The Race to Build Teeny Tiny Satellite Thrusters
You might not know it, but you’re really just one Kickstarter away from launching your own satellite.
Low Temperature Plasma Team thruster design. Image: Ane Aanesland
You might not know it, but you're really just one Kickstarter away from launching your own satellite. CubeSats and other small, cheap spacecraft platforms have reduced the cost of getting to orbit by a huge margin, allowing for a more democratized age of space exploration.
As exciting as this populist spaceflight movement is, there is still one major technological hurdle it has to clear before it will meet its full potential: Propulsion.
Take CubeSats—cubic nanosatellites that typically weigh around three pounds. These modules are extremely popular with both professionals and hobbyists, and hundreds of them have been launched since 2003.
However, once they make it to space, CubeSats are usually dumped into low Earth orbit, and they have no onboard engine that could propel them towards more ambitious targets like the Moon or Mars. They can't even adjust course in the event of an incoming collision or a mission failure. Bereft of propulsion, they are at the mercy of the trajectory into which they were deployed.
There's a good reason for this locomotive deficit. In spaceflight, propulsion usually involves storing combustible materials at high pressures, and there's always a risk that engines might leak or backfire. Considering that CubeSats are often launched in batches, the odds of accidents would be significantly multiplied if they were equipped with reactive engines. It would be like loading up a spacecraft with a bunch of tiny ticking time bombs, which is why the spaceflight community has issued strict regulations about onboard propulsion systems, rendering most CubeSats rudderless and immobile.
"The traditional chemical rocket is almost a no-no in small satellites, and not only because the technology is difficult to miniaturize," Paulo Lozano, director of the Space Propulsion Lab (SPL) at MIT, told me over the phone. "People are very scared about putting reactive containers that can fail and possibly damage the main payload."
But this limitation has turned out to be an opportunity in disguise for Lozano and his colleagues at the SPL. Over the last few years, they have been developing a miniaturized ion propulsion system for CubeSats that could endow the satellites with the power of motion while upholding the safety criteria.
In contrast to the heavyweight engines that steer larger satellites, Lozano's model is a nimble microchip about the size of a postage stamp. The surface of the chip is outfitted with 500 gridded tips, built on top of several layers of porous metal that mediate ion flow from a liquid plasma chamber at the base. When a voltage is applied to the plasma chamber, ion beams shoot out of the gridded microthrusters. This entire propulsion system weighs only 100 grams, and generates 50 micronewtons of thrust
Lozano and colleagues explain the microspray concept. Video: Massachusetts Institute of Technology (MIT)/YouTube
"The beauty about this is that you can modularize these or standardize them, so it's very similar to the CubeSat concept," Lozano said. "If you have a larger power capability in your spacecraft you can put more of them on the satellite, and do different things."
"What I'm most interested in exploring are the missions that would allow a little CubeSat, a shoebox-sized satellite, to actually leave the gravitational pull of the Earth and explore the Moon or asteroids or planets," he continued. "Right now, we're planning a Moon mission and an asteroid mission and I think we can do that with little satellites. What we want to demonstrate is that we can do this for a budget that is ridiculous compared to the budget of traditional missions."
Indeed, Lozano and his colleagues are not the only engineers who recognize that miniaturized propulsion systems are poised to radically change the landscape of space exploration. Many teams around the world are investigating different propulsion techniques geared for cheaper, more nimble systems for the small satellite market, from solar sails to water electrolysis.
"We are really in a technology race," said Ane Aanesland, head of the Low Temperature Plasma Team at École Polytechnique in France, in a Skype interview with Motherboard.
"What I have seen is that there are about eight technologies that are pushed. They are either based on what exists already and may be possible to miniaturize, or new concepts like the electrospray made for CubeSat systems," such as the one that Lozano is working on at MIT.
For her part, Aanesland is developing a propulsion system that combines ion thruster technology with techniques gleaned from the semiconductor industry. By doing away with the need for the bulky neutralizer component used in larger thrusters, her team has been able to scale down prototypes so that they can be installed on a variety of smaller spacecraft.
While this thruster could be fitted to CubeSats, it is ideally aimed for slightly larger satellites, in the range of 10 to 500 kilograms (by comparison, GPS satellites weigh about 2,000 kilograms each). Its dimensions and capabilities could also be modified on a case by case basis, depending on whether the satellite was destined for Earth orbit, or more exotic locations beyond it.
"I think it would be great to have an engine on a small satellite going to the Moon," Aanesland said, after I asked her how she'd like to see her prototype develop. "That is more the scientist speaking. To be able to power the small satellites for technology, imaging, and the Internet would also be quite an achievement."
"What small sats might also do [is] save missions that are already launched," she added. "Maintenance, repair, diagnosis of why a big satellite doesn't work anymore, or a realtime analysis of operating systems in space—these kinds of activities, for example."
Along those lines, mobile nanosatellites could also provide a crucial means of cutting down on space debris. "[CubeSats] that are in higher orbits are not beneficial at all because they become orbital debris, falling down over spacecraft," Lozano explained.
"Having propulsion can help you in both ways," he said. "One is allowing these satellites in higher orbits to reenter the atmosphere at the end of their lifetimes, and the other one is to push them back up, and do some exciting stuff in higher orbits." Both tactics would cut down on excessive space junk, which has long been noted as a downside to the rise of CubeSats and other cheap satellite platforms.
Both Lozano and Aanesland plan to have their propulsion concepts tested in space within the next few years, and once any kinks are worked out, they will be ready to literally propel the next generation of microsatellites to new frontiers.
Smaller, cheaper satellites have already brought low Earth orbit within reach of the average space dreamer. Soon, these scaled-down thrusters will extend that reach to the Moon, Mars, or even more distant destinations like Jupiter's moon Europa or Saturn's moon Titan. The nanosatellite revolution may have already begun, but now it's gearing up for the next big boost.