Jun 25, 1999

Venus Lends a Hand



Venus Lends a Helping Hand


En route to Saturn, Cassini flew just 370 miles above Venus on June 24 gaining a boost in speed from that planet's gravity
Venus and Cassini. Credit: D. Seal
June 25, 1999: The Cassini spacecraft, en route to Saturn, successfully completed its second flyby of the planet Venus yesterday, once again on time and on target.

Right: Artist's concept of Cassini as it flys by the planet Venus. Image Credit: D. Seal and JPL.

As planned, Cassini came within 600 kilometers (about 370 miles) of the planet at 1:30 p.m. Pacific time, with Venus' gravity giving the spacecraft a boost in speed to help it reach Saturn more than 1 billion kilometers away. The spacecraft, launched on its voyage October 15, 1997, remains in excellent condition as it travels its nearly seven-year trajectory to Saturn. Most of Cassini's scientific instruments were set to make observations during the Venus flyby. Scientific data from the flyby will transmitted to Earth over coming days.
Learn more about "Gravity Assist" Flybys
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Four flybys of planets -- two of Venus and one each of Earth and Jupiter -- give Cassini the speed it needs to reach Saturn. Cassini first flew past Venus on April 26, 1998 at a distance of 284 kilometers (about 176 miles). Today's Venus flyby will be followed by a flyby of Earth on August 18 (August 17 Pacific time at 8:28 p.m. PDT), then it's on to Jupiter for a December 30, 2000 flyby. 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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Cassini's mission is to study the ringed planet, its magnetic and radiation environment, and its moons and rings for four years. Cassini will also deliver the European Space Agency's Huygens probe to parachute to the surface of Saturn's moon Titan. Titan is of special interest partly because of its many Earthlike characteristics, including a mostly nitrogen atmosphere and the presence of organic molecules in the atmosphere and on its surface. Lakes or seas of ethane and methane may exist on its surface.

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?

Based in part on JPL's The Basics of Space Flight by Dave Doody and George Stephan, and on an article by Dave Doody in theMay/June 1995 issue of The Planetary Report (courtesy of The Planetary Society).

"Gravity assists" make it possible for a spacecraft to reach the distant outer planets without using vast amounts of propellant. Michael Minovitch, a student working at the Jet Propulsion Laboratory in the 1960s, helped develop this marvelous technique. Astronomers had long known that comets' orbits were altered by encountered with planets, but it was Minovitch who first recognized that the principle could be applied to spacecraft trajectories.

Right: This diagram from JPL shows Cassini's circuitous path to Saturn, featuring gravity assist flybys of Earth, Venus and Jupiter. Click for a larger image.
Cassini Trajectory from JPL
How can gravity assist a spacecraft? Consider the following:

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:

"Consider someone bicycling down a road into, and then up out of, a valley. [The hilltops on either side of the valley are the same height.] The cyclist will speed up approaching the valley floor, gaining momentum from the pull of gravity. But just as surely, all that momentum will be lost on the way back uphill, and the cyclist will slow down again when he reaches the crest on the other side. Well, in that case, what good is gravity for interplanetary travel?"

By analogy with the cyclist, a spacecraft would pick up speed -- gaining momentum from gravity -- as it approaches a planet, but it would slow down again as it departs with no net gain of energy. Right?

Not exactly.

The difference between the cyclist passing through a valley on Earth and the spacecraft whizzing by Venus is this: In the example of the cyclist, the bottom of the valley is stationary with respect to the cyclist's destination (the top of the hill on the other side). But, Venus is in motion with respect to Cassini's destination, Saturn. From Saturn's point of view, Cassini can gain energy from an encounter with Venus that will send the spacecraft racing toward the ringed planet.
Venus as viewed by the Galileo spacecraft
Right: Venus is a popular spot for Gravity Assist Maneuvers. This colorized image of Venus was recorded by the Jupiter-bound Galileo spacecraft shortly after its gravity assist flyby of Venus in February of 1990. Galileo's glimpse of the veiled planet shows structure in swirling sulfuric acid clouds. The bright area is sunlight glinting off the upper cloud deck. more information

Doody continues: "Gravity assist can slow you down, too. If you approach Jupiter [or Venus] from behind the planet in its solar orbit ... some of Jupiter's orbital velocity is added to the spacecraft, and the spacecraft receives a boost. On the other hand, if you fly more in front of Jupiter in its orbit, your spacecraft pulls Jupiter slightly in the other direction, causing the planet to speed up ever so slightly and causing momentum to be taken from the spacecraft, slowing it down."


Web Links

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

A Venus Landing -- Astronomy Picture of the Day, Jan. 24, 1999

Venus's Once Molten Surface -- Astronomy Picture of the Day, Jan. 10, 1999

NASA's Magellan Mission to Venus -- from JPL

The Planetary Society -- home of The Planetary Report

The Nine Planets: Venus -- from SEDS

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