Water, water, water. It’s essential for human settlement of space. Its uses are legion. Astronauts need water to drink. Permanent settlements beyond Earth will require water for crops, medicine, and washing. Water serves as an excellent liquid radiation shield. Or freeze it and use it as a structural material. Water, or ice, can bind other materials, for example producing mud bricks for construction in space. Water-soluble materials might be 3D-printed. Water efficiently, safely, and conveniently stores hydrogen and oxygen for use as rocket fuel. If we remove the oxygen from water—to breathe it—the leftover hydrogen can combine with carbon to produce methane, another rocket fuel and, in liquid form, an even better radiation shield. Water is the basis for batteries known as fuel cells. Water can be a coolant, a lubricant, and a hydraulic fluid. And it’s non-toxic. Use the water for all of the above, trading off among them as necessary.
In my opinion, if we are to become a spacefaring species, we must direct our exploration architectures, our space technologies, and our scientific investigation of the planets toward a sustainable and coherent vision for space exploration centered on using the resources that are already in space. Chief among these is water.
In the spirit of Spacecraft a Week, let’s think about a single spacecraft that could exploit this abundant resource: a robotic spacecraft that explores asteroids, refueling as it goes.
What makes this design possible is a water-based propulsion technology that Rodrigo Zeledon is developing at Cornell University. His is not the only one. Tethers Unlimited Inc. has done a great job with a related technology. In fact, the basics were well understood decades ago. What has changed is the rise of small satellites and the advances in fuel-cell technology, both of which we can now infuse into new space systems that haven’t been seen before. We expect Rodrigo’s solution to be uniquely mass and volume efficient thanks to a new flight-dynamics concept and simplifications. It would be capable of accelerating a 3U CubeSat by 1000-2000 m/s, unheard of in the early days of space exploration.
A brief aside about Rodrigo’s breakthrough research. The spacecraft has a water tank, where an electrolyzer from a fuel cell uses solar power to separate oxygen from hydrogen. The resulting gas has been given many names over the years. Let’s call it Zeledine. The electrolysis continues until the pressure in the tank reaches about 10 atmospheres (maybe higher in some applications). After that point, whenever the mission calls for thrust, the spacecraft can open a valve that sends some Zeledine into a combustion chamber. The valve closes, a spark plug ignites the gas, and the two reunite as water, shooting out the rocket nozzle with very high efficiency.
Unlike typical electric propulsion systems, there is no need for a battery to store energy. The water itself stores that energy. Unlike cryogenic oxygen/hydrogen propulsion, this technology doesn’t need heavy insulation, cryo pumps, or other hardware associated with keeping the separate oxygen and hydrogen as super-cold liquids. This propellant—water, remember—can be stored indefinitely and even transferred to some other spacecraft with the help of mundane terrestrial technology. And unlike the fuel in other propulsion techniques, the Zeledine is stored as a single fluid, kept separate from the water in the tank by the spin of the spacecraft, just like samples in a centrifuge. That spin has other benefits. It helps cancel out misalignments of the rocket nozzle, gyroscopically stiffens the spacecraft so that a little torque imbalance from the rocket doesn’t tip it over, and guarantees safe and reliable flight dynamics.
Here are some pictures that may help explain how this works on a 3U CubeSat, where one gram of water at a time combusts to produce half-second pulses of thrust.
3U CubeSat with Electrolysis Propulsion. Left: spinning spacecraft; right: cutaway view
But I have in mind something larger than a CubeSat. This asteroid explorer would be about the size of the brilliantly successful Hayabusa-1 spacecraft, which famously retrieved samples from the Itokawa asteroid a few years ago. Its mass was 530 kg. That spacecraft was able to visit an asteroid, with the help of an Earth-flyby gravity assist, and bring back a sample. It did so with 65 kg of xenon for its ion-propulsion system and another 50 kg of chemical bipropellant for attitude control. It used only 22 kg of that xenon.
For this asteroid explorer, I’ll trade in the 20 kg (or more) that Hayabusa dedicated to its sample return capsule for a drill, a heater, and a hose to melt ice and pump it into the propellant tank. Yes, ice can be found on some asteroids, including Themis (Itokawa, not so much, by the way). We’ll use that subsystem to refuel the spacecraft after it lands.
And I’ll also swap out the three tanks—xenon, monomethyl hydrazine, and nitrogen tetroxide—for one water tank. The Zeledine can serve both to change the orbit (where Hayabusa used xenon) and to impart reaction-control torques (where Hayabusa used MMH and NTO). Let’s guess that Rodrigo’s solution would need only half of the 70 kg mass of Hayabusa’s electric propulsion subsystem and about the same mass again for its 12 attitude-control jets. Furthermore, we won’t need the roughly 6.8 kg power-processing unit. I also suspect we’ll need fewer batteries than Hayabusa, but let’s leave them alone just for some mass margin. We’ll also keep the solar arrays, which can provide 1400 W to Hayabusa’s propulsion system, along with the rest of the spacecraft. And we’ll design our system to match Hayabusa’s roughly 1250 m/s velocity-change throughout the mission.
Remember, it did so with only 22 kg of xenon propellant. We’ll need a lot more because xenon ion propulsion is much more efficient than water propulsion: it offers a specific impulse of 3000 sec, while ours is about 300 sec. But we save a lot in other hardware. Conservatively, about 218 kg is now available as water storage. That figure neglects efficiencies gained in the tank, which needs not be pressurized as much as Hayabusa’s, and in general plumbing and mechanical bits. Thanks to those mass savings, it turns out we need only another 53 kg of water to achieve the same propulsion performance. The tank volume would have to roughly double, in part because xenon and bipropellant are denser than water, but that’s merely an increase in linear dimensions of 28%. At such low pressure, the tank doesn’t have to be spherical to be mass-efficient, unlike high-pressure propellant tanks one finds on typical spacecraft. So, these water tanks can be any old shape, whatever fits in the unused nooks within the Hayabusa structure.
If Hayabusa could make it all the way to Itokawa and back, I believe this design could make it to the asteroid belt as well but survey asteroids continually, reporting back to Earth when it finds something valuable. I don’t know, platinum group metals, perhaps? DNA? Water, for sure. Some day, NASA may be in the business of buying water from commercial entities, as some entrepreneurs hope. Such purchases are not in NASA’s current plan, but I argue that we need to think quite a bit bigger than the individual spacecraft that we discuss when we talk about space exploration. We need more than a flexible path. We need a sustainable path, a paradigm that ends our reliance on mass sent from Earth.
There’s nowhere you can be that isn’t where you’re meant to be. It’s easy. All you need is water.