On Sunday, September 21st, after a 14-year journey of about 2.8 billion miles, the Galileo spacecraft crashed into the planet Jupiter.
Galileo's instruments transmitted data up to its entry into Jupiter's shadow. Minutes later, the spacecraft was vaporized as it entered the planet's blazing hot (14,000 K) atmosphere.
|Remarkable frozen texture on Jupiter's moon, Europa.
The spacecraft was sent on a path to deliberately collide with Jupiter so that it wouldn't accidentally hit Europa, one of Jupiter's large moons. Europa has a subsurface water ocean, and thus the potential for life. Galileo, having come from bacteria-rich Earth, was a possible source of contamination. Galileo's waning supply of propellant meant that controlling the spacecraft was fast becoming impossible.
The Galileo project originally was approved in 1977, with a 1982 launch date. But problems caused the launch to be delayed year after year. Finally, Galileo was launched into near-Earth orbit in 1989.
"That was a great moment in my life, when we finally got it off the ground," says Jim Erickson, Mars Exploration Rover mission manager and former Galileo project manager.
Using gravity-assist flybys of Venus and the Earth, Galileo was sent like a billiard ball around the solar system, finally arriving in Jupiter's orbit in 1995. Galileo ended up orbiting the Jupiter system 34 times.
Jupiter has 61 known moons, and the largest of these are Ganymede, Callisto, Europa, and Io. The magnetic signature of Europa indicates there is a vast global ocean below the moon's icy surface.
"Europa is the star of the show," says Michael Belton, team leader of the Galileo Solid State Imaging Team. Before Galileo, Europa was just a fuzzy ball with indistinct markings. The images provided by Galileo showed Europa has a complex topography of ice rafting, and icy plains scarred with ridges and cracks.
|Giant Red Spot in background, one of Jupiter's moon in foreground with eclipse shadow cast on gas giant Credit: NASA/JPL Cassini
"[Europa] is more than a moon in many of our minds," says Belton. "What Galileo has done, by proving that there is a deep liquid briny ocean underneath the ice cap on Europa, is basically transform it from a mere moon into a prime candidate as a habitat for extraterrestrial life."
Galileo discovered that Callisto has a geologically active surface, with ices that evaporate and produce deep layers of debris. Like Europa, the magnetic signatures of Callisto indicate that there is a briny ocean deep down below the surface.
"Callisto has also been transformed from an uninteresting dead world to something that is rather exciting from a geological point of view," says Belton.
Ganymede is Jupiter's largest moon, and it's also the largest moon in the solar system. Thanks to Galileo, we now know that Ganymede has an ocean below the surface, as well as a substantial magnetosphere.
"Every single body in the solar system that has a magnetic field forms one of these magnetic bubbles - a magnetosphere - and trains charged particles within it, which in turn, react against the body and control part of its evolution," says Don Williams, principal investigator of the Galileo heavy ion counter. "The reason why it was such an unexpected result at Ganymede is that no one expected that a chunk of ice and silicon would be able to generate a powerful magnetic field."
Williams says that Ganymede's magnetosphere is even larger than the planet Mercury's. Ganymede's magnetosphere is caught within Jupiter's magnetosphere, which is so large that it stretches all the way to Saturn.
|The icy cracks of Jupiter's moon Europa continue to intrigue astrobiologists. The white sheen is likely frost and the moon's heat source is a combination of an underground ocean and tidal heating under the strong gravitational pull of Jupiter. Credit: Galileo Project, JPL, NASA
At the moon Io, Galileo discovered an unusual form of volcanism. Io is pockmarked with spewing volcanoes and covered in yellow sulfur.
"When we went there, because of the sulfur on the surface, we were talking about sulfur volcanism and we were also talking about volcanism like it is on the Earth," says Belton. "What we found is that the volcanism on Io is totally different to either of those two cases. It's much hotter, temperatures are up to about 2,000 degrees, and like the volcanism that used to exist on the Earth a billion years ago."
