In 1999, the California Polytechnic State University (Cal Poly) and Stanford University came together to develop a set of specifications for small Low-Earth Orbit (LEO) satellites. Their intention was to make it cost effective to launch small satellites into space. The intention of this program was to allow scientists and engineers to test things that would never get enough funding to build and launch a full-sized satellite; there was interesting but questionable scientific research to do and exciting but uncertain technology to demonstrate. A small, standardized satellite specification would make it more feasible to perform these tests.
The specifications that Cal Poly and Stanford came up with, the CubeSat standard, made space more accessible to scientists just as they had hoped. However, starting in 2013, an arguably more exciting purpose has come to dominate CubeSats: commercial and amateur applications. This is not a trend that started in 2013; however, since 2013 more CubeSats are being launched each year for non-academic purposes rather than academic ones. This is exciting because it means that CubeSats are giving smaller players a chance to participate in the exciting changes that are occurring in how people view space. New companies like SpaceX and Blue Origin are forcing their way into the growing commercial space market, fighting the traditional commercial giants in the area, like Boeing. Companies have been set up with the ultimate goal of exploiting space resources such as mineral-rich asteroids (although such ventures remain, for now, very highly speculative). CubeSats are letting small teams and small companies share in the developments, much like how small, affordable personal computers brought computation out of the realm of academics and large corporations, and into the realm of widespread use.
CubeSats are designed to piggy-back on other satellite launches; if a rocket has a payload but also some extra volume and mass to spare, they can generate extra revenue by selling rides to CubeSats. Since CubeSats have very specific dimensions and requirements that make them easy to add onto any mission, the rocket operator doesn’t have to worry about retro-fitting their rocket for the CubeSat or worrying about the CubeSat interfering with the primary payload. Cal Poly has even created the Poly Picosatellite Orbital Deployer or “P-POD,” a standardized system that will house CubeSats during a launch and deploy them safely. Any CubeSat should be able to fit in a P-POD, so any CubeSat can go in a rocket equipped with a P-POD.
The CubeSat standard recognizes 4 different types of CubeSats, each of which is a combination of 10x10x10 cm cubes: U1, U2, U3, and U6. U1 through U3 are straight chains of 1, 2, or 3 cubes and U6 uses a 2-by-3 configuration of cubes. A variety of specifications are prescribed in the standard, including maximum weight, a standard frame that all satellites must be built within, and various requirements that keep the CubeSats from interfering with other equipment. Given the practice of having CubeSats piggy-backing on multi-million dollar missions, the requirements for the CubeSats are fairly onerous.
For instance, CubeSats cannot have any pressurized fluids at more than 1.2 atm. They also cannot use any hazardous materials or materials that contain significant chemical energy. This is reasonable and necessary to protect the primary payload, but it particularly limits the ability of CubeSats to provide propulsion.
Propulsion is an important consideration for CubeSats. In an orbits of 500 km, there is enough air drag to de-orbit CubeSats in 4 to 7 years. CubeSats at lower orbits can deorbit in months. Some CubeSats require these low orbits to do their work, but could also benefit from longer-duration missions. Unfortunately, the requirement that CubeSats not have significant pressures or chemical energy makes it difficult to devise ways to maintain their orbits.
A few clever solutions are in the works to solve the propulsion problem. For instance, a Water Electrolysis Thruster contains benign and harmless water during launch. After launch, the CubeSat uses solar energy to split the water into hydrogen and oxygen, also known as rocket fuel. A second method is an Electric Field Solar Wind Sail, which uses electric fields to deflect protons from the solar wind, providing thrust, has been proposed but not proven.
A third proposed solution is a Photo Solar Sail, which uses a large sheet of material to reflect photons from the sun, providing thrust. The Photo Solar Sail technique has already been tested by the large space probes Mariner and Mercury, which used solar sailing to conserve their attitude (orientation) control propellant. The Japanese probe Hayabusa used solar sailing to help with its attitude control after it lost two of the reaction wheels that normally do the job.
In 2018, LightSail 2, a 3U CubeSat, will launch and hopefully demonstrate solar sailing with a CubeSat. This follows the relatively successful test of sister craft LightSail 1, which was placed in a low and therefore cheap orbit where solar sailing cannot take place, the atmospheric drag being more significant than solar radiation. While LightSail 1 did have a number of technical glitches, it ultimately achieved its goal of deploying its solar sail, giving the team confidence to move forward with the actual demonstration of solar sailing using LightSail 2.
These propulsion technologies, if found to be viable, will allow CubeSats to maintain low orbits for an extended period of time. However, they also open up the more exciting opportunity for CubeSats to move beyond LEO, granting small teams and companies the ability to send probes to other bodies in the solar system. Considering the boon that has occurred from CubeSats opening the tiny region of LEO to small players, the significance of reliable and effective propulsion on CubeSats could be huge.
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