PJS@GROUCH.JPL.NASA.GOV (Peter Scott) (03/17/89)
Excerpted without permission from NASA Voyager Bulletin, Mission Status Report
# 85, March 3, 1989 [stuff in square brackets is mine]:
It's not just a fuzzy tennis ball after all...
----------------------------------------------
A bright cloud feature on Neptune, similar to spots seen by planetary
astronomers using Earth-based telescopes, is visible in images taken by
Voyager 2 on January 23, 1989 when the spacecraft was about 309 million
kilometers (185 million miles) from the planet. The fact that distinct
cloud features are visible while the spacecraft is still so distant suggests
that pictures taken as Voyager 2 approaches its August 1989 flyby of Neptune
will show far more detail than was visible in the atmosphere of Uranus,
which Voyager 2 encountered in January 1986. (Due to the lack of visible
cloud features, Uranus has been lightheartedly described as a "fuzzy blue
tennis ball," and less kindly as bland.)
The cloud is at about 30 degrees south latitude, and its motion during the
time between images is consistent with the 17- to 18-hour rotation period
derived from observations with Earth-based telescopes. The January images
show details as small as about 6000 kilometers (3500 miles). The cloud
has not yet been confirmed to be any of the cloud features seen on Neptune
by Drs. Richard Terrile of JPL and Brad Smith of the University of Arizona
at Las Campanas Observatory, Chile, in 1983, or by Dr. Heidi Hammel of JPL
at the University of Hawaii's Mauna Kea facility in 1988. The features
seen from these Earth-based telescopes were best seen through methane filters
not available on Voyager 2, and imaging scientists have been somewhat concerned
that such features might not be visible to Voyager 2's cameras.
The mottled appearance of Neptune in these frames is likely to be "noise"
in the camera system. Color versions of these images, assembled from pictures
taken through violet, clear, and orange filters, show a dark band of clouds
encircling the planet's southern pole. [...] The natural color of Neptune
is a pale blue-green, caused by the absorption of red light by methane gas
in the planet's atmosphere.
[...] Spacecraft review
-----------------
[...] At launch, each spacecraft carried about 105 kilograms (232 pounds)
of hydrazine. During its 11.5 years in space, Voyager 2 has used about
60 kilograms (140 pounds) of hydrazine.
[...] Each Voyager is powered by three radioisotopic thermoelectric generators
(RTGs), which produce electrical energy throutgh the conversion of heat
generated by the radioactive decay of plutonium-238. At launch, the power
output of the RTGs was about 423 watts. The power output steadily declines
as the plutonium decays, and is now about 380 watts. The science instruments
require about 105 watts, or about the same wattage as a typical light bulb.
[...] Instrument Description and Health
---------------------------------
[...] Each Voyager spacecraft carries two imaging cameras: a 200-mm,
f/3.5 wide-angle camera using a refracting telescope and a 1500-mm f/8.5
narrow-angle (telephoto) camera using a reflecting telescope. Each camera
carries a one-inch selenium-sulfur vidicon to convert an optical scene into
electrical signals.
Each frame consists of 640,000 pixels, each of which is expressed as a level
of gray on a scale from 0 (black) to 255 (white). [...] The sensitivity
of the filters ranges from 3460 (ultraviolet) to 6184 angstroms (red-orange).
[...]
Voyager 2's narrow-angle camera has dust specks on the vidicon which result
in faint, doughnut-shaped blemishes in images [what do they expect, it's
been twelve years since the maid came]. In addition, the emission of the
vidicon cathode in the narrow-angle camera has decreased since launch.
[...] The photopolarimeter measures the way light is scattered from particles
in an atmosphere or on a surface. By studying the polarization of reflected
light as ther lighting geometry changes during a flyby, scientists can make
inferences about the nature of a planetary surface or atmosphere. The
photopolarimeter can also be used to study rings by measuring the intensity
of a background star as the starlight passes through the rings.
The photopolarimeter consists of a 200-mm Cassegrain telescope with filters,
polarization analyzers, and a photomultiplier tube to convert incoming light
into electronic signals. It covers three wavelengths in the region between
265 and 750 millimicrons. Five of the eight original filters and four of
the eight original analyzers are no longer accessible. [...]
Voyager 2's Health
------------------
Both Voyagers have experienced several health problems since launch, some
minor and some rather major ones. Nevertheless, mission controllers have
in every case been able to identify the problems and provide a way to continue
to meet mission objectives.
In September 1977, about a month after launch, Voyager 2 suffered a hardware
failure in the FDS [flight data subsystem - one of the computers]. As a
result, 15 engineering measurements can no longer be made (about 215
engineering measurements remain).
In 1978, eight months after launch, Voyager 2's main radio receiver failed,
and a tracking loop capacitor failed in the backup receiver. As a result,
Voyager 2 can receive signals in only a narrow "window" of frequencies--and
the window slides. The window is about 1000 times narrower than it originally
was, and temperature changes in the radio receiver of even 1/4 degree cause
the window to slide up or down in frequency. Temperature changes can be
caused by heat generated by the spacecraft's electronics. The flight team
has devised a rigorous routine for commanding the spacecraft. Signals are
sent several times at different frequencies to determine the receiver's
current frequency "window". Commands are then transmitted, after calculating
where the receiver's "window" will be, and taking into account how the signal
frequency will change due to the Earth's rotation and other motions.
[...] In August 1981, just after Voyager 2 passed Saturn, the scan platform
quit moving. Three years of intensive analysis and testing of similar parts
on Earth, and of the scan platform on Voyager 1, led to a failure model
and to guidelines for safe usage of the platform. The failure has been
attributed to a lack of full lubrication of the bearing area between the gear
and pin in the azimuth actuator. Lubricant has probably migrated back to
the bearing surfaces, healing the problem. Adherence to the guidelines
for safe usage permitted Voyager to complete a successful encounter with
Uranus in 1986, returning some of the highest resolution images ever taken
of solar system bodies. [...]
Just days before its closest approach to Uranus, Voyager 2 suffered the
loss of one word of memory in one FDS processor. As a result, bright and
dark streaks appeared in images. Only imaging data was affected, and a
software patch was sent to bypass the failed bit.
Despite a little arthritis, a little hearing problem, and some loss of memory,
Voyager 2 is still in excellent operating condition, and gaining rapidly
on Neptune and Triton.
Peter Scott (pjs@grouch.jpl.nasa.gov)