Williams says they also discovered very intense electron beams that are streaming up and down field lines that connect Io to Jupiter. These beams are so powerful they could supply the entire energy needs of Earth.
In addition to studying the large moons, Galileo provided a great deal of new information about Jupiter. A probe from the spacecraft was sent down into Jupiter's atmosphere, giving scientists direct measurements of an outer planet's atmospheric chemistry for the first time. We now know that Jupiter experiences powerful lightning storms, and has at least four different cloud layers, consisting of water, ammonia, hydrogen sulfide, and hydrocarbons.
In July 1994, Galileo was in the right place at the right time to observe comet Shoemaker-Levy 9 fly into Jupiter.
"Shoemaker-Levy 9 impacted Jupiter on the backside, and we couldn't see it on Earth," says Erickson. "But Galileo was positioned to see the actual impact on the backside and provide images and data concerning exactly the magnitude of those impacts."
Galileo also took photos of the ring of material that encircles Jupiter. The photos showed that the ring system is young and dynamic, with a structure that arises from the different masses and positions of the small moons.
|Fragments of Comet P/Shoemaker-Levy 9 colliding with Jupiter (July 16-24, 1994).
The gossamer ring is associated with the small moon Almathea. Almathea has a low density, suggesting that the moon may be a loosely conglomerated body with pieces nearby. According to Claudia Alexander, Galileo project manager at NASA's Jet Propulsion Laboratory, on a previous flyby of Almathea, Galileo's star scanner discovered some bright objects near the moon.
As Galileo passed by Almathea on its last journey to Jupiter, measurements were taken to try to confirm the presence of more of the bright objects.
"That might help us know if there's a ring of material at Almathea's orbit, going around Jupiter," says Alexander.
Galileo's journey through the cosmos has been far from trouble-free. In addition to the extremely late launch, Galileo scientists faced a crisis when they tried to open the main communications antenna. This antenna was needed so the spacecraft could transmit data back to Earth. But the antenna wouldn't deploy.
By reconfiguring the software and using a different antenna, the scientists figured out a way for Galileo to send back thousands of high-resolution images and other data back to Earth. Although Galileo could not study Jupiter as intensively as was hoped due to the main antenna failure, the technical glitch also gave scientists an opportunity to see if it was possible to make repairs from a distance of half a billion miles away.
After all the years of hard work and problem solving, many who worked on the Galileo project were sad to see it go.
"I almost think of it as a favorite car that you tinker with, and we're not going to be able to have fun with it anymore," says Alexander.
Galileo entered Jupiter's atmosphere about 1/4 degree south of the equator, traveling at a speed of 48.2 kilometers per second (nearly 108,000 miles per hour). At that speed, Galileo could have traveled from Los Angeles to New York City in 82 seconds.
Scientists were worried that they wouldn't be able to recover any data during Galileo's final hours, since the spacecraft would be passing through an especially high-radiation region as it approached Jupiter. But Galileo sent signals back to Earth until it passed into Jupiter's shadow.
NASA already has begun work on a new project to take us back to Jupiter's neighborhood: the Jupiter Icy Moons Orbiter (JIMO). This new mission will send a spacecraft to orbit the moons Ganymede, Callisto, and Europa.
Image Credit: JPL
"The Jupiter Icy Moons Orbiter will have 100 times more power than the Galileo mission had, and thus be able to actually orbit these moons, and investigate the possibility of life on them, and characterize them to an extent unheard of before," says Colleen Hartman, director of NASA's Solar System Exploration Division.
JIMO will launch sometime between 2012 and 2014, and will arrive in Jupiter orbit by the end of that decade. By orbiting the moons, JIMO will be able to determine the thickness of the outer ice layers, and see whether there are upwellings of salt water onto their surfaces.
"The Jupiter Icy Moon Orbiter will be so much more than Galileo, in that it will actually be able to spend months at each of these three moons, going from moon to moon with instruments that are much more capable than Galileo brought," says Hartman.