Subsatellite

A subsatellite, also known as a submoon, moonlet, or informally a moonmoon, is a "moon of a moon" or a hypothetical object that could orbit the moon of a planet.

Scientists think that subsatellites might be rare but possible parts of planetary systems, based on what we know about natural satellites in our Solar System. The large planets in our solar system have many natural satellites, and many stars far away have big planets too. One of these distant planets, called Kepler-1625b, might have a very large moon named Kepler-1625b I, which could, in theory, have its own small orbiting object.

However, apart from satellites we have sent to orbit our own moon temporarily, no actual subsatellite has ever been found in our solar system or beyond. This is because the strong pull of the planet would usually make such a system unstable over time.

Terminology

Scientists use different names for very small moons that orbit bigger moons. These names include "submoons" and "moon-moons." Some fun, informal names people have suggested are moonitos (similar to plutino), moonettes, and moooons.

Possible natural instances

Rhea

Scientists think there might be a ring around Saturn's moon Rhea. They thought these rings could let small moons stay in stable orbits around Rhea. But when the Cassini spacecraft looked closely, it didn't find any moons or big rings around Rhea. So, it's unlikely that Rhea has a ring system with moons.

Iapetus

Some people think Saturn's moon Iapetus might have had a small moon in the past. This idea helps explain why Iapetus has a strange ridge along its middle. A big crash long ago could have created this small moon. As Saturn's gravity changed Iapetus's spin, the small moon's orbit would have moved closer until it broke apart and fell onto Iapetus, forming the ridge.

Irregular moons of Saturn

Studies of Saturn's unusual moons suggest that one moon, Kiviuq, is very stretched out and might actually be two moons stuck together or orbiting each other. Other moons like Bestla, Erriapus, and Bebhionn might also be like this.

Artificial subsatellites

Historical

Many spacecraft have flown around the Moon since the first one in 1966 (Luna 10).

As of 2024^[update], no spacecraft has flown around any natural satellite except the Moon. In 1988, the Soviet Union tried to place two robotic probes around the Martian moon Phobos, but it did not work.

Current

The Lunar Reconnaissance Orbiter (LRO) was launched on June 18, 2009. It is a NASA spacecraft that orbits the Moon. It helps plan future trips to the Moon by finding safe landing spots and testing new technologies.

CAPSTONE was launched on June 28, 2022. It is made of small CubeSats and has been orbiting the Moon since November 14, 2022. CAPSTONE is checking if a special path will work for future missions.

Future planned artificial moon satellites

The JUICE spacecraft was launched in 2023. It will orbit Ganymede in 2034. It will be the first spacecraft to orbit a moon other than Earth.

The Lunar Gateway space station began building in April 2024. It will orbit the Moon in a special path to help with future trips to the Moon and beyond.