June 1982:the Voyager 2 Saturn flyby

Voyager 2 at Saturn

As 1980 drew to a close, life at the Jet Propulsion Laboratory
returned to normal. The teams of visiting scientists returned
home to begin the detailed analysis of the volumes of Voyager 1 data returned from Saturn. The excitement of the first flyby
was over, the planning for the Voyager 2 encounter was just
beginning.

On the basis of Voyager 1’s results it was decided to make
significant alterations to the plans for Voyager 2’s journey
through the Saturnian system. Voyager I had provided a
"broad brush" coverage of the system, while Voyager 2 would
be commanded to concentrate on specific targets in an attempt
to better understand the discoveries of its twin.

This replanning was made possible by the flexibility designed
into the spacecraft nearly a decade before. During the planning
stage it was realised that discoveries made by the first spacecraft
might require major changes to the encounter operations of
the second. An eight month gap between encounters was
therefore built into the flight plan. To provide this flexibility
and reliability three of Voyager's systems use doubly redun
dant, reprogrammable digital computers. These are the Command Control Subsystem (CCS), the Articulation and Attitude
Control System (AACS) and the Flight Data System (FDS).

The CCS is the brain of the Voyager spacecraft. It can Issue
commands to the spacecraft subsystems from its memory,
decode commands from Earth to update its memory and pass
those commands to other subsystems. The AACS controls
attitude and points the scan platform on which the science
instruments are mounted. The FDS controls the scientific
instruments and arranges all of the science and engineering
data for transmission to Earth. It also controls the data storage
subsystem, an eight track tape recorder capable of storing over
500 megabits of data.

Voyager 2’s trajectory past Saturn was determined by the
need to keep the spacecraft on course for Uranus (approval of
the extension of the mission to Uranus had been received in
November) but the complex series of experiments could be
changed in line with Voyager’s new objectives. These changes
were derived by JPL engineers and the mission’s principal
science investigators. New encounter sequences were then
prepared for Voyager's CCS. Before these could be transmitted
to the spacecraft they were run through complex computer
simulations to ensure maximum scientific return and spacecraft
safety. When Voyager 2 reached Saturn the round trip radio
time would have increased to nearly 3 hours and if a software
problem were to arise useful observing time would be lost
while ground control attempted to rectify it.

The main changes to the Voyager 2 science objectives may
be summarised as:

1. Voyager 2 would concentrate on studies of the complex ring system, searching for evidence of small moons which
might explain the proliferation of ringlets.

2. Additional studies of the braided F ring would be made
including stereo imaging and an occultation experiment using the star Beta Tauri.

3. Attempts would be made to view the B ring exactly edge-on to determine if the “spokes” observed by Voyager 1 were "levitated” above the ring plane.

4. Special studies of the two eccentric rings observed in the
B ring and in Cassini's division.

5. Reduced emphasis of imaging of Titan but a search for
aerosols in Titan’s atmosphere would be carried out. This
had been impossible for Voyager 1 because of a failure in
its photopolarimeter.

6. A detailed look at some of the small moons discovered bv
Voyager 1 and from Earth. Of special interest were the
small moons found sharing orbits with the classical satellites,
e.g. Dione B and the leading and trailing "’Trojan” moonlcts
of Tethys.

7. A study of electrostatic discharges discovered by Voyager
1. These were found to have a period of 10 hr 10 min,
similar to the period of any satellites embedded in the B
ring. 'These discharges may also be related to the B ring
spokes.

8. A systematic search on Saturn’s unlit side between equa¬
torial and polar latitudes for aurorae.

Voyager 2’s observations of Saturn's rings would be aided
by three factors. Firstly, the spacecraft cameras were more
sensitive than those of its predecessor. Secondly. Voyager 2
would pass closer to the rings and, thirdly, the rings would be
better illuminated as the Saturnian year continued towards
Northern Summer. Additionally. Voyager 2 would be on the
illuminated side of the rings at its point of closest approach.

Closing in on Saturn

Although some of the details of the near encounter sequences
of Voyager 2 had been changed, and the spacecraft was flying
a different trajectory, the general shape of the flyby was the
same as for Voyager 1.

Encounter operations began on 5 June 1981 with the observatory phase. Beginning 82 days before encounter and lasting
eight weeks, this period of long distance observations allowed
a second, short-term, history of the planet to be recorded.

The observatory phase began with a 43-hour session of near
continuous photography covering four planet rotations.
Throughout the remainder of the observatory phase Saturn
was imaged every 72 degrees of its rotation (so five photographs
covered a complete rotation). These pictures allowed the
assembly of a colour movie showing the approach. The spacecraft’s UV spectrometer scanned across the entire Saturn system mapping the intensity of the emissions from Titan’s
orbit. As with Voyager 1, radio emissions were sampled daily
with the planetary radio astronomy experiment and frequent
scans were made by the plasma wave instruments. Also during
this phase numerous calibrations of the scientific instruments
were made to ensure the best possible scientific data during
the near encounter.

