Mission Overview
================
The twin Voyager spacecraft, over the course of a dozen years,
drew back the curtain on nearly half of the solar system. From
launch in 1977 through the spectacular parting shots of Neptune
at the outer reaches of the solar system in 1989, this pair of
spacecraft explored four planets -- Jupiter, Saturn, Uranus and
Neptune -- as well as dozens of moons, and the rings and
magnetic environments of those planetary systems.

The Voyagers were designed to take advantage of a rare geometric
arrangement of the outer planets that occurs only once every 176
years. This configuration allows a single spacecraft to swing
by all four gas giants without the need for large onboard
propulsion systems; the flyby of each planet both accelerates
the spacecraft and bends its flight path. Without these gravity
assists, the flight time to Neptune would have been 30 years.

The second of the two Voyager spacecraft, Voyager 2, was
launched first, on 20 August 1977. It was followed on 5
September 1977 by Voyager 1, which was put on a faster, shorter
trajectory to Jupiter. Both launches took place from the Cape
Canaveral Air Force Station in Florida.

Eighteen months after launch, Voyager 1 reached Jupiter, 650
million kilometers away. The spacecraft made its closest
approach on 5 March 1979, while Voyager 2 followed on 9 July
of the same year. Images streamed back from the pair of
spacecraft showing the complex, swirling turbulence of
Jupiter's atmosphere in exquisite detail. A giant storm, three
times the size of Earth, raged in Jupiter's upper atmosphere,
surrounded by rippling currents that rotated about it.
Voyager 1 found nine active volcanoes erupting on Io, the
innermost of Jupiter's four major moons. Four months later,
Voyager 2 found that eight of the nine volcanoes were still
active. A thin, dusty ring was also discovered around Jupiter,
forcing revision of theories about origins and mechanics of
planetary ring systems.

At Saturn, both Voyagers took high-resolution images to help
determine ring composition and dynamics. The Voyager 1
encounter took place in November 1980 and the Voyager 2
encounter was in August 1981. Voyager 1 was targeted to fly
close to Saturn's largest moon Titan. This resulted in a
south polar passage of Saturn, which redirected the spacecraft
northward of the ecliptic.

Voyager 2 continued on to Uranus where ten new moons were
discovered in the Uranus system. The planet's magnetic
field was found to be significantly offset from the planet's
axis of rotation.

In August 1989, Voyager 2 flew past Neptune. Because Neptune
receives so little sunlight, many scientists had expected to
see a placid, featureless planet. Instead, Voyager showed a
dynamic atmosphere with winds blowing westward, opposite the
direction of rotation, at speeds faster than the winds of any
other planet. Neptune revealed its Great Dark Spot, a storm
system that resembled Jupiter's Great Red Spot, and a smaller,
eastwardly moving cloud, called 'scooter', which went around
the planet about every 16 hours. The blue planet was circled
by diffuse, dusty rings; six new moons were discovered.

Voyager 2 passed over the north polar region on Neptune,
using the planet's gravity to redirect the trajectory for a
final encounter -- with Neptune's largest moon Triton. It then
departed the solar system southward of the ecliptic.

At about the same time as Voyager 2 was encountering Neptune,
Voyager 1, continuing its journey to the edge of the solar
system on the north side of the ecliptic, turned its cameras
back to look at the planets and take one last parting shot.
Voyager 1's 'family portrait' illustrates the vastness of the
solar system and the huge expanses of emptiness within which
the outer planets lie.

Both Voyagers are now headed for the outer boundary of the
solar system, where the Sun becomes just one of many
contributors to the interstellar environment. That edge is
thought to be somewhere between 8 billion and 23 billion
kilometers from the Sun. Engineers are optimistic that the
Voyagers will still be transmitting data when that boundary is
encountered sometime in the first quarter of the twenty-first
century.

The spacecraft were assembled at and the mission was managed by
the Jet Propulsion Laboratory, Pasadena, CA. Early parts of
the mission have been described in more detail by
[MORRISON1982].