PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
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PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Franklin O'Donnell
FOR IMMEDIATE RELEASENovember 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
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
"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
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 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,"
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
"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
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: