The Cassini spacecraft, it might seem, is taking the long route to Saturn. The craft was launched in 1997, flew twice by Venus, and on Tuesday will pass 725 miles from the Earth. In 2000, it will swing past Jupiter, finally arriving at Saturn in 2004.
But actually such a circuitous route is the fast track to Saturn. Indeed, for all practical purposes, it's the only way to get there.
Similar to previous spacecraft that went to distant parts of the solar system, Cassini is making use of "gravity assists" -- boosts from the gravity of the planets that it passes. Such boosts, popularly known as the "slingshot effect," speed the spacecraft toward its ultimate destination.
What's actually happening is "a transfer of energy from a planet to a spacecraft," says Jeremy B. Jones, Cassini's navigation chief at NASA's Jet Propulsion Laboratory.
When a spacecraft is far from any planet, it travels in an orbit around the sun (much as a planet does). However, when the craft nears a planet, the planet's gravitational influence on it briefly becomes predominant. And since the planet itself is moving (relative to the sun), the craft is carried along for the ride. This speeds the spacecraft up (as well as altering its course in relation to the sun).
Cassini, for instance, will increase in speed by 5.5 kilometers (3.41 miles) per second as it passes Earth. And the total effect of the probe's four planetary flybys is an acceleration of 21.44 kilometers (13.64 miles) per second.
Without such boosts, sending spacecraft to the outer planets would take decades, or require rockets far more powerful than any that now exist. "We can do things that otherwise would be totally impossible to consider doing," says Jones.
The use of gravity assists in planetary exploration dates back to the early 1970s. The Pioneer 11 probe swung by Jupiter to speed it toward Saturn. In 1979, Voyager 2 took advantage of a planetary alignment that occurs only once every 175 years to slingshot past Jupiter, Saturn and Uranus. The Galileo probe did two Earth flybys, in 1990 and 1992 respectively, to bring it to Jupiter.
Moreover, speed is not the only benefit of gravity assists. The Ulysses probe was sent around Jupiter in 1992 in order to place it a highly oblique angle relative to the sun.
Incidentally, speeding a spacecraft up with a flyby also involves slowing the involved planet down -- a consequence of the law of conservation of energy. Not to worry, though. According to Jones's calculations, the Cassini flyby will change Earth's position by 15 inches over a period of one billion years.
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