Remember the near-Earth asteroid 2024 YR4, which for a while had a slightly uncomfortable chance of hitting Earth in December 2022? The chance peaked around 2.2%, but then dropped as more observations were made that nailed down its orbit better. Eventually the orbit was determined with sufficient accuracy to rule out an impact with Earth altogether.

But what about the Moon? YR4 is on a trajectory that brings it pretty close to the Moon, and an impact there is still possible. NASA just announced the chance got slightly higher, going from 3.8 to 4.2%. That’s still about 25-to-1 odds, so a longshot, but the chance isn’t zero. Interesting.

Right now the nominal (average, or most likely) miss distance is about 9,000 kilometers, but the minimum is only a few hundred kilometers! There’s still some uncertainty in these numbers, so they include an actual impact.

As I wrote before, if it does hit the Moon we’re likely going to see a bright flash of light, and leave a crater less than a couple of kilometers across. That would have a huge effect on Earth (an impact that size would create an explosion in the megaton range), but the Moon has so many craters that size it would get lost among them. There would likely be no issue with ejected material hitting Earth either, since any ejecta would be small and moving slower than the asteroid did when it hit.

I’d actually be pretty danged excited if it did hit the Moon. The physics of impacts is very difficult, since they happen at such large velocities that it’s hard to model theoretically or mimic in the lab even with high-speed guns. We’d learn a lot about impacts from such an event, at very little risk to us. And yeah, sure, I’ll admit it: it would be very cool to see that. I’ve seen a lot of amazing stuff in the sky, including the aftermath of a comet impact on Jupiter, but never an actual impact as it happened (and I had the chance once but looked away at exactly the wrong moment). I’d love to add a lunar asteroid impact to the list.

We tend to think of space as being empty, wide and vast, with plenty of elbow room. That’s true for deep space, millions of kilometers from Earth, but in orbit that’s not at all the case. We’re launching more and more satellites into low-Earth orbit (up to an altitude of roughly 2,000 km above the surface) all the time.

And it’s getting worse. A lot worse. SpaceX has already launched over 7,000 Starlink satellites, each the size of a car, and plans on launching tens of thousands more. Amazon, not to have an egomaniacal billionaire outpace another egomaniacal billionaire, is launching its own “megaconstellation” of Kuiper satellites with a total of over 3,000 planned.

The disastrous results for astronomy are pretty obvious, but there is a much larger danger: overcrowding in the limited volume of space in low-Earth orbit. Collisions aren’t just likely, they’re inevitable, and they happen at speeds many times faster than a rifle bullet. Debris expands away from the impact site, and every single one of those pieces can hit another satellite, causing more and more collisions. This is called the Kessler Syndrome, and if you’ve seen the movie “Gravity” you’re familiar with it (that movie had a lot of scientific inaccuracies in it, but the overall idea of the cascading collisions is correct). Something like this could make low-Earth orbit completely useless in short order.

I’ll add that there are over a hundred million objects in orbit bigger than 1 millimeter, when even a flake of paint can do serious damage if it hits a satellite at 25,000 kph.

Space agencies are taking this threat seriously. The European Space Agency issued a Space Environment Report for 2025 a few months ago, and reading it is pretty unsettling. Their key takeaways are many, but the overall message is that we need a way to decommission satellites at the ends of their lives, de-orbiting them or putting them in a higher “graveyard orbit” where they’re out of the way of low-Earth sats. I strongly urge you to take a look at it, and also suggest you curse Musk’s name as you do.

One statistic they report that really shocked me is that, on average, three rocket boosters or satellites re-enter Earth’s atmosphere every day. Every day! I would’ve thought it was much lower, and that number gives me the heebie-jeebies.

Not so much because it’s dangerous to us directly. The chance of one hitting something important on the ground (people, a building, and the like) is pretty low; most of Earth is water, and most people live between 40° north and south of the equator, and many satellites have orbits tilted more than that, so they spend some time over places with fewer people.

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