Earth’s quasi-moons, mini-moons, and ghost moons

Could quasi-moons or mini-moons hit Earth?

Scientists used to think certain quasi-moons had a significant chance of impacting Earth, but thankfully, that’s no longer the case. Since Earth’s current quasi-moons are around 10–300 meters (30–1000 feet) in size, some are large enough to cause massive damage to a region if they were to hit us. But there is no risk of a known quasi-moon colliding with Earth in the foreseeable future. The orbits of quasi-moons are predictable enough that astronomers should be able to see an impact coming well before it happens.

As for mini-moons, it’s not rare for them to hit Earth. About 1% of all mini-moons eventually do, but most are small, so they burn up in our planet’s atmosphere. A very dangerously large mini-moon, say 1 kilometer (0.6 mi) in diameter, is expected to hit Earth once every billion years at most.

What can these not-quite-moons tell us?

Quasi-moons and mini-moons are pieces of our neighborhood in space, and they carry information about where they come from. They might originate in the main asteroid belt, from impacts on the Moon, or from the break-up of larger objects on similar orbits — scientists don’t know for sure. Answering that question, and finding out what these almost-moons are made of, can help researchers learn more about asteroids and how they threaten Earth. 

Since quasi-moons orbit close to Earth and are often quite small, they would also be easier to visit and redirect than most other asteroids. This makes them ideal targets for missions like China’s Tianwen-2. Scheduled for launch in 2025, Tianwen-2 will visit the quasi-moon 469219 Kamoʻoalewa, collect samples, and explode part of the asteroid’s surface to see what lies beneath. In the future, quasi-moons around other worlds could even act as science platforms, offering a vantage point for monitoring whatever body they loop around.