Caoimhín's Content

Today we are finally going to take a look at Earth’s second moon or at least our temporary second moon, asteroid 2024 PT5.

In a recent previous piece, I talked about some of the classes of near-earth objects, like the Apollonian asteroids, the Aten group and more. There are a few objects that go around the Sun near the Earth’s orbit or with the ability to cross the Earth’s orbit. With a lot of near-earth objects, their orbits come close to the Earth’s orbit, but they don’t necessarily come very close to the Earth. This asteroid can get pretty close to the Earth, and it is pretty close at the moment. Most importantly, it gets close enough to the Earth to be under the effect of our gravitational pull.

Trying to take a closer look at this second moon in the real sky will be tricky. Luckily, Stellarium lets us see a little ball of rock once we zoom in close enough. This asteroid is very faint, just about 22nd magnitude at the moment, so it would take at least a medium telescope or a very big pair of binoculars to see it. Its listed size in Stellarium is 2 kilometers, which would be a pretty substantial asteroid. However, other sources are providing a diameter closer to 11 meters, which wouldn’t necessarily even hit the ground, it would likely burn up or blow-up in our atmosphere. Thankfully, based on current calculations, it isn’t going to hit us at all. It is orbiting the Sun, at a distance that brings it quite close to the Earth. By watching the asteroid over a period of time and checking its distance from the Earth, we can see how its distance to the Earth changes. Over the course of this year and into next year, the asteroid approached us, reached its closest approach, then drifted away again to the distance it is in October. It still isn’t free of our gravity, and it will get closer to the Earth again at the end of this year, before falling behind the Earth in early 2025. The reason this object has been called Earth’s second moon is because it does temporarily get affected by our gravity, it will temporarily orbit the Earth. Especially looking at it from above, or purely looking at the distances involved, it will move in such a way as that it seems to be going around the Earth. This is only temporary however, it doesn’t have a real normal orbit of the Earth. It will end up far above our position on the ecliptic, almost spiraling around us. This object will be under the effect of Earth’s gravity but it’s not completely getting trapped by our gravity.

The best way to see the orbit of the asteroid is from the outside, and soon we will shift our perspective out to space, but before that we will see how it looks from the countryside. Even with no light pollution, it is such a faint magnitude, it’s not going to be visible to the naked eye. Moving later into October will get the Moon, the main Moon, out of the sky, and darker conditions will help with spotting faint asteroids like this one. Nonetheless, a telescope is still necessary. It is a little further away from us now than it was earlier in the year, over a million kilometers away from us. It is roughly in North of the sky, close to the North Star. Having a look around the handle of the Plough, near the head of Ursa Minor, should put you close enough to the right place, but it will take significant magnification. It is really quite high up in the sky at certain times, which helps, but, because it is such a small object even when it gets to its closest to us it’s going to be incredibly difficult to see.

Stellarium lets you take a position far above the solar system to see how things orbit, so we will shift our view to the Solar System Observer. I discussed this recently in a previous piece about the orbits of other near-earth objects. From millions of kilometers above the Sun, asteroid PT% of course won’t be visible. We can at least see its orbit and how close it is to the orbit of the Earth. Zooming in on Asteroid PT5 and the Earth, we can see that they get quite close, but asteroid PT5 is still further from the Earth than the Moon. This means that anytime that I say PT5 is pulled by the Earths gravity, it is really the combined gravity of the Earth-Moon system. At its closest, PT5 comes within a million kilometers of the Earth. Stellarium measures the distance between the observer and the object, so from a position high above the solar system, we’re 29 AU from the Earth and this asteroid. By watching the asteroid move along its orbit, we can see that it gets pretty close to the Earth, and as we move through time we can see that its motion through space is influenced by the Earth’s gravity. Despite the Earths gravity being able to pull this asteroid and distort its orbit, it is still an independent object. However, over a short time span, it very much does look like its orbiting the Earth. It goes around the Earth, very nearly making a complete circle around the Earth. It goes through an orbit, pretty much.

Moving back to the beginning of 2024, Stellarium stops showing the asteroids orbit. Stellarium will only calculate orbits sometimes, usually when they are known to within a certain degree of certainty. This objects path is after becoming unknown, partly because we’re looking too far into the past. This object was only discovered in 2024, so anything too far in the past is going to be a little hypothetical, we didn’t observe it going through this motion as we hadn’t discovered it yet. Its orbiting around the Sun, and as it does so it comes pretty close to the Earth in its orbit around the Sun. Asteroid PT5 essentially caught up with the Earth in its orbit, and this is where it began to be effected by the Earths gravity. It’s essentially pulled around the Earth, over taking the Earth on the inside, closer to the Sun, as it passes. This is when it made its closest approach to the Earth. It then moves ahead of the Earth, but not for long. The gravity of the Earth slows it down, causing it to get pulled back around the Earth by our gravity. This will occur in the future, into next year. This brings it pretty close to the Earth again. It seems like it’s going to be almost in line with the Moon, potentially almost in line with the Full Moon, at this future date, but I would have to check the view from the Earth to confirm.