On 31 July, 26 days before closest approach, the narrow
angle camera could no longer capture the entire planet in a
single frame. The imaging was switched to two-by-two mosaics,
using four frames to build up each complete picture. At this
point Voyager 2 was a little less than 25 million kilometres
from Saturn and had entered the 12-day Far Encounter 1 phase.
During this phase, the observations were continued as the
detail visible on the planet increased day by day.

By 11 August the range to Saturn had reduced such that
two by two mosaics no longer provided full coverage of the
planet. The Far Encounter 2 phase had begun. At this time
Voyager 2 was 14.5 million km from closest approach and all
of its instruments were gathering data on the planet, satellites
and radiation environment. During this period, a series of
photographs of the' B ring were taken by the narrow angle
camera as more details of the elusive spokes were sought.
Spoke imaging took place over a period of about 30 hours with
the Voyager camera panning left to right to take the four
images necessary to ensure complete coverage of the ring
system. Each four image sequence lasted about 16 minutes.

On 18 August a final trajectory correction manoeuvre was
made to refine Voyager 2’s aim point. This was necessary
because it had been estimated that an error of a mere 20
seconds at encounter could compromise some of the scientific
results. In the end Voyager 2 arrived only 66 km off its aim
point and 3.1 seconds ahead of schedule.
Early in the Far Encounter Phase Voyager 2 made its closest
approach to lapetus, Hyperion and Titan. Just 18 hours before
closest approach the spacecraft began its near encounter.

Close Encounter

The close encounter operations were more complex than those
of Voyager 1 because it had been instructed to study specific
targets, requiring extensive use of the movable scan platform
carrying the scientific instruments. On the last day of the Far
Encounter 2 phase there had been 33 different observations
and the near encounter would involve rapid switching between
a number of targets.

On 22 August, while still 3V5 days from Saturn, Voyager 2
passed the mysterious, low density moon Iapetus. Iapetus has
long been known to have hemispheres of widely differing
reflectivity (or albedo). The closest distance was a little over
900,000 km but even this was 1.5 million km nearer than
Voyager 1. By monitoring the effect of Iapetus’ weak gravita¬
tional field on the trajectory of Voyager 2 the satellite’s mass,
and hence its density, could be determined. This showed
Iapetus to have a density of only 1.1 gm/cc. This very low
figure suggests a composition of 35 per cent rock, 55 per cent
water ice and about 10 per cent methane ice.

The next moon to come under intensive scrutiny was
Hyperion, a small body which Voyager 1 had hardly glimpsed.
As Voyager 2 imaged Hyperion the changing orientation of
the satellite caused it to appear in a variety of forms. Before
its true shape could be determined the satellite was described
as shaped like a peanut, a tuna-fish can and a hockey puck.
Eventually the semi-official description of hamburger-shaped
evolved with dimensions of about 210 by 360 km. Of great
interest was the discovery that the long axis of Hyperion points
out of the plane of its orbit - it had been expected that the
moon would be gravity gradient stabilised to point directly
towards Saturn. To have taken up this peculiar attitude
Hyperion must have been struck by another body, probably
within the last 100 million years.

As Voyager 2 sped towards Saturn it took a look at Titan,
although the range was over 600,000 km, a hundred times
further than its predecessors flypast. Even at this range the
cameras showed that the polar hood observed by Voyager 1
had changed. It was now more of a collar
surrounding the pole. Voyager passed by Titan about 18 hours
before closest approach to Saturn and shortly after it formally
entered its near encounter phase.

As the spacecraft closed in on the planet it continued to
photograph the rings and details on the planet itself. The
search for moonlets embedded in the rings continued in vain.
Four hours and 20 minutes from closest approach the newly
discovered moon 1980S26, known as Dione B, was glimpsed
at a range of 318,000 km. Shortly afterwards the first of two
planned occultation experiments began. The structure of Saturn’s rings was to be investigated with unprecedented accuracy.

Probing the Rings

The occultation of the star Delta Scorpii was one of Voyager
2’s most important ring observations. For 2 hours and 20
minutes the spacecraft's photopolarimeter was aimed so that
the star was viewed through the ring system. As the star
blinked on and off Voyagqr was'able to count the number of
ringlets and measure the size of particles down to about 500 m
in size. The star, magnitude 2.34, was chosen because of its
brightness and because its occultation by the rings took place
in Saturn’s shadow, minimising the interference from scattered
sunlight. By ingenious computer processing the results from
this experiment were reconstructed in the form of “photographs” showing the fine structure within the 70 km-wide F
ring strand. Resolution was at least 10 times higher than with
the best optical images.

