Pinning Tails on Satellites to Help Prevent Space Junk

Pinning Tails on Satellites to Help Prevent Space Junk

Low Earth orbit was already relatively crowded when only the big players were launching satellites, but as access to space has gotten cheaper, more and more pieces of hardware have started whizzing around overhead. SpaceX alone has launched nearly 1,800 individual satellites as part of its Starlink network since 2019, and could loft as many as 40,000 more in the coming decades. They aren’t alone, either. While their ambitions might not be nearly as grand, companies such as Amazon and Samsung have announced plans to create satellite “mega-constellations” of their own in the near future.

At least on paper, there’s plenty of room for everyone. But what about when things go wrong? Should a satellite fail and become unresponsive, it’s no longer able to maneuver its way out of close calls with other objects in orbit. This is an especially troubling scenario as not everything in orbit around the Earth has the ability to move itself in the first place. Should two of these uncontrollable objects find themselves on a collision course, there’s nothing we can do on the ground but watch and hope for the best. The resulting hypervelocity impact can send shrapnel and debris flying for hundreds or even thousands of kilometers in all three dimensions, creating an extremely hazardous situation for other vehicles.

One way to mitigate the problem is to design satellites in such a way that they will quickly reenter the Earth’s atmosphere and burn up at the end of their mission. Ideally, the deorbit procedure could even activate automatically if the vehicle became unresponsive or suffered some serious malfunction. Naturally, to foster as wide adoption as possible, such a system would have to be cheap, lightweight, simple to integrate into arbitrary spacecraft designs, and as reliable as possible. A tall order, to be sure.

But perhaps not an impossible one. Boeing subsidiary Millennium Space Systems recently announced it had successfully deployed a promising deorbiting device developed by Tethers Unlimited. Known as the Terminator Tape, the compact unit is designed to rapidly slow down an orbiting satellite by increasing the amount of drag it experiences in the wispy upper atmosphere.

A Real Space Race

Launched to space aboard a Rocket Lab Electron on November 20th 2020, Millennium Space System’s DRAGRACER mission consisted of two identical CubeSats which were released simultaneously into a 400 kilometer (250 mile) orbit above the Earth. The only difference between the two satellites was that one of them, called Alchemy, was equipped with the Terminator Tape device. The other satellite, referred to as Augury, had no active deorbit capability and served as the experiment’s control.

Once the two craft were safely in orbit, Alchemy unfurled the tightly packed 70 meter (230 feet) conductive tether stored inside the 180 mm x 180 mm x 18 mm Terminator Tape module. With the tether slowing it down, it was initially estimated that Alchemy would hit the denser sections of Earth’s atmosphere and burn up within 45 days.

The twin stacked satellites separated in orbit.

In the end it took approximately eight months for the Terminator-equipped vehicle to passively deorbit itself. This is considerably longer than the pre-mission estimate, but in a followup presentation during the SmallSat Virtual Conference, Tethers Unlimited President Rob Hoyt said the team was still gathering data to improve their predictions of satellite deorbit rates. For one thing, Alchemy was the first spacecraft to deploy the tether at a low enough altitude that it reentered the atmosphere as a result, so this was essentially uncharted territory. Hoyt also explained that the tether’s effectiveness is highly dependent on current solar conditions, which can make it difficult to determine how much it will slow the craft down until it’s actually been deployed and real-world data starts coming in.

Still, eight months is nothing compared to the time Augury is going to spend in space. Given its current velocity and altitude, it’s estimated that the control CubeSat won’t reenter the atmosphere until 2028 at the earliest. While the team obviously needs to improve their models for estimating deorbit time frames, there’s no question that the Terminator Tape is capable of greatly reducing the velocity of an orbiting satellite.

Growing a Tail

Take one look at your traditional satellite, and it’s pretty clear that atmospheric drag wasn’t of any great concern to the designers. Despite their large solar panels, haphazardly placed parabolic antennas, and general asymmetry, the drag imparted on most spacecraft is so slight that the occasional thruster firing is more than enough to compensate. Even the International Space Station, the largest and most ungainly vehicle humanity has ever put into space, only drops between two and three kilometers per month. As you’d expect the effect diminishes with increased altitude, meaning some satellites such as the Vanguard 1 launched in 1958, are expected to remain in orbit for hundreds of years.

The Terminator Tape works, at least in part, by greatly increasing the surface area of the satellite. Given the common 3U CubeSat is just 30 cm long, deploying the 70 m x 150 mm tether would increase its total surface area by a factor of roughly 150. Extended out from the satellite like the tail of a kite, the tether will passively reduce the craft’s orbital velocity so long as it’s at a low enough altitude to still experience significant atmospheric drag.

But it’s not just the increased surface area that will help bring the spacecraft down. A charge is built up within the conductive tether material as it moves through the Earth’s magnetic field, which in turn induces an electromagnetic drag on the system by way of a Lorentz force. The tether will essentially act as a retrograde propellant-less thruster, constantly pulling against the spacecraft and robbing it of momentum. This characteristic of the tether is especially important for craft in higher altitudes, where atmospheric drag alone may be too weak to have an impact.

Drag as a Service

To add the Terminator Tape to an existing spacecraft, there just needs to be a flat enough area to bolt the 180 mm square device onto and at least 808 grams available in the mass budget. It doesn’t even matter which face of the vehicle you attach it to or what orientation the craft is in at the moment of deployment, physics will handle all of that.

As a spacecraft designer, the only thing you really need to concern yourself with is providing it with the activation signal at the appropriate time. According to the datasheet that means applying 9 VDC to the unit’s shape-memory alloy (SMA) activator for 30 seconds, during which time the thermal device will pull around 1.9 amps. When using the smaller version of the tether, it only takes 300 mA @ 3 VDC. In either event, firing off the device at the end of a nominal mission should require little more than a free GPIO pin on the vehicle’s computer and a MOSFET.

The two Terminator Tape models currently offered by Tethers Unlimited.

Designers would also be wise to implement a secondary, automatic, deployment signal in the event of a vehicle failure. This could take the form of a dedicated battery, solar cell, and circuit that’s capable of providing the activation signal after a set period of time regardless of the vehicle’s status. For missions with relatively short lifespans, this contingency system could potentially even run on long-lasting primary cells.

In the future, such a system may not even require power to activate. As part of the growing “Design for Demise” initiative in the aerospace industry, research is being done into materials and adhesives which predictably break down based on time or external factors such as temperature and exposure to sunlight. Eventually, we may see a tether that’s deployed automatically once its cover plate has been deteriorated by the space environment; an autonomous and efficient reaper that makes sure no satellite stays around any longer than it needs to.

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