Tuesday, February 21, 2017

Surfing the Rings: Using Small Spacecraft to Explore Saturn’s Rings

MIT's ion micro-thruster, (developed by the Space Propulsion Laboratory - Photo: M. Scott Brauer) is a technological development which has contributed to MSc student Eric Shear's conception of what a small satellite fleet might accomplish at Saturn. Find out more, below!

By Eric Shear
Over the last several months, I have been working on a CubeSat mission proposal to Saturn’s rings as part of my master’s thesis, now called “Saturn Ice Ring Exploration Network” or SIREN for short. I wrote about the science rationale in my blog post “The Ring Paradox.” The first draft should be completed by the end of this month, and I’d like to use this blog to reflect on my experience developing SIREN.

The goal was not to “build” a spacecraft on paper, down to the last nut and bolt. That would take at least 100,000 person-hours - about a lifetime - which is why all interplanetary missions must rely on hundreds of specialists to execute successfully within a reasonable time frame. Rather, the idea was to assess the mission’s feasibility within my field of expertise while assuming current and near-term technology. That’s a job for a generalist, not a specialist. My undergraduate studies in mechanical engineering, then physics and astronomy, prepared me for this experience.

I have to admit, I was privately skeptical at the beginning about the feasibility of such a mission because small spacecraft already have challenges of their own and Saturn’s rings are one of the most difficult environments to operate a spacecraft in. But since Dr. Moores hired me as a research assistant for this project, I vowed to do this to the best of my ability and to identify areas of difficulty along the way.

One of the first things I tackled was propulsion. Since all three SIREN spacecraft would be deployed from a mothership orbiting near the rings, I had to calculate the change of velocity (delta-V) required to change the orbit’s inclination so the spacecraft can stay inside the ring plane during operations. It turned out to be only tens of centimeters per second. Returning to the mothership is more demanding, but after complex calculations the delta-V was found to be only 1-2 meters per second. The velocities of the ice boulders relative to each other is in the millimeters per second range, so it shouldn’t take more than a few mm/s to avoid them.

With the recent invention of ion micro-thrusters at MIT, which are the size and weight of a penny each, propulsion isn’t the biggest issue with SIREN. Thermal control and energy storage turned out to be the most important factors. With so little sunlight at Saturn, the spacecraft must draw its power from its batteries, which are in turn replenished by low-efficiency radioisotope power sources. Power management thus becomes the critical issue. Fortunately, all the components I selected would require very little power, and most of them won’t be on all the time. 

It’s easy to get cold around Saturn. The rings were found to have a temperature of minus 200 degrees Celsius. Most electronics would fail at this temperature. My approach was to design the whole spacecraft as a warm-electronics box, with all the temperature-sensitive components inside, kept warm by the radioisotope heaters and insulated with MLI blankets. The standardized 6U CubeSat form factor (like a large shoebox) made this a relatively simple task.

In my opinion, the biggest challenges to this proposal are autonomous operations and machine vision, which are outside my expertise. A signal from Earth takes one hour to reach Saturn, so the spacecraft must be able to perceive the surrounding ice boulders and their relative velocities
 and compute its own vectors to avoid (or dock) with them, all independently. I didn’t know how much computer hardware this would take, but according to Moore’s Law, the processing power of a CPU doubles every 18 months. With the work that’s being done in automation, I’m sure this problem will be solved within the next decade.

As hard as this project was, I’m grateful for the opportunity to sharpen my skills in space mission design and (possibly!) contributing to the future mission that will answer the riddle of Saturn’s rings!

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