Reporting on the #SciWri15 #TinyThrusters session.
At a glance, space exploration may seem like a game for the rich and powerful, but Professor Paulo Lozano of the Massachusetts Institute of Technology (MIT) wants to give start-ups, graduate students, and even high schoolers a chance to play. Lozano is sympathetic.
"Our hope is to give space access to people who do not currently have space access," says Lozano of his Space Propulsion Laboratory at MIT. Miniature satellites of roughly 1 liter in volume and 1 kilogram in weight have gained popularity in recent years due to their small size and, hence, small cost, allowing them to piggyback on larger launch vehicles to make their way into orbit. This would allow countries without large space programs, such as Lozano's home country of Mexico, to gain space access for purposes such as communication. Critical shortcomings, however, limit their utility. Traditional communication and propulsion systems are too large and heavy for their small bodies, leaving them to fend for themselves once in space. With no way to relay information or change trajectory, these miniature satellites are more likely to become overqualified space junk instead of useful tools.
Lozano's lab has devoted 10 years of research to solving the propulsion problem. Larger satellites chemically combust a fuel to provide the thrust required to accelerate; but, the large amount of fuel required is not feasible for miniature satellites. Such chemical propulsion also poses an explosion risk, a compromising quality when you want to hitch a ride into space on a more expensive and (arguably) more important launch vehicle.
Electric propulsion is the answer. Instead of the gaseous products of a chemical reaction expanding and pushing a satellite forward, charged ions emitted from an unreactive propellant provide the necessary thrust. Lozano's propellant of choice is an ionic liquid (composed of positive and negatively charged molecules that remain liquid in space) providing a dense, low volume source of ions under an applied electric field.
Lozano's goal is making space exploration accessible to nations without developed space programs
The voltage required to get these ions to jump out from an ionic liquid pool and provide the required push is much too high; but, if you funnel the liquid down to an area of a few nanometers, the required voltage decreases. Lozano's lab achieves this by using flat glass chips the size of a quarter that are coated on one side by hundreds of closely packed glass cones, much like a uniform sea of mini-mountains. As the ionic liquid wets the flat surface of the porous glass chip (known as a thruster) it gets drawn to the nanometer-sized tips on the other side, where the modest applied voltage can convince the ions to escape. Lozano likens the process to a candle, where the wick draws the liquefied wax to the tip to produce a flame.
Lozano now has thrusters that could possibly propel a miniature satellite; but, several questions needed to be addressed first. Would the ionized liquid cause corrosion? No, his research shows, as long as you alternate between emitting positive and negatively charged ions to keep the ionic liquid neutral. In the instant after emitting an ion, will the satellite become charged and attract the ion back? No, as long as you use an even number of thrusters and emit an equal number of positive and negative ions at all times. Still, one question kept Lozano up at night. "What kind of processes are happening in there that I can't even imagine?"
The only way to solve this lingering question was to build a prototype and give it a go. The task was taken on by one of Lozano's graduate students, who built a satellite with eight thrusters. Using a vacuum chamber and suspending the satellite to mimic the environment in space, Lozano's graduate student was able to fire up the propulsion system and make the satellite rotate on command without any measurable charge buildup. Further testing showed that even after 140 hours of work and complete depletion of the propellant ("Down to the last ion!" exclaimed Lozano excitedly), the thrusters showed no signs of degradation.
"This was the most exciting test we've done so far," said Lozano, who eagerly showed one of the undamaged thrusters after a 140 hour shift, its borders etched with the names of the students and postdocs who worked on the project. For its size, the amount of force generated by the eight thrusters was better than other systems reported to date. The lab's prototype yielded enough thrust to travel 800 kilometers which would allow a miniature satellite to deorbit and reenter the Earth's atmosphere after duty. You could think of this as the satellite cleaning up after itself and making sure it does not become space junk after service.
"Most [miniature satellites] are launched into orbits that cause them to naturally reenter Earth's atmosphere in a few months," explained Lozano, but such limiting trajectories may not be practical. The ability to reenter on its own terms increases the satellite's flexibility. To further improve mobility -- for, say, future missions to Mars -- while maintaining the same size, Lozano must sharpen the thruster tips to pack more onto a single emitter, thereby increasing the amount of force produced. This requires more advanced fabrication techniques, and may even involve moving away from glass towards a more suitable material.
With each advance in Lozano's propulsion system, he gets closer to his goal of making space exploration accessible to nations without developed space programs, as well as high schoolers wanting to get their hands dirty. Several of Lozano's previous students seem to share his dream, and have moved on to found satellite start-ups. Perhaps the future of space exploration looks like a tissue box-sized cube zipping through space.
References or Relevant Literature
J. Chu (2012) MIT-developed 'microthrusters' could propel small satellites, MIT News Office, August 17
P.C. Lozano, B.L. Wardle, P. Moloney, & S. Rawal (2015) Nanoengineered thrusters for the next giant leap in space exploration, MRS Bulletin 40(10): 842-849
The citation for this article is:
A. Landry (2015) Satellites: Coming Soon to a Lab Near You. DiverseScholar 6:14
Alexandra Landry is a chemical engineering Ph.D. candidate at the University of California in Berkeley. At Berkeley, she is a writer and editor for the Berkeley Science Review, a graduate student publication focused on innovative research at Berkeley. She is also actively involved in the Latino Association for Graduate Students in Engineering and Sciences, whose goals are to promote the recruitment and retention of Hispanic graduate students on campus. Landry attended the ScienceWriters 2015 conference on a DiverseScholar NASW Diversity Travel Fellowship. We thank mentor Matthew R. Francis, PhD for editing this article. Any opinions expressed in this article are solely those of the author.
Originally published 29-Dec-2015
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