The case for 24 Themis

Now that Dawn is approaching Vesta and sending back clearer and clearer images every day, not only Vesta but Ceres will be discussed about more and more, and along with that comes the question of whether they would be good locations for manned exploration.

The short answer is this: Vesta is not because (as far as we know at the moment) there is no water there, while on Ceres there is. Water is crucial for manned exploration because not only does it aid our survival in the first place (and drastically lessen the mass that we need to send into orbit in the first place), but it can also be used to make rocket fuel for the return trip. The moon has water and is certainly the easiest location to colonize first, but after that we have Mars, and Ceres, and...an asteroid called 24 Themis.

Before we get to 24 Themis we need to discuss the comparative difficulty of getting to Mars vs. Ceres. A forum post here goes into some detail on that. Here is how the two stack up:

-A journey to Mars takes about 6 months, Ceres somewhat longer. One big disadvantage to Mars, however, is how infrequent the launch windows are. Mars orbits the sun at just a fast enough speed that it takes Earth a long time to catch up; 780 days, in fact. In comparison with this, launch windows to Ceres take place every 467 days. The farther an object in the Solar System is from the Earth the more frequent the launch windows are, because they take so long to orbit the sun that they have hardly moved at all by the time Earth has rotated once and is now at closest approach again.

This video shows why launch windows to Ceres are more frequent than those to Mars.



Landing: one of the most interesting parts about Mars is its atmosphere and the weather and seasonal patterns it has. On the other hand, its atmosphere is not helpful when landing a manned mission: it's thin enough that it doesn't help much in slowing a craft entering it, while it is thick enough that one can't land without a heat shield. Using retrorockets is also tough as the difficulty involved in landing a craft on a planet with weather is much, much harder than a body without an atmosphere at all. An article here goes into detail on that.


Without getting into too much detail, the short explanation is this: Mars is doable mostly as a one-way mission. Send as many supplies as possible ahead of time, somehow manage to land successfully (an extremely large heat shield would be required to slow down enough in the thin atmosphere), and then go with that. Escape velocity from Mars is a full 5 kilometres per second, a bit less than half that of the Earth. Ceres by comparison is one tenth of that (510 metres per second), and without an atmosphere it is easy to both land on and take off from.

Ceres has one disadvantage: its inclination. While not drastically inclined (about 10 degrees), that is still enough to raise the delta-v (change in velocity) a great deal. Here is what Ceres looks like in comparison with the nearby planets:

If only there was another object kind of like Ceres, not too far away and with water, but with a less inclined orbit!

And there is (it's the title of this post): 24 Themis. Let's look at the case for manned exploration of this asteroid, certainly after the moon which remains the easiest target.

First of all, a general overview: 24 Themis is an asteroid in the asteroid belt, and as you can see from its low number (24 = the 24th asteroid discovered), it is quite large; it has a diameter of 198 km. A diameter of 198 km translates into a surface area of about 120,000 km², which is about the same as Greece or Bulgaria, or the US state of Mississippi. If you're Canadian, it's about Nova Scotia plus New Brunswick. 24 Themis is not a massive planet by any means, but it's the farthest thing from a tiny lump of rock. Walking around it once would take a full three weeks.

Surface gravity is about 1% that of Earth, and escape velocity is 87 metres per second. Extremely easy to break orbit with a rocket, but unless you can jump at 313 kph then you're not going to fall off.

Most importantly though: 24 Themis has ice on the surface, and lots of it. Its discovery made quite a bit of news in 2009, as ice on the surface of an asteroid that close to the sun was not thought to be possible. Nevertheless, it's there, and that's good for manned exploration.

With the above so far 24 Themis is about as good a location to explore as Ceres. The asteroid's orbit, however, shows why it may be an even better place to explore than Ceres. From the top it looks about the same:

but look at it from the side and we see how little inclination there is to the orbit. In fact, it is virtually nonexistent, and Mars is more inclined than 24 Themis.

That image doesn't really do justice though - check out the orbit for yourself here.

So that's the case for 24 Themis. It's at a similar distance to all the targets we are considering after the moon, with a journey time of months but not years (and once VASIMR is put into common use with a large enough power source (200 MW) this will turn into weeks), it has water, is large enough that it is a small world of its own, is easy to land on and leave. It also has a day of 8 hours and 23 minutes, so living and working there would be easy enough to plan: one is awake for two sunrises and sunsets, and sleeps every third day.

We will not get to 24 Themis soon, but it does deserve to be discussed, and it also merits an unmanned mission - at least a flyby, hopefully a dedicated orbiter or even a lander. The best way to make that happen is to begin talking about it. So remember the name, and let others know about it too when the subject of manned exploration comes up. All 24 Themis needs now is some name recognition.