Surrey Blog

OTB Blog Series: Deorbit Tethers...a Little Push from the Space Environment to Return Home Safely

Imagine this: you are driving on the Interstate in rush hour traffic with a lane all to yourself, and a law states that you must remain in your lane for the duration of your trip. If you had the means to never break this rule, you could go as far as you wanted, at the highest speed possible, and arrive at your destination in the shortest time—safely and consistently. This, of course, assumes that there are no other people in your lane.
In reality, there are disturbance forces and physical limitations at play that prevent our daily commute from being this simple. Maybe your neighbor’s car is 25 years old and no longer being maintained. His steering wheel goes slightly out of alignment over time. If your neighbor continues to drive this car, one day he might just veer into your lane, crash into you, create hundreds of pieces of debris and consequently a massive pileup. When this happens, none of us arrive at our destination.

NASA does in fact have rules they impose on spacecraft manufacturers to “move spacecraft off the road” when they are no longer capable of “staying in their lanes.” One of these rules states, in simple terms, that if you are going to put a spacecraft in low-Earth orbit, you must include in your design a way to remove it from this orbit within 25 years of the end of its mission.

This rule is typically difficult for spacecraft to adhere to at altitudes above 600 km without the use of chemical propellants. For larger spacecraft, the traditional approach is to point the spacecraft opposite its direction of travel and apply thrust to remove kinetic energy from the orbit—effectively causing the spacecraft to slowly spiral in towards Earth and “burn up” in the atmosphere. In the traditional sense, you would include additional propellant in the design above that which is necessary to keep the satellite on its trajectory. The problem is propellant consumes volume inside the spacecraft, adds mass, and isn’t always cheap; and since volume utilization, mass, and cost are critical considerations in any good spacecraft design, companies are actively seeking alternative solutions to the chemically driven “retrograde fire” approach.

We at Surrey are employing the use of a unique mechanism to accomplish deorbit for our Orbital Test Bed (OTB) satellite, which is currently scheduled for launch in 2016, and we will do so without the use of bulky chemical propellants. Tethers Unlimited Inc. (TUI) of Bothell, Washington, has devised the Nanosat Terminator Tape (nTT) Deorbit Module, which uses space plasma, the Earth’s magnetic field, gravity, and aerodynamic drag to remove enough energy from an orbit to accomplish the deorbit task. We selected the nTT to fly on our OTB smallsat to demonstrate its technology and to validate its lightweight, low-cost, and scalable design. In essence, the nTT uses existing resources available in space to generate sufficient forces on the spacecraft and remove energy from the orbit—as opposed to adding a relatively massive resource to the spacecraft to generate that force. Initially designed for satellites sized to 10 kg mass, a derivative design has been scaled up to a larger size for OTB (supporting up to approximately 180 kg). Upon activation, the nTT module will eject a cover physically connected to a specialized conductive tape. As momentum carries the tape away from OTB, Earth’s gravity will align the length of the tape to the direction of Earth’s center. At full extension, the tape will interact with Earth’s magnetic field and ionosphere to induce “Lorentz force” or “electrodynamic drag” on the spacecraft.

nTT Deorbit Module physics and conceptual design [credit Tethers Unlimited]
Here’s how it works: The tape draws in ions from the surrounding plasma in the ionosphere. The orbital motion of the satellite causes the charged particles to move in an electric field perpendicular to OTB’s direction of travel. The movement of these charged particles causes an interaction with Earth’s magnetic field, and this interaction produces an applied force in the direction opposite to that of OTB’s direction of travel. This force, along with neutral particle drag force (think, the force felt when sticking your hand out of the window of a moving car) will accelerate OTB’s orbital decay rate and send the spacecraft on a predictable inward spiral towards Earth.

Surrey engineer indicating approximate integration location of the Nanosat Terminator Tape (nTT) Deorbit Module with OTB
As a spacecraft mission designer and manufacturer, we realize the importance of following these “rules of the road.” Just as we are responsible for protecting the environment as a means of preserving our quality of life, all space-faring nations are responsible for protecting the “space” we occupy as a means of preserving the safety of other satellites. We are proud to fly technologies such as those developed by Tethers Unlimited and plan for many more OTB missions to bring these technologies to light. In the meantime, we will continue to do our part to “move our spacecraft off the road” and “stay in our lanes” when our 25-year-old vehicles are no longer roadworthy. Drive safely and responsibly.


23 March 20150 Comments1 Comment

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