Category Archives: Concepts

Gliese 581g (Hámnù, Pedak, Gaustan, or Estivama)

I finished a big new map! You can purchase a print here.

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The world known to humankind as Zarmina (catalog identifier Gliese 581g) is a habitable planet orbiting a red dwarf star. It is tidally locked to its dim sun, such that one face of the planet always points toward the sun. The most striking consequence of this orbit geometry is that the habitable region of the planet is a disk-shaped area roughly the size of an earthly continent. The center of this zone always sees a sun at high noon, while toward the edge of the disk, the sun sinks gradually away from zenith. Outside this region, Zarmina is encased in ice. As the sun does not define east and west, the cardinal direction convention on Zarmina refers to the planet’s orbit, instead: prograde (in the direction of the orbit), retrograde, normal (up from the orbit), and antinormal.

Zarmina does not exhibit evidence of plate tectonics. Surface features express several processes: large-scale rift graben form from tidal stresses, shield volcanoes build over mantle hotspots, impact craters and basins dot the planet, and erosion slowly whittles down the more ancient features.

The world hosts life with biodiversity similar to the Earth. One dominant intelligent species has settled across the landmass, with cultures reaching technological development levels roughly equivalent to 1300-1600 CE on Earth. There are three regions with large populations, indicated on the map in normal-retrograde (NR), antinormal-prograde (AP), and normal-prograde (NP) callouts. In the four major language families of Zarmina, the natives call their world Hámnù, Pedak, Gaustan, or Estivama.

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The NR region hosts two major linguistic and cultural families. The first is an empire ruled from the city of Hòmp Sīnkà (Port Sinka). Explorers and artisans populate this empire; though the political extent of the empire only reaches as far as Níngtòhús (Greencliff), speakers of the imperial language can be found all along the coast in the prograde direction as well as in coastal settlements on the other side of Fíkùm Pòst (The Normal-Direction Sea). The antinormal borders of the empire are more ragged and contentious, however – the imperial urge to spread its vision of culture and knowledge brings it into direct conflict with the city-states in that area. The people of Kivod Sev Adoso (Mountain Gate Town) dominate the substantial resources of Sev Skem (Mountain Channel) and have repelled several campaigns launched from Hútpòkā (Chasmtop). Hòmp Sīnkà rapidly loses its stomach for these campaigns, and so Kivod Sev Adoso holds back imperial expansion. A more fluid and contentious collision of cultures occurs in Pasken Gimet (Pasken Forest). Scattered settlements under the command of local chiefs raid imperial populations farming antinormal of Ngùsì Āmā (Wide River) while imperial reprisals prevent the Pasken peoples from incorporating large towns. The disparate kindgoms of Ogjapud (Grayrock), Katofa Petang (Retrograde City), and Fetva Zand (Calm Peninsula) maintain their own set of animosities and alliances.

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The plains of the AP region offer little shelter from the winds that blow in off the ocean. As the land rises, larger and larger plants cover the land until one encounters lush prairies between dendritic river networks. Roaming clans live on the prairie “kidan.” A few large settlements dot the kidan, most notably Jung a Uid Nakaun (the City of Two Rivers). The kida clans take pride in not pinning themselves to a particular place – many of their dwellings are portable, and they happily move their crops to new locations on the fertile plains when they tire of the old. The culture is leery of townfolk. The Ushtin clan is a splinter from the kida clans, and is more attached to their resource-rich homeland on the shore of Gaiju a Shai (Lake of Wind). On the other end of the cultural spectrum, the dramatically different Togui a Awaish (Chasm of the Forest) hosts a sect worshipping the sun god Dautwai. This sect possesses the settlements of Santiso (roughly, Above-the-Green) and Uigonja (named for the uigon trees), as well as a major urban center in Jung Togunau. From the isthmus of the Nakau Dautwai, dramatic views of the Audos a No (Mountain of the Sun) have inspired monuments throughout the city. The natural defenses of Togui a Awaish shield the people within from raiding kida clansmen.

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Lush lands and geographic barriers squeezed into a comparatively smaller area give rise to the warring city-states of the NP region. Though they share a common linguistic root, each of the population centers here represent separate nations. The largest are Evinbok and Neka Estag, both named for their original monarchs. Evinbok holds a position of strategic strength, with access to productive outlying farmland in Pantma Zhusti (the Upper Plains), while timber and easily quarried rock are in the ancient impact basin of Gesta Kazi (Broken Bowl). Kagzai (roughly, Blue-ton) and Ka Topi (Lower Town) are notable for practicing a form of representative democracy. Ka Shata Besi (High Cliff Town) is the center of a prosperous small nation of traders, who build ships from the timber of Tifa ko Pantma Shti (Forest of the Red Plain) and sail through Vimna Shti (Red Pass) as far antinormal as Sot Ushtin.

