Earth’s Many Moons

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It’s almost Moon Day! July 20^th (appropriately enough, falling on a Monday, or Moon-day, and yes that’s actually where the word Monday comes from, this year) marks the day of the year that humans first stepped foot upon the Moon in 1969. So when people say Moon Day, they clearly intend to refer to one, specific moon, the Moon.

But that’s not Earth’s only moon, if you’re willing to get technical about things. And sweet Carl Sagan am I ready to get technical about space-related things. I mean, that’s sort of my whole deal. So in honor of Moon Day we’re going to celebrate Earth’s most famous moon, but we’re also going to take a look at some other moon-adjacent things that rarely get the spotlight.

Let’s unleash some luna-cy (sorry not sorry)!

Lovely Luna

Okay, obviously we can’t start a Moon Day post without talking about, you know, the Moon. For all of human history from the first time something vaguely human looked up and started noticing lights in the sky right up until 1610 (when Galileo found things going around Jupiter and freaked everyone out) it was the only moon we were aware of.

I’ve already gone into how we think our Moon came to be in a previous blog post, but tl;dr it was probably born from the destruction wrought by a massive impact between a baby proto-Earth and a Mars-sized object named Theia. That makes it a native moon, to use a term I’ve seen to describe outer solar system moons that formed around their planets, as distinguished from captured moons, which form elsewhere and get gravitationally snagged by their planets later.

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Our Moon is unusually large for a moon compared to its parent planet. There’s no other planet with such a moon so relatively large compared to itself (yes, I know about Pluto and Charon, but Pluto isn’t a planet, don’t come at me Pluto people!). Among solar system moons overall, it ranks fifth in size.

All of that means our Moon has quite an effect on us (see this post on how tides work for some of that). Plus it’s large in our sky, and undeniably lovely with its silver sheen reflecting the golden glory of the Sun. There’s a theory that without it we wouldn’t have wound up developing a proper space program (also without it, life would possibly have never developed on Earth, but ignore that technicality) because it gave us something obvious to strive to reach.

Anyway, our Moon is awesome. It’s our one, true natural satellite. But we’re not going to let that stop us from discussing some…alternative moons.

Quaintly Quasi

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Let’s talk about quasi-moons! This is cheating just a little bit, because quasi-moons are not technically moons given that they are gravitationally bound to the Sun rather than to Earth. But they look like they’re bound to Earth, so for the sake of this post we are going to count them.

Quasi-moons are hunks of space rock that share very specific, special kinds of orbital configurations with a larger object, say a planet. A quasi-moon takes the same amount of time to orbit the Sun as the large thing it’s co-orbital with (it doesn’t have to be a planet, which is why I’m being technical with the language. I did warn you about me getting technical, you can’t say I didn’t), but has a differently shaped, more elliptical orbit. That means it’s going around the Sun in the same time as the larger object, but on a path that takes it in closer to the Sun and back out farther from the Sun than the larger object.

Let’s say the larger object in question is Earth. To a person standing on Earth it looks like this rock is moving around Earth as it paces our planet around the Sun but appears to move in closer and then move back father out. It looks like it’s orbiting Earth, despite actually being gravitationally bound to the Sun. This why it’s a quasi-moon and not a moon-moon.

These specific orbital configurations often don’t last. Eventually the quasi-moon’s movements can carry it enough out of tune with Earth so that it no longer appears to be moving around our planet. That said, sometimes quasi-moons stick around for a very long time.

Earth’s Favorite Quasi-Moon

While Earth has at least eight quasi-moons, it definitely has a favorite. Kamo’oalewa was discovered in 2016. It’s not close to us, shifting from 38-100 times the Earth-Moon distance away from Earth at various points in its movements. That means it takes about 45 years to appear to complete a single “orbit” around the Earth. Kamo’oalewa is the most stable of Earth’s quasi-moons, and modeling suggests that it’s going to be hanging out in our vicinity “orbiting” our planet for possibly another 500,000 years. 

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In an interesting plot twist, it’s possible that this quasi-moon actually has a lunar origin story. Studies of Kamo’oalewa’s surface suggest that it has an awful lot in common with the kinds of crust we see in the highlands of the Moon’s far side (the side Earth never sees). And it just so happens there is a crater there of the right size and age to have produced Kamo’oalewa.

So it’s entirely possible that Earth’s favorite quasi-moon is a lost piece of its actual moon, blasted loose 10 million years ago when something huge excavated the crater Giordano Bruno on the Moon’s far side. And then, having been sent flying off into the void, Kamo’oalewa managed to find just the right gravitational interactions to stick around its birthplace, rather than vanishing forever. You gotta love that kind of lunar loyalty.

As of this month we have an actual picture of Kamo’oalewa courtesy of China’s Tianwen-2 spacecraft, which just entered orbit around the quasi-moon on July 2. Perfect timing!

Marvelous Minis

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Then there are the minimoons. Technically speaking, these things are actual moons, captured and gravitationally bound to the thing they look like they’re orbiting—though only for a little while. That’s what distinguishes them from the captured moons of the outer planets. Those are more or less permanent residents of their planet systems. 

We’ve spotted a couple of objects over the years that were minimoons of Earth that completed at least one full revolution around the planet. We’ve also found a couple of others that we suspect were caught in orbit around the Earth but didn’t actually compete a full revolution before flying back off into space—in which case it’s hard to argue that it was any kind of moon at all.

We think that there may be many such minimoons around the Earth at any given time, they’re just usually quite tiny and hard to spot. Some of them, like Kamo’oalewa, might be debris knocked loose from our own Moon. Others are just random bits of rock passing through the system. And others…well. Sometimes it’s our own fault.

Did You Say “Paint”??

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In September 2002 an object was discovered and designated J002E3. Shortly after its discovery it was found that J002E3 was in orbit around the Earth, though likely only had been for a short while. Spectral observations revealed a mystery. J002E3 looked like it was made of…paint-covered aluminum? Specifically aluminum covered in black and white paint? What?

Backtracking J002E3’s path suggested it was last in the vicinity of the Earth around 1970, aka the Apollo years. You know, when we were launching the massive Saturn V rocket and littering Earth space with spent rocket stages. Black and white-painted rocket stages. Odds are that this particular minimoon is actually the third stage of the Saturn V rocket that sent Apollo 12 to the Moon in November 1969.

It’s not the only minimoon that we’ve seen that’s suspected to have a human-based origin story, but it’s the only one we’ve traced to a specific mission. J002E3 has since moved on, leaving its temporary Earth orbit in 2003, but the fun part about minimoons is that sometimes you meet them again. It’s thought we might re-encounter and re-capture J002E3 sometime in the 2040s.

Moons Galore!

It’s easy to get distracted by our one natural Moon, given its impressive size, undeniable beauty, fascinating science, and status as the only world other than Earth humans have walked on. But on this Moon Day, it’s perhaps worth taking a little time to appreciate those other weird moon-like things out in space: the real ones, the temporary ones, and the fake ones. They’re all cool space objects and therefore worthy of appreciation.

But, you know, also pay attention to our actual Moon. It’s really freakin’ amazing, and it is Moon Day after all.