The addition of Callirrhoe to the Pasiphae group of moons adds to the controversy over whether all the moons in this group were created from a single large asteroid moving into the orbit of Jupiter and breaking up into a collection of smaller moons.
The Discovery of Callirrhoe
The discovery of Callirrhoe is credited to a large team of astronomers working together as part of the Spacewatch Program at the University of Arizona. The team credited with the discovery included Jim V. Scotti, Timothy B. Spahr, Robert S. McMillan, Jeffrey A. Larsen, Joe Montani, Ariana E. Gleason, and Tom Gehrels on October 19, 1999.
Classification as an asteroid
The discovery of Callirrhoe was controversial as it was initially classified as an asteroid when it was discovered and given the name, 1999 UX18. Over the following years, the former asteroid was the subject of debate with the Minor Planet Center at the Smithsonian Astrophysical Observatory eventually calculating the object was a moon of Jupiter.
A member of the Pasiphae Group
The Pasiphae group of moons was originally thought to have been born from a single large asteroid, Pasiphae being dragged into the gravitational orbit of Jupiter. Eventually, the large asteroid collided with other objects in the orbit of Jupiter and splintered into many smaller moons. This theory is now disputed because of the fact the different moons are colored differently prompting many astronomers to believe these are from different asteroids entering the orbit of Jupiter.
Callirrhoe is a faint object
One of the reasons why the classification of Callirrhoe has proven so difficult over the years is because of how difficult it is to see the moons of Jupiter even with a telescope. One of the faintest objects in the galaxy is the dwarf planet, Eris but Callirrhoe is even fainter in the night sky adding to the difficulties in providing a positive classification.
The orbit of Callirrhoe
The orbit of Callirrhoe is a deep one sitting around 15 million miles from the surface of Jupiter. Like the other members of the Pasiphae group of moons, Callirrhoe spins in a retrograde orbit, meaning it moves in the opposite direction to the spinning of Jupiter. It takes around 759 Earth days for Callirrhoe to complete a single orbit on Jupiter
A Small Moon
Although it is not the smallest moon or satellite circling Jupiter, Callirrhoe is a small moon with a radius of just 2.7 miles. The moon is around 2.6 billion times smaller than the planet Jupiter it orbits.
The Shape of Callirrhoe
There is little known about the exact shape of Callirrhie because of the limited number of images that have so far been captured by the orbiters and telescopes created on Earth. What we do know is that Callirrhoe is not a classical spherical shape as the moons of the Pasiphae group are too small to be formed into the shape by the gravitational pull of Jupiter.
The Naming of Callirrhoe
A moon moving in a retrograde orbit is given a name ending with the letter e. The tradition for the moons of Jupiter is to name them after members of classical Greek mythology leading to S 1999/J1 being given the name, Callirrhoe. In Greek mythology, Callirrhoe was the daughter of the River God, Achelous.