This map is hand-drawn with Pigma Micron pens of various types, then colored in Derwent watercolor pencils. I finish the map by painting over the pencils to blend and soften the watercolors together. The last step is photographing the piece with a 60 mm macro lens. The entire thing is 17″ wide and 14″ tall.

Enjoy, everybody!

How to Get to Mars

NASA wants to go to Mars. Great!

The approach, as the agency has been publicizing with fancy graphics like the one below, seems to consist of the following:

NASA’s tentacles travel to Mars
  • Send astronaut Scott Kelly to the International Space Station for one year, to learn about the effects of zero gravity.
  • Perform the Asteroid Redirect Mission, moving a near-Earth asteroid into lunar orbit, to prove that solar electric propulsion works in space.
  • Assemble a Mars transfer spacecraft in distant Earth orbit out of components launched on the Space Launch System.
  • Pack everything astronauts need for a round trip to Mars on the new spacecraft and send them on their way!
  • Keep the spacecraft in space for future trips to Mars. Bring the astronauts and supplies back and forth separately.

Not that these aren’t good brainstorming ideas, but they are not how I would get to Mars. I am too confident and impatient for this plan.

For one thing, we can probably skip this one-year mission. In fact, NASA, I can help you out by zipping straight to the conclusions: Being in zero gravity for a year results in bone loss, muscle atrophy, a compromised immune system, radiation exposure, and changes to the shape of the astronauts’ eyes. We know all that already. Similarly, we already know that solar electric propulsion works – quite effectively, robustly, and scalably – in space. Commercial satellites are flying solar electric propulsion right now, with more on the way. Heck, NASA itself has been flying solar electric propulsion, on missions like Dawn, since the turn of the millennium! Nothing needs proving here. We can take the known technology and use it.

Now, assembling a Mars transfer spacecraft, sending it onward, and reusing it for further exploration – that I like. Here is how I would do it.

First, get one of the companies developing solar-electric propulsion satellites to build a number of spacecraft buses. They will probably run a few tens of millions of dollars each, and they can ride up to space on Falcons, Arianes, or Atlases. (That’s bargain basement stuff for NASA!) Then, tie them together. All I really want are the propulsion systems. Each spacecraft has a propulsion system with something around 10 kW power, and NASA wants to get up to around 100 kW to go to Mars. So, by my rocket science calculations, we need…ten satellites. Or maybe, if we strip out all the telecommunications payloads that these satellites usually carry but I don’t care about for this application, maybe we can get the number down to five-ish.

Four of 'em stuck together
Four of ’em stuck together (with obligatory blue ion engine exhaust)

Somebody would probably have to do some thinking about the best way to support all these stuck-together satellites. Maybe a truss of some kind. But I’m not too worried about that, because NASA has two decades of experience building modular things and sticking them to trusses in space. They can just do what they do best, using their own well-proven techniques.

Now we need a place to put our astronauts. Preferably a place that has some accommodations for solving the problems that Scott Kelly will be confirming. Many of the major physiological issues with space travel have to do with being in zero gravity. Too bad our Mars transit vehicle can’t bring gravity along with it.

Oh, wait! Science fiction knows the answer. It’s known the answer for decades! Spin the spacecraft. The astronauts get to live with a force akin to gravity, pulling them outward along the spin axis.

But building a giant ring-ship takes a lot of time, effort, energy, and resources. I have something different in mind. Something simpler:

My Mars transit vehicle is finished!
My Mars transit vehicle, finished!

On the right, that’s supposed to be an inflatable, cylindrical habitat. (Inflatable things would be terrific for space construction, because they only need a small launcher. Since everything on my vehicle is made of small components, we can launch them once a month instead of once every two years, if they needed a super-heavy launcher like SLS.) This inflatable habitat is tied to the central propulsion core by tethers, or maybe trusses of some type. The astronauts would feel “gravity” pulling them toward the right-hand side of this image (and a little bit downward, because of the thrust). On the left is a dumb counterweight: I’ve drawn it to evoke the empty upper stage of a rocket. It could maybe be long-term storage, but its main purpose is simply to be dead weight to make the spinning easier. The whole vehicle would rotate about the thrust axis, rapidly enough to give the crew at least lunar or martian gravity levels. (The illustration isn’t to scale!)

