BASED ON A NASA/JPL PRESS RELEASE
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
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.
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.
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.
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.
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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."
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