Monthly Archives: February 2014

Follow the Light

chipsat_iss

On a hot day, we might say that the sun beats down on us. It’s literally the case: those photons carry momentum, despite that they’re massless. As they bounce off a surface, they impart momentum, i.e. they provide a propulsive force without propellant. Because of this force, mere light can accelerate a spacecraft to high speed. There’s your interplanetary spacecraft, right there—a solar sail. Here’s the spacecraft design question: what opportunities present themselves when we consider very small solar sails? Can we do better than the classic approach to the problem?

The idea of a solar sail has been around for quite a while. Arthur C. Clarke wrote “The Wind from the Sun,” a story about a solar-sailing regatta in which large, diaphanous spacecraft race one another with light propulsion. Small, meter-scale solar-sails have been launched, with modest success. The basic physics are not really in question. In fact, NASA is now building a 1200 m2 experimental solar sail to investigate the technology, the largest ever, known as Sunjammer. It’s an inexpensive demo of a very lightweight structure, including inflatable booms and other wonderful innovations from L.Garde, Inc.

Solar sails accelerate in proportion to the surface area that reflects solar photons, and in inverse proportion to the mass of the total space system: i.e., the larger the faster, and the heavier the slower. So, an ideal solar sail is super-thin, yet rigid. Dupont now makes a 0.1 micron-thick material that is suitable for solar sails. But the sheer dimensions of typical sails require an explicit effort to stiffen the structure, e.g. with booms (hence Sunjammer’s inflatable components). Some have proposed spinning a thin sail so that centripetal acceleration keeps the sail stretched outward. Spin also helps keep the attitude fixed in a convenient way. To be precise, let’s say that there’s a linear dimension, L, that characterizes the width or diameter of the sail. That area goes with L2. And let’s say that its mass, or volume, changes with L3. The acceleration changes with area per mass, or L2/L3=1/L. So, as L decreases, acceleration increases, all things being equal.

Of course, they’re not quite equal. There’s a lower limit on the thickness of the sail, and larger spacecraft can have disproportionately thin sails. But remember that the larger the sail, the more mass is devoted to keeping it stiff. That’s where a satellite-on-a-chip comes in. It’s an idea that we’ve been working on at Cornell since early 2005. We call them chipsats. We even made some prototypes, thanks to collaborations with Draper Lab and Sandia Labs. In fact, in a few weeks about 128 circuit-board prototype versions will launch and will be deployed. They’re called Sprites (the name given them by Dr. Justin Atchison) and will be launched on the Kickstarter-funded KickSat led by Zac Manchester. By early April, we may know how well this basic concept works.

One thing a chipsat does exceptionally well is that it remains stiff despite being very thin. A single silicon wafer (maybe 25 microns thick), about 1 cm square, might be enough to transmit very small amounts of data from space. Its tiny antennas are essentially rigid, too, despite being thinner than a human hair. But it’s a got a great “lightness number,” comparable to typical solar-sail designs. Think about it. If you shrink a spacecraft uniformly, so that the acceleration due to pressure effects changes with 1/L, that little chipsat has a big advantage. As L becomes small, that acceleration grows. So, a chipsat may be a perfectly adequate solar sail, without any of the complexity that larger solar sails bring.

We can do better than the Sun, at least this far away. We can concentrate the light that acts on a chipsat, maybe with a parabolic mirror. Or maybe with a laser. Sure, let’s use a laser. How much power we need depends on how much momentum we want to impart. More precisely, the photon pressure P, in Newtons per square meter (N/M2), is simply the power per area (e.g. in Watts per square meter) divided by the speed of light: P=E/c.

For example, if a 1 cm2 chipsat is fully face-on to a beam of light that covers its surface, the area is A=0.0001 m2. If it’s essentially solid silicon, whose density is 2330 kg/m3, its mass is m=0.000005825 kg, i.e. 5.825 mg. Let’s say m=6 mg. The acceleration would be a=F/m, where F=PA. So, P=ma/A and E=cma/A. The power W would be W=EA=cma.

To lift off from the surface of the Earth, this little thing would have to feel a force just a little greater than gravity, whose acceleration at Earth’s surface is about 9.81 m/s2. The laser power would need to be W = 300,000,000 x 0.000006 x 9.81 = 17,142 Watts.

