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fundamental questions and the revelation of novelty, antic-
ipation was high.
4. Voyager 2Encounter
Future history will no doubt record theVoyagerproject as
one of humankind’s great journeys of discovery. Originally
conceived as a “grand tour” of all the giant planets and Pluto,
theMariner-class spacecraft that were eventually launched
in 1977 (and renamedVoyager) were only designed to en-
counter Jupiter and Saturn. If they worked, though, a highly
capable complement of remote sensing instruments for the
planets and satellites andin situdetectors for the magneto-
spheres and plasmaspheres would be carried into the outer
solar system for the first time. Two spacecraft allowed for
different encounter strategies, better satellite coverage, and
modification of the second flyby to reflect discoveries made
by the first.
At SaturnVoyager 1was targeted to pass close to Titan,
a trajectory that sent it out of the ecliptic plane afterward.
The trajectory ofVoyager 2was carefully chosen to preserve
the grand tour option, whereby each successive encounter
would boost the spacecraft to a higher velocity and in just
the right direction to reach the next giant planet, which were
fortuitously arranged in the 1980s. ThatVoyager 2would
reach Uranus, and then Neptune, was the decided wish of
the entire planetary science community.
There was no guaranteeVoyager 2would survive the
complete 12-year trip from the Earth to Neptune, many
years past its design life. Problems did develop. One radio
receiver went out and its backup was failing, and the
articulated scan platform, upon which the remote sensing
instruments were mounted, could no longer move as eas-
ily as before. Nevertheless, in August 1988, after successful
encounters at Jupiter, Saturn, and Uranus, and Neptune,
Voyager 2sent back images of Neptune and Triton that
were, for the first time, sharper than the best images taken
by groundbased telescopes.
Each newVoyagerencounter increased scientific and
public awareness of the richness of the Solar System. The
Voyager 2flyby of Neptune and Triton in late August 1989,
was going to be the last, and proved to be perhaps the most
exciting of all. But there was one last hurdle. In order to
get to Triton,Voyager 2would have to pass very close to
Neptune’s north pole in order for Neptune’s gravity to bend
its trajectory southward (Fig. 5). This would be dangerously
close (only 5000 km from the cloudtops) and in an unknown
and potentially dangerous environment. To everyone’s re-
lief,Voyager 2made it past Neptune without incident just
after midnight on August 25 (PDT), counting the more than
four hours it took forVoyager’sradio signals to reach Earth.
Five hours later it passed within 40,000 km of Triton, send-
ing back a sequence of beautiful, mind-boggling images.
These images form much of the basis for understanding, to
FIGURE 5 The trajectory ofVoyager 2through the Neptune
system. [From C.R. Chapman and D.P. Cruikshank (1995).In
“Neptune and Triton” (D.P. Cruikshank, ed.). University of
Arizona Press, Tucson.]the extent we do, Triton’s geology and surface-atmosphere
interactions.5. General Characteristics
Voyager 2determined Triton to be even smaller, brighter,
and hence colder than anticipated (Table 1). Its average
geometric albedo of≈0.7 is extreme even for an icy satel-
lite. Triton’s global appearance was revealed during the
approach sequence (Fig. 6). The view, mainly of the south-
ern hemisphere, showed extensive bright polar materials,
a bright equatorial fringe with streamers extending to the
northeast, and darker low northern latitudes. Radio track-
ing ofVoyageryielded a very precise mass for Triton, which
when combined with the size, gave a very precise density
of≈2.065 gm cm–^3. This density is essentially identical to
that of the Pluto–Charon system.
With size and mass known, internal structural models
can be created based on a set of plausible chemical com-
ponents; for bodies formed in the outer solar system these
would be rock, metal, ices, and carbonaceous matter. Such
models provide context and to some extent guide interpre-
tations of geological history. A calculation for Triton is illus-
trated in Figure 7. Given that little direct information exists
on the internal makeup of Triton, the model shown simply
matches Triton’s density and assumes the interior is hydro-
static (follows the fluid pressure–depth relation) and dif-
ferentiated (the major chemical components are separated
according to density). These last two assumptions are em-
pirically consistent with Triton’s surface appearance, which