Space has a trash problem, with defunct satellites, rockets, and smaller broken bits orbiting the Earth at high speeds. The amount of space junk is only increasing, raising the risk of collision with active satellites and spacecrafts, according to Kazunori Takahashi, associate professor in the Graduate School of Engineering at Tohoku University in Japan. Takahashi may have a solution, though.
"Owing to their uncontrolled motion and velocity exceeding that of bullets, space debris orbiting around Earth pose a serious threat by significant increase in the potential risk of collisions with satellites that support sustainable human activity in space," Takahashi said. "Most current space debris removal methods are direct-contact approaches and carry the risk of becoming entangled in the uncontrolled motion of debris. More recent work has focused on using a plasma thruster to decelerate the debris, forcing it out of orbit."
His approach, successfully demonstrated in laboratory experiments and published on August 20, 2025, in Scientific Reports, builds on the idea of slowing space debris down until it falls out of orbit and burns up upon re-entry to Earth's atmosphere. Previous research has suggested this concept, but it has yet to be effectively employed in practice.
The idea is that a removal satellite, deployed for the express purpose of removing space debris, could use a propulsion engine to spew plasma toward a piece of space junk. The force of the plasma would slow the junk, decelerating it enough to drop out of orbit ─ a process that takes about 100 days. The issue is that the force of the plasma emitted from the removal satellite has a harsh kickback, moving the satellite away from the target and lessening the deceleration effect.
To correct this, Takahashi developed what he called a "bidirectional plasma ejection type electrodeless plasma thruster." It is a propulsion engine that ejects two streams of plasma in two directions: one toward the target space debris and one in the opposite direction.
"This propulsion engine applies deceleration force to the target object by ejecting plasma, while avoiding excessive thrust on itself by ejecting another plasma plume in the opposite direction," Takahashi says, explaining that he also introduced a special magnetic field known as the "cusp" to enhance the deceleration force. Essentially, the cusp contains the plasma through a magnetic field, so it remains relatively contained to the direction of thrust rather than dissipating.
To see if the bidirectional plasma ejection could work as expected, Takahashi tested it in vacuum tubes meant to mimic the conditions of space. He found that not only did the bidirectional plasma ejection balance the engine as expected, but that the cusp configuration increased the deceleration force - meaning that targeted space debris could potentially reach the deceleration rate needed to deorbit much more quickly. Furthermore, operating the thrust at a high-power level under the cusp configuration tripled the deceleration force previously reported. Critically, the propulsion system can be operated using argon, which is cheaper and more abundant than conventional propellants.
"This achievement represents a significant technological advancement toward developing a propulsion system capable of efficiently and safely removing space debris," Takahashi explains.