I’d do one last thing before I send this to Mars with a crew. I’d pack the transit vehicle with enough food, water, and air to get the astronauts to Mars, and for their surface stay.

Not enough to get back, though.

Instead, I would bring seeds. When the astronauts land on Mars, the first thing they will do is become high-tech space farmers. They are going to grow all the food for their return trip on Mars’ surface.

Why would I want to do that? Well, for one thing, seeds are smaller and less massive than full-grown food products. They are probably less expensive – in an energy sense – to get to Mars than those food products would be. Then, on Mars, we can get water and carbon dioxide from the atmosphere, to fuel plant growth. So, over the whole mission, I’m actually saving time and money. There’s also a second reason, one I find more compelling. What’s the point of this whole endeavor if we don’t come out of it knowing how to colonize and explore other planets, and keep colonizing and exploring them? Learning to use the resources on other worlds is fundamental to the future of space exploration. We know Mars has water, we know it has oxygen, and we even know that we might be able to grow crops in its soil. We should focus on that idea and advance it. In other words, I think that – both pragmatically and philosophically – it would be shortsighted and silly to attempt Mars exploration using only what supplies we can bring from the Earth.

We need a space program that focuses on developing the technology to use the resources on Mars to support further Mars exploration. We need to do this in a modular, reusable, scalable manner. We need to make sure our astronauts – no, our pioneers – have the tools, the materials, the infrastructure, and the autonomy to solve their own problems. In other words, we need to stop thinking about how to put a few guys in spacesuits on Mars, and stop thinking about how to have astronauts do science on Mars, and instead think about how to colonize Mars. That requires a lot of little things to come together, with more than a few big things in the mix as well. But, for the most part, we have the technology. We’ve had it for my entire lifetime. We need a space program with the right stuff to use it.

That’s how to take a journey to Mars.

Back to the Future

This week, NASA launched the Orion spacecraft on a test flight. I have a conflicted viewpoint about this event. To me, Orion is both exciting and deeply disappointing.

On the one hand, Orion is NASA’s first entirely new spacecraft for human crews in over twenty years. There have been only seven in American history: Mercury (first successful flight in 1959), Gemini (1964), Apollo (1966), Skylab (1973), Space Shuttle (1981), Space Station (1992), and Orion (2014). Those dates have been getting unacceptably distant from one another, and it is wonderful to see NASA getting its game back on. Test flights are NASA’s business. The agency is supposed to operate in the proving ground of technology. I want to see it doing new things, and the Orion flight was a reasonable first step towards NASA getting back to its roots. Those roots, after all, were triumphant!

On the other hand, though, Orion falls far short of what NASA could, and should, do. The spacecraft is an improvement over the last capsule NASA designed – the Apollo Command Module – but it is an incremental improvement rather than a revolutionary one. As an example, one of the technological advancements NASA has been touting about Orion is its glass cockpit. But this is the era of the iPad: in such a mass- and volume-constrained environment as a space capsule, the glass cockpit is simply not innovative – it is obvious. (For examples of innovation in space vehicles, try inflatable habitatsrockets that fly back to their landing pads or crew shuttles that could land at any normal-sized airport runway.) To make matters worse, not only is Orion an incremental step in capability from the early ’70s, but its development is horrendously stretched. For reasons that are political, programmatic, and cultural, rather than technical, the next flight of Orion will be in 2018. A human crew will not use the capsule until after 2021, at which point it will be almost a decade old itself.

I think the most bitter disappointment is the concept of what Orion will do when it finally does have astronauts on board. The current plan is to build the world’s biggest rocket, the Space Launch System, and use it to send an Orion crew to asteroids and Mars. I’m all for visiting those places, but the “giant rocket with space capsule” architecture – the architecture of Apollo – is a recipe for one-shot visits to other worlds. After the first astronauts return from such “flags and footprints” missions, there is a very strong risk of program cancellation. I don’t want to see our space program cancelled.

An Orion capsule riding a Space Launch System rocket is not even close to how I would design a sustainable, long-term space program. A much better approach is to use many different spacecraft, and specialize them for individual purposes. Think of the Apollo Lunar Module: it was a flimsy, silly-looking vehicle that was exactly suited for the prospect of landing and taking off from the Moon’s surface. Instead of building on Apollo and Space Shuttle heritage to make an “all-purpose” space capsule, I would like to see NASA lean on its Space Station experience to design a set of interplanetary transport spacecraft. These vehicles would stay in space for their whole lives: we would assemble them in space, launch them out of Earth orbit rather than from Earth’s surface, and we’d never use them to carry astronauts up from and down to the ground. Whenever they need more fuel, we’d send only the fuel up to them – not a whole new vehicle. When we need to rotate astronaut crews, we could always hire SpaceX.

