CubeSats are tiny spacecraft with huge potential, but their small stature doesn't lend itself to typical propulsion methods. This has led scientists to get creative in working out ways of shifting them through space, including harnessing photons from the Sun and using regular ol' H20 as a propellant. NASA's OCSD mission has now leaned on the latter to pull off a first-of-a-kind maneuver between two of its CubeSats in low-Earth orbit.
Steam-based propulsion has been around for thousands of years in various forms, but is a particularly appealing option when it comes to today's tiny spacecraft. For one, water doesn't contain any volatile chemicals, therefore avoiding explosion risks at launch. It also has a low molecular weight and can be turned to vapor relatively easily.
And a few research groups are working on technologies for CubeSats that would harness water for propulsion through space. One prototype CubeSat from Purdue University back in 2017 was a particularly promising example, with its four onboard thrusters requiring just a few teaspoons of water for propulsion.
Since it entered low-Earth orbit in December of 2017, NASA's OCSD (Operations and Data Transmission Optical Communications and Sensor Demonstration), has been testing this out in space, among other things. The project involves three tiny CubeSats and at one point, the water-based propulsion systems have been used to bring a pair of them to within 20 ft (6 m) of one another.
They do this by taking water from their fuel tanks, warming it to produce steam and expelling it in short bursts to shift the vehicle's position in space. But the primary motivation for the mission isn't to actually demonstrate this propulsion system for CubeSats, but to explore new optical transmission methods that can relay data back to Earth more efficiently. Part of this involves exploring how swarms of small spacecraft can work together for such purposes when in close proximity to one another.
To that end, the team uses GPS receivers, optical proximity sensors and laser rangefinders to track the position of the spacecraft in space. They were around 5.5 mi (8.85 km) apart when the pair of CubeSats engaged one another via radio link. One of the tissue-box-sized spacecraft then issued a command to its partner to fire up its thruster and come into closer proximity, which it duly obeyed.
While this was choreographed by human operators ahead of time, NASA says it marks the first coordinated maneuver between two CubeSats in low-Earth orbit, with one issuing the command to another, and starts to demonstrate the potential of fleets of CubeSats to work cooperatively.
"The OCSD team is very pleased to continue demonstrating new technical capabilities as part of this extended mission, over 1.5 years after deployment," says Darren Rowen, director of the Small Satellite Department at The Aerospace Corporation. "It is exciting to think about the possibilities enabled with respect to deep space, autonomously organizing swarms of small spacecraft."
The short animation below illustrates how the maneuver played out.
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