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.
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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