[161] Pioneer 11, the second spacecraft to fly by Jupiter, returned approximately 460 images of Jupiter and its Galilean satellites during the period 18 November to 9 December 1974. As explained in earlier chapters the trajectory of Pioneer 11 past Jupiter was quite different from that of Pioneer 10. Not only did Pioneer 11 approach much closer to Jupiter's surface - to 0.60 Jovian radii compared with 1.82 Jovian radii for Pioneer 10, but also the spacecraft approached from south of the equator and left from above (to the north). This approach allowed the spacecraft to obtain many unprecedented images of the high latitude, near polar regions. And because the outgoing leg was highly inclined to the equator of Jupiter, several good images were obtained of the planet's north pole.
To distinguish these Pioneer 11 images from those of Pioneer 10, a "C" and "D" notation is used with the image numbers being sequential in time starting at periapsis. "C" images were obtained before periapsis, "D" images after. Full details of these images are given in Appendix 2.
On the facing page the two images taken about a day before and after encounter show the attitude of Jupiter during approach and departure. Unlike Pioneer 10, no rapidly changing terminator position is seen. In all "C" images the terminator is in a position similar to that seen in C22 and all "D" images have the terminator in a position similar to that seen in D17.
The closeness of the approach, and the high relative velocity of the spacecraft over the surface meant that very close-in images could not be obtained; the data would have been gathered too sparsely for later reconstructing into an image. Four pictures on each side of closest approach (C4 through D4) were taken in the step inhibit mode of operation, in which the spacecraft motion alone provided the sweep to build up the picture. Four of this series are shown later in Figs. 9-11 through 9-14. At this close range only partial views of the planet could be obtained.
At about one day before periapsis, a malfunction affected the stepping function of the telescope and a few images were partially lost before a work-around could be effected. Images C16-C10 were affected in this manner. Following this radiation damage the University of Arizona observing team worked for many days and nights to make the necessary corrections to the command sequences to ensure that no more images would be lost.
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Figure 9-11. Close up of Jupiter's Great Red Spot (Image C3). Obtaining this image of the Great Red Spot of Jupiter was one of Pioneer 11's most exciting prospects for planetary astronomers. The highest resolution image of the spot obtained by Pioneer 10 had been spoiled by radiation problems, but Pioneer 11 was successful in obtaining the unique image on the facing page (Figure 9-11a). The area covered on the planet is shown as the inset below the photograph.
This is the best view so far of the Great Red Spot of Jupiter. It was obtained at a range of 545,000 km (320,000 mi.) above the cloud tops.
The image contains more than 4000 individual pixels (see Chapter 7) of measurable data in the red area of the spot thereby providing a wealth of detail of the markings since each pixel represents an area of approximately 237 km (147 mi.) square.
Planetary scientists are deriving new interpretations of the Red Spot from this unique image. Despite the relatively high resolution obtained there is much less fine structure visible in the spot than in comparable images at other latitudes, for example, in Figures 9-12 and 9-14. The Red Spot appears to lie in the most quiescent zone of the planet, which may contribute to its stability.
There is very little internal detail in the blue image (Figure 9-11b), the main feature being the dark border encircling the periphery. A break in this border seems to exist in the northeast part of the spot, where some intermixing of the red material into the South Tropical Zone appears to be taking place.
Much internal detail is revealed in the red image (Figure 9-11c), but perhaps most significant are two circular outlines which cross over near the center of the spot. This same feature was also seen in Pioneer 10 images. No clear suggestion of motions within the spot is evident from this image. The image does not show direct evidence of flow lines from any single region inside the spot which might be interpreted as a source or a sink of the red material.
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The series of images Figures 9-15 through 9-20 shows Jupiter receding as Pioneer 11 leaves the giant planet and rises high above the ecliptic plane on its way to Saturn. Due to an anomaly which affected the rate at which the telescope swept across the planet the command sequence to obtain these pictures had to be changed day by day. Nevertheless all were obtained without any being lost, despite the fact that there was no time to verify the command sequence by computer simulations in advance.
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Figure 9-18. Image D12. Range 1,586,000 km (985,000 mi.), at 22 hours after periapsis. | |
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Figure 9-19. Image D14. Range 1,777,000 km (1,104,000 mi.), at 26 hours after periapsis.
Figure 9-20. Image D15. Range 1,847,000 km (1,148,000 mi.), at 27 hours after periapsis. |
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[181] With telescopes on the Earth, astronomers are afforded only the barest hints of what kind of markings appear on the Galilean satellites. Pioneers 10 and 11 recorded images of all four of these satellites which provide a much better idea of the albedo and color variations across the discs. Although all colors may not be represented properly since only red and blue data were recorded, it is certain that the yellow-orange regions are redder than white regions.
Only one good image of lo has been obtained. This is from Pioneer 11. The Pioneer 10 image of Io was missed completely due to the radiation environment of Jupiter. The Pioneer 11 image is a view from over the north pole of Io. From the Earth there have been observations suggesting that the polar regions of lo are reddish colored. On this Pioneer image there is orange coloration at the polar region, as contrasted with the whitish equatorial part of the satellite. lo is strongly affected by the radiation environment of Jupiter since its orbit is well within the radiation belts and the satellite sweeps up energetic particles from these belts. Also, its orbital motion affects the decametric radio emission of Jupiter. Although there is no indication of it in this picture, lo is the only Galilean satellite which is known to have an atmosphere although it is much less dense than that of the Earth or even that of Mars.
The single image of Europa recorded by Pioneer 10 has little color variation, but there is a broad dark region with some gross detail. Europa is among the brightest of 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 little color variation, but substantial albedo differences over the disc of this largest of the satellites. Ganymede's low density may be due to the presence of a high percentage of ices with some silicates from primordial material and from impacting material from space.
Several good images of Callisto show only small color differences and small albedo variations. The darkest of the Galilean satellites, Callisto has a low density which requires a high percentage of ices in its bulk structure. Two different views one a half-moon shape (reproduced here) and the other a gibbous shape-show the same prominent light region close to the terminator.
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Satellite No. |
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Jl |
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D7 |
(1974) 337:17:33 |
756,000 |
376 |
67 |
+60 |
184 |
|
JII |
|
A4 |
(1973)337:19:24 |
324,000 |
161 |
87 |
-24 |
290 |
|
Jlll |
|
C51/2 |
(1974) 336:19:38 |
739,000 |
367 |
44 |
-29 |
46 |
|
JIV |
|
C12 |
(1974) 336:09:37 |
787,000 |
391 |
82 |
-34 |
33 |
Note: DOY: day of year
LCM: longitude of central meridian
