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€G R A V I T Y    A S S I S T

What is the fastest means of travel from one planet to another? The first,
most obvious answer is rocket propulsion, the same force that lifts spacecraft
off the ground and allows them to escape the pull of gravity. Look closely,
though, at several of the planetary exploration missions of recent
years--MARINER, PIONEER, and VOYAGER--and you will find another kind of
"engine" at work.

By aiming a spacecraft so that it passes very close to another planet or
moon--actually within its gravitional field--it is possible to boost the
spacecraft on to still more distant planets, oftentimes with an even greater
velocity. The technique has been called the "slingshot effect" or more
properly, gravity assist, and it has become an established method of cutting
down on travel time between planets without spending additional rocket
propellant--the closest thing to a free ride there is for interplanetary robot
explorers.

While no ride here on Earth or in space is absolutely free, a gravity-assisted
spacecraft at least steals the relatively tiny amount of energy it needs to
change its velocity from a source that won't miss it: the orbital energy of
planets as they circle the Sun. Because everything in the Solar System is
moving with respect to the Sun, launching a probe toward another planet is not
like shooting at a fixed target, but is more an exercise in timing. When we
launch a spacecraft from our orbiting Earth, we place it into a new and
different heliocentric (Sun-centered) orbit, an orbit that in turn crosses the
path of another planet at a time when the spacecraft can "intercept" it.

As this well-timed spacecraft passes near to a massive body like Jupiter, it
is pulled by the planet's gravity. If it were to pass too close to the planet,
it would be drawn inward and collide with it, but if its trajectory takes it
instead through the outer part of what might be seen as a "gravity whirlpool,"
it can actually be propelled off in a new direction. Since velocity is defined
as both speed and direction, the spacecraft in this case receives a velocity
change with respect to the Sun.

Imagine that you are running in a straight line toward a merry-go-round, on a
tangential path that will take you only a meter or so from the edge of the
circling platform. As you pass very close, you grab the outstretched hand of
one of the riders.  Following the not quite perfect analogy with planets and
spacecraft, you, the spacecraft, are momentarily bound to the gravity field
of an "orbiting" body. Hold on too long and you will be drawn inward to ride
with your friend. But let go before the inward pull becomes too great, and you
will be hurled off in a new direction, and possibly pick up speed as well. The
energy you (the spacecraft) received for your velocity change was stolen from
the orbital energy of the circling rider (the planet).

Seen from the vantage point of Earth, the trajectory of the gravity-assisted
spacecraft is bent, because for any moving object, if there is an acceleration
not in the direction of the original motion (caused here by the pull of
gravity), a curved path results.

M A R I N E R    1 0

The idea of using gravity assists to navigate between the planets was
discussed in the earliest days of the space program, but it was not until the
MARINER 10 mission to Venus and Mercury that the technique was actually used.
Launched on November 03, 1973, MARINER 10 is still the only spacecraft ever to
have visited Mercury, the nearest planet to the Sun. Included in MARINER's 530
kilograms (1170 lbs) of hardware and science instruments were ultraviolet and
infrared sensors, television camaras, and other devices for measuring the
charged fields and particles in the vicinity of the two inner planets.

The spacecraft reached Venus first. On February 05, 1974, it passed within
5760 kilometers (3570 mi) of the planet, and returned the first close-up TV
pictures of its cloudy atmosphere. The Venus flyby was aimed very carefully so
that MARINER would swing around the planet to intercept Mercury in its orbit
around the Sun seven weeks later, on March 29, 1974. If MARINER had been sent
directly to Mercury, it would have required a Titan IIIC launch vehicle, but
the gravity assist from Venus allowed a smaller and less expensive Atlas first
stage rocket to be used for the mission instead.

