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Ice, Water, and Fire: The Galileo Europa Mission

Extracted from The Planetary Report (Jan/Feb 1998)
Courtesy of The Planetary Society

After a six-year journey from Earth, Galileo arrived at Jupiter on December 7, 1995. In moves designed to lock the spacecraft in orbit around the gaseous giant planet, Galileo swung by the moon Io, then fired its main engine, and in between, collected the precious data from the atmospheric probe it dropped five months earlier. For two years and 11 orbits during its Prime Mission, Galileo has revealed an array of fascinating details about Jupiter and its moons. While Jupiter's composition is reflective of the primordial mix, water rises and falls in the top cloudy layers, causing thunderstorm-like activity just next to dramatically dry spots. Ganymede is the first moon in the solar system known to have its own magnetic field. Callisto's covering of craters is layered with a fine dust. Io's surface has been changing since the Voyagers saw it in 1979. And scientists have now seen evidence that an ocean has existed in recent geologic history under Europa's crust of ice.

Originally scheduled to end its exploration on December 7, 1997, NASA and Congress have approved the extension of Galileo' studies through the last day of 1999, in three phases each with tightly focused objectives: the Europa Campaign ("Ice"), Perijove Reduction/Jupiter Water Study/Io Torus Passages ("Water"), and the Io Campaign ("Fire").

Europa Campaign. For the first eight orbits, spanning more than a year, Galileo will continue to search for further evidence of an ocean beneath the icy surface of the intriguing moon Europa, and determine if it sloshes still today. Scientists will scan the surface for spewing, active ice volcanoes and other direct evidence. They'll count craters which will help date the youth of the moon's smooth surface. They'll peek at Europa's layered interior by measuring the pull of its gravity, and look for variation in the thickness of the ice shell and in the depth of the ocean. A flowing, salty subsurface ocean can generate a magnetic field, so scientists will try to determine if the magnetic signals nearest Europa are generated within. Galileo will get detailed images and atmospheric data from around the globe, including Europa's polar regions, from closest approach heights ranging from 200 to 3600 km. With three times better resolution than in the Prime Mission, some planned images will show details as small as 6 meters (the size of a truck!). Heights of the relatively flat surface features will be determined from stereo imagery, and the distribution and composition of contaminants can be mapped as finely as 10 kilometers.

Perijove Reduction/Jupiter Water Study/Io Torus Passages. "Perijove Reduction" isn't some kind of fad diet, or a way to shrink the national debt - it's what we need to do to get the spacecraft in an orbit that is close enough to Jupiter to fly near Io. For six months in mid-1999, Galileo will use the gravitational pull of Callisto in four successive orbits, along with thruster burns for fine tuning, to halve the orbit's closest distance to Jupiter (called "perijove"). From the closest distances since Arrival Day, peering at Jupiter's atmosphere will reveal wind and storm pattern details, including the billowing thunderstorms that grow to heights several times those we have on Earth. Water circulates vertically in Jupiter's top layers, leaving large areas drier than the Sahara desert, and others drenched like the tropics. Mapping the distribution of water and its role in Jupiter's weather can also help us understand Earth's more fast-paced weather changes. Once each orbit, during this passage from "ice" to "fire", Galileo will shoot through the Io torus, a donut-shaped cloud of charged particles that ring the orbit of Io, and map the density of sulfur which streams from Io's spewing volcanoes and sodium and potassium that gets "sand-blasted" off the surface by sweeping particles caught in Jupiter's rotating magnetic field. Callisto will be studied very minimally.

Io Campaign. The closest Galilean moon to Jupiter, Io, is the most active body in the solar system, sizzling with dozens of molten sulfur and silicate volcanoes resulting from 100 meter high tides in its otherwise solid surface. But the close-up picture of Io's forbidding environment remains a mystery. Galileo's final two orbits in GEM will feature close flyovers from 611 kilometers, then 300, kilometers away. You might guess that scientists are trying to keep us all in suspense, waiting until the last part of the mission to glimpse fiery Io with breathtaking details (as small as 6 meters) in kamikaze style! And suspense will indeed be high as Galileo flies right over Pillan Patera's active plume of frozen sulfur. Waiting to explore Io until the end of the mission minimizes changes in perijove, leaving more time and resources for science studies. It also lessens the exposure of the spacecraft to Jupiter's intense radiation, which grows in intensity the closer to the giant we come, which in the vicinity of Io is strong enough to kill a human. Galileo has been exposed to different levels of radiation while it orbits Jupiter, and is expected to continue operating through the intense exposure of the Io campaign. However, it will be exposed to enough radiation to pepper the camera's light detector with blinding hits to many pixels, and potentially cause the computer's bits to flip in random ways, causing Galileo to "safe" itself until further commands are received from the ground. (It's hard to think with your bits flipped)!

Although engineers predict that through GEM, Galileo should have ample power from its radioisotope thermoelectric generators to power the spacecraft and its instruments, and plenty of propellant for its thrusters, the mission's essential tape recorder has already surpassed its design limit for stops and restarts. If it fails beyond repair, Galileo's on-board computer will be loaded with a program that allows the instruments to take and transmit a very limited amount of data in real-time, significantly reducing the mission's scope.

In keeping with NASA's vision of lower-cost space exploration, GEM's design takes advantage of an already orbiting spacecraft to perform a tightly-focused, lower-cost, higher-risk mission. To achieve a cost of $15 million per year, the resources used by the spacecraft and ground operations have been trimmed to a minimum. 20% of the original personnel will operate Galileo and analyze its reduced amount of data. Engineering and science teams have automated and streamlined operational processes and software. When Galileo passes closest to Jupiter and the target moon for each orbit, only two days of data will be taken (versus seven in the Prime Mission). In GEM, only minimal data on Jupiter's magnetic environment will be gathered while data is played back during the rest of the orbit. Only commands to Galileo which are prepared in advance are allowed, turns of the spacecraft are kept to a minimum, and Galileo's health will be monitored with the lowest possible number of bits to allow maximum return of science data. The GEM team will not contain expertise to deal with unexpected problems, so experts who've moved on to other jobs will be brought back in as a tiger team to assess serious problems and make recommendations. Costly repairs may be deemed not worthwhile to make. After GEM is completed, Galileo will no longer return science data, but will keep slicing through the intense radiation near Io's orbit, and regularly report on its health until it is silenced by radiation damage.

During the GEM mission of ice, water, and fire, Galileo will help pave the way for new investigations to these Jovian worlds of extremes, possibly confirming that an ocean presently exists on Europa, and locating some areas where the ice is thinnest. This big step supports possible future Europa orbiting or ice boring missions looking into a key question for the 21st century - is there life on Europa?

You can follow Galileo through its journey on the internet at http://www.jpl.nasa.gov/galileo.