Just Passing By Earth
Cassini has completed another flawless gravity assist
maneuver en route to the ringed planet SaturnBASED ON A
NASA/JPL PRESS RELEASE
August 18, 1999: Earthlings
bid farewell to the Cassini spacecraft last night as the Saturn-bound
mission successfully completed a highly accurate pass of Earth
at 8:28 p.m. Pacific Daylight Time (03:28 Universal Time August
18). The flyby gave Cassini a 5.5-kilometer-per- second (about
12,000-mile-per-hour) boost in speed, sending the spacecraft
on toward the ringed planet more than 1 billion kilometers (almost
one billion miles) away.
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
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. Learn more about "Gravity Assist" Flybys
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.
Cassini's mission is to study Saturn, its moons, elaborate
rings, and its magnetic and radiation environment 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 Earth-like
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/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
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 the May/June 1995 issue of
The Planetary Report (courtesy of The
"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
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
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
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.
Doody continues: "Gravity assist can slow you down, too.
If you approach Jupiter 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."
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
NASA Science News
Join our growing list of subscribers
- sign up for our express
news delivery and you will receive a mail message every
time we post a new story!!!More Headlines
return to Space
Science News Home