Slow down and speed up for satellites
Image credit: Dreamstime
Satellites must be able to de-orbit when they have fulfilled their mission. A plasma brake could be one answer.
Small Finnish company Aurora Propulsion Technologies is testing a radical new satellite de-orbiting technology this summer. The plasma brake technology was invented by astrophysicist Pekka Janhunen, one of the company founders.
The plasma brake is compact, lightweight, and relatively inexpensive. About the size of an old-style cassette tape, it can be installed on a satellite before launch, although in the future it may be possible to retrofit them to satellites of up to 600kg using a robotic arm mounted on another space vehicle.
De-orbiting satellites safely is critical to mitigate the generation of space junk by collisions between orbiting objects. A plasma brake unit that can safely de-orbit satellites from orbits up to 1,000km in altitude would weigh only 2kg and cost a fraction of competing technologies. The company does not claim to have the only de-orbiting solution out there, but it says the plasma brake comes in at a tenth of the cost of the competition. Aurora Propulsion Technologies’ chief executive officer, mechanical engineer Roope Takala, estimates that a likely sales price will be around €30,000 per unit.
The technology uses aluminium or titanium metal micro tethers, which are unspooled from the satellite to be de-orbited from the ionosphere. When a negative charge is applied to the micro tethers, the electric field generated presents as a barrier to positively charged particles in the plasma of the ionosphere.
The braking effects, or Coulomb drag, work best between 400 and 800km above the surface of the Earth, the altitudinal sweet spot where there are enough positively charged particles to interact with the electric field around the micro tethers and generate the drag. The interaction between the headwind of plasma that the satellite travels through at 8km/second and the negatively charged unspooled micro tether causes the satellite to slow down and lose altitude.
The drag from a single 300m-long tether is enough to deorbit a 4.5kg satellite by about 200km in 9-13 months. Longer, multiple tethers will brake larger satellites. On 3 May, the company said it successfully deployed two tethers to demonstrate the current iteration of the patented plasma brake design during its AuroraSat-1 mission, which launched from New Zealand.
According to the company, over the next few weeks, its demonstration satellite, known as The Flying Object, will deploy to low-Earth orbit to validate its proprietary next-generation propulsion technologies enabling collision avoidance and de-orbiting. Aurora’s satellite will deploy to low-Earth orbit in a demonstration of how the company’s resistojet thrusters and plasma brakes can “provide efficient propulsion and deorbiting capabilities for small satellites”.
In the future, the technology will only be deployed once the satellite has reached the end of its working life. “After all, why slaughter a milking cow,” says CEO Takala, laughing. “The business drive will always be to keep satellites up there and productive for as long as possible.” Unlike similar devices that use atmospheric drag to decelerate satellites, the micro tether is thinner than a human hair, making it safe to other spacecraft as it will not cause any damage to them. Coulomb drag is also orders of magnitude stronger than atmospheric drag. This difference in decelerating forces is even larger as the orbital altitude increases. AuroraSat-1 includes a twin Aurora plasma brake module, which for testing purposes has two independently deployable spools of aluminium micro tether, in addition to all the control electronics.
In the near future, efficient de-orbiting will become a prerequisite of launching and integral to the initial design of any satellite. The company expects international legislation designed to limit space junk to extend to smaller satellites and other pieces of space debris. The current regulations governing end-of-life requirements to deorbit are set out by the UN; these only apply to satellites of 500kg and above, which would not completely burn up during their descent through the atmosphere, potentially posing a danger to life as they crash to Earth. The US Federal Aviation Authority is proposing new international regulations that would require even the smallest of satellites to be equipped with a means to return them safely to Earth at the end of their useful lives. Called Designed for Demise (or Death), D4D regulations will require satellites of any size to have a means of deorbiting technology designed into the satellite pre-launch, so that new launches never contribute to near-Earth orbiting space junk in the future.
“We are confident that this technology will be a gamechanger in the deorbiting area, especially on orbits where aero drag deorbiting is infeasible,” Takala states.
Electric sailing on the solar wind
The plasma brake tests in low-Earth orbit will pave the way for a novel form of space propulsion: the electric or e-sail. The e-sail, invented by Pekka Janhunen, could propel a small craft to the heliopause – the edge of our solar system, in as little as 10 years, turning it into the fastest human-made object ever. It took Voyager-1, a conventionally powered probe, 35 years to reach the same point.
Where the plasma brake works in low Earth orbit, the electric sail can only be deployed once a spacecraft leaves the Earth’s magnetosphere. The sail will be formed by unspooling several 25-micron-thick wires or micro tethers, 20km long, and rotating the spacecraft to keep them extended. The wires are then positively charged using a solar-panel-powered electron gun, creating a positively charged electric field a hundred metres across around each nanowire.
Our Sun throws out a constant stream of charged particles as well as light. The new propulsion system takes advantage of what happens when the negatively charged particles flowing from the Sun in the solar wind encounter the positively charged electrical umbrella-like structure of micro tethers attached to a small spacecraft. Each encounter adds an infinitesimally small push to the e-sail, which builds up over time, adding speed as the spacecraft moves through space.
The beauty of the system is that it is lightweight, as it requires no chemical propulsion other than that required to boost the probe beyond the magnetosphere, according to the researchers.
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