Astronauts on the International Space Station (ISS) are testing a new propulsion system ... inside the station. While this might seem like the height of recklessness, this particular system doesn't use rockets or propellants. Developed in the University of Maryland's Space Power and Propulsion Laboratory, this new electromagnetic propulsion technology called the Resonant Inductive Near-field Generation System (RINGS) uses magnetic fields to move spacecraft as a way to increase service life and make satellite formation flying more practical.

Formation flying is a new field in spaceflight that allows for tackling large jobs without large satellites. By having satellites flying in a coordinated pattern, they can be turned into sensor arrays in the same way as astronomers use separate of telescopes to create one gigantic scope. It’s a technique with a large potential, but suffers from the fact that it requires a lot of propellant to keep the satellites in position. This makes the spacecraft heavier and shortens their working life. The use of rockets also risks the danger of other craft in the formation getting caught in the backwash, and the flash and heat can blind instruments.

Electromagnetic formation flight (EMFF) gets around this propellant problem by turning the satellites in a formation into electromagnets. By using a combination of magnets and reaction wheels, spacecraft in formation can move and change their attitude and even spin without propellant. Satellites can change their polarity to attract or repel one another, turn, or shift their relative positions in any manner that doesn't require changing the center of gravity for the entire formation.

According to an MIT study [PDF], when EMFF is perfected, it will have a wide number of applications including interferometers, space telescopes where each satellite carries a section of mirror, generating artificial gravity, creating a magnet shield against solar radiation storms, and clearing space debris by using their spin to toss the debris into a safer trajectory. However, there is still a great deal of work to do because EMFF will need superconducting wires, high-velocity reaction wheels, cryogenic cooling, and other critical technologies to be developed before they become practical.

The University of Maryland's RINGS is one version of EMFF. It was developed by a team led by Associate Professor of Aerospace Engineering Ray Sedwick and the experimental prototype was sent to the ISS aboard the Japanese HTV-4 Cargo Ship on August 3. It consists of two separate units, each made of a polycarbonate ring containing a coil of aluminum wire, though in a practical version this would be a superconducting material. The magnetic fields are regulated by microcontrollers that allow the units to maneuver about one another.

The RINGS system has already undergone 2D bench tests and undergone freefall tests in a NASA plane flying a parabolic trajectory. The ISS experiments will allow the system to be tested for longer periods.

"While reduced gravity flights can only provide short, 15 – 20 second tests at a time, the cumulative test time over the four-day campaign provided extremely valuable data that will allow us to really get the most from the test sessions that we’ll have on the International Space Station," says Sedwick.

The ISS tests will see RINGS connected to a pair of SPHERE robots developed by MIT as a test bed for miniature satellite operations. Four test sessions are planned aboard the ISS and data collected will be transmitted back to Earth for analysis.

The tests will also allow the team to put a second technology called the wireless power transfer (WPT) through its paces. This will allow the units to be remotely recharged and in practice, it will make maintaining a satellite formation fleet easier.

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