In a recent article by Duncan Forgan posted on Discover Magazine’s website and IFLScience, claims were made about Earth having a second moon. Now, if you’re like this writer, you might initially be curious about that concept, perhaps asking that if there’s a second moon, why don’t we see it sitting by our regular one each night?
The thing is, moons don’t have to be able to be seen by the naked eye. Moons are simply defined as an astronomical object that qualifies as a natural satellite to its planet.
Forgan’s article claims that the astronomical object known as 3753 Cruithne is our planet’s second moon. Cruithne is about three miles across with a gravity of 67P, which is considered very weak — walking too quickly on its surface would send you floating off solid ground. This so-called moon is a firmly mid-sized non-planetary body.
This size and gravity doesn’t disqualify Cruithne as a moon. However, according to an article posted on Earthysky last July, Cruithne is not a true satellite, but a quasi-satellite for Earth — in fact, one of many similar astronomical bodies. This classification relies solely on the objects orbit.
Cruithne does orbit the sun at the same rate as the Earth, however, it doesn’t orbit the Earth as our actual moon does. Instead, astronomers discovered in 1997 that it runs around the inner solar system in what’s known as a “horseshoe” orbit.
This orbit path is unique as the object moves toward its planet, turns round and moves away. After moving away, it approaches the planet from the other side and it turns round and moves away again. Cruithne is special as it makes it’s horshoe orbit very sloppily, taking nearly 800 years to make a ring around Earth.
Astronomers have created computer models that show Cruithne may spend another 5,000 years in its current orbit, then enter a true orbit around Earth and finally qualifying as a real second moon for our planet. However, this wouldn’t last too long as these same astronomers estimate that 3,000 years later Cruithne would be flung back into its old orbit around the sun.
So no, Earth doesn’t really have a second moon, at least not for now. What we do have is a pretty cool pseudo-moon that shows us a unique characteristic of certain astronomical bodies. It’s given astronomers new information on orbiters and has plenty of other knowledge left to share, such as forensic evidence regarding how planets were assembled. Currently, scientists are hoping that its strange orbit will give insight on how the solar system evolves under gravity.