Caoimhín's Content

In today’s piece we are going to talk a little bit about asteroids and a little bit about comets. Firstly however, in the most recent piece I showed off some of the basic features of Stellarium, it was more of a walk through of the different kinds of things that Stellarium can do than an article about objects in the sky. I will mention a couple of features here as well, including lists of objects that can be found in the search window. Unfortunately, the lists I have don’t seem to have a particular class of object, the class of object that I’m looking for. Centaurs don’t seem to be laid out here in its own separate list, unlike many other objects.

Luckily there is a list of comets and a list of asteroids. Asteroids are usually given both a number and a name, such as (1) Ceres. The Stellarium list goes up to pretty high numbers, so there are a lot of asteroids available to take a look at. Asteroids are generally thought of as the rocky things that go around the Sun, and comets are usually thought of as the objects with those big beautiful tails. Comets do have amazing tails when they’re near the Sun, but when a comet is further from the Sun it’s not going to have that big beautiful tail. Proximity to the Sun is what causes a comet to form a tail. Using the list of comets, I can easily select objects that are not visible to the naked eye. This lets me use the comet C67P/Churyumov-Gerasimenko as an example.

This is the comet that the European Space Agency probe Rosetta traveled to and the lander Philae landed on. As the only comet to have something landed on it, this is a comet that we know a surprising amount about, thanks to a probe landing on it and taking samples. i was hoping that it would be dramatically far away, bit it seems to be 6 AU away at the moment, which isn’t really that far away for the solar system. However, it is more than far enough for this comet to lose its tail. It is a short period comet, so it won’t go as far from the Sun as some of the longer period comets. It has produced a tail in the past, but right now it’s just a cold ball of rock and ice, much like an icy asteroid. It’s orbit will take it back in towards the Sun eventually and its tail will return, it is definitely a comet.

Most of the asteroids that we see in the asteroid belt, those aren’t going to be producing a tail and probably wouldn’t produce a tail even if you brought them closer to the Sun. Most asteroids in the asteroid belt are rocky objects, The Asteroid Belt runs pretty much along the ecliptic between Jupiter and Mars, so most asteroids will be found along the same region of the sky where we see planets. By getting rid of the atmosphere and zooming in a little on the right region of the sky, smaller bodies like asteroids will start to be highlighted. There’s a lot of empty space between asteroids, but some an appear quite close in the sky thanks to our perspective. Very quickly, a bunch of these distant objects get highlighted. One had a fairly small number, (20) Massalia, and smaller numbers on an asteroid usually means it was discovered longer ago. The longer we’ve known about an asteroid, the more likely it is that we’ll have reasonable pictures, and Stellarium does indeed provide an image for this one. Even at reasonably low levels of magnification, Stellarium shows a little bright dot where Massalia should be, indicating that you can probably see some sign of this object without a massive telescope. Taking a closer look, this object even looks pretty spherical. Given that it is listed as an asteroid, I must assume that it’s not big enough to be under hydrostatic equilibrium. Some things can look like a ball, but without their gravity pulling material together to close any gaps, pushing out gases and changing their interior so that the heavier things kind of sink towards the center and the lighter things float towards the surface, it isn’t in hydrostatic equilibrium. A loose aggregate ball of material is still going to be an asteroid even if it looks pretty round, that’s why hydrostatic equilibrium is the actual term for things like dwarf planets and planets, they’re not just round, they’ve reached hydrostatic equilibrium. There is a bit of a difference there in the meaning of those things, even though saying “big enough to be round” is a fair simplification. A quick further look around reveals a bunch more, including fainter ones like (138) Tolosa. As this one has a much higher number, it must have been discovered comparatively recently and that means it’s probably not as well photographed or imaged. Certainly, there is no image provided in Stellarium, but with so many asteroids there won’t be an image for all of them. There are hundreds of thousands of asteroids, millions if we count all of the asteroid outside the asteroid belt.

Although we may think of the asteroid belt as the place for asteroids, there are many more scattered around our solar system. Many of these objects are further out, including a big number between Jupiter and Neptune. These objects known as centaurs. Just like the mythical centaur was half-man half-horse, the astronomical centaur is half-asteroid, half-comet, or at least shows some features of each. Centaurs are very similar to asteroids in that they go around the Sun pretty much in a circle, so they don’t usually get close enough to the Sun to produce tails. They are however outside of the ice line, also known as the frost line or snow line. They’re out by the giant planets like Neptune and Uranus, Jupiter and Saturn. Many of them could, if they flew in towards the inner solar system for any reason, they could develop tails, thanks to the volatile compounds they contain. By volatile, I just mean able to sublime in space, turning from ice to vapour, and potentially forming a tail.

