A persistent thunderstorm over a receiving station in Australia cost scientists more than three hours of data from the Voyager spacecraft today, including observation of a cloud of sodium surrounding Io, one of Jupiter's moons.
The storm literally washed out three hours and 20 minutes of photographs and observations made of Jupiter and Io by Voyager, which at 6 p.m. EST tonight was less than 1.3 million miles from Jupiter and approaching the planet at 35,000 miles an hour. Also lost in the Australian storm was a mosaic photograph taken of the Great Red Spot, the permanent hurricane at the tops of the clouds in the southern hemisphere of Jupiter.
"We lost all our science data during the storm over the tracking station near Canberra," Voyager Project Manager Robert Parks said at the Jet Propulsion Laboratory here where the Voyager flight is being directed. "Not only was the storm intense but it lasted for more than three hours and seemed to sit right over the tracking station."
Voyager transmits its photographs and measurements over a radio frequency called the X-band, whose high speed transmission can be absorbed by rainwater, and thus blocked from the receiver. At the time the mosaic of the red spot and the observation of the sodium cloud over Io were being sent to earth, the spacecraft was lined up with the antenna at Canberra, where it is now summer and sudden squalls are frequent.
Parks explained that Voyager could have switched to a slower transmission rate to override the storm but that would have prevented the observation of Io anyway. So scientists gambled that the storm would subside, he said. Instead, it got worse.
Parks said a tape recorder on Voyager could not be used to store pictures today because it is already "booked up" to handle 96 pictures of Io and the red spot when the spacecraft flies by the planet Monday morning.
Unfortunately Voyager will make its closest approach to Jupiter Monday when the spacecraft is again transmitting through Canberra, rather than the other two stations outside Madrid and in California's Mojave Desert. This is why precautions have been taken to store many close approach pictures on the spacecraft's tape recorder.
Parks said there is no chance for Voyager to make another observation of the sodium cloud over Io, since the ultraviolet instrument that made the measurement today is booked solid for the rest of the trip. A second voyager spacecraft will attempt the measurement when it flies by Jupiter in July.
After Voyager began transmitting through the Madrid station later today, its pictures of Io and the Great Red Spot were more detailed than any taken before.
Io was photographed three times, once with Jupiter serving as an enormous backdrop and twice against the blackness of space. For the first time, surface features could be picked out on Io.
One feature on Io looked like a bull's-eye, with a bright spot in its center that a few scientists though might be the remnants of a volcano. Others thought it looked like a basin formed by the impact of a huge meteorite and then filled in with lava from below the surface.
"There are large features, whatever they are,' said Dr. Torrance Johnson of the Jet Propulsion Laboratory. "The outer boundaries of this feature are 1,500 kilometers long and the large bright area is 600 kilometers across. This is of the same scale as Olympus Mons," the vocano on Mars thought to be the largest in the solar system.
The photographs of the red spot clearly showed counterclockwise currents that form the hurricane and have kept it in almost the same place in the planet's South Tropical Zone for the last 300 years. Winds of 10 to 200 mph can be seen moving clouds of red, brown and white in opposite directions north and south of the red spot.