Jupiter's large moons - Ganymede, Callisto, Io and Europa - number four. But Saturn has just one large moon, Titan. Why? It may be the planets' magnetic fields.
For the first time, astronomers have direct evidence of a close pair of supermassive black holes in a galaxy. These black holes could merge in 100 years.
The most distant spacecraft from Earth is Voyager 1. Launched in 1977, Voyager 1 is a few billion miles farther from Earth than its sibling, Voyager 2.
Researchers in Europe are testing a prototype for walking robots on Mars or the moon. Robots like this would be faster and more autonomous than current rovers.
Juice reveals Comet 3I/ATLAS is highly active, with vast gas and dust clouds. It behaves like solar system comets and offers clues to its interstellar origins.
For the first time, Hubble observations reveal that a tiny comet has stopped spinning in one direction and has reversed to spin in the opposite direction.