The degree of unreasonableness of this estimate has to do with several issues. First, can you get a laser this powerful? Sure. Off-the-shelf industrial laser diodes are available at 10 kW. (The Army even put one on a truck as a weapon.) Doubling or tripling that number should be within the reach of current technology. How about the fact that the laser will be a little sloppy, or in any case it won’t exactly cover the 1 cm2 chip? OK, let’s gang together a half dozen of these diodes and point them upward in parallel, maybe with the help of an array of precisely steered mirrors. That’s less than six trucks worth of electronics, if we extrapolate from the Army’s design. What about the fact that directing nearly 20 kW worth of laser power would probably vaporize the little chipsat in an instant? Now that’s a good question.

The answer lies with the technology of dielectric mirrors. It turns out that reflecting off metal always incurs losses. The photon that reaches the surface of the metal induces a little electromagnetic response in the metal, which then releases a new photon. That process involves some loss of energy because of the unavoidable electrical resistance of the metal. A dielectric mirror literally reflects the same photons, with no such electromagnetic response (they can’t respond that way; they’re not electrical conductors). Dielectric mirrors reflect better than 99.999% of the incident light, but only in narrow bands. Well, conveniently, laser light has exactly one wavelength. That’s as narrow as it gets. This near-perfect reflection leaves only 0.17412 Watts behind, only about 70% more power than sunlight at Earth’s distance to the Sun. In short, I’d expect the chipsat to get less than twice as hot as if we just left it sitting out in the sun.

It’s also hard for this system to deal with Earth’s atmosphere. Wind gusts would blow the chip around, forcing it off the beam. (Dr. Leik Myrabo’s Lightcraft had a related problem during their White Sands tests, although the physics are entirely different from what I’m proposing here.) Setting aside atmospheric disturbances for a moment, let’s consider the problem of beam divergence, the tendency of the beam to spread out. It’s going to happen. And it happens in proportion to the wavelength. Say we use a 500 nm (green) laser. After a 1 cm narrowest point, which the laser folks call the “waist,” the beam diverges with an angle of 0.000016 radians. After 628 m, the beam has spread out to twice that thickness, so that its intensity is only a quarter of what it was when the beam was 1 cm wide. The beam becomes so weak that the initially high acceleration peters out, and the chipsat reaches only about 117 m/s. Maybe more important, the beam attenuates while the spacecraft is still close to the Earth. That is, the beam weakens faster than gravity does, which means that the chipsat falls back to Earth (in fact, it would hover at some distance above the ground, where gravity balances the photon pressure of the laser).

Two ways to address this problem. Assuming that only modest adjustments in wavelength are possible (so, we’ll assume they don’t help), we adjust the optics, widening the beam as much as possible. If it can be widened to 10 m, the photon acceleration drops slower than gravity. However, this approach is flawed because the intensity is much lower, and 1 g can’t be achieved to begin with.

The second way is to start in space (and now we don’t have to deal with the atmosphere), with a wide beam, say 10 m (a hundred-fold increase). Now the chipsat can accelerate for much longer, reaching 10 times that speed eventually. Another hundred-fold increase in beamwidth (not really possible) provides another factor of 10 increase in speed.

Increasing the beam width won’t get us what we want. It attenuates the power too much. To make the chipsat more effective, we have to change its lightness number, i.e its surface-area-to-mass ratio. Let’s add a skirt, a sort of tiny solar sail, made from that Dupont material, perhaps supported by the chipsat’s own 5 cm antennas, for a 28 cm square patch of thin material. With a density of about 1420 kg/m3, that patch adds only 0.11 mg, an increase so tiny that we can neglect it in the following calculations. And let’s set the beam width at the same number, 28 cm.

Now the chipsat accelerates to LEO escape velocity (11030 m/s) after about 3 hours’ laser firing. And we really don’t need a half dozen lasers after all. The diverging beam actually helps in that regard, bathing the chipsat in ever-widening laser light as it accelerates along the beam, despite whatever imprecision there may be.

So, that’s one way to create an interplanetary satellite-on-a-chip, using what amounts to current laser technology, if we can afford to launch such a laser into orbit and power it. Again, it should be about the size of a large truck, with a solar array not far from what NASA will be building for the asteroid rendezvous mission. But once you do, you can send an unlimited number of chipsats on interplanetary trajectories. And chip the light fantastic.

Advertisements

Lunar Xistera

xisterawithcrater

What if you could land on the Moon without a rocket motor—in fact, just by landing on a runway and rolling to a stop with more-or-less familiar mechanical brakes? And what if you could take off again without a rocket, simply by using an electric motor?

