Cassini's Earth fly-by
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August 18, 1999
Web posted at: 12:31 p.m. EDT (1631 GMT)

(CNN) -- The plutonium-powered Cassini spacecraft successfully skirted within 727 miles (1,171 kilometers) of Earth late Tuesday, picking up momentum for its long voyage to Saturn and dispelling fears of a nuclear accident for at least this mission.

NASA's most expensive unmanned spacecraft arrived within two miles of its target and six-tenths of a second late -- all inside an acceptable margin of error.

"It was right by the book," said Bob Mitchell, Cassini's program manager at NASA's Jet Propulsion Laboratory. The craft passed over the Easter and Pitcairn Islands in the southeastern Pacific Ocean.

Concerns over the safety of spacecraft stemmed from the 72 pounds of on-board plutonium used to power Cassini's instruments.

Anti-nuclear activists say they will protest any future missions that use radioactive materials.

Activists feared that an accident during the Tuesday fly-by or during Cassini's launch nearly two years ago could rain its carcinogenic cargo down to Earth. NASA managers said the risks were extremely low and the scientific benefits of the mission are very high.

The Cape Canaveral launch in October 1997 went off without a hitch, and NASA NASA put the risk of a fly-by accident at less than one in a million.

Cassini roared into space on October 15, 1997

Anti-nuclear activists 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."

The robotic probe reached its closest point over the southeastern Pacific Ocean as expected at 8:28 p.m. The spacecraft's closest point was nearly 110 times higher than commercial jets normally cruise.

The swing-by, as with two previous close encounters with Venus and a future fly-by of Jupiter in December 2000, was designed to give the spacecraft momentum for its trip to Saturn -- about 800 million miles away from Earth on average.

Cassini approached Earth at about 35,000 mph. Its speed increased by about 5.5 km per second (12,000 mph) after the swingby.

Cassini is set to arrive at Jupiter in 2004 for a study of Saturn, its rings and its moons.

NASA first used planets' gravity to fling its probes through space in 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 spacecraft requires plutonium not for propulsion but to power its dozen scientific instruments. The decaying plutonium generates heat, which circuitry converts into electricity.

The craft's nuclear units were built especially strong to prevent rupture in the event of an accident during launch or fly-by. Each pellet is boxed in layers of heat- and corrosion-resistant iridium and graphite.

The Associated Press contributed to this report.