Yesterday, around 4:02 p.m. Eastern (US) time, Earth acquired a second moon.

Kinda. And not for long.

2024 PT5 is a small asteroid, about 10 meters across (so, house-sized), that orbits the Sun on an extremely Earth-like orbit. Its average distance to the Sun (really, its orbital semi-major axis for physics pedants) is 1.01 times Earth’s, or 150.15 million kilometers. PT5’s orbit is very slightly elliptical, so sometimes it’s closer to the Sun and sometimes farther away.

But it gets weird. On average, Earth orbital velocity is slightly faster, so they spend a long time apart. But every 30 years or so Earth catches up to PT5, and for the next few years they’re close together. Because PT5’s orbit is more elliptical, when it’s farther from the Sun than Earth it slows down a bit relative to us, and when it’s closer it speeds up. So the two stay close together, but from our point of view it almost seems to orbit the Earth, following a horseshoe-shaped path. This makes it what we call a quasi-moon or quasi-satellite.

But a funny thing is happened this time around. PT5 is getting so close to Earth, and approaching us so slowly, that for about two months it will actually be a satellite of Earth! Technically speaking, its orbital energy with respect to Earth will be negative (it would take energy to break free), meaning it will be bound to us gravitationally, actually orbiting us instead of the Sun [link to journal paper].

That happened at 20:02 Universal time on Sept. 29. It will slowly revolve around us, finally getting far enough away that the Sun’s influence will take over again on Nov. 25, 56 days later (too bad it didn’t get here earlier and break away on September 13). It’s sufficiently far from us that it won’t even complete a full orbit in that time. More weirdness: It’ll actually be closer to us after it gains enough energy to break free from Earth, but it’ll be moving fast enough at that time to keeping moving on. Closest approach (or perigee) will be on January 9, 2025, when it’ll be about 1.8 million km from Earth, nearly 5 times farther from us than the Moon. Heck, it was closer when it was discovered in August 2024, at a distance of a little under 570,000 km.

Orbital mechanics is bizarre.

Mind you, the whole time it’s a second moon it’ll be pretty far from us, over 2 million km. At 10 meters across, it’ll be far too faint to see without a powerful telescope. Of course, despite that awkward reality, I’m seeing tons of really chuckleheaded claims of it being big and bright in the sky, and on TikTok I’ve seen lots of obviously hoaxed videos showing it as big as the moon already.

There’s nothing to worry about here. Technically, this asteroid could impact Earth in the future, but it’s so small it won’t do anything except put on a really amazing light show and maybe drop a few meteorites (for comparison, the Chelyabinsk asteroid that blew up over Russia in 2013 was 19 meters across, roughly 7 times the volume and mass of PT5). Also, because its orbit is so Earth-like it approaches us very slowly, and the energy of impact would be far lower. As it is, it’s not predicted to impact at least for the next century, and probably far longer.

This kind of thing probably happens pretty often, but it’s only been recently our telescopes and tech have been good enough to spot these tiny rocks. The last one seen was 2016 HO3, less than a decade ago. There may be dozens more out there, hundreds even, depending on how small an object you’re willing to consider a moon (Venus has quite a few known, including 8 new ones just discovered recently). That’s pretty cool!

And scientifically very interesting indeed. Because they move so slowly relative to Earth, it’s easier to send a probe to these asteroids than others. I’d love to see something like this; because a quasi-moon is close to us the probe wouldn’t need a big sophisticated communication rig either, so I imagine a CubeSat might be sufficient to catch one, and those are extremely cheap to make. 

I hope someone out there takes this as a challenge!

It’s taken 20 years, but scientists and engineers have completed work on the jaw-droppingly huge LSST Camera, which will eventually be installed on the Vera Rubin Observatory telescope.

LSST stands for Large Synoptic Survey Telescope, the name of the telescope before it was changed to the Simonyi Survey Telescope, after the billionaire donor who made the construction possible.

The camera is immense: 3.2 GIGApixels, making it (I believe) the largest astronomical camera ever built. It will also have a very wide 3.5° field of view, about 7 full Moons across, which will allow it to see lots of sky at a time. It weighs 3 metric tons and is nearly 4 meters long. 

I mean:

That’s a big camera. It will observe distant galaxies, stars, and asteroids in our own solar system, producing 20 terabytes of data every night for astronomers to swim through and find all kinds of celestial goodies.

Speaking of big kilometerstones for the observatory, the telescope’s 8.4-meter primary mirror just had its reflective coating completed! This is a very big deal, turning the mirror from just a monstrous slab into a precision astronomical device. This video shows it, and should give you a sense of scale; 8.4 meters is big.

I can’t wait until this monster goes online!

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