That sort of multi-vehicle concept offers some big advantages. First of all, it’s much more flexible than giant, infrequent, one-shot missions. Second, it’s far more efficient and cost-effective. Think about this: 20% of Orion’s mass is its heat shield, a component only needed for the last ten minutes of its mission. If Orion is returning from Mars, then that means our Mars return rocket needs to be huge in order to push that heavy heat shield on its half-year-long journey back to Earth. And if the heat shield and Mars return rocket need to be huge, then the Earth departure rocket needs to be enormous. Instead, though, we could forget sending the heat shield to Mars in the first place. Forget having the vehicle that goes to Mars also be responsible for landing on Earth. When the astronauts come back from Mars, you just send a little rocket with a little capsule into Earth orbit – just enough to bring the astronauts to the ground. The lightweight interplanetary spacecraft stays in space.

In 2011, when Congress ordered NASA to begin work on the Space Launch System, internal NASA studies came out that agree with my assessment. Multi-vehicle approaches that we can re-fuel and re-use in space are more efficient and cheaper than SLS. They will also get astronauts to other worlds much sooner. Most importantly, such approaches won’t be reliant on one-and-done missions. They will be much more likely to keep our explorers in space.

That is why, while I applaud NASA’s successful demonstration of the ability to launch new vehicles, I hope the agency moves on quickly from Orion, and begins work on a new fleet of exploration vehicles to stay in space.


The Space Review has a fantastic article that invaded my whole way of thinking this morning while I was trying to get into my groove for work. It casts the golden age of space exploration – the Space Race – as a contest of two visionary dreamers against their employing superpowers. It also goes a long way towards explaining the allure of SpaceX! The arguments presented therein may or may not be right, but they certainly form an interesting view to read.

It’s a fantastic and different historical perspective. Plus, some of the author’s writing includes delicious indictments of the use of space technology for evil.

…how about half an elevator?

If you’ve paid any attention to science fiction in any form, you’ve probably seen the concept of the space elevator. A super-strong tether or tower extends upward from the surface of the Earth, past geostationary orbit, and beyond; to get into orbit you just need to ride a car up the elevator to the geostationary point and…step off.

The space elevator solves a fundamental problem with access to space: speed. Getting height up from the Earth is fairly easy – just point a rocket up. But to get a spacecraft to stay in orbit, you also need to accelerate your vehicle to orbital velocity, which is at least 7 km/s. That’s where all the big booster rockets come from. The elevator, though, lets you get this speed without even trying. Since the whole structure remains oriented radially out from the Earth at all times, as your car climbs up the tether you automatically gain rotational kinetic energy. At the geostationary point, you will have enough energy to simply push out of the airlock and remain in orbit. Easy!

(This energy is easy to get, but it doesn’t come for free. Every time you go up the space elevator, you slow down the rotation of the Earth.)

Space elevators have some problems of their own, though. For one thing, we need materials and technologies sufficient to support the tether against the forces of gravity and rotation. For another, the Earth’s troposphere has some pesky disturbances that we call weather, and the space elevator has to be near the equator – where tropical storms happen. And then there’s…politics.

Great concept art from DVICE's article about the partial elevator. (Tony Holmsten)
Great tangentially related concept art from DVICE’s article about the partial elevator. (Tony Holmsten)

There was an article the other day about a paper examining a “partial” space elevator. The idea is to place a station at geosynchronous orbit, and run a tether only partway down to the Earth. The tether doesn’t have to deal with cyclones or touch the surface. Rockets bring payloads just to the bottom of the elevator, where they can ride the rest of the way up.

The idea reminds me of Robert Forward’s “rotovator,” which involves placing a long tether in orbit and making it rotate in the same sense and at the same rate as its orbital motion. Each tip of the tether traces a cycloid around the Earth: a trajectory that momentarily stops (relative to Earth’s surface) at the low point where it can pick up a payload, and swings back up to a high point where it flings the payloads forward much faster than the orbit velocity. It also has some similarities with cyclers, which are hypothetical objects in orbits that visit two (or more) celestial bodies at regular intervals without propulsive maneuvers. (Buzz Aldrin is a fan of these; he has an Earth-Mars cycler orbit named after him. That vehicle would alternately visit the Earth and Mars, with a 146-day transit time.)

