Millions of pieces of space junk, many only the size of a fleck of paint, orbit the Earth. NASA

Australian researchers are developing “hunter-killer” satellites designed to neutralise potentially catastrophic space junk.

In a paper published in the journal Scientific Reports, a team led Rod Boswell from the Australian National University describe a craft that uses a single electric propulsion device to create two jets of plasma. The jets not only propel the satellite, but can be directed to fire at orbiting debris, slowing it until it falls gently to a fiery end in the Earth’s atmosphere.

Not all pieces of space junk are large, like those that dramatically spoil the day for Sandra Bullock and her fellow cast members in the 2013 movie, Gravity.

Although there are tens of thousands of dead satellites and debris larger than a football in low-Earth orbit, there are also hundreds of thousands of pieces larger than a fingernail, and likely over hundred million tiny grains the size of a paint fleck.{%recommended 3841%}

All these objects travel at orbital velocity, nearly eight kilometres per second, and have enough kinetic energy to damage any functioning satellites in their path. This in turn releases yet more pieces from the collision, increasing the chance of more crashes, causing a runaway chain reaction known as the Kessler Syndrome that would turn low-Earth orbits into shrapnel-filled no-go areas.

Removing space junk, especially larger pieces before they fragment, is critical to preventing this doomsday scenario. The easiest way to do this is to slow the junk, causing its orbital altitude to decrease. When it drops in altitude to 500 kilometres above the Earth the atmosphere is sufficiently dense to slow it yet further and ultimately burn it up in a year or two.

Several creative ways have been proposed for slowing space junk down. The most advanced techniques include harpooning it, or casting a net around it, and then slowing it by an attached drag sail until both captured and capturer burn up.

This solves space junk one piece at a time. The challenge is that most bits of it are uncontrolled and likely spinning, making physical contact a dangerous exercise. Ideally the cleaner would sit some distance from the target and never touch it, and if that cleaner could remove many pieces of junk instead of just one then the entire exercise becomes more cost effective.

Enter the plasma-powered satellite designed by Boswell and colleagues. It uses superheated, and superfast, ionised gas to propel itself in orbit but also, when directed at the space junk, to push it and slow it down in accordance with Newton’s Third Law.

Boswell explains how his team’s “tests show you can push plasma out one end of a satellite to thrust it towards the junk, and then push it out the other end to send that junk in the right direction”.

Honing in on fast moving debris in the first place poses a challenge.

“I call this the hunter-killer satellite because it would have to hunt out the space junk,” Boswell says. “From the ground you can calculate the orbit trajectory, so you know within a certain number of kilometres – but then it would have to find the junk with its own radar.”

The bi-directional plasma ejection means that that the satellite can fire a plasma blast at the space junk, which would move it away from its prey, but at the same time fire a jet in the opposite direction to keep its distance the same.

Once the target has been slowed sufficiently, the hunter-killer is free to move off in search of new prey, using the same single plasma jet set up. The mission ends when the hunter runs out of fuel.

So far, Boswell and his colleagues have demonstrated the satellite design in a laboratory space simulation chamber. Work is already underway to create a prototype that can be launched and tested for real.