For the first time, NASA's Galileo spacecraft flew close to Jupiter's moon Callisto this morning (Nov. 4), passing within 1,104 kilometers (686 miles) of the stark, crater-studded natural satellite at 13:34 Universal Time (5:34 a.m. Pacific Standard Time).
The flyby was by far the closest any spacecraft has ever come to Callisto, the outermost of the four big moons orbiting Jupiter that were first discovered by the Italian astronomer for whom the mission was named, Galileo Galilei. Signals confirming the event were received on Earth 46 minutes later.
Data from this Callisto flyby and another one next June should help resolve questions about why this seemingly inactive, pockmarked moon is so different from its vastly more active siblings -- tectonic Ganymede, volcanic Io and Europa, which may have an ocean beneath its cracked, icy crust.
Callisto is the outermost and, apparently, least active of Jupiter's four major Galilean satellites. The 4,800-kilometer- diameter (2,400-mile) moon is the second largest of Jupiter's 16 known moons. Its aged appearance is its most distinctive known feature. It has the oldest, most cratered face of any body yet observed in the solar system.
"With data from this encounter, we'll know more about why Callisto is so different from Jupiter's more lively moons," said Galileo Project Scientist Dr. Torrence V. Johnson of NASA's Jet Propulsion Laboratory.
"Some of the most interesting aspects of the Callisto flyby are actually the observations we're making of other bodies, such as Jupiter and Europa. We're coming to within 34,000 kilometers (about 21,100 miles) of Europa and 658,000 kilometers (about 409,000 miles) of Jupiter. Looking at their dark sides, we should get some very good data from atmospheric measurements and auroral searches," he said.
Science instruments on the spacecraft were pre-programmed to take measurements of Callisto's surface to determine its composition and history, to look for evidence of any activity such as tectonism, and to search for hints of any magnetic field that may be generated by the moon. While some of the data are sent back to Earth immediately, much of it, including all the images, is being recorded on the spacecraft tape recorder for playback to Earth over the next few weeks.
Like Jupiter's biggest moon, Ganymede, Callisto seems to have a rocky core surrounded by ice. Unlike the other large moons of Jupiter, however, the surface of Callisto is completely covered with scars left by tens of thousands of meteoric impacts. Although the exact rate of impact crater formation is not known, scientists estimate that it would require several billion years to accumulate the number of craters found there, so Callisto is believed to have been inactive at least that long.
The Callisto flyby marks the start of a new telecommunications capability created to maximize the amount of data that can be received from Galileo. The giant ears that listen to NASA's exploratory robots in deep space have been augmented this week with the inauguration of a new link between the agency's Deep Space Network telecommunications stations in California and Australia and Australia's Parkes radio astronomy antenna.
NASA's intercontinental link-up -- or "arraying" -- of giant antennas was developed to retrieve the maximum amount of data possible from NASA's Galileo spacecraft, whose planned high- speed, high-power telecommunications voice was reduced to a whisper when its main antenna failed to open four years ago.
For several hours a day, large collecting areas of these big antennas will be devoted to concurrently receiving the spacecraft's faint transmissions as Galileo nears Callisto and returns data from its flyby. The Callisto encounter occurs with the spacecraft at one of its most distant points from the Earth, which makes receipt of Galileo's weak signal even more difficult. The arraying technique, however, allows more of the spacecraft's signal to be captured, thereby enabling a higher data rate. The arraying will be used daily throughout most of the remainder of the mission.
The debut of routine arraying of the Deep Space Network antennas represents the final installment of several imaginative engineering solutions that have allowed the Galileo Project team to carry out its mission despite the loss of the spacecraft's main telecommunications antenna.
"With our spacecraft software and ground receiving station improvements already in place, this new arraying capability is the icing on the cake," said Galileo Mission Director Neal Ausman of JPL. "The new array is critical to getting Galileo's scientific data from the Jupiter orbital tour back to Earth."
Arraying, together with other improvements in the space-to- ground communications link, increases by 10 times the quantity of raw data received from Galileo than would otherwise be possible. Changes in the way the Galileo spacecraft edits and compresses data result in an additional factor of 10. When taken together, these improvements enable Galileo to meet 70 percent of its original science goals.
Software changes on the spacecraft now ensure that every bit of science and engineering telemetry from the spacecraft is crammed with as much information as possible. Consequently, while the data amount received from Galileo is comparatively small, all of it is highly valued.
Galileo's high-gain antenna was to have provided a 134- kilobit-per-second real-time data rate from Jupiter. Had no improvements been made in the Deep Space Network, only a 10-bit per second data rate would have been possible with Galileo's small low-gain antenna for most of the mission. With these improvements, however, along with the changes made on the spacecraft, further increase the downlinked data to an effective rate of 1,000 bits per second.
"As the Earth turns relative to Galileo's position in the sky, different arrayed antennas will 'hand-off' the receipt of data from Galileo over a 12-hour period," said Leslie J. Deutsch of JPL, one of the principal innovators behind the solution for Galileo's communications problem. The array electronically links the stadium-size, 70-meter (230-foot) diameter dish antenna at the Deep Space Network complex in Goldstone, CA, with an identical antenna located at the Australia site, in addition to two 34-meter (112-foot) antennas at the Canberra complex. The California and Australia sites concurrently pick up communications with Galileo. The Parkes radio telescope joins in with the Canberra station for six hours each day.
"For two hours each day, a total of up to five antennas are pointing in unison to receive transmissions from Galileo," Deutsch said.
The new hardware, software and operations that make antenna arraying possible for Galileo represents a major improvement in the world's deep space telecommunications system for other missions as well, said Paul Westmoreland, director of telecommunications and mission operations at JPL. The effort cost $30.5 million.
"The methods used and much of the equipment will be especially useful for the new era of our faster, better, cheaper interplanetary missions," Westmoreland said. "This opens the way for mission developers to reduce future spacecraft and operations costs by using smaller spacecraft antennas and transmitters."
In the future, after the Galileo mission, other antennas may be added to routine arraying for spacecraft communications and radio science experiments. Among them are the twenty-seven 25- meter (82-foot) diameter antennas that make up the Very Large Array of radiotelescopes in Socorro, NM, and the Japanese 64- meter (210-foot) radio telescope facility at Usuda.
The new arraying capability sprang forth as an emergency measure to mitigate the loss of Galileo's high-gain antenna, but the effort now represents a permanent change and improvement in the way deep space telecommunications will be conducted from now on, said Joseph I. Statman, telecommunications specialist at JPL, who engineered the system.
"Galileo gets credit for giving arraying a huge push," Statman said. "This is the first time we will see routine arraying of antennas for spacecraft communications day in, day out. The Deep Space Network is now implementing arraying at Goldstone, as part of its standard configuration."
Several 34-meter (112-foot) antennas at Goldstone are to be outfitted with the equipment needed so they can operate together as an array, he added. "This represents the wave of the future because no more 70-meter antennas will be built for the Deep Space Network; only 34-meter antennas will be added to the network from now on," Statman said. "When faster, better, cheaper spacecraft with small transmitters and antennas require a higher downlink data rate, we will have a field of antennas with which we can manage and satisfy our customers' demands for telecommunications resources."
Galileo was launched aboard Space Shuttle Atlantis on Oct. 18, 1989. The mission is managed by JPL for NASA's Office of Space Science, Washington, DC.
Additional information on the Galileo mission and its results can be found on the World Wide Web at: http://www.jpl.nasa.gov/galileo/
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