The final destination I’d like to consider is the asteroids. But before we get into the wherefore and whyhow, we should spend a little time dispelling the most common myths about the asteroids, simply because those myths frequently distort the arguments.
Myth #1: All asteroids are located between Mars and Jupiter.
There are actually asteroids located in just about every region of the Solar System, from inside Mercury’s orbit to outside Pluto’s. Though asteroids are more plentiful between Mars and Jupiter, there are also significant families of asteroids that bounce between Earth and the Sun (the “inner Earth Objects”, at least 6), those with nearly circular orbits that cross the Earth’s orbit (the Apollos, at least 5,000), those with elliptical orbits that cross the Earth’s orbit (the Atens, at least 500), and those that cycle between the orbits of Earth and Mars (the Amors, at least 2600) . All told, there are more than 620,000 asteroids and 3200 comets  known to be in the Solar System.
Myth #2: All asteroids are big/small
Asteroids have a wide range of sizes, from as large as a planet (1 Ceres, 590 miles across) to as small as a grain of dust. Though most of them are very small, due to a combination of how they formed and how they collided, there are more than 10,000 known asteroids with a diameter of more than ½ mile . The trick is to find an asteroid that is big enough to be useful but not so big that it cannot be handled.
Myth #3: All asteroids are identical rocks.
To put it simply, asteroids are the leftovers of the Solar System. They are the pieces that either never got incorporated into a planet or got knocked apart during planet building. That’s why they come in such a wide range of sizes. And that’s why they come in such a wide range of compositions, as well. Some asteroids (the C-group) were just barely cooked; they fell together and generated just enough heat to form small inclusions (“chondrules”) but not enough to drive off the water. Based on the meteorites we’ve observed on Earth, C-class asteroids can be up to 22% water along with organic molecules and primitive rocks. Based on the spectral composition of asteroids, some 75% of them are C-group .
If you cook the asteroids a bit more, either by slamming them together at higher speeds or by making larger asteroids out of them or by bringing them in closer to the Sun, then the amount of water goes down and you are left with a stony or S-type. Because the process takes time and energy, only about 17% of asteroids are S-class.
If you make the asteroid large enough and hot enough, then it may differentiate and form an outer stony crust and an inner metallic core; current thinking has this happening within the first 100 million years of Solar System formation. If you then break those large asteroids apart through collisions, you’ll get E-type asteroids (named for the mineral enstatite in their rocks) and M-type asteroids (named for the metals that they hold); together, these represent about 5% of all asteroids.
Most of the remaining asteroids are P-type, which are believed to be rich in organic materials and various ices . The interesting thing about P-type asteroids is that their compositions are very similar to those of comets. In addition, there are groups of comets that are known to have very short periods (called, logically enough, “short period comets“). One famous set of 470 known comets has a maximum distance from the Sun near the orbit of Jupiter and are known as the “Jupiter comets“. And then there are minor planets that have characteristics of both comets and asteroids, such as the Centaurs.
The point here isn’t to confuse you with a barrage of technical terms, but merely to point out that asteroids have a wide variety of compositions, ranging from pure metal to nearly pure ice, and to remind you that nature rarely puts hard boundaries on categories so that dismissing something as “an asteroid” or “a comet” does the science a disservice.
Myth #4: Asteroids stay in one place or one orbit.
Something that frequently gets lost on those not in the planet-building biz is that orbits are dynamic things. As the various chunks of stuff wing through the Solar System, they interact with each other. Sometimes, the interaction is subtle, as in the 8:13 orbital resonance between Venus and Earth . And sometimes, the interaction is pretty obvious, as in the time that Jupiter tore apart a comet and ate it. Orbital resonances are common in the Solar System and play an important part in how it has changed over time. Currently, most planetologists think that an orbital resonance between Jupiter and Saturn in the early stages of formation was responsible both for their migration to their current locations and for the Late Heavy Bombardment that gave the Moon its pock-marked puss.
