New space junk removal idea: Using ion engine exhaust to knock debris out of the sky

Over 14,000 pieces of junk are clogging up low Earth orbit, but this number could be reduced by a new invention that involves a satellite using the plasma exhaust of its ion engine to knock dangerous chunks of space junk into the atmosphere where they can burn up safely.

The huge amounts of space junk in orbit, ranging from nuts and bolts to rocket fairings and dead satellites, pose a serious hazard to satellites and the International Space Station, which has to regularly take evasive action to dodge space shrapnel, much of which is moving faster than a bullet.

Therefore, scientists and engineers have been working on ways to clean up low-Earth orbit. Most solutions proposed have included spacecraft that could grab space junk with robotic arms, nets and tethers. The problem with these ideas, however, is that much of the junk is tumbling. This means there's a risk of the spacecraft getting caught up in the chaotic motion.

A non-contact method would be safer, and one proposal is to use the exhaust of a spacecraft's ion engine to gradually but firmly push against a piece of space debris, slowing the debris down until it falls out of orbit and into the atmosphere.

However, an ion engine is designed to push a spacecraft forward. Firing an ion engine's plasma exhaust at a piece of space junk would therefore push the removal satellite away from the junk.

Now though, Kazunori Takahashi of Tohoku University in Japan has developed a solution. Rather than having just one exhaust, a removal satellite could have two exhausts pointing in opposite directions. The thrust from each exhaust would cancel the other out, allowing the removal satellite to hold station while it goes about its job of deorbiting space junk.

Takahashi calls his system a "bidirectional plasma ejection-type electrode-less plasma thruster." An ion engine works by using an inert gas as a propellant; often this gas is xenon, but Takahashi used argon.

"It can be operated using argon to a similar efficiency as with xenon, but providing a reduced cost for the propulsion device," Takahashi told Space.com.

The gas fills a chamber, with a cathode on one side producing a cloud of electrons that are then attracted to a positively charged discharge wall. On their way, they interact with the gas and ionize it. Ionized gas is called a plasma, and it is electrically charged such that it can be directed and accelerated by electromagnetic fields out through a thruster nozzle, producing thrust.

In Takahashi's system, "the plasma can flow along the field lines towards both sides, providing the bi-directional plasma ejection," he said.

The thrust from an ion engine is initially small compared to a chemical rocket, but over time it can build up and become comparable. However, with so many pieces of space junk to remove, time is of the essence.

To deorbit a piece of space junk a meter across and with a mass of 1 ton in less than 100 days requires the constant application of 30 milli-Newtons (mN) of thrust. This is greater than the thrust provided by the ion engine on board the Japanese Aerospace Agency's (JAXA) Hayabusa2 mission to the asteroid Ryugu, for example. Hayabusa2 achieved 10 milli-Newtons (mN) of thrust from 300–500 watts of electrical power produced by its solar arrays.

Takahashi's bidirectional system is far more power hungry. "It needs to have a few kilowatts," he said.

In order to boost the power of his system, Takahashi introduced a "magnetic cusp" that is able to keep more of the plasma away from the discharge wall and force more of it out through the thruster nozzle.

"The specific shape of the cusp provides a geometrical separation of the plasma from the wall, reducing the plasma loss," said Takahashi.

This means more of the plasma is available to be spewed onto the space junk, and more plasma means more thrust. In laboratory experiments testing his system with large vacuum tubes mimicking conditions in space, Takahashi was able to reach 25 mN, tripling the power output compared to previous experiments.

Takahashi's bi-directional thruster is designed to be able to take out larger pieces of space junk because they are the ones most likely to lead to a frightening scenario called Kessler syndrome. It describes how a collision between a large satellite and a piece of space junk could cause a chain reaction as shrapnel is scattered throughout orbit, instigating more collisions in a runaway effect that could leave huge swathes of low-Earth orbit inaccessible because of the sheer amount of debris would be too hazardous for spacecraft or satellites to pass through.

Long-term, it could completely block our access to space, which is why a means of removing space junk grows increasingly vital as more and more debris begins to encircle the Earth.

Takahashi's work on the bidirectional plasma thruster was published on Aug. 20 in the journal Scientific Reports.