Galileo project

The Galileo project was an ambitious American robotic space program. Its primary goal was to study Jupiter and its many moons. It also examined several other bodies within our Solar System.

The mission was named after Galileo Galilei. He was the Italian astronomer who first observed Jupiter through a telescope in 1610. The spacecraft was composed of two distinct parts. One part was an orbiter designed to circle the planet. The other part was an atmospheric entry probe. This probe was designed to dive directly into Jupiter's clouds.

To reach the giant planet, the spacecraft followed a complex path. It was delivered into Earth orbit on October 18, 1989, by the STS-34 mission. To gain enough speed, it used gravity assist flybys. The spacecraft flew past Venus and Earth to use their gravity for momentum.

This journey allowed it to arrive at Jupiter on December 7, 1995. Upon arrival, it became the first spacecraft to ever orbit Jupiter. Once in orbit, the orbiter acted as a relay station. It received data sent from the atmospheric entry probe. The orbiter then transmitted that information back to scientists on Earth.

Developing the mission required overcoming many technical hurdles. NASA's Jet Propulsion Laboratory (JPL) served as the lead center. In the late 1970s, engineers realized they needed better technology for the probe. Specifically, they needed a heat shield that could survive Jupiter's atmosphere. Testing facilities for such a shield were not available until 1980. Scientists also had to account for intense radiation. Earlier missions like Pioneer 10 and Pioneer 11 provided data on these effects.

The orbiter used a Mariner-style attitude control system. This system used gyroscopes and nitrogen jet thrusters to maintain its position. This allowed the spacecraft to take very high-resolution images of its surroundings.

Planning the launch was also a difficult process. NASA originally intended to use the Space Shuttle to launch the project. They planned to use an Interim Upper Stage (IUS) to push the craft toward Jupiter. However, the orbiter was very heavy. A two-stage IUS was not powerful enough for the mission. NASA considered a three-stage IUS, but the cost was too high. The cost for the three-stage version was $179 million. This was $100 million over the original budget.

Eventually, NASA decided to use a Centaur G Prime upper stage instead. This change ensured the mission could proceed on a reasonable schedule.

Galileo achieved many scientific firsts during its mission. It performed the first asteroid flyby of 951 Gaspra. During this encounter, it also discovered Dactyl, the first known asteroid moon.

In 1994, the spacecraft observed Comet Shoemaker–Levy 9 colliding with Jupiter.

It also mapped the structure of Jupiter's magnetosphere. This is the region of space dominated by the planet's magnetic field.

Additionally, Galileo discovered that Jupiter has a faint ring system. This ring is made of dust from impacts on the four small inner moons.

One of the most significant discoveries involved the moons of Jupiter. Galileo studied the composition of the atmosphere and the ammonia clouds.

The spacecraft observed volcanic activity on the moon Io.

It also found evidence of exospheres around Europa, Ganymede, and Callisto. Most importantly, the data supported the theory of a liquid ocean under the icy surface of Europa.

There were also indications of similar saltwater layers under Ganymede and Callisto.

Ganymede was even shown to possess its own magnetic field.

The mission lasted much longer than its original design. The primary mission ended on December 7, 1997. This was followed by the Galileo Europa Mission (GEM). This extended mission ran until December 31, 1999. Later, a second extension called the Galileo Millennium Mission (GMM) was authorized. By the end, the spacecraft had absorbed three times the radiation it was built to withstand.

On September 20, 2003, the mission was finally terminated. NASA sent the orbiter into Jupiter's atmosphere at high speed. This was done to prevent the spacecraft from contaminating the moons with Earth bacteria.