And they help change asteroid orbits. As a simple example, consider an asteroid in the main belt that is jiggled outward by getting too close to another asteroid. Soon enough, it falls into one of the gaps in the belt caused by orbital resonance with Jupiter. Every time that Jupiter gets near the asteroid, its gravitational pull tugs on the little rock, distorting the orbit more and more until it finally is thrown farther out or forced inward. And it isn’t just Jupiter that causes this sort of interaction; those asteroids that come close enough to Earth may find their orbit perturbed in weird and wonderful ways. Indeed, orbital resonance is one of the reasons that Earth typically has a temporary, small, second Moon drawn from the asteroids that pass near it. So, though the asteroids may have been in the Solar System since it formed, they haven’t been where they are for all that time.
Myth #5: Asteroids are hard to get to
This is one of the more persistent myths about asteroids. Unlike the other myths, there is a tiny shred of fact buried deep in it. You see, many asteroids are both easy and difficult to get to. In terms of the amount of delta vee required to get to an asteroid, there are at least 10 known asteroids that require less than 4 km/s, along with 461 more that need between 4 and 5 km/s, and another 1,804 that would require a delta vee between 5 and 6 km/s. All told, there are some 2,275 asteroids that are known to require less energy to get to than the Moon.
But, as we saw when discussing the Moon, energy is only half of the problem; the other half is time. It takes just three days to get from LEO to the Moon. If you want to get to the lunar poles, it will take a little longer (how much longer depends on how much energy you want to spend). Getting to an asteroid takes much longer. Unless the asteroid is one of Earth’s temporary moons (and we can’t really plan a mission based on a temporary phenomenon), the travel time is likely to be between three and six months for a Near Earth Asteroid . Traveling to an asteroid in the Main belt could take anywhere from six months to several years. If we restrict ourselves to a round trip that lasts less than a year, has a delta vee of less than 6 km/s, and includes a stay at the asteroid of at least a week, then there are 79 different known places we could go. If we want a trip that lasts less than six months with a delta vee of less than 6 km/s and a stay of more than a week, then it drops to just fifteen different known asteroids . (NB: These numbers are based on the assumption that we want a human to actually go out to the asteroid. If we instead decide to send a DAWN-type probe, then the travel time problem becomes much less onerous.)
Myth #6: Asteroid have no (usable) water
There are two roots to this myth. The first is the misperception that asteroids are mostly stony or iron instead of being largely carbon- and volatile-rich; fortunately, the spectra that we’ve recorded provide some pretty strong proof that a typical asteroid should have (or have had) water. The second is the belief that water ice is incredibly unstable under typical asteroid conditions and will quickly sublimate away. There is some science behind that statement, but it doesn’t bear up to scrutiny.
We need to start with the idea of the “snow line” or “frost line”; it is the innermost location at which hydrogen-rich compounds would have condensed into ices during the formation of the Solar System. Most experts put the snow line at about 3.5 AU , which is well beyond where most asteroids now live. So if the asteroids formed in their current location, then they couldn’t have a lot of water because the combined heat from the Sun and from collisional accretion would have driven it off. However, the current view which is based on observations of “hot jupiters” that have been seen around other stars is that the asteroids didn’t form in situ but were pushed into their current location by the inward migration of Jupiter and the other planets, though the exact amount of inward movement is (to put it mildly) a topic of some debate. As evidence of this, there is the example of asteroid 24 Themis, which has an orbit stretching from 2.7 AU to 3.6 AU and is known to contain water ice, along with that of 1 Ceres, which has an orbit going from 2.5 AU to 3.0 AU and a thick layer of water ice. Given that they couldn’t have formed where they are and have ice, they must have formed farther out and moved into place.
And it is that movement that matters now. It is indisputably true that as an asteroid moves closer to the Sun, it gets hotter. We can even use the black body temperature equation calculate how warm it should be , or, more correctly, how hot the surface of the asteroid should be. But an asteroid isn’t all surface and doesn’t heat all the way through instantaneously. If you’ve ever walked into a cave or used a heat pump then you know why: there is a thermal lag between the temperature at the surface and that in the interior. In general, it takes about three months for a temperature change to make its way from the surface of an asteroid to a point one meter deep and the amount of the temperature change is just ~1/3rd that of the temperature change at the surface. So if you were to move a ten-meter asteroid from 4 AU to 2 AU, its surface temperature would increase from 130 K to 194 K. If you left the asteroid there for three months, then the surface would still be 194 K and the temperature would quickly drop to 154 K a meter in and 138 K two meters in and remain at 130 K in the center. If you then moved the asteroid back out to 4 AU (as orbits are wont to do) and left it there for three months, then the surface temperature would be 132 K (the extra is from the interior heat flow) and the temperature would rise to 136 K one meter in and drop back down to 132 K two meters in and remain at 130 K in the center. Because the asteroid would spend more time at the farther end of the orbit, the average temperature would end up being closer to that at the far end than that at the near.