Yes, I’m aware that there is no air on the Moon, and only a few centimeters of what one might call atmosphere (and is in fact electrostatically levitated dust particles). I’m not proposing that an aircraft would lift off the surface with aerodynamic effects. I’m considering the possibility of a spacecraft that lands at orbital velocity and slows down from there. To take off again, the spacecraft must reach orbital velocity as it travels along this runway.

Why bother? Quite simply, a rocket takes a lot of fuel. For reference, each Apollo mission that landed on the Moon comprised both a lunar descent stage (10,149 kg) and an ascent stage (4,547 kg), for a total of 14,696 kg. About half of that total was propellant. In our example, let’s assume that we want to land 20,000 kg on the surface of the Moon. And why that number? It’s the low end of the total mass NASA calculates a human Mars mission needs to land on the surface of Mars. So, that total might make sense for a lunar landing as well—for instance, the case of a mission with a longer duration than any of the Apollo landings. For a spacecraft to be in orbit just above the surface of the Moon—assuming it’s at the average lunar radius of 1,740 km—it has to be traveling faster than 1,679 m/s. So, for it to stop by the end of the runway, it has to get rid of 1,679 m/s worth of energy. And for it to take off again, it has to attain that speed (or more). Unlike Apollo, our spacecraft may be able to use the same hardware—the wheels or something else mechanical—to land and take off. It does not necessarily expend propellant in the process. So, right there, I anticipate some significant mass savings. And this same vehicle can land and take off repeatedly, without refueling. Nice.

How long this runway would have to be involves some guesswork. Here are some of the numbers involved in my guesses. The FAA says that 1,829 m is sufficient for most aircraft under 90,718 kg. Longer runways exist, e.g. O’Hare’s runway 28, at 3,963 m. However, different issues are in play here, notably that gravity is lower on the Moon and likely doesn’t let the brakes decelerate the vehicle as quickly as they would on Earth without skidding. And the absence of atmosphere gives greater determinacy to the spacecraft’s touchdown, i.e. landing precisely at the beginning of the runway. So, I take it that the overrun area and other forms of margin are less necessary. Of course, most aircraft land at speeds far below that of our orbiting spacecraft. In fact, they land at speeds lower than that of the Space Shuttle, which used to land on Earth at 366 km/hr (104 m/s). Good automotive brakes can decelerate at about 1 g, and let’s assume that that principle holds for lunar gravity, where g=1.622 m/s2.

Decelerating at that rate from orbital velocity therefore requires at least an 869 km-long runway. That seems very long. Inconveniently, expensively long. Let’s try scaling up from those FAA guidelines, using the ratio of kinetic energy as a scale factor. For the same mass, that ratio is (1679/104)2=260.6. So, that basic runway distance would scale up to 477 km. Still quite long!

My dad, a former Marine Corps pilot, has a better answer, or I think he would if I were to ask him. Naval aviators extend a so-called tailhook from the back of their aircraft to snag a cable (one of several available attached to hydraulic cylinders) that slows down the aircraft over a relatively short distance: from 240.8 m/s to zero in about 104 m. Those are high gs, and I doubt it feels very nice. Nevertheless, at no more than 23,000 kg, those aircraft are about the mass of our spacecraft, albeit with about 1/50 of the spacecraft’s kinetic energy. So, how about a series of arresting gear, like those cables, over a distance of about 25 km? The spacecraft would decelerate at a withstandable 56.4 m/s2 (5.7 g, or 5.7 times Earth gravity), and it would take about 7.7 seconds. Quite a ride, but nothing the Navy hasn’t seen, and actually quite a bit more gentle than landing an aircraft at sea. But let’s add 20% margin to that length and call it 30 km.

Making a lunar runway requires something like concrete. We have an estimate of its length, but the volume of concrete depends on its thickness and width, too. Now, width seems not to be very important, judging by the lessons learned from aircraft carriers. Let’s say 10 m, which is more generous than what naval pilots require. As for thickness, we need enough concrete to prevent the landing from crushing the runway and to resist deformations of the Moon’s surface (yes, there are moonquakes). Our spacecraft is traveling a lot faster than an airliner, but its weight is far less—both because it has less mass and because the Moon’s gravity is only a sixth of Earth’s. While airport runways range between 25 cm and 1 m in thickness, I don’t see why ours needs to be any thicker than the most extreme terrestrial runway, at 1 m. Even that is probably excessive, but the point here is to come up with an upper limit of how much material is needed.

