Just Passing By Earth
Right: These images of Earth were captured by the Galileo spacecraft in 1990 as it flew by our planet on its way to Jupiter. [more information]
Engineers at NASA's Jet Propulsion Laboratory confirmed that the spacecraft flew past Earth at an altitude of about 1,171 kilometers (727 miles), passing most closely above the eastern South Pacific at -23.5 degrees latitude and 231.5 degrees longitude. Cassini may have been visible from small islands in that area, such as Pitcairn Island or Easter Island.
The spacecraft remains in excellent health as it continues along its seven-year-long journey to Saturn. Having completed its cruise among the inner planets, Cassini's future now resides in the cold, dark realm of the outer planets. The spacecraft will pass by Jupiter on December 30, 2000; the giant planet's gravity will bend Cassini's flight path to put it on course for arrival into orbit around Saturn on July 1, 2004.
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The Cassini/Huygens program is a joint endeavor of NASA, the European Space Agency and the Italian Space Agency. The Cassini orbiter, built by NASA, and the Huygens probe, provided by the European Space Agency (ESA), were mated together and launched as a single package from Cape Canaveral, Florida, on October 15, 1997. Cassini's dish-shaped high-gain antenna was provided for the mission by the Italian Space Agency. At Saturn, the Huygens probe will detach from Cassini to parachute to the surface of Titan on November 30, 2004.
Nine of Cassini's 12 science instruments were turned on to make observations of the Earth/Moon system. Scientific and engineering data from the Earth flyby will be transmitted by Cassini to receiving stations of NASA's Deep Space Network over coming days.
The Cassini mission is a joint effort of NASA, the European Space Agency and the Italian Space Agency. The mission is managed and the Cassini spacecraft built and operated by NASA's Jet Propulsion Laboratory, Pasadena, CA. JPL is a division of the California Institute of Technology.
Gravity Assist Maneuvers
or... What happens when a ping-pong ball hits an electric fan?
At point 4 in the diagram pictured right, the spacecraft flies behind Venus. The planet, of course, pulls Cassini with its gravity. But the spacecraft has gravity too, and it pulls on the planet a tiny amount! This causes Venus to lose a little energy from its solar orbit, while Cassini gains the same amount. A small change in energy for massive Venus causes a minute reduction in the planet's speed, but the same energy applied to a tiny satellite causes a great change in speed.
The resulting red arc extends out past the orbit of Mars (Mars's orbit is not depicted). You can think of it as a ping-pong ball hitting an electric fan. The fan blades, whirling around the motor, have lots of angular momentum (as do the planets as they go around the Sun). When the ping-pong ball hits a fan blade, it slows the blade a very small amount, but the ping-pong ball gains lots of speed from the encounter. The ball connects with the blade mechanically, while a spacecraft connects with a planet via mutual gravitation.
Two months after the June 1999 Venus flyby, Cassini proceeds to point 5, where it steals energy from Earth's solar orbit, and the spacecraft's resulting arc reaches all the way to Saturn. The Jupiter flyby simply reduces travel time to the ringed planet.
You say tomato, I say tomato...
Gravity assists are well-grounded in classical Newtonian physics, but they can appear paradoxical, as illustrated by this thought experiment posed by JPL's Dave Doody in the May/June 1995 issue of The Planetary Report:
Cassini: Voyage to Saturn -- Cassini Mission home page
The Basics of Spaceflight -- from JPL. Includes basic information about gravity assist maneuvers.
Venus: Just Passing By -- Astronomy Picture of the Day, May 1, 1998
Venus's Once Molten Surface -- Astronomy Picture of the Day, Jan. 10, 1999
The Nine Planets: Venus -- from SEDS
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