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$Unique_ID{bob01171}
$Pretitle{}
$Title{Pioneer
Chapter 9: Jupiter Revisited}
$Subtitle{}
$Author{Fimmel, Richard O.;Allen, James Van;Burgess, Eric}
$Affiliation{Ames Research Center;University Of Iowa;Science Writer}
$Subject{pioneer
images
jupiter
image
io
obtained
periapsis
spacecraft
galilean
radiation}
$Date{1980}
$Log{}
Title: Pioneer
Book: Pioneer: First To Jupiter, Saturn, And Beyond
Author: Fimmel, Richard O.;Allen, James Van;Burgess, Eric
Affiliation: Ames Research Center;University Of Iowa;Science Writer
Date: 1980
Chapter 9: Jupiter Revisited
Pioneer 11, the second spacecraft to fly by Jupiter, returned
approximately 460 images of Jupiter and its Galilean satellites during
November 18 through December 9, 1974. The trajectory of Pioneer 11 past
Jupiter was quite different from that of Pioneer 10 Pioneer 11 approached much
closer to Jupiter's cloudtops (0.6 Jovian radii compared with 1.86 for Pioneer
10), inbound from south of Jupiter's equator and outbound toward the north.
This trajectory allowed the spacecraft to obtain many unprecedented images of
high latitudes of the planet. The views obtained on the incoming leg reached
latitudes near the south polar region. Also, because the plane of the
trajectory was highly inclined (51.80) to the Jovian equator, several good
images were obtained of the planet's north pole.
To distinguish the Pioneer 11 images from those of Pioneer 10, the image
numbers, sequentially arranged around periapsis, were given a "C" and "D"
notation - "C" images were obtained before periapsis, "D" images after
periapsis.
Two images taken about a day before and after periapsis show the attitude
of Jupiter during approach and departure. Unlike those of Pioneer 10, the
series of Pioneer 11 images did not show a rapidly changing position of the
terminator.
Because of the closeness of the approach and the high relative velocity
of the spacecraft over the cloudtops, good images could not be obtained near
periapsis; the data gathered would have been too sparse to be assembled later
into a satisfactory image. Four pictures on each side of closest approach
(images C4 through D4) were taken in the step-inhibit mode of operation of the
imaging photopolarimeter, in which the motion of the spacecraft provided the
sweep of side-by-side scans needed to build up the picture. At close range,
only partial views of the planet could be obtained.
About one day before periapsis, a malfunction caused by radiation
affected the stepping function of the telescope; because of this, a few images
were partially lost before a workaround could be effected. Images C16 through
C10 suffered from this problem. As soon as the problem became known, the
observing team from the University of Arizona worked to correct the command
sequences to ensure that no more images would be lost.
The resolution of the images increased as Pioneer 11 approached Jupiter.
The structure in the belt below the spot changed considerably between the two
Pioneer encounters.
With telescopes on Earth, astronomers are afforded only the barest hints
of the markings on e Galilean satellites. Pioneers 10 and 11 recorded ages of
all four of these satellites, which provided a much better idea of the albedo
and colorations across their disks before larger spacecraft at followed the
trail-blazing Pioneers through the Jovian system later produced detailed
pictures of their surfaces. Although all colors could not be represented
properly since only red and blue data were recorded, it was certain that the
yellow-orange regions were redder than white regions. Only one good image of
Io was obtained - D7 from Pioneer 11. Pioneer 10 did not obtain an image of
Io because of the radiation environment of Jupiter. The Pioneer 11 image was
a view from over the north pole of Io. From Earth, previous observations had
suggested that the polar regions of Io are reddish colored. On this Pioneer
image, there is orange coloration at the polar region, as contrasted with the
whitish equatorial region of the satellite. Io is strongly affected by the
Jovian radiation environment since its orbit is well within the radiation
belts and the satellite sweeps up energetic particles from these belts. Also,
its presence in the radiation belts affects the decametric radio emission of
Jupiter. Although there was no indication of it in this Pioneer image, Io is
the only Galilean satellite known to have an atmosphere, although it is much
less dense than those of Earth and Mars.
The single image of Europa recorded by Pioneer 10 (A4) has little color
variation, but there is a broad dark region with some gross detail. Europa is
one of the most reflective satellites and is thought to have a crust of mainly
water ice.
Two excellent images of Ganymede were recorded by the Pioneer spacecraft.
These images show very little color variation, but there are substantial
albedo differences over the disk of this largest Galilean satellite.
Ganymede's low density may result from the presence of a high percentage of
ices with some silicates from primordial material and from material impacting
from space.
Several good images of Callisto showed only small color differences and
small albedo variations. The darkest of the Galilean satellites, Callisto,
has a low density that requires a high percentage of ices in its bulk
structure. Two different views - one a half-moon shape and the other a
gibbous shape - show the same prominent light region near the terminator.