It certainly would take a lot of effort to bring concrete from Earth. And this runway requires 300,000 m3 of material. Naturally, we would build this runway from local materials—the lunar regolith. There are myriad recipes for lunar concrete, although the only solutions I’ve encountered require bringing quite a bit of material from Earth. So, here’s my recipe, which doesn’t:

Let’s begin by considering small rovers that can create regular, comparatively smooth bricks by sintering lunar material, as has been demonstrated at the Johnson Space Center (JSC) and elsewhere, or perhaps some other process. For sintering, the rovers will need solar panels and time, and an insulated (ceramic) mold that compacts the regolith (like WALL-E compacts trash), but not much else. And they can be at it for years as part of a robotic precursor mission before anyone is ready to land on the runway. Say each brick consists of a liter of material (10 cm on a side), with about 3.1 kg of mass each. There are 1,000 in a cubic meter and therefore 300 million bricks in this runway.

There’s a classic solution: melt regolith into bricks, whether with a 3D printer or some sort of oven and a mold. A 400 W rover could create a brick every eight hours—one hour during which it collects the material and later places it, and seven for heating up the sample (as in the JSC tests) from 350°K to 1,100°K. The bricks need to reach 1000°C for the silica to fuse, and although the JSC tests held it there for three hours, this slower heating would require less hold time. How much? I don’t know. This is just a ballpark figure. The specific heat of the regolith varies with temperature and was reported by some folks at Harvard to be about 1 J/g/degK at room temperature—i.e. 1,000 J/kg/degK in useful units. I modeled the specific-heat variation as linear, extrapolating from that Harvard data. This estimate is based on 1 m2 solar panels at 30% efficiency, for about 400 W of power during the 14 1/2 Earth-day-long lunar day. And I assume that about 20% of this available power is lost in electronics, imperfect insulation in the mold, et al. So, about three bricks per Earth day per rover—at least, while the sun is shining. Half the time it doesn’t shine, even on the Moon, simply because the Moon rotates as it orbits the Earth. (Incidentally, this sintering could all be done a lot faster with nuclear power, but I’d rather focus on a readily built system.) So, this approach would take 54,800 rovers 10 years to complete.

Forget that! Way too many rovers, obviously. I say fabricate the bricks out of frozen mud: regolith+water, which is far more brittle but could be repaired every time with far less effort than building a sintered-brick runway. With a supply of water, and in the cold, each brick would take maybe 5 minutes. And at that speed, we’d need only 570 rovers. That still seems like a lot of rovers, but at 50 kg each (the scale of the Violet spacecraft), that’s about 28,550 kg—one landing’s worth of rovers. Maybe plan for three landings, since we’ll need all that fuel etc. before the runway is built. Still, three landings and 10 years gets us permanent infrastructure for lunar transportation.

In preparation, a strip of the lunar surface is cleared of boulders. Also not hard for some small-scale rovers to achieve.

An adhesive mortars the bricks together. Again, proposed solutions for such mortar are legion. A particularly appealing one is the use of sulfur as a binder and as mortar, and that’s not hard to find on the Moon. That solution would also require no materials to be brought from Earth. However, if the mud bricks are solid enough to resemble terrestrial bricks, I would propose that we use water again as mortar—simply freeze the bricks in place, which is a solution that both uses in-situ material and also lends itself to straightforward repairs, as long as the ice is not exposed to the sunlight. So, protecting the surface of the mortar would be necessary, again using local material. Regolith itself—a thin layer of dust—might be enough. And this principle raises an important issue: sunlight would soften these bricks, turning them into mud. The simplest solution would be to land at night. Another, less simple, solution would be to build this runway in a permanently shadowed crater, of which there are several. The constant shadow ensures constant sub-freezing temperatures, which would keep the runway solid. In fact, these locations are also where water is found on the Moon’s surface. However, the location would be near a lunar pole, which may be limiting (although there are many reasons why a polar outpost could be a good idea, such as the availability of permanent sunlight and shadow).