Fundamentally, what all these concepts are trying to do is establish infrastructure in space – infrastructure that lets us offload some of the delta-v requirements from individual spacecraft, at the expense of an initial investment.

A more near-term such architecture would be an orbital propellant depot: a place where space vehicles could pause, after launch, and “top off” before they proceed onward to destinations beyond Earth orbit. Lots of technologists and policymakers have given thought to these depots, with many concepts nowadays revolving around the Falcon 9 and Falcon 9 Heavy launchers.

I’m a fan of these ideas. Any infrastructure that lets us explore space freely, without our launches being tied to landing requirements or our excursions on other worlds being limited by how we take off from the Earth, will only help our efforts to discover our place in the universe and establish humanity on other worlds. I think it’s high time our space program got back to thinking about the nuts and bolts of working in space and building the space-based vehicles that will take us to other planets and moons.


The National Ignition Facility – a humongoid stack of lasers all aimed at a tiny target to try and compress it until it fuses – announced today that they had “positive fuel gains,” meaning that fusion happened and that more energy came out than went in.

Clearly, this is big news for power generation on Earth.

But, with this breakthrough, I want to do something slightly different.

The ignition rocket!
The ignition rocket!

Jove’s Moon Shot

Here’s a bit of wild speculation for the weekend:

Suppose there is intelligent life in the globe-spanning ocean on Europa. Given how small our space exploration budget is, and our generally declining investments in R&D, how likely is it that the Europan life would discover us before we discover it?

An artist’s concept of Europa’s structure. (Britney Schmidt/DEAD Pixel VFX/University of Texas at Austin)

Any life or societies that evolve on Europa would do so underneath a shell of water ice. A human would no doubt find the environment claustrophobic, whether near the bottom of the ice shell or at extreme depths. Native Europans, though, would live comfortably with the perpetual presence of hydrostatic pressure. The creatures’ science would be familiar with the concepts of temperature and pressure. Some intrepid Europan theorists may even have extrapolated their equations to pressure = 0, but it’s likely that none of the creatures would have any firsthand experience with vacuum. The surface environment of the moon would therefore be totally alien and inhospitable; possibly, many of the creatures would have died in attempting to breach the ice before any succeed.

Would they even think to go up to the surface? I think so, as there is a strategic rationale. Without viscosity to slow them, and with about the same gravity as Earth’s moon pulling them down, a Europan army could move much more quickly over the surface of the moon than through the ocean. Europan kingdoms could launch surprise attacks if they were able to access the surface. Of course, the notion of doing this requires that the creatures realize that the ice over their heads has a surface. There may be ways to determine this from below, perhaps by watching for minute changes in lighting conditions, or even by direct observation of one of Europa’s surface cracks during its process of formation. Or maybe the Europans will just have to rely on explorers analogous to Earth’s Ferdinand Magellan.

Still, even with that strategic rationale, I think penetrating to the ice surface would be the Europan equivalent of the United States’ moon shot in the 1960s. That is, there would have to be a certain high level of technology, as well as a sufficiently well-organized political organization to support a successful attempt. The creatures of Europa would need to figure out how to support their high-pressure life requirements in the vacuum of space, not to mention figuring out how to tunnel or otherwise travel through anywhere from one to one hundred kilometers of ice. If the creatures aim for one of the cracks which may provide surface access from the ocean, then they would have only about a quarter of a Europan day (7/8 of an Earth day) to make the traversal before Jupiter’s tides close the crack again. Any way I look at it, I think that the creatures getting to the surface represents a tremendous achievement of technological prowess.

Once they get to the surface, the creatures would make a stunning array of discoveries. They may already know that their own world is spherical, but suddenly and immediately they would become aware – for the first time in their history – of Jupiter, the Sun, and the stars. In short order, they would discover the other large moons of Io, Ganymede, and Callisto. After spending some time making astronomical observations, they would see other Jovian moons, followed quickly by objects that orbit the Sun rather than Jupiter: Saturn, Mars, Earth, Venus, Uranus, comets. It would be an astonishing and groundbreaking time to be a Europan, as their worldview would experience revolution after revolution.

However, the downside of all this rapid discovery is that the Europan creatures’ science may lag behind the science of Earth. They would not have the long history of looking at the stars that we do. They might not have very good models for gravitational force. Until they get very good telescopes trained at the Sun, the very idea of fire or explosions might be foreign to them.

This is important for my main question, because without the concept of a gaseous explosion, the creatures would find it difficult to conceive and build rockets to begin a true space exploration program.

