Galileo Less Than Three Weeks From Jupiter
From the "JPL Universe"
November 17, 1995
After a journey of more than 2,200 days and 3.7 billion kilometers (2.3 billion miles), the Galileo spacecraft and its atmospheric probe are now less than three weeks away from their long-anticipated Dec. 7 encounter with Jupiter.
Project Manager William O'Neil reported in a Nov. 9 press conference that Galileo's orbiter is in "generally excellent health" and that the probe--which has been on a solo flight to the giant planet since July--is "believed to be in excellent health." He added that the orbiter's current trajectory is so good that a scheduled trajectory correction maneuver on Nov. 17 was cancelled.
O'Neil also addressed the two anamolies the mission has had to endure in recent months. He said a problem with an oxidizer check valve, first detected after an orbiter deflection maneuver in July, was being carefully controlled by propellant temperature management. "We have taken every possible precaution," he said. "We feel we are quite safe and that all propulsion functions can be successfully performed."
Regarding last month's malfunction in Galileo's tape recorder, O'Neil said the "leading candidate" for the problem was the tape sticking to adhesive on a "dummy" erase head, which is used as a tape guide. The speculated source of the adhesive, he said, is the laminate step on the tape at the leader. The problem appears to be manageable, however, and should not jeopardize return of the full complement of images and other data from Jupiter and its moons that are to be stored on the recorder for playback over the course of the orbital mission.
Pictures of the Jovian moons Io and Europa will not be taken on Dec. 7, including the scheduled closest encounter of Io from a distance of 1,000 kilometers (600 miles). "We are limiting tape recorder operations before Jupiter arrival to the atmospheric probe mission and Io torus," he said. "There is not enough time before arrival to put the safeguards in the central computer that will enable safe tape recorder use in orbit."
On the final days of approach, O'Neil said the Galileo team was anticipating an impressive "celestial ballet" that will be performed as the effects of Jupiter's tremendous gravity will actually stop the forward, sun-centered motion of the probe and orbiter and cause them to, in effect, "fly backward" to the giant planet.
Probe scientist Dr. Rich Young of Ames Research Center in Moffett Field, Calif. called the probe's mission the most difficult atmospheric entry ever attempted, and noted that Jupiter "in some senses is the Rosetta Stone of early solar system development."
The 339-kilogram (750-pound) probe will enter the atmosphere at a top speed of 170,000 kilometers per hour (106,000 mph), or about 50 times faster than a bullet shot out of a rifle. The probe will experience deceleration forces as high as 230 times Earth's gravity. In about two minutes, the orbiter's speed will be slowed to about 1,600 kilometers per hour (1,000 mph) as it begins its 75-minute mission to measure the planet's atmosphere and clouds, while descending into the dense atmosphere under its parachute. It's possible that the probe will also encounter lightning and rain.
The Galileo orbiter will record the probe's measurements from about 214,000 kilometers (133,000 miles) overhead and then begin its two-year tour of the Jovian system by carrying out a 49-minute firing of its main rocket engine that will slow the spacecraft to allow it to be captured into orbit around Jupiter. Three months later, another rocket firing will lift the spacecraft's orbit out of the high-radiation environment of the planet's charged particle belts, which could damage Galileo's electronics.
Young, summing up the anticipatory mood of scientists as orbit insertion approaches, said "it will definitely be white-knuckle time" when waiting for confirmation that the orbiter has locked onto the probe signal.
Noting the two-year duration of the Galileo orbiter's primary mission at Jupiter and years of study to follow, Project Scientist Dr. Torrence Johnson said, "The most exciting discoveries to come from Galileo will be those we haven't thought of."