Today’s factismal: Twelve years ago, the Galileo probe made the closest approach ever to Jupiter’s moon Io; it was just 112 miles away from the surface of the moon!
The four largest moons of Jupiter (Io, Europa, Ganymede, and Callisto) have a special place in the minds of all planetologists. They were the first new planets to be discovered in modern times (they were originally called planets; the astronomers only started calling them moons when there were too many to count on the astronomers’ fingers). They helped establish the validity of the Copernican model of the universe and destroy the Earth-centered one. And they helped to establish Galileo’s reputation as a scientist, which then helped change science from a descriptive endeavor to an experimental one.
Though the four Galilean moons are easily visible with a pair of binoculars, it wasn’t until Voyager 1 and 2 passed by that we got our first good look at them. And perhaps the most surprising of the four was Io. A small, rocky world, it was rapidly revealed to be the most volcanically active body in the Solar System. Covered by lava flows and sulfur frost, it was unlike any of the other moons we’ve seen. Obviously, it needed to be explored in more detail.
And so we sent out another probe. Launched in 1989 and named Galileo for the discoverer of the four moons, its primary mission was to map Jupiter’s moons and to discover the secrets of Jupiter’s atmosphere using a secondary probe. And, from the time it arrived at Jupiter in 1995 until its final plunge into Jupiter’s atmosphere in 2003, it returned over 14,000 images of Jupiter and its moons that have forever changed the way that we see planetary formation.
If you’d like to learn more about the Galileo probe (or any other planetary probe), then why not join the Association of Lunar and Planetary Observers? They’ve got lots of information on every planet (even Earth), along with several citizen science projects that you can get involved with!