Into Orbit Around Jupiter ...

Party Time!
 
So Much To Do!

The 24 hours of Arrival Day—December 7, 1995—were the busiest of the whole mission. The orbiter flew close to two of the Galilean satellites, listened as the probe plunged into the atmosphere, and performed a large burn to slow itself down and get into orbit.


Io Encounter
 
Encounter

Gravity assists can be also used to decelerate a spacecraft, by flying in front of a body in its orbit, donating some of the spacecraft's angular momentum to the body. When the Galileo spacecraft arrived at Jupiter, passing close in front of Io in its orbit, Galileo experienced deceleration, helping it achieve Jupiter orbit insertion.

Two Moons

The orbiter passed about 32,500 kilometers from Europa; less than 5 hours later, the orbiter flew by Io at about 900 kilometers (559 miles). Because of the tape recorder problem, no pictures (which require high tape recorder rates) were taken of either moon.


The "Hot" Seat
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About 4 hours after leaving Io, the orbiter made its closest approach toJupiter. The radiation dose was intense, perhaps 25 times more than is deadly for humans. In fact, a third of the total whole 2-year tour at Jupiter might have been absorbed on this one day. There is no evidence of any radiation-induced anomalous effects; the spacecraft and its instruments have been and continue to perform normally.

(Now after two years, there is at least one circuit that has started behaving badly and the engineers do believe that the integrated circuit function has degraded because of radiation.)


Probe Telemetry
 
Eight minutes after closest approach to Jupiter, the tape recorder started again in preparation for storing probe data and engineering data from the Jupiter orbit insertion (JOI) maneuver. The orbiter was 215,000 kilometers (134,000 miles) above the probe—ready for the radio relay.

Descent

Three minutes later, the probe slammed into Jupiter's upper atmosphere at 106,000 mph, experiencing a structural load 230 times Earth gravity and temperatures twice as hot the sun. The probe's aeroshield saved it from burning up and two minutes after entry, its speed had been reduced to 1,500 mph and the parachute deployed. For fifty-two minutes, data was transmitted to the orbiter which stored it for re-transmission later on to Earth.


Ode to JOI
 
Jupiter Orbital Insertion or JOI was a huge maneuver that required the Orbiter's 400 Newton main thruster to be fired for forty-nine minutes. Everything had literally to be perfect - any mistake and Galileo would have flown past Jupiter and its moons and on into space.

The orbiter spun up to 10.5 rpm as soon after completion of the probe relay as possible to guarantee orientation and stability during the burn. The orbiter then fired the engine to slow its speed by 643 meters per second.

A perfect JOI placed Galileo in the desired orbit.

Nine hours after engine cutoff, Earth disappeared behind the disk of Jupiter and Galileo plunged into radio silence. Finally, after 3 and 1/2 hours of radio silence, the Earth reappeared to the orbiter, and contact was reestablished. The orbiter, alive and well, was on its 7-month first orbit in the Jovian system!


Orbital Mechanics: Petal Power
 
At Jupiter, the next two years' worth of flight mechanics had been worked out in advance. There would be 22 moon flybys with Orbiter doing CCD imaging and collecting data through its other instruments, including infra-red detector, magnetometer and particle sensors.

This would all occur over a 23-month tour comprising an 11-orbit tour of the magnetosphere and the Galilean moons, including ten close satellite encounters.


The Big One: Jupiter
 
Jupiter

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The Galilean Moons
 
Moons

The four large Galilean Satellites of Jupiter (from left to right in mosaic: Io, Europa, Ganymede, and Callisto) resemble a miniature solar system. The global density, relative amount of rocky material, and duration and intensity of geologic activity on each satellite all decrease with increasing distance from Jupiter. Each satellite is very distinctive in appearance and geologic history. Io, the densest and innermost major satellite, is volcanically active today. Europa has a highly fractured, young icy outer shell. Ganymede has a complex volcanic and tectonic history, and Callisto, the least dense and outermost satellite, is heavily cratered and rather bland by comparison. The geologic histories of these satellites are related to tidal deformation and heating, which is more intense closer to Jupiter.


Io
 
Io

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Europa
 
Europa

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Ganymede
 
Ganymede

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Callisto
 
Callisto

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The Extended Mission
 
GEM

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