Caoimhín's Content

The Geminid meteor shower is upon us, or very nearly at least. I mentioned the Geminids in our overview of December, and there is a whole video dedicated to meteor showers as well. However, unlike many other meteor showers, which are caused by comets, this one is caused by an asteroid called Phaethon or 3200 Phaethon. Comets are famous for their tails, and it makes sense that they would leave behind a trail of dust and gas. It is this dust and little bits of rock that burn up in our atmosphere to create the shooting stars or meteors. Although comets are a common source, these little bits can come from asteroids as well.

Some asteroids spew out material in a way reasonably similar to comets and a bit like volcanoes on Earth. Asteroids are too small to have molten rock inside them, but they might have frozen or slushy liquids. Changes in temperature and pressure can cause these materials to come squirting out, a phenomenon known as cryovolcanism. Asteroids like that are reasonably rare and sometimes called active asteroids. Even if they lack cryovolcanoes, asteroids are often quite dusty, covered in small bits of dust and rock that their gravity has attracted. Being rather small, this gravity is usually quite weak, meaning that this loose surface covering can be lost, providing the same trails of dust necessary to trigger a meteor shower here on Earth.

3200 Phaethon is an active asteroid, and it travels quite far from the Earth, drifting from nearly the asteroid belt all the way in to Sun, even closer than Mercury. This is one of the reasons it has such a high number, it wasn’t one of the early asteroids to be discovered due to the difficulty in observing it. Other asteroids are much closer to us and easier to see.

Vesta or 4 Vesta is one such asteroid, the only one usually visible to the naked eye. It is only visible under ideal conditions, under dark and clear skies, and it isn’t something I’ve managed to see myself, however, unlike Uranus, I do believe that it is possible. Vesta is a particularly bright object due to its reflectivity, it’s full shiny reflective minerals such as pyroxene. In fact, Vesta has such a different composition to other asteroids, it is put in its own class of Vestoids. It is only in the asteroid belt, so it is just a bit further from us than Mars. It may be very bright, but it’s not quite the biggest. That title goes to Ceres.

Ceres or 1 Ceres is the big one, Queen of the Asteroid Belt. Ceres is so big it is classified as a dwarf planet, but is usually given a dual status, it is a dwarf planet and an asteroid. Ceres is nice and round, very spherical, and this spherical nature is an important part of the classification of planets and dwarf planets as distinct from asteroids. At about a 1000km across, Ceres is by far the largest thing in the asteroid belt, even if it is a bit smaller than some of the more distant dwarf planets like Pluto. Some people were saddened by Pluto’s reclassification from planet to dwarf planet, but as a result Ceres got an upgrade. Named for the goddess of agriculture, especially grains and cereal, the Roman goddess Ceres is where we get our modern word for cereal. Cere’s is given the the number 1 even though it wasn’t the first discovered, so we have skipped a couple since 4 Vesta.

Pallas or 2 Pallas is one of the ones we skipped. It is also a particularly large and quite round. Every large solid object in space is round, and even galaxies can become round if their spiral shape breaks down (that’s been covered in a previous video as well!). This is due to the effect of gravity. As an objects mass grows, so does its gravitational pull. This gravity pulls the surface of the object so that every point is almost equidistant from the gravitational center, forcing it into a round shape. Known as hydrostatic equilibrium, this phenomenon is part of what makes planets and dwarf planets separate from asteroids, which can be much more lumpy. Ceres is the only dwarf planet in the asteroid belt so far, but as definitions shift and get refined we may gain more.

Juno or 3 Juno is a bit smaller and we can see that is a little lumpier as well, but still in the top 20 largest. It was, roughly, the third to be discovered, despite not being very large, and the early discovery of the asteroids, when they were initially called planets, was a little complicated and involved many separate astronomers, often in different countries, usually collaborating my letters.

All of those asteroids are in the asteroid belt, and are big enough to be seen with a telescope. Asteroids can orbit in different areas, and we’re going to look at one very close to Earth. Using the Solar System Observer, a point high above the Sun’s pole, we can look down on the solar system, and see how things orbit. This is an imaginary point, other than potentially some members of the Oort cloud, there isn’t really anything up there. It is however a very useful feature of the Stellarium software, affording us a view that gives a very different perspective on the planets.

Cruithne or 3753 Cruithne is the object that we are looking for, a small asteroid that shares its orbit with the Earth. This might seem strange, as planets normally need to clear out other objects or control their orbit. Sharing your orbit with other objects is part of the reason that Pluto lost its planetary status, but here Earth is in control. The Earth and Cruithne are in orbital resonance, with the gravity of the Earth, and other planets, pulling and pushing Cruithne as it orbits the Sun. For part of Cruithne’s orbit, it races ahead of the Earth, before slowing down so the Earth can begin to catch up. Over a period of hundreds and hundreds of years, Cruithne completes a complex horseshoe shape as it orbits the Sun with the Earth.

Cruithne is a tricky word to pronounce, even for an astronomical object. Rather than coming from Latin or Greek, Cruithne is an Irish, or Old Irish, name for a tribe that lived in the north of Ireland in the past. If you’d like to hear how its pronounced, have a listen to the video, I go through a couple of versions at the end.

Here, we only touched on a few of the biggest, and one particularly interesting, asteroids, as you might be able to tell from the numbers on 3200 Phaethon and 3753 Cruithne, there are thousands. Really, there millions of asteroids, minor bodies or minor planets known to exist, our solar system is full of little chunks of rock. Hopefully you’ll join me back here in the future to look at more of them.