Wednesday, April 01, 2009
The best image we have so far of Ceres, a (dwarf) planet located between Mars and Jupiter, which we'll be seeing up close for the first time in 2015.
When it comes to manned exploration the human race has yet to decide on just where we intend to begin human settlement outside of Earth, and for good reason: each and every location within reach of us has its positives and negatives, and there is no perfect destination within reach of us that is the obvious choice. The Moon, Mars, Venus (Venus? I'll explain why in a bit), simply building a colony in LEO...but there's one other possible destination that gets almost no attention at all, even though it should: Ceres.
Before explaining why Ceres should be given more attention as a destination though, it's good to have some background on the positives and negatives of other proposed destinations, so let's go over them quickly.
The first proposed destination is obvious: the Moon. The biggest advantage for the Moon can be summed up in one word: location. The Moon is only 384,000 km away from us and takes about 3 days to reach, and even more important than this is the fact that it orbits us instead of the Sun, meaning that we don't have to wait phenomenal lengths of time for launch windows in order to reach it as we have to do with other destinations. In fact, it's so close that even an image of our planet and the Moon made to scale still shows a fair amount of detail. It looks like this:
However, there are some serious negatives about colonizing the Moon: elementwise, it's lacking in pretty much everything we need to colonize it (carbon and nitrogen), meaning that humans would have to take everything they need from Earth - almost nothing can be made on site. The Moon also has a weird day and night cycle with 14 days of day followed by 14 days of night, and so anything on the Moon needs to be able to keep itself powered for a full 14 days without the Sun. In short, there really isn't anything on the Moon that we particularly want or need. The Moon is kind of like a barren and rocky island located nearby that is easy to get to but doesn't have any soil or resources worth exploiting.
Next destination: the cloudtops of Venus. This is actually quite a simple concept: 50 km or so above the surface of Venus the temperature and atmospheric pressure is about the same as that on Earth, and Venus' atmosphere is much heavier than ours, meaning that breathable air floats similar to the way helium does here. An aerostat carrying breathable air for humans would then float, and thus it would be possible to stay afloat for as long as one needs. Venus is also the second-closest object to us after the Moon, and launch windows are more frequent than a destination like Mars. Gravity on the "surface" of Venus is also about the same as that on Earth, so there would be no need to worry about the physiological effects of extremely low gravity. For a pdf giving more detail on exploration of Venus (from a lecture given by Geoffrey A. Landis at MIT last year), see here.
Here's a quick chart of the temperature and atmospheric pressure on Venus (from here) that shows how the temperature and atmosphere lower until they reach a very earthlike level by the time you reach an altitude 50 km above the surface:
Negatives: these are obvious. The first is that any colony is necessarily limited to the air, and so no exploration of the surface could be done. Also, we still don't know enough about the atmosphere at that level, regarding important aspects like the frequency of lightning. The air is also acidic, and the winds are extremely high due to something called super-rotation, and they reach speeds of 400 kph. The environment there is perfect for an unmanned solar flyer which could stay in the sun for 24 hours a day (because of Venus' slow rotation), but for manned exploration we're not ready yet. Finally, there seems to be quite a bit of resistance to the idea of exploring a planet without being on terra firma. Perhaps psychologically the idea of staying above the surface in an artificial aerostat all the time is a little bit depressing.
The logical next destination would then seem to be Mars. It's the next farthest out, it has an atmosphere (but a very thin one), the length of day and night is almost the same as our own, water exists to a certain extent. But Mars has some serious drawbacks for human exploration and since we're comparing the two it's best to begin with a quick introduction to Ceres.
Ceres: Ceres is an object of some 950 km in diameter (since it orbits a star and has hydrostatic equilibrium I believe it should be called a planet without all the nonsense about "clearing its orbit", but that's a different subject) located in between Mars and Jupiter. Here's what its orbit looks like:
Ceres has one important detail that makes it much more interesting than one might expect: apparently it has lots and lots of water:
This 100 km-thick mantle (23–28 percent of Ceres by mass; 50 percent by volume) contains 200 million cubic kilometres of water, which is more than the amount of fresh water on the Earth. This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modelling.When you take a look at Ceres compared to some other bodies like the Earth it seems like quite a small place to explore, such as in this image:
(that's Ceres on the bottom left)
This looks tiny, but there's still a lot to explore: the actual surface area of Ceres is some 2.8 million km2, which is the equivalent of the surface area of either Argentina or Kazakhstan, or the total surface area of the largest three states in the US put together: Alaska plus Texas plus California. Ceres isn't just some tiny asteroid with nowhere to explore.
Now let's compare Ceres with Mars as a destination for colonization. How do they stack up?
