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.

32 thoughts on “Pinning Tails On Satellites To Help Prevent Space Junk

  1. So the proposal is to massively increase the surface area == target area of each satellite. So collisions with the tapes will become the next thing, and loads of snapped-off conductive tape floating about up there ready to wrap around the antennas and solar panels of perfectly good satellites.

      1. Not all antennae are dishes. Probably LEO satellites will use smaller ones, but nevertheless almost any antenna will have its performance degraded by being wrapped in conductive film.

        So – what powers starlink satellites (for example?) I found this, albeit engineering speculation: “The next estimate is related to solar panels. There are 12 segments, where longer edge of each segment equals width of the satellite (3.2m). The segment appears to have 32×4 solar cells, where each solar cell has 2:1 aspect ratio.”

        So -my points seem to stand.

  2. Heh, dependent on solar activity… wonder if next time there’s CME glancing off the atmosphere, there’s a bunch of these tailed birds just coming around and they catch it as an up elevator to a higher orbit. Not to mention that while the Lorentz force is a steady drag, the more extreme solar magnetic events might hoist it by the tail or accelerate it (I know you cannot push something with a string, but if the force acts steadily on the string, the string will move to the other side of what it is attached to, and then it will be pulling)

    1. TL;DR: the odds of a CME occurring at exactly the right time and vector relative to the orbit are exceedingly low. And, even in the case of a golden BB, for any CME that humans can survive, the magnitude of the change is miniscule. Also, in almost all cases, even that potential minor increase in apoapsis would be offset by significantly greater decreases in periapsis. At most, it might delay a reentry fractionally, but it’s far more likely to accelerate reentry significantly. Same thing is true of satellites without tether tails, just at a much reduced magnitude.

      It’s actually theoretically possible to use a tether and lorentz force to increase (or decrease) the diameter of an orbit via electrodynamic forces. Not practical; you have to induce a lot of amps of static charge into a very long tether (20 km or longer) with equal masses on both ends, and the combined body needs to rotate end-over-end in the plane of the orbit, and static-charge isn’t an effective way of creating a magnetic field in the first place.

      The (1960s) design I studied in detail involved very large electron sources in dumbbells on each end of the tether, taking turns energizing the tether. This created a charge imbalance in the tether that could be dynamically tied to the phase of the dumbbell’s cartwheel along its orbit path. The design specifically noted that particle damage on the satellites due to the charge imbalance was an open problem, and noted that the maximum force demonstrated in a magnetic field in a vacuum lab was not promising.

      Simulation(using ideal maximum forces) has shown some promise for orbital phasing adjustments, but much less promise for actual orbit raising. Potential force (in any vector) drops as inverse square of apoapsis, so decircularizing an orbit for phasing purposes is easy, but efficiency is poor for raising periapsis.

      The Terminator Tape approach is an unpowered tether, and the Lorentz forces are solely due to back-emf from induced current. It can only result in a drag/decelerative force. It’s basically the same effect that causes “cogging” in magnetic generators. In generators, its something to be minimized, but in an orbital tether, it’s unavoidable, and convenient to boot.

      During a CME, the CME itself doesn’t induce current into the tether. Rather, the Earth’s magnetic field gets distorted by the CME, and the field moves past the tether. The resulting distorted drag vector will be dragging against the vector of the magnetic distortion. A CME that could shift the Earth’s field enough to create net acceleration on an orbital tether has already burned out the Earth’s power grid, so nobody’s going to care about what happens to the satellite at that point :)

      Remember, an exoatmospheric nuclear explosion within Earth’s magnetosphere is equvalent to a very small CME in terms of magnetic field distortion, and the much-feared “nuclear EMP” is just current induced in the power and telecommunication grid when the Earth’s magnetic field distorts and springs back. The difference between a nuclear explosion positioned for maximum EMP and a random CME is simply this: the CME produces a MUCH LARGER change in the earth’s field, but a nuke can potentially create a greater delta field strength within a much smaller volume.

      If humans set of a high-megaton nuke at exactly the optimal north/south magnetic field line, at an altitude to produce the optimized conductive plasma in a near vacuum, they could produce a potentially militarily useful amount of damage (although much more damage would result if the nuke was merely detonated at a conventional low altitude). However, pretty much ANY CME has the potential to do far more damage. In both cases, the mechanism is the same: current induced in conductors swept by the field as it distorts and rebounds. A CME is likely to do more than 3 orders of magnitude more damage to a large fraction of the earth than a nuclear EMP would cause in the 20 mile radius of maximum intensity.

      The tether is far less effective at coupling to the magnetic field, so the back-emf caused by even a big CME will always be small relative to that induced into the global power grid by the same CME.

      The tether’s back-emf results in a drag deceleration against the apparent velocity in the magnetic field, always.

      And no survivable CME can create a sufficient magnetic field distortion of the right vector to meaningfully augment the satellite’s real orbital velocity.

      A powered tether would have a substantially more complicated effect during a CME; While the total force wouldn’t change, it’s a lot more complicated (and less intuitive) to determine the net change in velocity vector. For a powered tether, the resulting change in force is net-positive/non-drag (but depending on vector, it could of course actually decrease orbital velocity and thus orbital diameter).

      Exactly what happens would depend on the orientation of the orbital plane vs the sun and exactly where in orbit the satellite is, as well as the total duration of the CME.