Among the scientific returns from MARINER 10 were the discovery of Mercury's
atmosphere and the first photos of its heavily cratered surface. The March 29
flyby brought the probe within 800 km (500 mi) of the planet's night side, and
took advantage of yet another gravity assist, this time from Mercury itself,
to put MARINER into a heliocentric orbit that would bring it back for two
more close passes of the planet. On September 21, it flew past the sunlit
hemisphere of Mercury at a distance of 50,000 km (31,000 mi). On March 16,
1975, the spacecraft returned for its final, closest pass at a distance of
only 327 km (203 mi). The first gravity-assisted planetary mission ended eight
days later, when MARINER ran out of attitude control gas.

P I O N E E R    1 1

Before the PIONEER 10 and 11 missions, no one was certain that spacecraft
could survive passage through the swarm of asteroids circling between the
orbits of Mars and Jupiter, but the PIONEERS proved that such a journey was
safe. The two probes, launched on March 02 and April 05, 1973, respectively,
were the first to reach Jupiter and the first manmade devices to achieve
sufficient velocity to leave the Solar System.

The nearly identical spacecraft carried instruments to measure the magnetic
and energetic fields of the giant gas planets, and had onboard a light-sensing
photopolarimeter, but no TV cameras. PIONEER 10 reached Jupiter on December
03, 1973, after which it was boosted by the planet's gravity out of the solar
system, crossing the orbit of Neptune (then the most distant planet) on June
13, 1983.

PIONEER 11, however, was targeted to pass by Jupiter at a distance of 43,000
km (27,000 mi), three times closer than Pioneer 10 had come to the cloudtops.
As a result, it received a greater boost from Jupiter's gravitional field as
it passed under the planet's south pole, and shot upward at an angle to the
ecliptic plane (the plane of Earth's orbit around the Sun). The spacecraft
velocity nearly doubled at Jupiter, reaching a rate of 173,000 km (107,000 mi)
per hour, some 55 times the muzzle velocity of a high speed rifle bullet.

It took nearly five more years for PIONEER 11 to reach Saturn, because it
traveled an arced path that took it as high as 164 million km (102 million mi)
above the ecliptic plane, then back "down" again to intercept Saturn. On
September 01, 1979, it became the first spacecraft to reach the ringed planet.
As PIONEER crossed the plane of the rings, it was moving at 112,000 km/hr
(69,600 mph). Two hours later, accelerated by the planet's gravitional field,
it reached 114,000 (70,800 mph) as it passed Saturn at a distance of 20,930 km
(13,000 mi) from the cloudtops.

Among PIONEER 11's discoveries at Saturn were the outer "F" ring and the
planet's magnetosphere. By using Saturn's gravity to shape the spacecraft's
trajectory, PIONEER also made the first flyby of the large moon Titan, at a
distance of 354,000 km (220,000 mi) on September 02, 1979.

V O Y A G E R    1  &  2

Sometimes called the "Grand Tour" of the outer planets, the VOYAGER mission
took advantage of gravity-assist techniques and a once-every-175-years
alignment of Jupiter, Saturn, Uranus, and Neptune, to fly past all of the
large planets beyond the asteroid belt in one curved trajectory. VOYAGERS 1
and 2 were launched on September 05 and August 20, 1977, respectively. VOYAGER
1, launched later but followed a more direct route, reached Jupiter on March
05, 1979.

Each 815 kg (1800 lb) spacecraft carried 11 experiments, including
high-resolution television cameras. It was VOYAGER 1 that returned the first
close-up pictures of the swirling atmosphere of Jupiter and its four colorful
moons Io, Ganymede, Callisto, and Europa; discovered a thin ring of material
around Jupiter; and found active volcanoes on Io.

VOYAGER 1 flew closer to Jupiter--270,000 km (168,00 mi)--than VOYAGER 2, and
so received a larger gravity boost for its trip to even more distant Saturn.
While the spacecraft had a four month lead on VOYAGER 2 at Jupiter it
increased that lead to nine months at Saturn, which it reached on November 12,
1980. Passing 124,000 km (77,000 mi) from the cloudtops, VOYAGER 1 revealed
for the first time the complexity of Saturn's rings and the character of its
moons. After the Saturn encounter, VOYAGER 1's planetary studies ended, but
the spacecraft continued to monitor the interplanetary environment as it
headed out of the Solar System.