After each landing, rovers inspect the runway. They seal cracked bricks and dribble water into the interstices as needed. Or they remove bricks entirely and replace them. Now for an interesting adaptation. What if the runway were not straight but, in fact, curved and banked—for example, along the edge of a crater. Some of the larger craters, like Tycho, are wider than 50 km. The tighter the radius and greater the bank angle of this runway, the higher the centripetal acceleration that would keep the spacecraft from skidding, thanks to increased friction at the wheels. So, how about a runway with an initial, flat landing region that curves into a lower, circular track? With no atmosphere, the Moon doesn’t require that the runway be entirely horizontal. The spacecraft’s initial approach simply allows the vehicle to begin tracking the runway’s kinematics, its path, which I suggest should be tilted so that the vehicle’s path is parallel to the lunar surface, but curved, like a jai alai xistera.

extrathickxistera

Let’s say the spacecraft decelerates at 5 g (Earth gravity, again), i.e. 49.05 m/s2. The force it feels would be inward, i.e. toward the runway surface, to keep the wheels in contact, as in the case of terrestrial runways. At this deceleration, the runway spirals inward over its roughly 28 km length, and there’s no need for the arresting device. This banked runway’s shape is a little harder to build than that of a flat runway, but it’s likely easier to operate, is a lot shorter than the alternative, and requires no hardware sent from Earth other than the rovers that build it. The image above is an exaggerated view of the runway: too thick and wide (I am showing it that way for clarity), but the curvature and other parameters are exactly what would accomplish this goal.

The runway merely needs to withstand 49.05 x 20,000 = 981,000 N inward, toward its surface as the spacecraft travels along the curve. It also has to withstand that same amount as a shear, along its surface, as the vehicle brakes. Even doubled (2,774,000 N), that force is far lower than commercial airliners apply to terrestrial runways.

Incidentally, for cargo only, a much higher g-load would be possible. Say 15 Gs. In that event, the spiral runway could be less than 10 km, as long as the runway could withstand the force.

Now, let’s free up our thinking even more. Do we really need those bricks? If landing in soft regolith—powdery sand—is possible, all that may be necessary would be for microrovers to clear the large rocks and boulders. The motivation for this banked spiral is to avoid a large number of bricks. So, one might return to the 800 km or longer runway, if it’s possible to find such a stretch of open, flat area on the Moon. A quick look at a lunar map suggests that it may be. Such a runway may be even shorter, given the drag of the regolith on the landing gear. The downside is that the drag of the regolith may be hard to predict and may overturn the landing vehicle.

So, in summary, we have several concepts here. There’s a long, flat 800 km runway that may have a sintered-brick surface or may simply be a soft, rock-free area. There’s a thoughtfully curved, banked runway that is much shorter—10-30 km. And there’s a 30 km runway with arresting gear, like on an aircraft carrier.

The spacecraft that lands on this runway needs wheels, or maybe skis. But if the goal is to take off again, a set of wheels makes more sense to me. Could they withstand the landing? A key issue is that the wheels must come in contact with the ground without too much relative velocity. Aircraft are able to land with wheels that spin up as they contact the runway, but the orbital speed is much higher. A 1 m radius wheel is not far from what large commercial aircraft use. Such a wheel, rotating at just over 16,000 RPM, would contact the runway without skidding. 16,000 RPM is fast, though. The tensile strength of the material must be quite high for the wheel not to tear itself apart at that speed, let alone the other forces associated with landing. A carbon-fiber composite wheel is necessary here.

Spinning up these wheels is not trivial. The International Space Station uses control-moment gyroscopes (CMGs) for attitude control, and their rotors require a long time to spin up—many hours. That’s because the spin motors typically are used only to keep the rotors spinning. On the rare occasion that a spin-up is necessary, they go as fast as they can. But that’s not very fast. So, I would anticipate that the wheels for this spacecraft need to begin spinning up many hours before landing. The power would come from solar energy, though, not propellant. A really useful feature of establishing that much angular momentum in the three-or-more wheels is that the spacecraft would have a high momentum bias, stiffening its attitude dynamics and allowing for a lower-risk approach to the runway, with little or no pitch or yaw motion. The landing gear need some sort of shock absorbers, like the oleo stroke gear on other aircraft. That’s a largely off-the-shelf component.

At this point I need to acknowledge that I’m not the first to consider all of these ideas. Some, perhaps, but not all. After reading the first draft of this post, a key member of my vast editorial staff introduced me to the work of Krafft Ehricke. Ehricke was a futurist and visionary technologist. He came up with the notion of a lunar runway long before I did, and lots of other great ideas besides. He had in mind several permutations, roughly along the lines of two of my three architectures: a long, dusty runway cleared of boulders and a paved surface. He put quite a bit of effort into the former, looking into the behavior of regolith for a vehicle that might land in it. But I have to say that his thinking, like mine, was driven by the spirit of his age. For him, a nuclear powered system to gather regolith and produce concrete was not much of a stretch. But for me, having seen how money is spent and how work is prioritized in Washington, I am focusing on a much leaner design that involves readily launched technologies with comparatively low cost. And, perhaps most important, I have benefited from recent discoveries, from Clementine to LCROSS, that confirm that the Moon is simply loaded with water. Ehricke had no idea. In fact, most of us assumed that the Moon was simply bone dry. Until about a decade ago, our exploration-mission architectures had humans bringing all the water they would ever need. That fundamental principle even shapes today’s architectures. It’s time for a re-think.