Continuing into the future, it’s orbital properties become unknown again, but it looks like it will begin to fall behind the Earth again. it continues its orbit around the Sun, but it seems to be slower than the Earth, causing it to get further and further away from us. This is what makes it only a temporary satellite. We can see for that couple of months it is going around the Earth, it is influenced by gravity, but it’s not completely captured by our gravity, it will fly away again into space. How does an asteroid like this, that for a while appears to circle the Earth, break free? It’s easier to see that from the Earth. Coming back to today’s date, looking down on the solar system from above, it looks like the asteroid is between us and the Sun. From this perspective, looking down at the solar system as if it were a flat plane, it looks like this object is closer to the Sun than we are. Based on the concentric rings mapping out the orbits of the planets and asteroids, if we were measuring straight out from the Sun on a flat plane, PT5 is definitely closer to the Sun then we are. Looking at the distances, it is a fraction of an AU closer to the Sun than we are. However, this object isn’t orbiting the ecliptic perfectly, its orbit is a little bit more inclined than the orbit of a lot of the planets. This means that, although it is closer to the Sun, it isn’t really between us and the Sun.

Returning to the view from Cork, at around 9:15 the Sun will be down. Looking at a dark sky, we can see that from our perspective here on Earth, it is very high in the sky and far from where we would expect to see a planet. Especially if we come back to day time, it’s so high off the ecliptic it’s going to look like it’s way way above the Sun for us here in the Northern Hemisphere. In fact, during day time at the moment, it almost looks like it is at the zenith, directly over head. It is between us and the Sun, but way higher off the ecliptic than we’d expect for, for example, a planet. This means that the asteroid isn’t as close to the Earth as a 2 dimensional, top-down view would make it seem. Even if its path doesn’t appear to take it very far from the Earth, it ends up above the Earth as well as being off to the side. Returning to the night time view, in order to follow this temporary moon over a few months we’ll get nice and close to midnight. If we look at the sky to early in the night, then moving back towards summer time will bring us back into daytime as the sunset then was later. The closer we are to the middle of the night here, the better, so that the sky will remain dark even if we look at June or July. This will let us go back to the closest approach PT5 had earlier in the year.

Asteroid PT5 had its closest approach back in August, just as it was catching up to the Earth, before our gravity started pulling it out of its normal orbit. The asteroid still didn’t get as close as the Moon, but just outside the Moons maximum distance, well under a million kilometers. Even then, it seemed to peak at a brightness of magnitude 18.3. Even at its closest it, was still super super faint. Taking a closer look with a reasonably powerful telescope, it certainly looks like an asteroid. Thanks to our view, we can see that lovely half-shape almost like a Half Moon. This is because we’re pretty much looking up at its underneath, looking up at its sort of bottom side while its front faces the Sun. Of course, as I’m sure you know, up/down and top/bottom doesn’t really matter that much when you’re in space. I don’t know the rotational axis of this temporary second moon that we have, so I don’t know which way is “up” from its perspective either.

If you missed asteroid PT5 at its closest approach to us, it will reach another close approach, though the next approach will not be its closest approach. It will be significantly further away than the closest approach it had earlier in the year, but it will get closer to the Earth than it is now again. It looks like this temporary moon, after breaking free of our gravity, will drift past us reasonably close in early January 2025, on it’s way away from us, but closer than it is at the moment. All in all, we are going to have this temporary mini-moon, for quite a while, it will be near us for a couple of months, which is a long time for something to be temporarily affected by the Earth. It will only temporarily be considered to be in orbit around us, for about a month, but especially for an object that’s not going to be stably orbiting us, an object we haven’t actually captured. that’s quite a while. Asteroid PT5 will fly away again, but thankfully we’ve got a little bit of time to observe it. It’s currently up in the Northern Sky, just off the head of Ursa Minor, the scoop part of the Little Dipper.

I hope you enjoyed this little piece dealing with our temporary second moon. If you did enjoy it please make sure to like this piece and you can subscribe to this website and YouTube channel to support future articles and videos. Hopefully I’ll see you back here next time.