There is a way the creatures could start to explore their local system without developing rocket propulsion, though: it is conceivable to build large-scale catapults capable of accelerating objects to Europan escape velocity, which is only 2 km/s compared to the Earth’s 11 km/s. (“Only,” though that’s still very fast…Wolfram Alpha tells me that a reasonable comparison to the speed of 2 km/s is the X-15 rocket plane, though, which suggests to me that 1960s-equivalent Europans might have some hope to reach that speed technologically.) Surface-based accelerators would give the creatures the ability to explore the Jovian system.

Maybe a surface catapult plus a gravitational slingshot around Jupiter would allow the creatures to explore the wider Solar System. But I think that they need to develop some kind of rocket propulsion to have control over their efforts – or to return again. They also need to develop the sciences of orbital mechanics and Newtonian motion. These are disciplines that humans have been studying since the dawn of civilization. Our quantitative study of orbits and classical physics goes back to the 1500s.

I think the bottom line is this: if the Europan civilization has the same 7000 years of recorded history that human civilization does, and they reached the surface of their moon around the 1950s-1970s, even if they are more inherently curious and more willing to put forth exploration efforts than we are…they aren’t going to discover us first.

Unless we sit on our hands for a few hundred more years.

Orbiting magnetic balloons

I recently got the following question:

Let’s say I have a 500kg balloon floating in the stratosphere at fixed altitude with solar cells collecting 10kw from the sun, then my computation shows that if this energy can be converted to horizontal magnetic propulsion by repelling against the earth’s magnetic field at 100% efficiency then it could reach escape velocity in about one month. This is possible because at this altitude the air resistance is quite small so it is almost like pushing at an air hockey which does not require much force to get it to speed up horizontally. … My question for you here is that in reality how close to practicality is the design of this ‘spacecraft?’

Launch costs are one of the big drivers in the space industry, and the propellant required to get a spacecraft up to orbital speed is a major part of that cost. If we could use some sort of “propellant-less” means to get a vehicle into orbit, we could revolutionize the whole space industry. In fact, this is an idea that my grad school research group once brainstormed about during a lab meeting: push on the Earth’s magnetic field. If we start pushing from high altitude, where air resistance is small, then we just have to wait long enough to accelerate our spacecraft up to at least low Earth orbit speed (about 7 km/s). Launches might take a long time, but they would be far cheaper and easier.

As long as we can push on the spacecraft with a net force in the direction of its velocity, then it will accelerate. So, the first question we come to is this: how much drag force do we need to overcome? That force will provide us with an estimate of the minimum force our electromagnetic device needs to produce.

Air resistance causes a force in the opposite direction to an object’s velocity. For a sphere moving through the air, this force has a magnitude equal to 1.1 d A v2, where d is the air density, A is the cross-sectional area of the sphere (pi r2), and v is the object’s velocity. Let’s suppose we mount our 500 kg spacecraft on a high-altitude balloon that can get all the way up to 30 km altitude before we engage the magnetic propulsion device. At that altitude, the atmospheric density is in the ballpark of 0.02 kg/m3. (I’m reading off of the 1962 US Standard Atmosphere graph on Wikipedia, since I can’t look at NASA’s web resources. Thanks, Tea Party!) Now we have d.

Next question: how big is the balloon? Way back in Ancient Greece, when Aristotle had the original “eureka!” moment, he realized that objects float in a fluid when they displace a weight of fluid equal to their own weight. (Equivalently, they displace a mass of fluid equal to their own mass.) So, our 500 kg balloon-based vehicle has to displace 500 kg of air – and if it’s floating at a level where the air pressure is 0.02 kg/m3, then that means the balloon takes up a volume of at least 25,000 m3. That’s a sphere 36.3 m in diameter. (Note that here I’m assuming that the mass of the vehicle includes the mass of the balloon and of the gas we pumped in to inflate the balloon. What finally gets to orbit will be less than 500 kg.) So: A is about 1035 m2.

Now we have an estimate for the drag force magnitude on our electromagnetic launch vehicle at 30 km altitude, of about 22.77 v2. If we start our electromagnetic devices pushing, the spacecraft will start to move – but it will eventually settle on a steady-state speed at which the drag force and propulsive force balance each other. Here’s the bad news: even though the atmosphere is not very dense 30 km up, that v2 in the drag equation will really get us as we reach higher and higher speeds. If the balloon gets going at 1 m/s, the drag force will be 22.77 N. If we reach 10 m/s (about normal human sprinting speed), the drag force is 2,277 N. If we tried to accelerate the balloon all the way up to 7 km/s at this altitude, putting the vehicle in orbit, then the drag force will get to over one billion newtons! It’s not feasible to build a compact device that could push on the Earth’s magnetic field and generate this kind of force.