First of all, getting there in the first place. Mars is closer to us, Ceres is farther out. It would seem that this would result in us not being able to send missions to Ceres as frequently as to Mars...but in fact the opposite is true. Keep in mind that the only time we can reach a destination is when a launch window opens up, and frequency of launch windows is determined by the synodic period (basically the amount of time it takes for an object and the Earth to line up with each other). This image of Spirit's flight path to Mars shows how a launch to a destination like Mars works. The launch window opens up when the two planets are fairly close to each other but the outside object is ahead, then the probe launches, and meets up with the destination planet a few months later on.
So you can see that launch windows occur when two objects are fairly close to each other but not at their closest point, and the frequency in which two objects approach each other (the synodic period) determines how frequent they will be.
Here are the synodic periods for some destinations in the Solar System (:
|Synodic period (days)|
Notice something about Mars? That's right, it has the longest synodic period of them all. On one extreme with an extremely fast object like Mercury it catches up with us in no time at all, and on the other extreme objects far out at the edge of the Solar System have extremely slow orbits which means that every time the Earth moves around the Sun it has almost "caught up" with the other object (since it has only moved a tiny bit around the Sun during that time) and we have an opportunity to make use of this to launch a probe to the planet. But Mars' orbit is exceptionally badly placed in that even though it moves more slowly around the Sun than Earth does, it still moves fast enough that it takes Earth a long time to pull away and finally catch up with Mars again. Since Ceres is located farther away and thus has a slower orbit, launch windows are much more frequent. 780 days vs. 467 days is a huge difference in terms of frequency.
Once a launch window has opened up it does take longer to reach Ceres than Mars, but since at the moment a journey even to Mars takes around 6 months, we're talking about a very long-term journey in any case. With new propulsive technologies like VASIMR, travel time to a destination like Mars will eventually apparently be brought down to something on the order of 3 weeks or so, and if we have reached this point technologically then even a trip to Ceres should take less than 3 months, half the time it takes to reach Mars now.
Also bear in mind that human exploration of Mars at the moment just isn't possible with our current technology and funding, so any assumptions on exploring these planets should be made keeping in mind the fact that any human exploration of these objects will necessarily require better technologies than the ones we have access to now. So in short, once we have reached a level where we can seriously think about exploring one or the other we're looking at a tradeoff of a few extra weeks in space when going to Ceres instead of Mars, but with launch windows every 467 days instead of 780 days...which means that Ceres is the destination we'll be able to get to and from more frequently.
Next: getting back. Ceres is much smaller and less massive than Mars:
...which gives it a much lower escape velocity. To escape the gravity of Mars you need to be moving at a velocity of 5.027 km/s (almost half that of Earth), but for Ceres this is a much much lower 0.51 km/s. With a much lower escape velocity not only do you need much less fuel to escape the planet to return to Earth, but the initial mission from Earth (since the vessel and fuel needed to get back also needs to be launched from Earth in the first place) will then require that much less fuel to launch from Earth, which lowers costs dramatically.
To show how big a difference this makes, I've made up a very quick and rough example. It doesn't take into account delta v, wind resistance or anything else, but even as rough as it is it serves a purpose in showing just how huge the difference is in the velocity needed to escape Ceres vs. Earth, Mars and the Moon.
Edit: apparently the rocket images below are somewhat inaccurate, but inaccurate in the right direction (I'm happy with that as that was my intent in the first place, merely to show how ridiculously small a rocket you send off from Ceres would be ); they could all break the gravity of their respective bodies with a fairly smaller size than that shown below. Keep that in mind as you look at the images.
Let's take the Delta II rocket on the launch pad before it sent up the Kepler telescope in early March. It's one of the most reliable rockets we have. This rocket is capable of sending spacecraft either into or out of Earth orbit, and it's around 38 m in height. There are people milling about below in this picture, but you can barely see them.
So let's take a closer look at some of the people at the bottom there. They look like this:
Next we'll use that guy in black next to the tower to show the size of the rocket required as we reduce the size by location. First let's take a look at Mars. Escape velocity is 45% that of Earth, so we'll shrink the rocket to 45% of its size but leave him as is:
So there he is standing next to a rocket about the size one might need to break Mars' orbit. It's still a pretty hefty rocket, 17 metres in length so just a bit smaller than SpaceX's Falcon 1. Something akin to this (in size/heft) would need to be available to a group of humans on the surface of Mars to get back.
Now let's take the Moon as the next example. You need 21.25% of the velocity you would need on Earth to break the Moon's orbit. Now the rocket is only 8 metres in height.