      The forces are small (less than a small ion thruster), so even a long-duration CME would only be vector-augmenting the satellite during a very small fraction of the orbit. Even if the CME was in-plane (the orbital plane directly aligned with the sun), the statistically dominant factors would be the parts of the orbit where the augment vector was radial-in, retrograde, or radial-out. All of those cause significant lowering in some part of the orbit. Orbits not aligned with the sun would have this effect exaggerated, similar to what would occur with an uncompensated orbit-normal burn.

      Basically, although the details can differ in a myriad of ways, almost all outcomes result in exaggerated de-circularization of the orbit. Statistically, the net result is always to the periapsis.

      Any CME that doesn’t last for a complete orbit would only at most minimally raise the apoapsis, and a perfectly timed CME might not shrink the periapsis. However, any apoapsis growth would be offset by increased electrodynamic and aerodynamic drag at periapsis.

      Most CMEs wouldn’t occur at the correct orbit phase to grow the apoapsis. The majority of short CMEs and all long CMEs would guarantee to lower the periapsis along with whatever other random changes they made to the orbit.

      If you think of it in Kerbal terms, an intuitive scenario covering the simple in-plane case would be:
      * point a very small solar-powered ion thruster directly at the sun. Don’t change this vector as you orbit.
      * whenever you’re in sunlight, it’s powered. In terms of the body you’re orbiting, this will result in thrust during retrograde, radial-in, and prograde orbital segments, but no thrust during radial-out.
      * as such, retrograde and prograde will mostly cancel, and the net effect will be to lower the orbit segment during which the craft is retreating from the sun and raise the orbit segment during which the craft is approaching the sun.
      * since the rate of orbit decay is enhanced as soon as the periapsis is low enough to begin inducing atmospheric drag, reentry will be more rapid than otherwise.

      In reality, the specific vectors will be totally different than the kerbal scenario presented above, but the asymmetric nature remains present, resulting in the same kind of enhanced orbital decircularization. Even if this increases the apoapsis, the decreased periapsis eventually deorbits the satellite. And remember, whenever the satellite is at the higher portion of its orbit, electromagnetic force effects decrease as the square of the altitude.

      A milder form of this decircularization happens naturally due to solar activity, even without a tether. In addition, expansion of the earth’s atmosphere drag boundary occurs at the same time, making the reentry tipping point easier to achieve. This is a (small) part of what prevented Skylab from surviving to be refurbished…

      At altitudes where a tether is most effective, this asymmetry is magnified because the Earth’s shadow subtends a greater fraction of the orbit. At altitudes where this is not true, inverse square law reductions in Earth’s magnetic field make tethers much less effective. Even so, the asymmetric deformation of Earth’s magnetic field will bias the tether’s effectiveness to favor the down-solar-wind side of the orbit, once again resulting in asymmetric effects on the apoapsis and periapsis that decircularize the orbit.

  3. The Space satellites mention are deliberately placed in a low orbit that is unstable. The Spacex satellites need to periodically use trusters to maintain orbit. So if the satellite stops working or runs out o fuel it will debris in about one year. And even if a spacex satellite hits another satellite most of the debris will deorbit in months with the small amount remaining deorbiting in about a year.

    Small cube satellites are placed in low orbits that decay in a few years with only a few lasting about 10 years. All due to atmospheric drag.

    The satellites that are a problem are all in higher orbits were atmospheric drag doesn’t occur. So if one fails it stays there. All geostationary satellites are in high orbits. When they are near their useful life they are typically moved to grave yard part orbits and left there. Any collision of high earth orbiting satellites will result is debris that will stay there. Fortunately there are fewer of these high orbit satellites.

    1. Slight correction: SpaceX satellites need to fire their engines continuously (not periodically) for the entire time they are in orbit (planned life of five-ish years). A SpaceX sats that looses it’s ability to fire enignes will deorbit naturally in under a year and will completely burn up in the atmosphere. Also, there is not much debris in those orbital planes due to atmospheric drag. Source: I work there.

  4. If it’s an SMA surely it’s possible to trigger it with a chemical charge instead of needing a battery to keep charged. I’d have thought even a Mylar balloon and a CO2 canister would do the job to create a large draggy tail?

  5. What about plasma thrusters? They can work off of electric power alone. They are very low thrust, but they could work over an extended period of time and they are fairly small and reliable. You could set them up to automatically deploy at end of life.

    1. You’d need to stabilize the satellite to do the burn, which may or may not be possible depending on how functional it is. The beauty of the tether is that it will naturally stabilize the tumbling satellite like a kite’s tail.

  6. I was amused by the movie Wall-e where the rocket leaving the abandoned earth has to smash its way out through a thick layer of UNMOVING satellites. But as the article shows, the reality is that after we humans managed to kill ourselves off (or just leave), it will become a cascading crash fest.

    But on the positive side, it will catch the attention of future passing aliens who will wonder why this planet is surrounded by a cloud of refined metals and silicon! A giant tombstone in space? Or maybe our “Kilroy was here” sign?

  7. It’s written that air has metal debris and it’s bad for us… passes through a cigarette hitting 4000°. Amazing. I’d buy, 400, but that’s what I read. Anyway, I’m not comfortable letting things vaporize upon reentry… esp metals. I’d rather it not be allowed. Collect old stuff. Force a price to do so. Once the garbage scow is full in a space shuttle, bring it down.

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