PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

Contact:  Franklin O'Donnell                  November 4, 1996

GALILEO MAKES CLOSE PASS BY CALLISTO

     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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11-4-96 MBM
#9684