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This newsletter is issue #1060, which seems like an unremarkable number, until you realize it’s equal to the sum of all the prime numbers between 1 and 100!

2 + 3 + 5 + 7 + 11 + 13 + 17 + 19 + 23 + 29 + 31 + 37 + 41 + 43 + 47 + 53 + 59 + 61 + 67 + 71+ 73 + 79 + 83 + 89 + 97 = 1060

So tautologically it’s literally remarkable since I’m remarking on it, but also that’s just kinda fun. Math is cool.

Let’s hope I make it to issue 76,127, which is the sum of all primes from 1 to 1000! I may employ a medium to dictate the newsletters by then.

It was a pretty good week for asteroids: the Japanese mission Hayabusa2 flew past Torifune, and the Chinese mission Tianwen-2 approached 2016 HO3, aka Kamo'oalewa, in preparation to take samples.

Torifune is a near-Earth asteroid about 450 meters end-to-end, and Hayabusa2 passed it at a distance of just 800 meters, an incredible precision given it was moving 5 kilometers per second! This was done to test such precise targeting, in case we ever need to whack an asteroid that is headed toward an Earth impact.

Torifune is weird, isn’t it? Except it really isn’t. It’s a double-lobed rubble pile, which we’re learning is actually a not uncommon shape for small asteroids. Rubble piles are literally that; collections of rocks of all sizes held together by their own gravity. If you stood on one and started throwing rocks off it, eventually there would be nothing left. That’s all it is.

There are competing ideas for how they get that peanut shape. A likely one is that it may have been more round at some point, then either suffered a big impact or spun itself up so much it started flinging away rocks (I think the latter is now considered unlikely). The material coalesced into two lumps, which then fell toward each other slowly and stuck together like two snowballs. The comet 67P/Churyumov-Gerasimenko looks very much like this, as do other small asteroids. Whatever mechanism makes them this way happens often. 

Hayabusa2 flew past Torifune never to return. The plan is for the spacecraft to meet up with the tiny 10(ish)-meter-wide asteroid 1998 KY26 in 2031.

Kamo'oalewa is also a near-Earth asteroid (meaning it can get closer than about 45 million km from our planet), but is much smaller, only 27 meters long. Unlike Torifune, it looks more solid, almost like a shard of rock chipped off something bigger. Tianwen-2 is approaching it slowly, and will study the wee rock for about nine months. The plan is to retrieve samples from the surface and return them to Earth in 2027. From there the spacecraft will get a gravity assist from Earth to flyby the comet 311P/PanSTARRS sometime in the mid-2030s.

Kamo'oalewa is an odd one. It’s a quasi-satellite of Earth, meaning it has an orbit around the sun very similar to Earth’s, so from our point of view we see it following us, sometimes getting a bit closer and sometimes farther away on its elliptical orbit. The orbit is unusual, and some astronomers think it may very well be a chunk of the moon blasted into space by a big impact (maybe the one that created the crater Giordano Bruno). That would explain its appearance, too. We don’t know, but with Tianwen-2 studying so intently (and sending us a sample!) we’ll almost certainly know for sure in the coming years. 

The shot of the asteroid above was taken from 20 km away — it’s the smallest asteroid ever visited and imaged up close — so it’s not terribly sharp. However, the spacecraft will get much closer, so stay tuned for better images soon.

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