In addition to creating thermal lag, the blanket of soil would also act to reduce the sublimation rate because it would have lower permeability than open space (i.e., it would slow down the escape of any sublimated gasses) along with a higher tortuosity (i.e., the escape paths would be longer, which again decreases the rate of loss).Some experts think that just s thin blanket of loose soil could reduce the loss of water by a factor of ten to twenty. But is that enough? Well, given that one expert has found that a 1 km sphere of pure water ice would last 3,000 years at 1 AU and 3,000,000,000,000 years at 3.5 AU before the effects of thermal lag, permeability, and tortuosity are taken into account, the answer appears to be yes.
Of course, even if all of the water ice is gone, there will be plenty of water left in the form of hydrous minerals, such as gypsum and kaolinite. This is especially true of the asteroids that stay entirely within the Earth’s orbit. Will this bound water be no better than the water trapped in concrete? No. Simply grinding and heating gypsum to 425 K will extract ¾ of the water locked up in the mineral (this is how they make sheet rock). Similar methods work on other hydrous minerals. So, even if there is no water ice, there will be plenty of water on most asteroids.
Myth #7: Mining on an asteroid will be just like mining on Earth or on the Moon
One of the more interesting things that they teach in business school is that the first company to jump into a new technology almost always goes broke. Why? Because, instead of adapting their work to the new technology, they adapt the technology to their work. For example, when electric motors began to take over for steam engines and water mills as a power source in factories, the factory owners would usually buy one electric motor and place it at the end of a long belt that it would run; when workers needed power, they would link to the belt. It was done that way because that was the most efficient way to do it for a water wheel. But electric motors allowed each tool to have its own power source instead of working off of a common one; using it like a steam engine lowered the efficiency of the factory and made many of them go broke. When they began to use tools that were individually powered, the factory’s efficiency went up and the costs went down.
The moral of the story is that mining on the Moon will be different than mining on Earth. And mining on an asteroid will be different from either mining on the Moon or mining on Earth. For example, electromagnetism to separate ores is used on Earth but not extensively because the magnet has to fight gravity and only finds the highest iron content ores. On an asteroid, finding the iron ores may be as simple as running a magnet over the surface. Similarly, one could use a mass spectrometer to separate elements from an asteroid; the same could be done on the Moon but only much less effectively.
But the biggest difference may be how the ores are collected for processing. On Earth, it is generally not cost-effective to carry ores very far for processing ; it is likely that the same will hold true on the Moon. For example, the Spudis plan for mining the Moon requires 16 robotic missions to bring all of the gear simply because there are limits to how much one lander can carry. But because many asteroids require less delta vee to go to than the Moon and because you don’t land on an asteroid so much as co-orbit, that limitation is no longer in force; what takes 16 missions on the Moon can be accomplished with one mission to an asteroid – everything can be assembled in LEO  and then launched to an appropriate asteroid in one large package .
Or, if we’d rather not expose astronauts to the tedium and dangers (and dangerous tedium) of a long voyage, we can bring the asteroid to Earth for processing. Though logic would place the asteroid in LEO for ease of access, NASA has proposed a plan that puts it into cislunar orbit as a way of giving Orion something to go to . Placing a twenty meter C- class or S-group asteroid into LEO would pose no risk to Earth as it would break up in the atmosphere in an event like the recent one in Russia . We could send out a robotic tug that would either capture the asteroid in a giant air bag or lasso it or use the tug’s gravity to slowly shift the asteroid’s orbit. The good thing about sending robots is that it would be less expensive and easier than sending people; the bad thing is that we’d have to either work with an abominable time lag (due to the amount of time it takes each signal to go and return) or develop autonomous robots that could work semi-supervised. (Of course, if the robots are good enough, we could have them process the asteroid and just send back the good bits.) The only sure thing about an asteroid mission is that trying to run it like a lunar mission will ensure its failure while running it like an asteroid mission will make it more likely to be successful.
