2020 CD3 (another temporary moon for the Earth)

How many moons does the Earth have?

I've been watching QI for years. It's got to be one of the favourite shows for the "well, actually" crowd. One of the themes in the show is that Stephen (lately Sandi) asks a plain question, and one of the panel gives the answer that everyone thinks is right. This is a trap.

One? Wrong!

A while back, Stephen Fry talked about Cruithne, and claimed it was a moon of the Earth's. I wrote about this about a year back in one of my Facebook posts.

So two? Wrong!

The question has come up a number of times on the show, always with different answers. It's turned into a standing joke.

But - for the last few months, Earth has had another companion, and it's absolutely tiny. It doesn't have a name, apart from 2020 CD3, and appears to be a captured asteroid, of between two and four metres in size.

Because it hasn't been under observation for long, we really don't know when it showed up, but the orbital models indicate at least five passes that we might (at a stretch) call an "orbit". Have a look at this diagram - its path has been chaotic, and it's expected to wobble about the Earth until May before wandering off to find something more interesting to do.

Image: NASA

The physics of this is something that eludes me. I thought that if the satellite doesn't have the velocity (kinetic energy) to escape the Earth's gravitational well, it doesn't leave. Of course, it's not as simple as that. You have to factor in gravitational potential energy, which fluctuates a lot if the "orbit" is highly elliptical.

But... doesn't the decrease in potential energy balance the increase in kinetic energy when the satellite approach the larger body?

Or did I just break VCE Physics? Where's Kepler when you need him?

Because it's new, and so small, we're not really sure what it is. There's a suggestion going around that it might actually be an old lunar spacecraft or a rocket stage. After all, we've been throwing these guys out there for some time.