You might get the idea that as we accelerate, we can also gradually increase the balloon’s altitude. After all, if the air gets less dense, that drag force will decrease. With less resistance opposing our spacecraft, we don’t have to work as hard to accelerate it.

There are two problems we’ll run into if we follow this idea. First, while going up in altitude makes our spacecraft encounter less atmospheric density, it also has a weaker magnetic field to work with. At these high altitudes, atmospheric density is very much like an exponential decay. But the magnetic field from a dipole (like the Earth’s) falls off with distance from the dipole as 1/r3. How do the two functions compare?


This is good news. While at first, the magnetic field is lower than the density, eventually we come to a point where the magnetic force will be stronger than the drag force for fixed velocity. (This makes sense, because some spacecraft use magnetic forces to orient themselves when they are well above the levels of appreciable atmospheric density.) Suddenly, this idea doesn’t seem so crazy.

The second, problem, though, is tougher. Remember buoyancy? Once we get up to about 34 km altitude, according to that graph, the air will be about half as dense – which means our balloon will need to take up twice as much volume in order to stay afloat. The higher up our spacecraft goes, the bigger than balloon has to be. Eventually our balloon is going to need to be kilometers in diameter, since we won’t yet be up to orbital speed and gravity will just pull the spacecraft down unless we keep our spacecraft buoyant. (This is why high-altitude balloons always eventually pop!)

Because our vehicle has to solve both problems simultaneously – staying afloat and accelerating – I don’t think it’s feasible to get a large satellite into orbit this way.

However, if we move to a size scale where some of the physics behave differently – say, if we make our spacecraft very small – then perhaps we won’t run into this problem with the balloon. A few years ago, one of the researchers in my old lab took a look at some of these very questions of drag and magnetic forces on tiny spacecraft, though not with the goal of launch in mind. But one could, theoretically, make tiny spacecraft capable of accelerating to high speed by interacting with a planetary magnetic field. One could also, theoretically, make spacecraft tiny enough to flutter down through an atmosphere unharmed. Combining and reversing these ideas would be an interesting long-term research challenge!


Spacecraft Research is at it again

It’s been a little while since I checked in with the goings-on back at my Cornell research lab. Totally unsurprisingly, some very cool things are happening there!

One is that the Sprite and KickSat project has gone all the way from a back-of-the-envelope concept when I was at the lab to a flight manifest! Sprites are tiny spacecraft – think the size of a coin – that consist of little more than a solar cell, a little CPU, and a diminutive radio. They are pathfinders for an idea that, rather than relying on a single monolithic (and super-expensive) spacecraft, instead we could just run off a batch of a million tiny satellites and fling them all out into space to cooperatively complete a mission. Some of the applications we talked about included integrating basic lab-on-chip functionality to test for biomarkers, and then rain a bunch of the Sprites down onto Mars or Europa. They wouldn’t return the same wealth of data of a NASA flagship mission, but they would tell us where the interesting things are. Another reason why tiny spacecraft are cool is because they interact differently with Solar System objects than large vehicles do – so they might be able to take advantage of light, magnetism, or planetary atmospheres in different ways.

The KickSat project was the brainchild of grad student Zac Manchester. It’s a simple CubeSat design with a spring-loaded deployer, designed to release a couple hundred Sprites. On the ground, Zac can then track the intermittent radio signals from all these mini-spacecraft, and evaluate how well their unshielded components survive in space. Radiation will eventually kill them, but with many copies of the same spacecraft, we’d expect to see them die out statistically. They’re spacecraft with a half-life, and as long as the half-life is long enough to complete the mission, we don’t care that a huge number of Sprites burned out.

When I left the lab, Zac was applying for grants to build the KickSat hardware. But – despite the cool concept – there weren’t any takers. Eventually, he decided to turn to KickStarter to see if he could crowd-fund some spacecraft research. He ended up raising almost three and a half times as much money as he asked for, and become something of a pioneer for crowdfunded space activities! Zac is now working at Ames Research Center to perfect the Sprite and KickSat designs. They will be launching on the same SpaceX Falcon 9 rocket that will carry supplies to the International Space Station in September. This is actually the first CubeSat from my lab to make it all the way to launch, so I say: Go Zac!

Second, a project that is perhaps a little less flashy but a little closer to my heart has been making some great strides. Ben Reinhardt has been squirreled away in the same basement lab I remember, working on what he calls “eddy-current actuators.” The more fanciful – and very nearly accurate – name for the devices he is working on would be “tractor beams.” He wants to use these to grab onto defunct satellites, the outside of the Space Station, or maybe even some asteroids and comets, all without mechanical contact.