Mining the asteroids
OK, with all of that in mind, what will an asteroid mission look like? Well, just as has been the case for the Moon and Mars, we’d start by finding where we want to go. We need to do a full-sky survey that will locate, track, and spectrally identify every asteroid or other object more than five meters across in the inner Solar System. NASA has been working on that survey for nearly twenty years now, but it has always been underfunded . Though NASA now has the orbital elements for about 230,000 asteroids and has identified another 390,000, that is by no means a complete catalog! Fortunately, private industry may be stepping up to help. Recently Planetary Resources announced the funding of their first space telescope dedicated to finding asteroids; they hope to have several dozen of them in orbit before they are done.
Once several targets are identified, the real work begins. They would want to have several options because work schedules may slip. Thus, they wouldn’t train to go to “asteroid 23456” or “asteroid 31415”; instead they’d train to go to an asteroid. As noted before, there are at least 79 different possible asteroid targets that are known right now. So the number of targets isn’t a big problem, nor is the frequency of launch windows. Indeed, there are more launch windows for asteroid targets than there are for lunar and Mars missions combined!
If we are sending people, then they would launch into LEO along with their living quarters, propulsion unit, and mining equipment. Once in orbit, the whole mass would be joined into one ship that was launched toward an asteroid. The crew would head toward the asteroid and then extract the water and other valuable materials  before heading back to Earth. Net time, about a year. Net gain, damfino.
If we are sending robots then we would still want to assemble everything in LEO before shipping it off to the asteroid. But that is a much simpler problem as we don’t need shelter or food for the robots (unless you count batteries). The robots could be sent on a slower trajectory than people, which would allow for longer mission times. The real question with robots is do we process the asteroid in place or bring it back home?
The real sticking point for an asteroid mission is that we haven’t really done one yet. Right now, we are in the same place that the US was with Apollo 6 or 8: we’ve sent probes to the general area that we want to go (Surveyor and Ranger for Apollo, DAWN and Hyabusa for asteroids) but haven’t actually put boots on the ground yet. As Apollo 13 showed, there can be a lot of unexpected things that go wrong with any space mission and a long duration deep space mission has a much slimmer margin for error than just about any other type of exploration.
So, with all of that in mind, how does exploring the asteroids look on our four point checklist?
What do we learn?
This is perhaps the best part of going to the asteroids. Where going to the Moon or Mars teaches us about mature planets, going to the asteroids teaches us about planets before they formed. We get samples of the primordial muck that formed the Solar System (C-group) along with samples from planets that almost formed (S- and M- types) and possibly even some prebiotic matter that could have kick-started life on Earth (the P-type). We would learn at least as much as we would from going to a planet, and possibly much, much more. In addition, a six-month or year-long visit to an asteroid would help us as an intermediate step between the short week-long jaunts out to the Moon and the much more arduous three- to five-year trek to Mars. Finally, learning to move asteroids would help us the next time that a dinosaur killer takes aim at Earth.
What can we do next?
This is a at least as great a strength for the asteroids as it is for the Moon. The asteroids are as rich in resources as the Moon and have a much lower delta vee budget (though a longer travel time). If the asteroid resources are used in space (e.g., water for fuel, metals for manufacturing), then they save the cost of shipping everything up from the Moon’s gravity well and back into the ecliptic plane, albeit at the cost of longer delivery times. That cost can be minimized by having multiple asteroid missions running concurrently.
What does it prove?
This isn’t as great a strength as it was for Mars, but better than it is for the Moon. As with Mars, asteroids are implicitly but not explicitly mentioned in the OuUter Space Treaty so exploiting them for resources may not be forbidden (just very, very difficult – I see committees – lots of them). And going to the asteroids also demonstrates that our technology has continued to improve in a way that returning to the Moon won’t.
What does it cost?
