponents say the government is downplaying the odds of an accident and its consequences. They say any amount of plutonium is too much and even the slimmest chance of a radioactive accident is not worth the risk.

"NASA is lying," said Bruce Gagnon, head of a Gainesville-based group that has long fought against the use of nuclear material on space missions.

The May and June liftoffs of Mars Exploration Rovers A and B will mark the first time plutonium has been launched from Florida aboard a NASA spacecraft since 1997.

That's when the agency launched Cassini toward Saturn. Protesters picketed the gates of the Cape Canaveral Air Force Station and hung fliers on residents' doors warning of a potential nuclear disaster over the beaches.

There are some big differences this time, not the least of which is the amount of plutonium involved. Cassini bore about 72 pounds of plutonium in generators needed to power the spacecraft on its journey to a close encounter with Saturn, which will happen sometime next summer. Each rover will carry about 30 grams of the radioactive element to keep them warm during subzero Martian nights.

"Eight pennies in your hand, that's how much plutonium we're talking about," said David Lavery, NASA's program executive for solar system exploration.

The analysis of the risk by NASA and the Department of Energy is spelled out in a 200-plus page document the agencies were required to make public under federal right-to-know laws before launching the missions this summer.

The Environmental Impact Statement, which is based upon hundreds more pages of analysis by the government and contractors, details how the radioactive particles could be released over the coast if one of the Delta 2 rockets explodes before or during launch.

The report says the particles, if enough of them get into the body, can cause cancer and birth defects in humans. However, the government says its worst estimates show the most exposed person would get 3 percent of the radiation he or she might get in a year's worth of regular activity.

Anti-nuclear forces are gearing up for Cassini-like protests, including one planned at the space center on May 3. They've mailed fliers advertising the protest, claiming the government analysis relies on formulas and assumptions that are unrealistic at best and fraudulent at worst.

Gagnon and others also say NASA's movement toward more nuclear-powered ships is going to mean more launches, andcould potentially pave the way for space-based nuclear weaponry that could mean more dangerous launches on the Space Coast.

"As you increase the number of launches, you increase the percentage chances of failure," Gagnon said. "We are always playing Russian roulette. There is a bullet in the chamber, but so far, we've been lucky.Accident odds, resultsThe accident odds the government is citing for the rovers are fairly similar to those provided for Cassini back in 1997. However, this time there are two separate launches set to go off just weeks apart.

NASA and the Department of Energy calculate the probability of a radioactive release while either of the Rovers are perched atop their Delta 2 rockets on the launch pad at 1 in 16,000. They say the likelihood of a radioactive accident in the moments immediately after liftoff is 1 in 1,100 for the May launch and 1 in 1,600 for the June shot. The analysis included an extensive review of the Delta 2 rocket, including a January 1997 accident during launch.

By comparison, the chances of a plutonium-releasing accident with Cassini were 1 in 19,200 on the launch pad and 1 in 1,490 during its early flight, according to similar government studies done before its Oct. 15, 1997 launch.

While the Cassini analysis talked about a land cleanup effort that could include removing contaminated vegetation and destroying buildings over a several-square-mile area, the new documents say the tiny amount of plutonium aboard the Mars craft would fall over a fraction of a square mile and be so minimal that, after a year, the level of contamination would subside to the point that little, if any, cleanup might be necessary.

"I'm bringing my family and my kids down for the launch and I do not have a realistic concern that I am putting my family in danger," said Lavery, who said in the event of a launch accident people along the coast could go inside but even that is not really necessary.Even in a worst-case scenario, the NASA and Department of Energy scientists predicted all radioactive particles will be confined within the boundaries of Cape Canaveral Air Force Station and Kennedy Space Center -- falling to the ground in an area about one-fifth of a square mile near the pad.

The plutonium is not weapons grade. It's not the highly explosive stuff of nuclear bombs. It does become a health hazard if its containers are somehow broken and is reduced to dust-like particles. The likelihood in this case is remote because the pellets are a sort of ceramic material that is hard to vaporize, but NASA concedes it can happen.

In addition, the pellets are encased in layered protective canisters that have been put through tests to ensure they could withstand the forces of an explosion during ascent. The government documents say it's not the force of the explosion or even the impact from debris in an accident that poses the biggest threat. It's the heat of the rocket fuel fireball. Extreme temperatures could weaken the canisters and allow them to leak radioactive particles.

But NASA's report says the amount of plutonium -- and humans' potential exposure to cancer-causing particles -- for these missions is minuscule. Far more radioactive particles already have been released in the atmosphere from practices such as above-ground nuclear weapons testing.

The bottom line, according to the government scientists, is that the worst possible plutonium release resulting from a mission accident would not cause even one extra case of cancer during the next 50 years.Why plutonium?NASA says the plutonium is critical to the landmark rover missions.

The 2.7 gram plutonium pellets are part of eight nuclear heating units tucked inside each rover. The decaying plutonium will generate heat needed to keep the rovers' fragile electronics warm while they're parked overnight on the dark side of Mars, where temperatures plunge to minus-150 degrees. Solar power won't work then because there's no sunlight.

Anti-nuclear activists such as Russell Hoffman of Carlsbad, Calif., wonder, "Did they forget about batteries?"

Batteries won't work either because Mars is so far from the sun, reducing its power. The electricity generated by the rovers' solar panels will be needed to power its scientific gear and to send data and pictures back to Earth. There's simply not enough to also recharge batteries big enough to keep the ship warm overnight on the dark side of Mars.

Counting Cassini, the U.S. has launched at least 27 other space ships carrying plutonium since the 1960s. There have been three publicly acknowledged accidents. However, the worst known accident actually involved a Russian spy satellite that crashed through the atmosphere and spread radioactive mess over the Canadian wilderness. The cleanup cost a reported $14 million, but the area was not populated so it is not believed there were human health impacts.

But Cassini was special. It was the large load of plutonium that generated so much angst. NASA has since been working with DOE and contractors on more efficient systems that will require far less plutonium. Still the safety and procedural lessons learned from it and other past missions only enhanced expertise on the safety front.

"On Cassini, a new era in environmental sensitivity was dealt with," said Al Herzl, a Denver-based subsystems manager for Lockheed Martin, which made the nuclear generators for Cassini. "The Lockheed Martin team, Department of Energy and NASA worked together to try to resolve all of the issues involved with safely launching that amount of plutonium, and there were very few issues."Nuclear proliferationThe debate arises again just as NASA Administrator Sean O'Keefe is saying the White House has signed off on "substantial" increases in funding for a $1 billion Nuclear Systems Initiative.

The reason: meaningful exploration of the deeper parts of the solar system with unmanned probes is far more feasible and cheaper using nuclear power. Furthermore, the continued research into better systems could lead to nuclear technologies that will someday enable manned trips to places such as Mars.

"This is the enabling technology that lets us make decisions about destinations in 5 to 10 years, about where to go in the solar system," said Brian Chase, head of the National Space Society.All of the pro-nuclear sentiment has people like Gagnon worried. The groups warn of a space-age nuclear proliferation. They say the U.S. is using its civilian space program to develop technology that could easily be used for nuclear weaponry in space. And they say that would mean more and more launches of dangerous nuclear material.

"With a growing succession of nuclear launches on rockets with 10 percent failure rates, you're going to have an accident and you're going to have a release," Gagnon said.