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Today we are going to take another look at asteroids, focusing on a couple of particular subgroups of them. In a recent piece we looked broadly at comets and asteroid, as minor planets or small solar system bodies, as two groups that almost blend into each other. Today, we’re going to be looking at a couple of groups within the category of things normally called asteroids rather than comets. The groups we will be looking at are of particular interest because of how close they are to the Earth, or how close they can come to the Earth.

One of the reasons I am talking about these objects in this video is the temporary moon, asteroid 2024 PT5, that the Earth will have over October and November. The objects that I’m about to discuss, these particular asteroids, are likely to become temporary moons. Of course, a temporary moon isn’t really a true natural satellite, they are sometimes called quasi-satellites. Whether they fully orbit us or otherwise end up bound to the Earth by our gravity, we only temporarily capture them, we don’t usually get to keep them. The asteroid 2024 PT5 will technically orbit the Earth, however it won’t complete a full orbit before it flies off again into space. The reason that the Earth will able to capture this asteroid is because of how close it’s going to come to us

To see how the orbits of these asteroids relate to the Earth, we will need to imagine the solar system from above. We can look down on the solar system with the Solar System Observer location in Stellarium, and we will see that Venus is bright enough to be visible from a few hundred AU above the Sun, but the other inner planets are too faint. The planet Earth is invisible from this location, luckily a search in Stellarium will bring it up. With the Earth selected and the orbits set to display, we can see the orbit that it takes around the Sun and how close to circular it looks. The orbits of Mars, the Earth and Venus from three concentric almost-circles at the level of zoom we’re working under here.

The first asteroid we will look at is Apollo. Apollo is the name of a couple of different objects, it is one of the other names for the star Castor in Gemini for example. The Apollo asteroid gives its name to a whole class of asteroids. Apollo at its perihelion gets almost as close to the Sun as the orbit of Venus, but its aphelion is outside the orbit of the Earth. Apollo’s orbit is eccentric enough that it’s able to cross the orbit of the Earth. The Apollonian asteroids, the group of asteroids named after the Apollo asteroid, they are all Earth Crossing Asteroids, and they’re not the only ones. The Apollonian asteroids would all stay outside the Earth if their orbits were circular, their orbit has a semi-major axis greater than the Earth’s, but they’re eccentric enough to come closer to the Sun, crossing the Earth’s orbit as they do so. Another type of orbit is occupied by the asteroid Aten. Aten is another Earth Crossing asteroid, but its orbit has a semi-major axis smaller than the Earth. It would stay inside the Earths orbit if it’s orbit wasn’t so eccentric. The Aten group is another group of Earth Crossing asteroids, so these are asteroids whose orbit crosses the orbit of the Earth from the inside, rather than the Apollonian asteroids crossing in from the outside.

There are a couple of other groups of very Near Earth Asteroids, that come close to our orbit but do not cross it. Asteroid Amor is one of these kinds. Amor was a god kind of like Eros or Cupid, a god of love. Amor has an orbit that is a bigger than the Earth’s orbit, but it does come very close so the Earth’s orbit. The Amor group or Amorian group of asteroids are asteroids that have bigger orbits in the Earth’s, they orbit the Sun further, but still come quite close to the Earth, or at least their orbit can get close to the Earth’s. Finally, moving on to the last group with the asteroid Atira. Atira is an asteroid who’s orbit is completely inside of the Earth’s, but who also comes quite close to the Earth. This gives us the four major groups of Near Earth Objects, the Atira asteroids between us and Venus, the Amor asteroids between us and Mars, and then we have the Apollo and the Aten asteroids that cross from inside the Earths orbit to outside and back again.