Much smaller than the last one. Now how about Ceres? Escape velocity is 4.6% that of Earth, so we'll shrink the rocket down to 5% its size and now we have this:
With this it's now barely larger than the person standing next to it.
For one more example of how easy it is to reach escape velocity from Ceres, take this rocket for example in the video below. It was launched by a private team in 2004 and reached a velocity of 1.5 km/s. That's far below the velocity needed to escape Mars, still not quite enough to escape the Moon...but three times the velocity needed to escape Ceres.
So for a rocket to escape Ceres you wouldn't even need the velocity of a fairly small rocket that a group of private space enthusiasts can construct all by themselves. That's a huge difference. Furthermore, looking here you can see a list of rockets built by some other rocket enthusiasts, some of which have maximum velocities over half that required to break orbit from Ceres. These rockets as well are usually only 1-3 metres in height. In short, the difference between the velocity needed to escape Mars vs. Ceres really is the difference between night and day.
Day length, climate, seasons, and gravity:
- A day on Ceres lasts 9 hours compared to 24.6 hours for Mars.
- Mars has a thin atmosphere whereas Ceres (probably) has none
- Mars has an axial tilt similar to that of the Earth, giving it seasons similar to our own
- The gravity of Ceres is only 3% that of Earth whereas Mars is 38%.
Granted, Mars may feel more like Earth given all that. However, considering ease of exploration Ceres wins out on all three. A shorter day means equipment doesn't need to be built to withstand longer nights. Mars is closer to the Sun and so the amount of power that can be gathered during the day is greater, but the thin atmosphere is actually no help here: it's thin enough that Mars is a void in terms of human physiology (i.e. take off your helmet and you still die) but still enough that dust storms can and do often blow up for weeks at a time, obscuring the Sun and suddenly making it harder to gather power. A dust storm in 2007 darkened the sky in almost the entire southern hemisphere and almost doomed the rovers there, so these dust storms are nothing to take lightly.
So when planning electricity gathering for a mission to Mars one has to take into account yearly changes in sunlight depending on the season as well as the possibility of bad weather over months at a time, whereas on Ceres there's very little variation at all.
Finally: terraforming. Yes, Mars can be terraformed, eventually. However, terraforming is an extremely long process though that is so completely beyond us at this point that it's not even worth talking about. That is, by the time we've obtained the ability to completely terraform a planet we'll have long since surpassed our current problems re: launch windows, fuel to get to and from a destination, protecting astronauts from radiation during the journey, etc. Besides, terraforming can also be started without a human presence.
On a slightly off-topic note, could Ceres ever retain an atmosphere? The short answer is no: even though it's farther from the Sun than Mars is, it's much too small (in terms of mass) to retain its own atmosphere, although remember that this is over a long period of time; that is, were Ceres to suddenly be given an atmosphere tomorrow it wouldn't just be blown away by the solar wind the next day. I've seen some estimates on forums before that Ceres should be capable of retaining an atmosphere for a few hundred years (tinier than tiny in geologic terms, but not quite so tiny from a human colonization point of view), but those were unsourced so take that number with a large grain of salt. There are some pretty creative ideas out there on giving Ceres an atmosphere such as constructing a coil around the equator to give it a false magnetic field, and using a lot of SF6 (sulfur hexafluoride) in place of nitrogen considering how heavy it is in comparison. Some have proposed just a tiny atmosphere (perhaps with an atmospheric pressure just 1% or 0.1% that of the Earth) to keep colonists safe from micrometeorite impacts. The technology to do any of this though is way beyond us though, so the possibility of terraforming or the creation of an atmosphere doesn't really have anything to do with the potential for colonization in the short term.
Back on topic: so where to start? That's easy - we've already begun. A probe called Dawn is on its way right now and will arrive at Ceres in 2015 after orbiting Vesta for a few months. Once Dawn arrives and we are able to see Ceres close up then we'll be able to determine whether it's worth exploring in further detail with rovers and eventually with humans. Since there is no perfect destination near the Earth to explore there will naturally be differences of opinion over where we should explore first, but Ceres thus far has been more or less ignored by the public at large, and deserves more attention than it has gotten. So in the meantime as we wait for Dawn to arrive, just keep in mind that there's a (dwarf) planet out there called Ceres that is likely a very worthwhile candidate for us to explore, and deserves our consideration as well.
For more info on Ceres, see here, here, here, and here (this last one is a pdf). If you're interested in seeing some other informal discussions on the subject check out here, here, and here, and finally there's a fairly interesting discussion on the subject on the space.com forums here as well that actually goes into a fair amount of detail on exactly what steps we could begin taking over the next few years to begin the process.