Voyager scientists anxiously awaited the first views of Io that would show whether the volcanic eruptions seen in March were still active. This picture was taken on July 4, at a range of 4.7 million kilometers, about the same as that of the volcano discovery picture on March 8. One large plume is clearly visible, rising nearly 200 kilometers above the surface. At the time of release of this picture on July 6, the scientists wrote, “The volcano apparently has been erupting since it was observed by Voyager 1 in March. This suggests that the volcanoes on Io probably are in continuous eruption.” [P-21738B/W]
Thursday, July 5.
(Range to Jupiter, 4.4 million kilometers). The press room at Von Karman Auditorium opened and the members of the press, most of them veterans of the first encounter, arrived at JPL. Meanwhile, the spacecraft continued to measure fluctuations in the magnetospheric boundary. By noon, JPL had reported at least eleven crossings of the bow shock as the solar wind flirted with Jupiter’s magnetosphere. Apparently the solar wind was much more variable in July than it had been in March. At times the bow shock seemed to be thicker than that experienced by Voyager 1; one Voyager 2 crossing took ten minutes, whereas the longest Voyager 1 crossing was only one minute long. Even though the processes affecting the magnetosphere seemed more complex, the magnetosphere was less compressed; when Voyager 2 actually entered the magnetosphere at a distance of 62 RJ, it was much farther from Jupiter than Voyager 1 had been at its final crossing (47 RJ).
Photos obtained the day before from over 4 million kilometers showed that at least one of Io’s volcanoes was still active. A total of eight ongoing eruptions had been seen by Voyager 1, and scientists were anxious to see how many of these were still erupting four months later.
| VOYAGER 2 BOW SHOCK (S) AND MAGNETOPAUSE (M) CROSSINGS | |||
|---|---|---|---|
| Boundary | Day | Distance (RJ) | |
| Inbound | |||
| S | 7/02 | 99 | (multiple) |
| S | 7/02 | 97 | |
| S | 7/03 | 87 | |
| M | 7/04 | 72 | (multiple) |
| M | 7/05 | 71 | |
| S | 7/05 | 69 | |
| S | 7/05 | 67 | |
| M | 7/05 | 62 | |
| Outbound | |||
| M | 7/23 | 169 | |
| M | 7/23 | 173 | |
| M | 7/24 | 174 | |
| M | 7/24 | 175 | |
| M | 7/24 | 176 | |
| M | 7/24 | 177 | |
| M | 7/25 | 184 | |
| M | 7/25 | 185 | |
| M | 7/27 | 213 | |
| M | 7/31 | 253 | |
| M | 8/01 | 258 | |
| M | 8/01 | 262 | (multiple) |
| M | 8/03 | 279 | (multiple) |
| S | 8/03 | 283 | (multiple) |
Although Voyager 2 did not come as close to Io as had Voyager 1, some changes in the surface during the four months between encounters were so large that they could still be easily seen. These two pictures of the region of the volcano Pele were taken in early March and early July, respectively. The most dramatic change was the filling in of the indentation in the ejecta ring, turning the hoofprint into a symmetric oval. The oval is about 1000 by 700 kilometers in outermost dimension, and the area that changed amounts to more than 10 000 square kilometers. [260-687AC]
While attention at JPL focused on the unfolding drama of the Jupiter encounter, many members of the world’s press seemed more interested in the fate of Skylab, which was nearing its death plunge into the Earth’s atmosphere. Launched in 1973, Skylab had been one of NASA’s more successful projects. Three crews of astronauts had visited it, carrying out intensive studies of the Sun and breaking one record after another for the duration of manned space flight. Since the departure of the final group of three astronauts in 1974, Skylab had been sinking gradually lower as a result of friction with the extreme upper atmosphere of Earth. During the past year, higher temperatures in the atmosphere had increased this drag, and now the end was near. With a strange fascination, the world watched the end of this old spacecraft, almost seeming to forget the spectacular new results being transmitted from Jupiter. To the frustration of the Voyager team and the press “camped out” in Von Karman Auditorium for the second encounter, the exaggerated stories of a possible Skylab disaster took precedence over Voyager news. Ultimately, Skylab fell over the Indian Ocean and Australia on Wednesday, July 11, just as the major findings of Voyager 2 were being released.