It seems that the single biggest problem with exploring the solar system is, simply, fuel.
It takes a lot of fuel to get a rocket into space, and then even more to go where you want to go. You have to save some if you want to slow down, and even more if you want to orbit, then even more if you want to land. It adds up.
There are some interesting solutions, like ion drives. These have been used in many probes; they use gas (typically xenon), ionize it using electricity, then use a very strong electric field to accelerate the ions and throw them out the back end. They move so quickly that you don’t need nearly as much fuel, and carrying even a few hundred kilos can get you to the asteroid belt, as it did with the Dawn mission.
But even using the fuel miserly, Dawn still eventually ran out. An idea that’s getting more interest is using asteroids themselves for fuel. A probe can land on one, dig in a little bit, find what it needs, mine it, process it, tank up, and away you go to wherever you’re headed next.
In general, water is what’s being looked at. It can be broken down into hydrogen and oxygen, then recombined into, say, hydrogen peroxide, which in nearly pure form makes a good propellant.
However, a team of scientists and engineers took this one step further … or, really, one step back. Why not just use the water itself?
That’s what their prototype spacecraft — called, oddly, World Is Not Enough, or WINE — does. In a nutshell, it can extract water from asteroidal material (where it’s relatively abundant), store it, then heat it up to create steam. Once the pressure builds up enough the steam is directed out the back of the spacecraft for propulsion.
WINE is the brainchild of University of Central Florida scientist Phil Metzger, who worked with Honeybee Robotics and Embry–Riddle Aeronautical University to build the test WINE prototype. Honeybee put together this brief video explaining how it works:
Of course comets have water in them, but so do many asteroids (including, for example, Bennu, currently being investigated by NASA’s OSIRIS-REx spacecraft). They have water in them either as ice or trapped in phyllosilicates (clays). WINE drills down into the surface, using heated drill bits to warm up the water. The water vapor moves up the hollow drill bit and into a cold trap inside the probe, where it’s stored. It then uses either solar panels or a radioactive power source (both are used routinely in many space probes — RTGs are useful when you get too far from the sun to use solar panels) to vaporize the water as needed to use as propellant. The prototype even has legs if it needs to move around slowly to extract more water from different locations. In tests, it was able to extract water from simulated lunar and asteroid material, store it, and then generate enough thrust from steam to move:
The beauty of this is that water is so plentiful in space that the spacecraft, theoretically, never runs out of fuel. It’s not really that easy, of course; some asteroids are dry, and traveling through space isn’t like driving down a highway and stopping whenever you need to tank up. Everything out there is moving, and quickly; it’s more like you’re on a highway when your fuel light comes on and the nearest gas station is moving perpendicular to you at 10 kilometers per second.
That’s actually my biggest concern with this idea here. Assuming it all works, the problem with steam is that it tends to be a pretty inefficient propellant. Rockets work by throwing stuff out their back ends, which then (by Newton’s Third Law) pushes the rocket forward. The faster you throw something out the back, the more efficient your rocket is.
Chemical rockets combine various molecules together to generate a lot of very hot gas, which expands rapidly and creates enormous pressure. The gas is then thrown out the back of the rocket at high speed (depending on the engine, usually a few kilometers per second), which then moves the rocket forward. Ion drives use much less material, but make up for it by flinging the ions out the back at much higher speed, like 30 km/sec.
Steam usually lacks that oomph. But, to get off a typical asteroid, even a decent-sized one, you don’t need a lot of kick. The escape velocity of Bennu, for example, is less than 1 kilometer per hour, so you could jump right off it! Bennu is small (about a kilometer across), but even an asteroid a few kilometers in diameter has a pretty low escape velocity. So WINE could easily land on one, fuel up, and leave again.
The problem then is getting somewhere else. You need to be able to accelerate enough to match velocities with some other asteroid moving relative to you faster than a speeding bullet, and that’s hard to do with steam.
… but maybe not impossible. This is basically an engineering problem, either finding a way to store enough water to build up speed, or increasing the efficiency of the engine so that the steam leaves the back end faster. I’m not saying it’s easy, I’m saying it’s probably not impossible.
That’s why I find this technology interesting. It’s far too early to dismiss it — it would be like dismissing Robert Goddard’s first tests of liquid fuel rockets because they only went up a dozen meters or so. Sure, they couldn’t leave the Earth, but less than half a century later such rockets were more than capable of doing so. We won’t be launching rockets using steam — you still need a lot more thrust to escape Earth — but once out in space, steam might work pretty well.
Or maybe it won’t. This is why we experiment, folks: To see what works and what doesn’t. Maybe this will never get past prototype, or maybe in 20 years we’ll have little steampunk robot hoppers flying from asteroid to asteroid to study them, map them, do mineralogical surveys of them, then flying away to their next target on pillars of steam (or maybe even using those steam engines to push the asteroid out of harm’s way, should one be headed toward Earth on an impact trajectory).
I’m OK with this either way. It’s good to try different things, and we know there are lots of different ways to explore space. I say let’s try them all and see what works best for the mission at hand.