On September 20, the U.S. Naval Research Laboratory (NRL) will launch a small, spherical satellite called SpinSat. It's a multifold mission, says Andy Nicholas, the Primary Investigator, but the primary mission is demonstration of a new thruster technology.
The thrusters, which were developed by
Digital Solid State Propulsion, Inc., burn only as long as they are receiving electrical current. If you increase current, it throttles up; if you use less current, throttles down, says Nicholas. We're interested in it from an attitude control system or a delta-v maneuver type technology for small satellites, like CubeSats or PicoSats and NanoSats.
He adds, Of course we can't just do one thing, so we're [also] using it as a test of the
space surveillance network, which are all the ground assets that do orbit determination and characterization of spacecraft. Because SpinSat's a near-perfect sphere and the thrusters can alter its movements and spin, It's a good calibration object for them to say, 'Okay, we know this thing's going by. Can we do a maneuver detection, can we do a change detection, how small of a rotation can we see, how small of a shift in the orbit can we see?'
And finally, NRL will monitor the drag on SpinSat as it flies through space to better model the density of our atmosphere.
SpinSat is straightforwardly beautiful. It's simple, simple, simple, says Nicholas. A near-perfect sphere, It's about 50 kilograms, think of it as the size of a beach ball. The aluminum is coated with gold iridite and black anodized in four sections. The two surfaces take heat in from the sun differently—and let it reflect heat, emit heat differently. So using that beach ball pattern on a spinning satellite keeps it in a good thermal stability.
On September 20, NRL will launch SpinSat to the International Space Station from
Cape Canaveral aboard a SpaceX Falcon 9 resupply mission. A few weeks later, says Nicholas, The astronauts are going to unpack it out of this big block of foam that we ship it in. They'll install the arm plugs and then put it on a plate attached to a spring, called CYCLOPS. SpinSat/CYCLOPS will go into a sealed-off airlock, then the astronauts use a robotic arm to pull them outside the station and orient SpinSat for deployment. They point it to where [they] want, and then hit the button—and off goes the satellite. A tiny little push.
Phase I: demonstrating the thrusters
The first phase of our mission, says Nicholas, that'll be the characterization of the thrusters. On SpinSat, We've got 12 sets of thrusters on board; each set has 6 tiny thrusters on it. We're going to fire them in pairs to spin the satellite up and down, and SpinSat has on board instrumentation to monitor the spin rate.
With just primary batteries and only 4.8 grams of fuel on the spacecraft, Nicholas expects this phase to last three to six months.
It's a multi-use propellant; it's very green, it's got low volatiles, no noxious fumes, he says.
Phase II: testing the space surveillance network
While the onboard instruments are collecting data, the
International Laser Ranging Service will track the satellite from ground stations all over the world. We've got retroreflectors all across the spacecraft, says Nicholas. These are corner cubes made of three flat mirrors, so whatever light goes in reflects off all three [mirrors] and comes back out the same exact direction.
When a ground station fires a laser at the spacecraft, the light gets reflected back. Triangulating, scientists determine where in time and space the satellite is passing overhead. They know the laser light's moving at the speed of light, says Nicholas, they know where they were pointing the laser, and from that get very accurate orbit positions—down to the millimeter level.
The stations can also determine the spin rate. And that's the cool part, says Nicholas. So think about a sphere. You've got all these little retroreflectors, and there's space between them, so the one that's here versus the one that's slightly farther away from you, and as it spins it's coming in range, so you can actually see that range change as it goes overhead.
Researchers have already performed ground truth measurements of SpinSat, looking at it optically and determining what its different optical characteristics are at different wavelengths, so that
Haleakala Observatory will be able to do optical space object characterization during the mission.
Phase III: improving atmospheric density models
When scientists observe a satellite in space and want to determine what path that satellite is on, they use an orbit determination model. There's a term in there, called the coefficient of drag (CD), that they let the model adjust so they get the best fit that that they can, says Nicholas.
Because of SpinSat's unique design, The terms that go into that drag aren't changing much. You know you're not changing your frontal area, like you would with a big long rocket body; you're not losing mass, you don't have any propellant coming out of the sky; and we know the CD very well by the way we designed it. How the orbit determination software changes the CD term to find the best fit is actually a good monitor of the difference between the software's model atmosphere and the real atmosphere.
Says Nicholas, We know that's just a climatology model, a 30-year average of those conditions and what you should expect. They probably have 25 percent error in them.
To better characterize the atmosphere, NRL flew four satellites very similar to SpinSat off the Space Shuttle in 2006 and in 2009. Called the
Atmospheric Neutral Density Experiment (ANDE), We're really doing that experiment again, during a more active portion of the
solar cycle, he says.
After two years, SpinSat will fall back into the atmosphere and burn up and be gone.
Thrusters for three-axis attitude control of small satellites?
We're really excited about it, says Nicholas of the SpinSat mission. One of the next steps is we want to do a [three-axis] CubeSat attitude control system. Unlike traditional solid propellants, which, once lit, burn until all the fuel's gone, the electric current allows for more precise adjustments. You can do a scaled version that would do bigger orbital maneuvers. [Digital Solid State Propulsion, Inc. has] a hockey puck sized version that's a one-fire thruster that would deorbit a CubeSat.
Nicholas studied Space Physics at the University of Alaska, Fairbanks, and has been a research physicist at NRL since 1993. He studies satellite data to better understand Earth's thermosphere and ionosphere, improving space weather forecast models. This is important for military operations, because how signals are transmitted or reflected influences the reliability of radar and of communication and navigation systems.
One reason Nicholas came up with the SpinSat experiment is because, after the ANDE missions, NRL had two spheres left over. But, because SpinSat will launch from the ISS, the satellites had to be remade bigger and lighter. The space station safety people said, 'Well, we want you to be a 22 inch sphere, not a 19 inch sphere, so that your ballistic coefficient changes.' They were worried that we would come back around and contact the space station, says Nicholas.
So what about those two gorgeous spheres still in Nicholas' lab? I know, now I've got to come up with another idea! he says.
See also:
- NRL to Launch SSULI on April 3rd; Will Measure Ionosphere Electron Density