An innovative and tiny thruster design could prove the future of interplanetary exploration with the tiny satellites known as CubeSats.
Palm-size satellites have become commonplace in near-Earth space — roughly 100 of these oversized Rubik’s cubes packed with instruments and devices have launched every year since 2013.
So it should come as no surprise that CubeSat scientists are turning their ambitions beyond Earth orbit to interplanetary space. The vision of tiny, inexpensive satellites flitting around the solar system by the dozen is appealing to be sure, but is it doable?
Among the many challenges facing CubeSats today, perhaps the two biggest hurdles, as well as the biggest areas of research and development, are propulsion and communication. Paulo Lozano directs the Space Propulsion Laboratory at the Massachusetts Institute of Technology. For the past several years his team has been developing an innovative technology that will help propel CubeSats beyond Earth orbit.
Driven by a Mosquito
Any team building a CubeSat today has limited options for propulsion. In fact, many CubeSats don’t carry any means of movement at all. Those that do rely on technologies that are inefficient compared with the ion propulsion that carried the Deep Space 1 and Dawn spacecraft out to interplanetary space.
Ion propulsion involves splitting a gas apart into protons and electrons. Then an electric field sends those ions flying out the back to propel the spacecraft forward.
Lozano’s team is essentially miniaturizing that method into ion electrospray thrusters. Rather than using xenon gas under high pressure for propellant, as Dawn does, Lozano’s thrusters carry molten salts. These unique liquids don’t evaporate in a vacuum, so they don’t need thick-walled vessels to keep them stable. Instead, the propellant is stored in a tiny chamber the width of a thumbnail.
On the top of this cube rests a thin slab of porous material. Early versions, such as the demo resting on Lozano’s desk, are made of porous glass. Cradle the cube in the palm of your hand and you’ll just make out 500 microscopic tips etched onto its surface. Newer slabs are made of more versatile carbon aerogels and can contain up to 5,000 tips (which wouldn’t be visible to the eye).
These tips are the innovation that make the microthrusters work. The molten salt propellant wicks upward through the porous material to the tips, where the ions feel an electric field drawn from the CubeSat’s power supply. And that’s when the magic happens.
“If there’s a thunderstorm and you are standing on a flat field, you probably want to lie down,” Lozano jokes. “If you’re standing up, you’re the sharpest point on the field.” Similarly, the tips etched onto the porous material intensify the electric field to a colossal billion volts per meter. Then “voom!” the ions begin to fly out of each tip in a thin, continuous stream.
All of this microscopic action results in 10 micronewtons of force from a single thruster, about the weight of a mosquito. “But if that mosquito is applying its weight for long times, it can move pretty fast,” says Fernando Mier Hicks, a graduate student in Lozano’s lab. For a cubesat in low-Earth orbit, these microthrusters could easily effect a velocity change of 100 meters per second, enough to boost a satellite's orbit by 200 kilometers, extending its lifetime from months to years.
Back in 2012, MIT featured the Space Propulsion Laboratory in a video that demonstrated how the team is testing and refining the technology:
The team is now coming close to realizing the ultimate goal: “We could definitely put out right now a propulsion system that maybe cannot reach Mars, but could reach the Moon,” Lozano says. “We feel very confident that within the next year we could do this.”
Earth to CubeSat
Even if a CubeSat can get to the Moon, it still faces another key challenge: communicating what it finds (or even just where it is) back to Earth.
Most CubeSats use radio antennas for their communication needs, but since a radio signal gets broadcast in every direction, it has to be a strong signal to be caught by a receiver back on Earth.
Not only does a CubeSat face limited bandwidth (it may only be able to communicate a few bits at a time), but its signal also grows weaker as it goes farther out. A university ground station can pick up the signal from a CubeSat in low-Earth orbit, but an interplanetary signal would need the big dishes of the Deep Space Network, which NASA uses to communicate with bigger missions such as Dawn, Kepler, New Horizons, the Voyager spacecraft and many more.
“That’s very expensive,” Lozano says. “And even if you have the money, which would be more expensive than the [CubeSat] mission, you probably won’t get the time because of scheduling constraints.”
So what’s the solution? Any good (or evil) scientist would tell you: lasers.
Lasers have the advantage of transmitting targeted information — rather than spreading their signal in every direction, their focused signal achieves high bandwidth and uses little power. But that boon also poses the greatest challenge to their use: the laser must point in exactly the right direction or it will miss the spacecraft or the ground station entirely.
Since any lasers or sensors would be built onto a CubeSat’s body, propulsion turns out to be as key to communications as it is to reaching destinations. Angling the CubeSat properly will help maintain contact with Earth.
NASA has been developing laser communications for some time. These projects include experiments on LADEE as it orbited the Moon in 2013 and the recent Optical Communications and Sensor Demonstration (OCSD), a CubeSat that launched on October 9th. A second demonstration with two CubeSats is slated for a February launch.
Corporations such as SpaceX and Google have plans for laser communications, too, though they’re secretive about concepts. But the technology will be vital in supporting the dreamed-of constellations of hundreds of CubeSats for global communication networks, which won’t work if the mini satellites can’t hold or target precise positions.
But these projects may be successful far sooner than we’re accustomed to with other space endeavors. While NASA typically plans a mission to Mars a decade in advance, a CubeSat can go from concept to launch within a year or so, Lozano explains.
“If someone [right now] said they had the first interplanetary CubeSat, I wouldn’t be surprised at all.”