The exploration of the outer solar system can be divided into two eras: before and after the Galileo Mission. While probes like Pioneer and Voyager only performed brief fly-bys, Galileo was designed to stay. As the first artificial satellite of Jupiter, the mission delivered data for over eight years that fundamentally changed our understanding of gas giants and their moons.
Named after the Italian polymath Galileo Galilei, who discovered Jupiter’s four largest moons in 1610, this NASA mission was a milestone in spaceflight history – marked by technical challenges, brilliant engineering solutions, and scientific sensations.
The Long Road to the Gas Giant (1989–1995)
The Galileo Mission launched on October 18, 1989, aboard the Space Shuttle Atlantis (Mission STS-34). However, the path to Jupiter was not a straight line. To achieve the necessary velocity, Galileo utilized a complex trajectory known as the VEEGA maneuver (Venus-Earth-Earth Gravity Assist).
The probe gained momentum through targeted fly-bys:
- Once past Venus (February 1990)
- Twice past Earth (December 1990 and 1992)
Encounters En Route: Asteroids and a Comet
Even before reaching its destination, Galileo made history. It provided the very first close-up images of asteroids: 951 Gaspra and 243 Ida. During the fly-by of Ida, the probe discovered a tiny companion – Dactyl, the first confirmed moon of an asteroid.
Another historic event was the impact of Comet Shoemaker-Levy 9 on Jupiter in July 1994. While Earth-based telescopes could not see the impact sites directly (as they occurred on the far side of the planet relative to Earth), Galileo had the perfect vantage point to observe the massive explosions in the Jovian atmosphere directly.
Technical Drama: The Antenna Problem
No mission description is complete without mentioning the probe’s greatest crisis. The umbrella-like High-Gain Antenna, intended for rapid data transmission to Earth, failed to fully deploy. It was jammed.
This forced NASA engineers into one of the greatest rescue operations in spaceflight history. They reprogrammed the onboard computers from millions of kilometers away to compress data more efficiently and utilized the much weaker Low-Gain Antenna. Despite this massive limitation, Galileo was able to achieve approximately 70% of its original scientific objectives.
Arrival and the Atmospheric Probe
On December 7, 1995, Galileo reached the Jovian system. But before the orbiter began its primary work, it released a 340-kilogram atmospheric probe.
This capsule plunged into Jupiter’s cloud deck at a speed of 47,000 km/h (about 29,000 mph). It withstood deceleration forces of up to 230 g and transmitted data for 57 minutes while gliding down on a parachute before being destroyed by the immense pressure and heat. The surprising result: The probe measured less water and fewer lightning strikes than expected—likely because it happened to enter an unusually dry “hot spot” in the atmosphere.
Scientific Yield: A New View of the Moons
The orbiter itself circled Jupiter in elliptical orbits that brought it repeatedly close to the Galilean moons. This is where the most spectacular discoveries were made:
1. Europa: An Ocean Beneath the Ice
Perhaps the most consequential discovery of the mission concerned Europa. Magnetic field data and images of the cracked icy surface provided strong evidence that a global ocean of liquid saltwater exists beneath the ice crust. This instantly made Europa one of the most important targets in the search for extraterrestrial life (astrobiology). The probe Europa Clipper is currently on its way to Jupiter.
2. Io: Pure Volcanism
On Io, Galileo documented the constant reshaping of the surface. The probe observed active plumes (volcanic fountains) and lava lakes hotter than anything known on Earth. This confirmed that tidal forces from Jupiter generate enormous heat within the moon’s interior.
3. Ganymede: A Magnetic Moon
At Ganymede, Galileo became the first probe to detect a distinct magnetic field around a moon—an absolute anomaly. This suggests the presence of a liquid iron core deep inside.
4. Callisto: The Witness of the Past
Callisto revealed itself as an ancient celestial body with no significant internal geological activity, its surface perfectly preserving the impact history of the early solar system.
The End of the Mission (2003)
After nearly 14 years in space and 35 orbits around Jupiter, fuel reserves were running low. Since the probe would no longer be controllable, there was a risk it could crash into Europa and contaminate it with terrestrial bacteria.
To protect the potential ocean on Europa, NASA decided to destroy Galileo in a controlled manner. On September 21, 2003, the probe was steered into Jupiter’s atmosphere, where it vaporized.
Conclusion and Legacy
The Galileo Mission paved the way for modern exploration of the giant planets. It proved that moons can be geologically active and complex worlds. Its data is still being used today and formed the foundation for current missions like NASA’s Juno and the ESA mission JUICE (Jupiter Icy Moons Explorer), which will return to Jupiter in the 2030s.