VOYAGER 2, though, continued onward to Uranus after its follow-up studies of
Jupiter and Saturn in July 1979 and August 1981. The dual mission had been
designed so that VOYAGER 2 could be targeted either to fly close to Titan (in
case of VOYAGER 1 failure), or receive another gravity boost to bend its
trajectory toward Uranus, where no spacecraft had ever visited. Since Titan
was no longer a priority--VOYAGER 1's study of this fascinating world had
been a success--the Uranus option was chosen. Flying around the planet's
middle at a distance of 101,000 km (62,800 mi) on August 25, 1981, VOYAGER 2
received yet another gravity boost that sent it onward for a January 1986
encounter with Uranus, and from there to an encounter with Neptune in 1989.

I N T E R N A T I O N A L    C O M E T A R Y    E X P L O R E R

One of the more innovative uses of gravity assist technique was the diversion
of the International Sun-Earth Explorer 3 (ISEE-3) spacecraft in 1983 to a
new orbit and a new life as the International Cometary Explorer (ICE).
Originally launched in August 1978, ISEE-3 spent more than three years
monitoring the solar wind from a vantage point between the Sun and Earth. In
1982, the spacecraft's onboard jets were fired to send it on a data-gathering
trip through Earth's geomagnetic tail, then on to an even more ambitious
mission--a flyby of Comet Giacobini-Zinner in September 1985 and Comet Halley
in October 1985 and March 1986.

To accomplish these cometary encounters, which were not part of ISEE's
original mission, and also to keep the spacecraft in the magnetic tail region
for a longer time, project scientists and engineers designed a complicated
series of five swingbys of Earth's Moon to shape a new orbit for the
spacecraft. When ICEE was maneuvered to pass in front of the Moon's orbital
path it lost energy, but when it passed just behind the Moon as it circled
Earth, it received a gravity boost. By alternating the two types of swingby
(lunar passes occured on March 30, April 24, September 28, and October 21,
1983), and supplementing the trajectory changes with well-timed rocket
firings, ISEE-3 was positioned for one last gravity boost out of the
Earth-Moon system. On December 22, 1983, the spacecraft passed within 110 km
(68 mi) of the lunar surface, and received a gravity boost into orbit around
the Sun, on a course that would take it 3000 km (1800 mi) from the nucleus of
Giacobini-Zinner in 1985, and through the tail of Halley at a distance of 5000
km (3000 mi) on its second pass in 1986.

The spacecraft, now called ICE, carries no televison cameras, but it
investigated the electrical and magnetic properties of Halley and
Giacobini-Zinner and studied the solar wind in their vicinity. By taking
advantage of lunar gravity assists, this rerouted explorer became the first
spacecraft in history to encouter a comet.

G A L I L E O    A N D    U L Y S S E S

Two more missions of the late 1980s and early 1990s--GALILEO to Jupiter and
ULYSSES mission to orbit the Sun's poles--will rely heavily on gravity assist.
GALILEO, to be launched in 1989, is actually two spacecraft in one: a probe
into the atmosphere of Jupiter, and an orbiter that will take close-up
photographs and perform scientific studies of the planet and its moons over a
period of 20 months. During that time, the orbiter will make a series of loops
around Jupiter, its trajectory bent and shaped by the gravity of the giant
planet and its moons, in order to make close flybys of the large satellites
Callisto, Europa, Io, and Ganymede.

ULYSSES formerly called the International Solar Polar Mission, is a joint
project with the European Space Agency. Although its target of study is the
Sun, the spacecraft--to be launched in 1990--will first fly past Jupiter.
There, the planet's powerful gravity field will propel ULYSSES into an orbit
that will take it out of the ecliptic plane, up and over the poles of the Sun.
The five-year mission will be the first opportunity to study our parent star
from this new vantage point.