Given the importance of how and where something orbits in its definition, being able to see the orbits would help. Stellarium allows this, but from the Earth it’s hard to tell what orbits are near and which ones are further out. It’s also difficult to see which ones are eccentric and which are more circular. Thankfully, Stellarium provides a solution to this as well, the Solar System Observer. the Solar System Observer is a location in the solar system, accessible from the same part of the location window as other planets and moons. However, it isn’t a real location, it’s a point about 600-700 AU above the North Pole of the Sun. Moving there brings us up to a location where we’re essentially looking down on the Sun and the Solar System as a whole. Of course with all of the stars in the background, it’s going to be hard to even spot the planets. We can get rid of the stars as well, using the hotkey S, for anyone who looked at the last piece about how to use Stellarium and how Stellarium sort of works, that is one of the hotkeys I failed to mention.

Looking down on the solar system with no stars to get in the way, it’s easy enough to find the planets closer to the Sun. Once you find Mars, looking roughly between Mars and Jupiter you might get to see some asteroids popping up if you look in the right place. Asteroids are pretty small compared to the planets and spread over a large area, so it can take a bit of searching before you stumble upon one. If we could see the orbits of all of these objects then we could at least see the ellipse that they move along, even if we can’t see exactly where on the ellipse it is. Turning on the orbits of the objects in the Solar System using Stellarium lets us do just that. It’s not really all of the objects orbiting the Sun, it doesn’t show everything in the Solar System, but it’s still quite a lot of orbits. Stellarium can only keep track of so much of course, but there are still a lot of objects in our solar system that Stellarium is aware of, enough to occasianly cauase Stellarium to freeze, on my computer at least. Despite the amount of orbits being traced, it’s worth taking a look. Seeing all of the orbits together, some are clearly very close to circles, see some appear to be almost straight lines. One of the practically straight lines might be `Oumuamua, the interstellar object that passed through our solar system without actually orbiting anything. The dense set of orbits between Jupiter and Mars makes the asteroid belt very visible. There’s also some wide orbits pretty far out, around and beyond the orbit of Neptune. Neptune stands out against the orbits of asteroids thanks to the orbits of all of its moons, it looks like a squiggle rather than a point.

The orbits of objects that would be the comets stand out thanks to how eccentric they are. We can see that the paths of some objects keep them out as far as Neptune, as far away as Pluto when Pluto’s close to the Sun for most of their orbit. If these orbits are eccentric enough, the object will fall in towards the Sun and heat up, losing gas and releasing volatile chemicals to form the expected tail. That is enough to make a comet, as long as the object is going to have that beautiful tail, it’s what we’d imagine of a come. Even objects that just barely cross Jupiter’s orbit, only getting a little further from the Sun then Jupiter does, can be comets. As long as their orbit takes them far enough from the Sun to be icy. It also looks like one of the orbits is passing very very close to the Sun, which is generally bad news for any object. Comets that get too close to the sun, sometimes called sungrazers, tend to break up and fall apart. Even looking at the asteroid belt, we can see that some of the orbits are almost circular, while some of the curves look pretty extreme. All of these orbits are not circles, they’re definitely ellipses, but some are much more eccentric than others. Some of these asteroids are orbiting the Sun eccentrically, but usually not like a comet, not as hyperbolically or parabolically as a comet would orbit. The asteroid belt is stabilized by the gravity of Jupiter, which keeps most asteroids in roughly the same belt, but they’re definitely not orbiting the Sun in exactly a circle. Even if these asteroids fell in towards the Sun, the chances of them getting a tail are pretty small, simply because they wouldn’t have as many of those icy volatile compounds. Many of them formed too close to the Sun, are at least have been too close to the Sun for too long.