I claim that this banked and pitched, mud-brick runway built by robotic rovers in a permanently shadowed lunar crater is a new idea. It allows a spacecraft to land in a short distance, seems feasible to build on a useful time scale, and requires no fantastical technological breakthroughs.

Taking off again requires some more attention. I’ve given some hints at how it might be accomplished already. However, since this post is already quite long, and I’ve already offered about four new ideas here, I’ll save an analysis of this maneuver for a future post.

Spacecraft a Week

757243main_06-01_graphic_Peck_full

I’m going to design a new spacecraft every week. Then I’ll blog about it.

One reason for my starting this blog is a conversation I had with a former chief technology officer for a large government agency. No, not NASA. A different one. He asserted that his time at the agency had already addressed the remaining open problems in space technology, that there was nothing fundamentally new to be done. Only incrementalism remained. We agreed to disagree.

Another has to do with the two years I recently spent at NASA. I served as the agency’s Chief Technologist from late 2011 until the end of 2013. From time to time I would encounter a similarly dreary perspective among some members of the world’s space community. But, as I had hoped, I also found the opposite. There is a lot of enthusiasm out there for innovating in space. Many of us believe that the best days are ahead, that there are inventions to make, adventures to experience, and science to discover. NewSpace companies are popping up all over California. Some have their sights set on asteroids, mining them to create a self-sustaining space economy and space infrastructure, and some are going to image the Earth in unprecedented temporal and spatial detail. Some of my academic colleagues are setting out to explore space on their own: discovering new planets, understanding the Earth, and maybe heading to Mars in the next few years, all without waiting for the science community to catch up.

Space is possibility. As Carl Sagan said, “The surface of the Earth is the shore of the cosmic ocean. From this shore we have learned most of what we know. Recently we have waded our way out, maybe ankle deep, and the water seems inviting. Some part of our being knows this is where we came from. We long to return, and we can.”

In fact, there are so many “open problems,” so many completely new ideas that have never seen the light of day, that I’m confident I could blog about a new space system every week—each week a system that no one has seen before. In fact, I’ll go so far as to blog a year’s worth of these spacecraft a week. I’ll do so to hint at the vastness of that cosmic ocean and the ships that might sail it.

There is an awful lot of incrementalism in the technology world, so much near-term thinking. We need that, sure. But there is so much that has not yet been said. I think someone needs to say it.  Let me offer an even broader point.  Humanity needs a grand vision for space exploration, one that we commit to.  There are many such visions out there, but we only infrequently see the big questions addressed strategically.  More common are modest tactical investments in near-term capabilities that address part of a large vision that is not fully articulated.  I’m confident that there are creative solutions to some of the fundamental problems of space—access to orbit, how to live and even thrive beyond Earth, how to extend humanity across the cosmos.  But we need to think big.  I’ll try to prove my point with these posts.

I’m not concerned about people adapting these ideas for their own creative purposes. In fact, I’d be flattered. I want these things to exist out there somewhere. Maybe I’ll be the one to bring them into existence, or maybe it will be someone else. As an academic, I’m used to the idea that my work shows up elsewhere. Typically, people cite your work. Every now and then they don’t, but that sort of thing would distress me only if I thought I was out of ideas. In fact, when I start complaining about not receiving credit, you’ll know I’ve lost confidence in my ability to create new things.

But just to be clear about all that, I’m going to say that the technology concepts in these Spacecraft a Week posts are covered by a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). According to the Creative Commons website, this license means that you may copy and redistribute the material in any medium or format. You may transform and build upon the material. As the licensor, I cannot revoke these freedoms as long as you follow the license terms: you must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. You may not use the material for commercial purposes.

Why not commercial? After all, I’m just as much a believer in the power of self-interest to incentivize progress in things like space exploration as anyone. I’m OK with the idea that self-interested pursuits, such as making money from lunar exploration or asteroid mining, will hurry along the expansion of humanity into the solar system. It’s simply that I don’t think that using a license to cover my 52-or-so blog posts about spacecraft is going to hamstring the commercial space economy. There are enough commercial opportunities out there. Instead, what’s at stake is the democratization of space. Use these ideas to help you extend your presence into the cosmos.