These are all asteroids that are generally speaking Near Earth Objects, their orbits generally follow the Earth’s quite closely. The asteroids that orbit inside the Earth’s orbit or outside the Earth’s orbit but can still pass close to us, the Earth can grab on to them with its gravity and they could end up following us for a little bit before flying off again. There are asteroids that orbit the Sun just outside the Earth’s orbit, and when they are just outside the Earth they’ll stay with us as we orbit the Sun for a little while, but their speed doesn’t match ours perfectly and they’ll eventually fly away again. if they are outside the Earth’s orbit, usually it’s more a case of us catching up to them, staying level with them for a while, followed by the Earth continuing off on its orbit. Objects outisde our orbit will usually orbit the Sun more slowly than us. All of these asteroids, the ones that stay inside the Earth’s orbit and the ones that stay outside the Earths orbit, they can come close enough to us to be affected by our gravity, but even more interesting are the ones whose orbits cross ours. The Atenian and Apollonian asteroids, those asteroids who’s orbits cross the Earth’s, they sometimes have the potential to impact us. Luckily, most of these asteroids cross the Earths orbit while we are at some other point in out orbit, often millions of kilometers away.

Cruithne is one Earth Crossing asteroid that we are certainly safe from. Cruithne is an asteroid that’s often called Earth’s second moon as well, I have even called it Earth’s second moon in the past. By looking at Cruithne’s orbit, we can see that Cruithne is orbiting the Sun in a slightly more eccentric elliptical orbit than the Earth, but it does go around the Sun just like we do. As Cruithne reaches it’s furthest form the Sun, where it moves its slowest, the Earth begins catching up. As Cruithne’s orbit takes it in towards the Sun and across the orbit of the Earth, we have a reasonably close approach. Cruithne then speeds off in towards the Sun towards its perihelion, before crossing the Earths orbit again and beginning to slow down. This gives the Earth a chance to catch up, and the cycle repeats. This cycle is thanks to Cruithne’s strange orbit, because of how elliptical it is compared to ours. Any object will move a little faster at its perihelion and a little slower at its aphelion, but this is exaggerated for more eccentric orbits. Cruithne’s eccentric orbit causes it to slow down and speed up again. This means it can end up coming reasonably close to the Earth, or I suppose the Earth can get reasonably close to it, sort of catching up to it, but then Cruithne speeds up again and ends up zipping around the Sun away from us. It’s orbit is influenced by the gravity of other planets like Venus and Mars as well, speeding it up and slowing it down. This oscillation means that at different times of the year it can be closer or further from us, but in a cycle that repeats much quicker than a comet.

Cruithne has a reasonably close approach to the Earth every year, I will use the year 2026 as an example, but this close approach is coming up near the end of 2024 and again in 2025 as well. Coming back to the Earth and Ireland in particular, we can see roughly what the sky looks like when Cruithne is relatively close to us. As the Earth almost catches up to Cruithne, Cruithne will be ahead of the earth in it’s orbit, putting it above the horizon as we approach morning time. Cruithne is about 5 kilometers across, and even at it’s potential closest approach of about 12 million kilometers from us, it remains pretty distant. It looks like we’ll be seeing Cruithne at magnitude 17.23 during its 2026 approach, so definitely pretty faint, certainly not visible with the naked eye but with a sufficiently big telescope you might be able to see Cruithne. Stellarium is showing it as only a 100-odd million kilometers away from us, but as time moves on that number will increase as Cruithne flies around the Sun, eventually reaching almost the opposite side of the Sun to the Earth. This distance will reach a peak just a little past the orbit of Mars, and then it will decrease again as Cruithne slows down and the Earth catches up yet again.

We can look forward to that in 2026, and each year as Cruithne makes its closest approach, which is part of why this object sometimes gets called Earth’s second moon. The asteroid PT5 will actually orbit us, even if it’s only for a partly, whereas Cruithne never actually orbit us, it just passes pretty close regularly.

I do hope you enjoyed this closer look at a couple of classes of asteroids and where they move through space, all in honour of our temporary second moon. Near Earth objects, even those that could come very close to the Earth, are usually quite safe, being more interesting than threatening. If you found this piece interesting then feel free to like it and subscribe to this website and my YouTube channel. Hopefully, I’ll see you back here next time.