This is where the asteroids wins, hands down. Where a return to the Moon would cost somewhere between $93 and $280 billion, and going to Mars would cost in the neighborhood of $91 to $911 billion, going to the asteroids could be done for free. More exactly, it could be done in such a way that it makes money for the taxpayer. If NASA goes to the asteroids, then it would likely cost between $2 and 20 billion for a robotic mission. And most experts believe that the costs for a private mission would run about the same. However, unlike NASA, a private mission could sell the water and minerals extracted from the asteroid and make several billion dollars , almost all of which would be taxable. So the company would make $10 billion and pay $2 billion in taxes to the US government, meaning that we would get the mission for free.
The next post will summarize everything we’ve discussed and try to come to some conclusions.
 Please notice the “at least” in each of those statements. Those are the asteroids that we know about. But asteroids are tricky little buggers to catch. They are small, dark, and fast moving, all of which make it very hard to identify them and track them. The numbers that I give are based on the known asteroids; there are probably at least 1/3 more in each group that haven’t been discovered (i.e., I wouldn’t be surprised if we found out that there were actually 6700 Apollo asteroids out there).
 Why I include comets will be obvious in a moment.
 This is actually a bit of a circumlocution because the actual situation is much murkier. Because asteroids are so small, we cannot measure their sizes directly. Instead, we measure the amount of light that they reflect (the “H magnitude”) and assume that the asteroid is spherical (a bad first approximation for the smaller asteroids) and has an albedo of .15 (a good first approximation); from that, we can relate the amount of reflected light to the asteroids size and distance. But an oddly shaped asteroid can have a magnitude that is too big or too small, and the asteroid’s composition can change the albedo dramatically. But this works for a first approximation.
 There are lots of subdivisions and overlaps that show two things: (1) we haven’t learned enough about asteroid formation to develop a consistent system, and (2) planetologists are splitters not lumpers.
 Please remember that, to a planetologist, an ice is any solid material that would be liquid or gaseous at Earth normal conditions. So you can have water ice, ammonia ice, and even formaldyhyde ice.
 The 8:13 tells you how many orbits the outer planet makes (Earth, 8) compared to the inner planet (Venus, 13).
 I’m assuming a free-fall Hohman-style orbit, simply because they are the current best way of getting between planets in space. As mentioned on the Mars Next post, if we had a better ion drive then we could speed from Earth to an asteroid in under a week but that will require drives at least ten times more powerful (and efficient) than we currently have.
 You may have noticed that the word “known” keeps popping up. That’s because, as noted previously, we haven’t discovered all of the asteroids. If you increase each number by about 1/3, you’ll probably be in the right ballpark (though some have suggested that a factor of ten may be more appropriate).
 Just a quick reminder for the non-science geeks reading this: an AU is an “astronomical unit”, equal to the mean Earth-Sun distance and set at 149,597,870,700 m. Just as parsecs and light years are handy yardsticks for interstellar distances, AUs are useful for comparing distances in the Solar System.
 That equation is one of the strongest proofs of the greenhouse effect as it predicts the average temperature of the Moon almost perfectly but not that of Earth.
 The Keystone pipeline is an exception, not a rule. If it were easier to build a refinery in Canada or the northern US, then the pipeline wouldn’t be built and the ores would be processed there and the oil shipped here.
 Just as God and Werner von Braun intended…
 How large? Consider NASA’s notional ion drive to Mars, which was assembled in LEO before heading off to the red planet. The fleet had five ships, each massing 360 tons, that all set off for Mars at the same time.
 As of right now, that plan appears to be a non-starter. The House wants to cut NASA’s budget severely, the Senate wants to keep the budget where it is but use it to build a rocket to nowhere, and the White House wants to send people to an asteroid and not asteroids to the people.
 I’m assuming a density of 3000 kg/m3 and a relative velocity of 17 km/s (i.e., way too high). If you’d like to play with the parameters yourself to find out how large a rock we can drop before things get ugly, head over to the Imact: Earth! website.
 At one point, the search for asteroids was paid for by the US Air Force because NASA stopped funding it!
 This is one place where the asteroids have the Moon and Mars beat all hollow. A single asteroid such as Eros could have enough precious metals and other valuables to be worth $20 billion!
 Some folks put the estimates into the trillions for some of the more exotic asteroids. This may be where some of the frustration from the Moon First and Mars Next crew comes from; no economic analysis of those missions ever shows them as doing more than breaking even – which is why it is so hard to get private companies interested in those missions other than as NASA contractors.