Boeing

For the past half-century, chemical rockets have propelled most spacecraft, because this is the simplest and best-understood way to do it, and it is also the fastest way, with current technology, to reach one's destination (particularly important if one is a human being). But chemical rockets require a huge amount of propellant, and so some engineers have long envisioned a future powered by other means, such as ion propulsion.

It's coming. Boeing recently announced the first communications satellite to use no chemical propulsion at all. This will allow much more of the satellite mass to be devoted to transponders and solar panels, rather than propellant, propellant tanks, and engines. And each transponder generates revenue—the more Boeing can fit onto a satellite, the more money it makes.

Why Ions?

Chemical rockets provide a lot of power, allowing for rapid acceleration, but at the cost of a lot of propellant. This is because the lower the exhaust velocity, the more propellant you must expel to achieve a given velocity change, and chemical combustion doesn't produce high exhaust velocities. For instance, one of the best chemical combinations—liquid oxygen and hydrogen—provides an exhaust velocity of about 14,000 feet per second. That sounds pretty fast but is actually a slouch compared to other possible methods. The rocket equation is exponential, so if engineers could use a propellant with twice that velocity, they could cut the amount of propellant needed by much more than half.

That's why ion propulsion has always excited many spacecraft designers. In ion propulsion, electrons are stripped from or added to a noble gas such as xenon to give it a net electrical charge. The ion engine then accelerates them with an electric or magnetic field and blasts them out the back of a thruster. This can provide exhaust velocities greater than chemical propulsion by 10 times or more, meaning a mission would need only a small fraction of the propellant a chemically powered vehicle must carry.

So what's the catch? Ion systems are very low-thrust. While chemical systems provide their own energy in the propellants, which can result in very high power when they're mixed and burned rapidly, an electrical system's propellant supply has no energy of its own. It has to be provided externally, so its thrust is limited by the power available.

One could solve this problem with a nuclear reactor, as was planned for the now-cancelled Jupiter Icy Moons Orbiter. But no space-rated reactor exists, and the cost and politics of getting one are currently prohibitive. So, for now, electric propulsion is limited to what's available from solar cells, which means that the thrust levels are orders of magnitude below that of chemical thrusters. That in turn means that the trip times for vehicles using them can be very long. For this reason, their primary application has been for station-keeping communications satellites in geostationary orbit—gently shoving them to keep them in place against slight gravity variations, so that fixed dishes on the ground, such as the one for your satellite television, can find them.

Boeing Goes Ahead

Now, Boeing has taken the next step. Not only will it use ion thrusters to keep itself in the right spot, but it will also use them to move itself to geostationary orbit (GEO) from low Earth orbit (LEO), a task most satellites accomplish by burning chemical thrusters.

It won't be easy. First of all, using ion thrusters means it'll be a slow trip. Rather than hours, it will take the Boeing satellite months to actually spiral out to its operating altitude. That reduces (or at least delays) the return on investment, so the satellite's owners and financiers will need to be more patient. Second, it means a long traverse through the radioactive Van Allen belts a few hundred miles above LEO, so the electronics may have to have additional shielding to survive the trip. But apparently it's worth it, because Boeing has already signed up two customers, in Asia and Mexico, with prospects for more.

And, most importantly, the fact that Boeing is willing to stake a commercial venture on ion-propulsion signals that this long-researched technology could finally be ready for a major step forward. Pushing a satellite into a higher orbit is a far cry from deep-space exploration, like some have dreamed for ion propulsion. But there are some new applications on the horizon for ion propulsion that will dramatically reduce the cost of not just satellite communications, but perhaps human space exploration.

Consider the dream of propellant depots in space. The Saturn vehicles of the Apollo era had to be enormous to carry enough propellant to take astronauts on a lunar round trip. If there had been a refueling station near the moon, the flight there would have required much less than half as much propellant and the launch vehicle could have been much smaller, while allowing the delivery of more payload to the surface. But the propellant has to get to that depot somehow, and if we used chemical propellants to move it there, we wouldn't be saving ourselves much.

But with electric propulsion, one could imagine routine, relatively low-cost back-and-forth trips to the moon. Imagine a propellant depot in low Earth orbit that would fuel up a spacecraft for a trip to the moon, and another in low lunar orbit, or in a stable point such as the L-1 or L-2 Lagrange point, supplying the fuel for the return trip. We could move propellant between the two points on slow trips by using ion thrusters like the ones Boeing has on its new satellites (but probably more and bigger ones). Some of the electric power would be devoted to propulsion, and the rest to ship-board systems and cooling of the propellant to avoid boil-off (assuming the propellants are cryogenic and not the less efficient but more easily storable types). There might be several such tankers continuously in transit at various stages of the journey, just as there is an oil tanker about every 10 miles between Japan and the Persian Gulf. It would be a propellant pipeline to the moon.

Ultimately (and not too far in the future) the same concept could reduce the cost of trips beyond the moon. Slow electric tankers could go ahead of human missions to Mars or asteroids, dramatically reducing the amount of propellant needed to launch crews to those destinations. Eventually, propellants could be manufactured on-site (for example, manufacturing methane and liquid oxygen from the Martian atmosphere). But early propellant caches could create an "interplanetary highway system," similar to the gas stations that allow the terrestrial interstate system to function—and hasten the day that travel throughout the inner and outer solar system becomes routine.

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