I was still active in the lab when this project got off the ground. In fact, I put together one of our first tabletop demonstrations of the principles involved: a changing magnetic field generates eddy currents in conductive materials; these currents have their own magnetic fields which we can push or pull with magnets. That’s where I left the project, though…a quick video where I waved a magnet around, some rough number-crunching to show that the induced forces were feasible for applications, and then I was out to let other members of the lab hash out the details. (That’s the fifth-year grad student for you!)

The cool news is that Ben has gone from my rough video to a much more carefully controlled demonstration. He’s generated attractive and repulsive forces in a bare piece of aluminum (not unlike the skin of a spacecraft), without touching it, and he’s working on characterizing the design space of his device. This is a critical step in figuring out how to go from proof of concept to a useful technology, and it’s a step I remember quite well. While Ben’s twitching pendulum might not look to you like the tractor beams from Star Trek, it is a clear and measurable experiment illustrating the device. I went from similar experiments in my first two-ish years of grad school to flight demonstrations in my third and fourth; I hope Ben follows a similar trajectory. And who knows – if some companies or space agencies take an interest, we may soon see spacecraft grappling asteroids and assembling components with eddy-current tugs!

Ben and some of the other Cornell Space System Design Studio grad students are keeping a blog about their technology research projects, which you can read here. I think it’s very cool to see what’s going on in the lab!


Way back when I was looking at grad schools, I visited an MIT space propulsion lab where students and faculty were developing something called an electrospray thruster. This is a device consisting of a plate covered in tiny spikes, with a tiny grid layered on top. You feed an ionic liquid onto the plate, where surface tension wicks it up to the tips of the spikes. (Ionic liquids – and this kind of boggled my mind when I first learned about them – are salts that are in their liquid state. They’re just a bunch of sloshing positive and negative ions. Wild!) Then, you apply a voltage to the grid sitting above the spikes. The potential difference between the spikes and the grid yanks ions up and hurls them out through the holes in the grid, and voila – ion thruster.

The MIT Space Propulsion Lab has been developing these as little patch thrusters that they can put on CubeSats. The thrusters are 1×1 cm patches and seem to generate forces in the range of ten or so micronewtons. (That would be, say, 1% of the weight of a postage stamp.) These are very small forces, but we are talking about very small satellites and we can leave the thrusters on for a very long time.

The idea that stuck in my head when I learned about these devices, though, is that they are mechanically very simple: all we have to do is texture a surface appropriately, touch the ionic liquid to it, and energize part of it. We could probably develop a fabrication method to print the thruster “texture” onto a flexible membrane or fabric of some kind.

And then we could deploy it like a sail.

10 micronewtons from a 1×1 cm thruster gives a thrust density of 0.1 N/m^2. So a 1×1 meter sail would produce a thrust force of about a tenth of a newton. On a standard 3U CubeSat, this corresponds to an acceleration of 3.4 milligees – which is actually getting up to the acceleration regime of the chemical thrusters on large spacecraft! With such acceleration, it would take five minutes for the CubeSat to add one meter per second to its velocity. Starting from low Earth orbit, this miniature sailing vessel would need only twenty minutes to hit Earth escape velocity!

3U CubeSat with 1 meter "pusher" sail
3U CubeSat with 1 meter sail

What probably makes the most sense from a propulsion perspective is to deploy the membrane engine behind the spacecraft, maximizing the engine area and minimizing any adverse effects of the ion exhaust. (All those high-energy ions might eat away at the spacecraft’s solar cells or other surfaces!) However, there might be some challenges in running the ionic liquid down to the sail.

A good compromise would be a sail mounted to the back or middle of the CubeSat – think of  the NanoSail-D configuration – where a reservoir of ionic liquid could supply a steady stream of propellant to the membrane and most of the zooming ions will miss the back of the spacecraft. The forward-facing part of the membrane might also be usable area, for things like solar cells. Or CCDs.

Ion engines caused a shift in the way spacecraft engineers thought about propulsion: instead of brief, impulsive maneuvers, they could use a gentle but steady acceleration for a long period of time. The ability to spread an ion engine over a large area might be a way to create a high-efficiency thruster that also produces a large force, and with few moving or complex parts. That’s the kind of device we might use to send a small spacecraft to the outer Solar System. Of course, we’d need a lot of electrical power, but that’s why the DOE is starting plutonium refining again…