Hundreds of protesters had chanted slogans before Nasa's launch of Cassini to Saturn in 1997 carrying 33kg of plutonium fuel.

On Tuesday the New Horizons mission to Pluto is due to liftoff with 11kg of the radioactive substance on board. Yet the most raucous the planned demonstration got was when protesters tied colourful origami birds to the fence of the Cape Canaveral Air Force Station.

The organiser Maria Telesca of the Global Network Against Weapons & Nuclear Power in Space said that "folks tend to forget."

Regular usage

Plutonium fuel has been used on two other spacecrafts taking off from the Cape Canaveral area since Cassini's launch. The two Mars rovers, Spirit and Opportunity, sent up in 2003, had much smaller amounts of plutonium, which creates energy from natural radioactive decay. The amount of nuclear fuel aboard New Horizons will create less energy than two 100-watt lightbulbs.

The plutonium is located in New Horizon's radioisotope thermoelectric generator, an aluminum-encased, 56-kilogramme cylinder, one metre long and 46 centimeters wide, that sticks out of the spacecraft like a gun on a tank.

Inside the cylinder are 18 graphite-enclosed compartments, each holding 600 grams (1 1/3 pounds) of the plutonium dioxide. Similar generators previously have been used to power six Apollo flights and 19 other U.S. space missions.

Nasa and the US Department of Energy have put the probability of an early-launch accident that would cause plutonium to be released at one in 350.

Unspecific cost

Nasa last year estimated the cost of decontamination, should there be a serious accident with plutonium released during the launch, at anywhere from $241 million to $1.3 billion per 1.6 sq km, depending on the size of the area.

The space agency is setting up two radiological control centers and deploying 16 mobile field teams that can detect radiation around the launch site. Medical personnel at local hospitals also have been trained in the treatment of patients exposed to radioactive materials, and the launch required the approval of the White House.

The emergency plans are ready for Tuesday, "if need be, but hopefully not," Nasa launch director Omar Baez said on Sunday.

Some Nasa safety managers had raised concerns about the New Horizons mission when a fuel tank similar to the one expected to be used failed a pressure test during factory evaluation.