If you are too close to the Sun, you will be inside the ice line. The ice line is the line in space where everything inside of it is too hot to be ice, it has to be water vapour, or H2O as a gas, and outside it’s too cold to have water vapour, all of the water is frozen as ice. Liquid water is usually not an option if you’re not on the surface of something with sufficient gravity. Floating in the vacuum of space, ice or water vapour are you only options, once ice starts to melt, in space it will turn directly into molecules of water flying around the vacuum or whatever sufficiently Low Gravity environment you’re on. Pressure is what allows water to exist as a liquid, and this is often provided by an atmosphere held in place by gravity. It’s only when gravity is high enough that ice can melt, loosen up it’s inter-molecular bonds, but remain loosely bonded together as a liquid without flying away into a vapour. If you’re in the International Space Station there’s air pressure in a pressurized space even with only micro-gravity, and this can still keep water liquid. If the ISS was a hard vacuum, the astronauts wouldn’t explode. It would be bad for them but they wouldn’t explode if they were in a vacuum, it’s not quite that dramatic. The lack of oxygen in a vacuum will cause the same problems a lack of oxygen would cause anywhere. As long as you’re not in a vacuum there can be enough pressure for water to be liquid. In the vacuum of space, ice and vapour are the only options. A lot of things can be ice or vapour, here I do mostly mean water ice and water vapour, but the principle is true for many molecules at different temperatures and pressures. In the asteroid belt and closer to the Sun, the only option for exposed water is water vapour, if you’re not bonded to the surface of a planet and under pressure, water vapour is the only option. These asteroids that are close to the Sun, if any of their ice is exposed they’ll sublimate it off into space, the way a comet would when it flies in close to the Sun. Asteroids in the asteroid belt have been doing this for billions and billions of years, they’ve lost most of the water on their surface. Ceres might be an exception, Ceres might have some ice on it’s surface but this is likely to be recently exposed.

The ice line might have been somewhere inside the Asteroid Belt in the past, but it is much further out today. The asteroid belt spans from around 2 AU to about 3 AU from the Sun, while the ice line today is closer to 5 AU out, close to the orbit of Jupiter. The younger Sun was a little cooler, but the solar system was also a lot more dusty, which helped cool the ambient temperature quicker closer to the Sun. The movement of the ice line has left some icy things in the asteroid belt, but collisions can expose this ice and cause it to sublimate away. Outside the ice line the ice sticks around even if it is exposed, and this is where actual comets seem to originate from. These objects spend most of their time far outside the warmth of the Sun, but fall in towards the Sun occasionally, whereupon the ice sublimates as expected.

Of course there are plenty of these things that are just orbiting the Sun at that the correct distance to stay icy. Their orbits are too far from the Sun to get heated up to produce a tail. If they ever fell in to the inner solar system they would likely develop one, but something would have to disturb them, like the pull of Jupiter or the combined pull of the inner planets. This does happen, and is believed to be cause of most of the comets we do have. There are also a lot of objects that cross inside the orbit of Neptune, including Pluto, but never come much further and so never develop tails even with orbits that are much less than circular. Thanks to the former location of the ice line, some asteroids do contain ices, there are what are known as active asteroids which produce the jets of vapour that we would associate with forming a comets tail, though usually not as much. Comets really only look like comets because of their strange orbits and technically asteroids could fall into an orbit like this as well, or any rocky body that wouldn’t produce a tail. If anything from the inner Solar System got flung outwards and then eventually fell back in again, if it didn’t produce a tail it may not get called a comet, even if it fit the orbital characteristics.

Asteroids and comets, they’re a complicated bunch. As well as the simple cases, the asteroids that are just balls of rock with nothing volatile on their surface to boil, and the comets that are icy bodies with sufficiently eccentric orbits to form tails, there are edge cases. Any distant rocky objects, if there are any, that wouldn’t produce a tail if they fell too close to the Sun, asteroids that are active and do have volatiles but probably won’t pass too close to the Sun to produce a tail, Then there are objects that would be comets if they fell close enough to the Sun, but they may never, they may always stay far outside those orbits, as plutinos and cubewanos out past Neptune. This includes the less distant centaurs for the ones that are close to the massive planets like Jupiter and Saturn. These definitions, as you may remember from previous videos, can be a little bit tricky. We can call some of them small solar system bodies or minor planets, there are various other classifications that try and tidy these things up.

If you would like to hear more about little things in space, then you can subscribe to this website or my YouTube channel, I’m sure I will talk about this more in the future. If you enjoyed this article discussing these different objects, make sure to like the article, and the attached video if you’re feeling generous. Hopefully I will see you back here next time.