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.

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.

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?

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.

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!

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!

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…