During the occultation experiment over 700,000 data points
were recorded at a rate of about 100 per second. Initial analysis
suggested that resolution could be as high as 100 m in places,
and that the thickness of the outer edge of the A ring was less
than 200 m.

A search was made for fluctuations in ring particle density
since spiral density waves within the rings had been proposed
to account for the complex structure. Wave features were
found with peak to peak distances ranging from 100 km to the
100 m resolution limit.

The Inner Moons

During the occultation experiment Voyager 2 had passed
Dione with a maximum possible resolution of 12 km, some
four times lower than Voyager 1 because of the increased
distance. Soon after, Mimas was passed at a distance of about
310,000 km and Saturn closest approach was less than an hour
away.

During this hour Voyager made its closest approach to three
of the newly discovered small Saturnian moons 1980S25 (trailing Tethys), 1980S28 (orbiting outside the A ring) and 1980S26 ( orbiting outside the F ring). Both 1980S25 and I980S28 were imaged at resolutions of about 5 to 10 km before Voyager 2
skimmed 100.000 km above the cloud tops of Saturn.

Closest approach occurred at 8.21 JPL local time, as Voyager
2 swept down on the rings from above their illuminated side.
As seen from Earth Voyager 2 proceeded around the right
hand side of the planet, the gravitational attraction of Saturn
turning the trajectory through almost 90 degrees and swinging
the craft onto its course for distant Uranus.

Nine minutes after closest approach Voyager passed
I980S27, another small moonlel. before its rendezvous with
Enceladus. Voyager 1 had passed 200,000 km from Enceladus
and had shown that at a resolution of 12 km it was unusually
smooth. As Voyager 2 closed in. on Enceladus it was revealed
as an exciting new world. In general terms, it resembled
Ganymede, although with a diameter of about 500 km it was
only a tenth of size of the Jovian satellite.
Some regions of Enceladus showed impact craters up to
55 km in diameter while other areas, those imaged by Voyager 1 are smooth and uncratered. Like Ganymede and Europa,Enceladus is also criss-crossed by linear grooves several
hundred kilometers long. These probably represent faults
resulting from crustal deformations. The large uncratered areas
on Enceladus must be young, probably less than 100 million
years old, suggesting recent geological activity.

Voyager 2 was still working well 15 minutes after Enceladus
closest approach as it disappeared behind Saturn to begin the
90 minute Earth occultation phase (Sun occultation occurred
6 minutes later). While out of sight from Earth the spacecraft
crossed the ring plane about 745 km outside the C ring. Once
out of contact with Earth, Voyager’s scientific observations
were the responsibility of the spacecraft’s CCU which would
use commands stored in its memory to control the complex
array of instruments. Mission controllers waited expectantly
for the spacecraft to reappear.

When contact was re-established it was obvious that all was
not well with Voyager 2. Instead of Saturn, photographs showed
only empty space. The 220 lb scan platform carrying Voyager’s
cameras, spectrometers and photopolarimetcr had stuck and
was pointing uselessly into space. The platform was jammed
at 260 degrees in azimuth and 20 degree elevation and would not respond to commands to slew in azimuth. Immediately ground controllers commanded a slew in platform elevation to
keep the sensitive instruments pointing away from the Sun.
This was followed by an instruction to the scan platform not
to respond to azimuth commands from the spacecraft. Several
azimuth scans had already been sent by the CCU to the
platform but the platform had failed to respond. Later in the day ground controllers were able to move the platform about 10 degrees in
azimuth but further attempts to free it were delayed to allow
time for a full evaluation.

As Voyager began to leave Saturn ground controllers strove
to bring the scan platform back into action. Photographs were
obtainable but only at a cost - slewing the spacecraft was using
precious attitude control gas needed for the journey to Uranus.
On 28 August, three days after closest approach, the platform
was successfully moved by ground command to point the
instruments at Saturn once more. Early tests showed that the
platform could be moved, although response was sometimes
hesitant and slow. Response improved steadily over the next
few days and the platform was returned to the.onboard computer sequences for the imaging of Phoebe, on 4 September.
Voyager -2 flew no closer to Phoebe than 2 million kilometres but even at that range was able to return useful results. Phoebe
was shown to be about 200 km in diameter and very dark.
Earth-based measurements had assumed a more reflective
surface and deduced a diameter half that measured by Voyager.
The rotation period was determined as about 9 to 10 hours,
the only one of Saturn’s moons which does not always show
the same face to the planet. Phoebe orbits Saturn in a retrograde
direction, and in the ecliptic plane rather than the plane of the
equator.