Galileo Starts Two-Year Europa Extended Mission
From The "JPL Universe"
January 9, 1998
After yielding a rich harvest of science results in 1997, NASA's Galileo spacecraft wrapped up its primary mission on Dec. 7 and began a two-year follow-on journey, known as the Galileo Europa mission.
The transition from primary to extended mission brought a change in management. Bob Mitchell, who served as mission director for the last year of Galileo's primary mission, was appointed project manager for the Galileo Europa Mission, taking over from Bill O'Neil, who served as Galileo project manager for the flight to Jupiter and the two-year primary mission at Jupiter. O'Neil will serve as a consultant on the senior staff of JPL's Telecommunications and Mission Operations Directorate pending his next assignment at the Laboratory.
"A great bounty of Jupiter system science has been obtained and the continuing study of these data will surely add many more important discoveries," O'Neil said of the mission. "I've been involved with the Galileo mission since its beginning in 1977, and have been at the helm since 1990 for the flight to Jupiter, the first-ever outer planet entry and orbit insertion, and throughout the two-year primary mission tour of the Jovian system. I feel extraordinarily fortunate to have had this priceless, truly unique experience. But it is time for new challenges. I am delighted to turn the reins over to Bob Mitchell. Having worked closely with Bob for more than 25 years, I know that he will do a superb job leading the team."
"Accomplishing what we have set out to do with such a small team is going to be a real challenge," Mitchell said. "But we have an excellent team in place, and I'm looking forward to it."
The first flyby of the Galileo Europa Mission took place on Dec. 16, when the spacecraft swooped past Europa at an altitude of 200 kilometers (124 miles), making it the closest Europa encounter of the entire Galileo mission. The extended mission will include seven more Europa flybys, four encounters with Callisto, and one or two close flybys of Io, depending on spacecraft health.
Pictures and other data returned by Galileo during its primary mission continued to fascinate the public in 1997. New images of Europa revealed evidence of ice flows, a complex network of crisscrossed ridges, chunky ice rafts and relatively smooth, crater-free patches. The areas of rafting added to the mounting evidence of liquid oceans under Europa's icy crust at some point in its history. The presence of oceans would increase the odds that life could have existed there.
"We're intrigued by these blocks of ice, similar to those seen on Earth's polar seas during springtime thaws," said Dr. Ronald Greeley, an Arizona State University geologist and Galileo imaging team member. "The size and geometry of these features lead us to believe there was a thin icy layer covering water or slushy ice, and that some motion caused these crustal plates to break up."
Galileo investigators discovered a hydrogen atmosphere around Ganymede and both hydrogen and carbon dioxide in an atmosphere on Callisto. The spacecraft also found that Europa has an ionosphere, produced by ionization of its tenuous oxygen atmosphere. This finding came after a series of six occultation experiments, when the radio signal was interrupted while Europa was positioned between Galileo and Earth. These experiments were performed during Galileo's encounters with Europa in December 1996 and February 1997.
"While this discovery does not relate to the question of possible life on Europa, it does show us there are complex surface and atmospheric processes occurring there, and Europa is not just some dead hunk of material," said lead investigator Dr. Arvydas Kliore of JPL.
Galileo also transmitted new evidence of numerous high- temperature volcanoes on Jupiter's volatile moon, Io. One recent discovery revealed a new dark spot the size of Arizona on Io. The visible change occurred between Galileo's seventh and tenth orbits of Jupiter, and produced a dark area about 400 kilometers (249 miles) in diameter, surrounding a volcanic center named Pillan Patera.
"This is the largest surface change on Io observed by Galileo during its entire two-year tour of the Jovian system," said Galileo imaging team member Dr. Alfred McEwen, a University of Arizona research scientist.
Other significant results from Galileo this past year included the confirmation of the spacecraft's 1996 discovery of a magnetic field and magnetosphere on Ganymede, and the discoveries that all the Galilean moons except Callisto have a core. Callisto did show signs of surface erosion and blanketing at fine scale.
"Before Galileo, we could only make educated guesses about the structure of the Jovian moons," said Dr. John Anderson, a JPL planetary scientist. "Now, with the help of the spacecraft, we can measure the gravitational fields of the satellites and determine their interior structure and density. We can determine how the matter is distributed inside."
Galileo's instruments also detected some interesting, Earth- like phenomena on Jupiter, including the presence of lightning and aurora. Recent findings confirm the suspicion that the thunderstorms provide energy for the low pressure centers on Jupiter, which in turn feed the Great Red Spot, white ovals and other large storms.
In 1997, Galileo also found clusters of volcanic vents and hot spots in greatest concentration on Io in the areas closest and farthest from Jupiter. Other discoveries include evidence of salt and carbon dioxide in Europa's icy crust and landslides on Callisto.
While the spacecraft was busy making scientific history, Galileo team members made history of their own in January. O'Neil, Johnson, and others met with Pope John Paul II during a trip to Italy.
"None of us ever anticipated that Project Galileo would result in a papal audience, "O'Neil said. "The Pope seemed very interested in learning about Galileo results. He encouraged continuing exploration of the universe."
O'Neil and Johnson received honorary doctorates from the University of Padova and attended the Three Galileos Conference, a meeting designed to honor Galileo the man, Galileo the mission, and Galileo the new national telescope of Italy.
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