Kilometer-Long Space Tether Tests Fuel-Free Propulsion

A massive cloud of space junk—containing more than 23,000 pieces larger than 10 centimeters across—is currently zooming around Earth with an average speed of about 36,000 kilometers per hour. And as companies such as SpaceX and OneWeb plan to launch tens of thousands of new satellites over the next few years, this hazardous clutter will likely pose an increasing threat to space missions and astronauts. One possible solution may be an electrodynamic tether, a device that could help prevent future satellites from becoming abandoned wrecks. The U.S. Naval Research Laboratory plans to test this technology in the next few weeks.

In early November the Tether Electrodynamic Propulsion CubeSat Experiment (TEPCE), already in orbit, is set to make its move under the watchful gaze of telescopes on the Hawaiian island of Maui. The Earth-bound control team is waiting for an ideal 10-minute period at dawn or dusk, when the dim sunlight will offer the best possible view of the shoebox-size spacecraft involved. Once the crew triggers the process, TEPCE should separate into two identical minisatellites joined by a kilometer-long tether as thick as several strands of dental floss. If deployment goes smoothly, the mission can observe how the tether interacts with Earth’s magnetic field in the ionosphere (where much of the space junk orbits) to change the satellites’ velocity and orbit; the results could possibly enable future spacecraft to move around while orbiting Earth—without having to carry unwieldy chemical propellant.

“In other words, it is the sailing ship of space,” says Enrico Lorenzini, a professor of energy management engineering at the University of Padova in Italy, who is not involved in the TEPCE mission. But instead of wind, the electrodynamic tether technology moves thanks to the physical laws that govern electric and magnetic fields. A tether in Earth’s ionosphere—an upper atmospheric layer filled with charged particles such as free electrons and positive ions—can collect electrons at one end and emit them at the other, generating an electric current through itself. The electrified tether’s interactions with Earth’s magnetic field produce an impetus known as the Lorentz force, which pushes on the tether in a perpendicular direction.

Several TEPCE components work together to create the necessary current. After separating, each of the two mini-satellites deploys a five-meter steel tape that can collect free electrons from the ionosphere. Each satellite also has a tungsten filament that uses power from onboard solar panels to heat up until it reaches a temperature at which it can emit those electrons. This produces a current that runs from the electron collector on one satellite through the tether to the electron emitter on the other.

Illustration of TEPCE with its tether deployed. Credit: Sarah Peterson U.S. Naval Research Laboratory

Because each tethered satellite contains both components, the current can flow in either direction—depending on which end of the tether is gathering electrons, and which is releasing them. When the current flows in one direction the Lorentz force pushes in a direction opposite to the spacecraft’s motion, producing drag that eventually slows the satellite down and reduces its orbital altitude. Run the current in the opposite direction and the direction of the Lorenz force will reverse as well, creating propulsion instead of drag. The resulting increase in velocity could help maintain or increase orbital altitude, enabling more complex maneuvers without requiring any additional fuel.

This mission will focus on testing the fundamental technology, rather than moving the satellites significant distances; the Lorentz force TEPCE generates will be small, due to the weak electric current involved and the reliance on solar panels—which barely supply enough power for an ordinary lightbulb. For this reason, mission planners do not expect ground-based radar and telescopes to have an easy time detecting small changes in the tether’s velocity. So the team will work on measuring the tether’s current flow and check positional data from GPS receivers on each satellite.

“The actual maneuver to be able to demonstrate that the orbit actually changed requires a lot more current than we can flow,” says Shannon Coffey, principal investigator for TEPCE at the Naval Research Laboratory in Washington, D.C. “So this is an experiment to show that we have all the parts working for an electrodynamic system.”

TEPCE’s power (and goals) are limited by size—the team had to cram all the onboard electronics, sensors, tether deployment devices and other mechanisms into two identical satellites with a combined capacity of about 3,000 cubic centimeters, roughly half the volume of a soccer ball. By using such tiny satellites, often called CubeSats, mission planners kept launch costs low and gained more opportunities for TEPCE to ride into space as a secondary payload. TEPCE hitched a ride aboard a SpaceX Falcon Heavy rocket in June 2019.

“TEPCE would demonstrate electrodynamic tether propulsion under extremely hard volume and weight constraints,” says Gonzalo Sánchez-Arriaga, an aerospace engineer and astrophysicist at the University Carlos III of Madrid in Spain, who is not involved in the current mission. Sánchez-Arriaga notes that this technology is not new; the 1990s represented a “golden decade for tethers in terms of funding level and missions” conducted by space agencies, he says. Altogether, missions successfully deployed more than 65 kilometers of tethers, demonstrating they could produce both thrust and power. But funding and interest tapered off after NASA cancelled a particularly ambitious mission due to the tragic loss of the space shuttle Columbia in 2003.

Now TEPCE is among a new wave of space-tether missions testing improved versions of the technology, and how it could help remove retired spacecraft. Sánchez-Arriaga is leading a separate project called Electrodynamic Tether technology for Passive Consumable-less deorbit Kit (E.T.PACK), funded by the European Commission. That mission aims to test how a tether, attached to future satellites, could deploy as a passive brake that brings down dead or decommissioned spacecraft at the end of their lives.

A successful TEPCE demonstration could also pave the way for a larger tether spacecraft. The aerospace company Star Technology and Research, based in Mount Pleasant, South Carolina, is developing a craft called the ElectroDynamic Debris Eliminator (EDDE) that is designed to act like an orbital tugboat: it could grab dead satellites or space junk with nets and a mechanical device somewhat akin to a catcher’s mitt. This company helped design the electron collector and emitter components for the TEPCE mission, and sees the current test as a precursor to EDDE. “This is basically a mini EDDE test because it’s based on our EDDE spacecraft,” says Jerome Pearson, an aerospace engineer and president of Star Technology and Research.

Once the TEPCE tether deploys, the countdown toward the mission’s end starts immediately. This is because the long tether will create additional drag that will lead the spacecraft to burn up in Earth’s atmosphere within approximately 60 days. But if all goes well, TEPCE’s sacrificial end could mark the beginning of a new era in decluttering Earth’s orbit—and preserving humanity’s access to space.

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