The durable U.S. spacecraft Galileo has performed its last scientific investigation at Jupiter and has begun its 35th and final orbit around the giant gas planet. During the past seven years, Galileo has made spectacular discoveries, including hints that some of its moons may have liquid environments hospitable to life. It is headed for its demise in a crash landing on Jupiter.
Do not cry for Galileo. Although it will plunge to its destruction late next year, it has lasted five years and 24 orbits beyond its expected two-year, 11-orbit mission.
At the end of its next to last orbit on Tuesday, November 5, Galileo visited three intriguing features of Jupiter's neighborhood a small, red, egg-shaped moon named Amalthea; a dusty ring; and the inner region of Jupiter's high energy magnetic environment.
Why study Jupiter so closely?
"One can look at the Jupiter system as a mini-solar system," said the manager of the Galileo project, Eilene Theilig.
She said the planet holds clues to the formation of our solar system. "Just like our solar system has the denser planets formed closer to the sun, on Jupiter the denser moons formed closer to Jupiter. But we do not have a good estimate of the material that may have condensed out of that nebula in close to Jupiter. It is important to understand the composition of this material that can tell us more about the dynamics about how these systems form and how the moons form," she said.
As a result, Galileo came the closest it ever has to Jupiter, inspecting one of the four small inner moons, Amalthea. At just 160 kilometers from the moon and 7,200 kilometers from the planet, its instruments measured Amalthea's gravity, which will help scientists calculate its mass and density to provide clues to its composition.
The fly-by brought the spacecraft through Jupiter's wispy inner ring, a unique opportunity to study a planetary ring from inside. It detected the size and movement of dust particles that make it up.
Galileo also measured the fierce radiation around Jupiter. NASA planetary scientist Claudia Alexander said the radiation's intensity threatened the spacecraft's electronics, which during the mission have endured four times the dose they were designed to withstand. "The high energies near Jupiter are so monumental that the spacecraft was never designed to handle this kind of energy. So we will be lucky and grateful to get any kind of measurements at all," she said.
If they do, Ms. Alexander saidd the data from the close encounter will be useful. "It is speculated that in many respects Jupiter resembles a star - a planet that is massive but did not quite make it to be the size of a star. As we get closer and closer to Jupiter, many of the processes begin to resemble what we think happens close to a star. It may be that we will make the kinds of measurements that will enable us to understand a little bit more about how stars behave," she said.
What scientists learn from Galileo's last scientific rounds will add to the impressive data the craft has already returned. During its six-year journey to Jupiter from a U.S. space shuttle, it became the first spacecraft to fly by an asteroid and discover the moon of an asteroid.
After arriving at the massive planet, it found that Jupiter is a stormy, windy place with lightning strikes up to 10,000 times more powerful than on Earth. It also documented extensive volcanic activity on the moon Io and found evidence for subsurface salt water on three other moons - Europa, Ganymede, and Callisto.
Now, Galileo is heading for oblivion. It will be instructed to dive and burn up in Jupiter's atmosphere next September. Project manager Eilene Theilig said NASA wants to control the plunge rather than taking the chance that the spacecraft contaminates Europa with terrestrial organisms.
"We wanted to protect this because it is a prime site for future exploration searching for life in the solar system," she said.