August 11, 1999. Copyright, 1999, Graphic News. All rights reserved 

CASSINI SPARKS PLUTONIUM PANIC 

NASAÕs controversial nuclear-powered Cassini mission will hop aboard the EarthÕs gravitational field Ð like a stowaway on an ocean vessel Ð and ÒstealÓ precious energy to speed it up on its 2.2 billion mile (3.5 billion km), seven-year voyage to the ringed planet Saturn.
    On Monday (August 16) the space probe will hurtle at more than 42,500 mph (68,400 km/h), just 729 miles (1,173 km) above the EarthÕs surface, in a manoeuvre condemned by anti-nuclear activists, pacifists and academics who fear the tiniest miscalculation of its trajectory could kill or maim thousands. 
    The 6-ton (5,650kg) robotic probe Ð developed by NASA, the European Space Agency and the Italian Space Agency Ð is carrying a record amount of 72.3 pounds (32.8 kg) of plutonium on board.
    Because Cassini coudnÕt be launched with sufficient speed to head straight for Saturn, mission analysts designed a unique trajectory which involves flybys or Ògravity-assistsÓ from Venus, Earth, and Jupiter. This allows it to pick up enough speed from their gravitational fields to reach Saturn by June 2004.
    Since Saturn is so far from the sun, solar panels are impractical as a power source. Electrical power for the spacecraft and its instruments is provided by three radioisotope thermoelectric generators (RTGs) which contain plutonium dioxide. It is these RTGs that have caused the furore.
    RTGs have provided electrical power for some of the U.S. space programÕs greatest successes, including the Apollo lunar landings and the Viking landers that searched for life on Mars. RTGs made possible NASAÕs celebrated Voyager explorations of Jupiter, Saturn, Uranus and Neptune, as well as the Pioneer missions to Jupiter and Saturn.
    But three of AmericaÕs 23 nuclear-powered space missions have failed since the first such craft was launched in 1961. NASA stresses that in all cases the malfunctions did not involve the RTGs which performed as designed.
    In 1964 a U.S. military navigation satellite carrying two pounds of plutonium plunged to Earth. The RTG, burned up in atmosphere releasing radioactive material.
    Four years later a U.S. weather satellite carrying 6 pounds of plutonium crashed into the Pacific Ocean off California shortly after liftoff. Both RTGs remained intact and were retrieved.
    The fated Apollo 13 lunar lander carrying 8 pounds of plutonium was discarded prior to the crewÕs return from the aborted moon mission in 1970. The RTG sank in the South Pacific near Fiji where it is believed to be intact.
    NASA says there is a less than one-in-a-million chance of Cassini re-entering the atmosphere and releasing plutonium during MondayÕs Earth flyby. Mission analysists say they can control the flyby altitude within an accuracy of 2 to 3 miles (3 to 5 kilometers). Opponents say the chances are much higher.
    NASA says in the worst possible accident, during the Earth flyby, the number of cancer deaths worldwide would increase by 120 over a 50 year period. Critics say that number could be 200,000 or more.
    The three $50 million RTGs are the latest, safest version, providing up to 885 watts of electric power.
    Each of the 54 plutonium modules is encased in heat-resistant iridium and graphite shells that can resist temperatures of about 2,360 ¡C Ð typical of rocket fuel fires and much higher than the 1,140 ¡C experienced by the Space Shuttle on re-entry.
    Opponents point out that this is just 65 degrees below the melting point of the iridium casing and during the fireball of re-entry there may be severe rupturing of the RTGs.
    Assuming CassiniÕs flyby is successful, the craft will then spend four years gathering data on the solar systemÕs second largest planet, its rings, magnetic environment and icy satellites. A sub-probe, Huygens, will be dropped by parachute on to the largest moon, Titan, described as a Òprimordial Earth in deep freeze.Ó
    It is hoped the information Cassini sends home, enough to fill 300 CD-Roms, will provide clues to the creation of the solar system and pre-life conditions on Earth.
/ENDS  
Sources: NASA, ESA, Aviation Week and Space Technology, Reuters, Associated Press