Wednesday, April 20, 2016

Scientists Observe Plasma Waves That Could Help Prevent Space Debris Collisions

Scientists at the Institute for Plasma Research have observed a phenomenon in the lab that could help predict collisions between satellites and space debris in the Earth’s ionosphere. Bits of dead and disintegrating satellites, spacecraft, and spend rocket stages clutter lower Earth orbit. The amount is growing at an alarming rate. Traveling as fast as 17,500 mph, even a piece the size of a penny could cause serious damage in a collision with a live satellite. The serendipitous story of new research that could help detect, and therefore prevent such collisions takes place at the intersection of basic research and practical need. A good place to start is back in 1834.

Space debris (size exaggerated) in lower Earth orbit. Space debris is an increasing problem, with likely millions of objects below 1 cm in size and hundreds of thousands of larger pieces of junk.
Image Credit: David Shikomba. Rights Information
Watching as a ship was drawn rapidly though Scotland’s Union Canal, the naval engineer John Scott Russell was captivated by a 30-foot long, solitary wave that swelled up in front of the ship. He chased the smooth, round “heap of water” on horseback for a mile or two before losing it in the winding channel, but was intrigued that the wave retained its shape and speed during his pursuit.
Despite the fact that they were once thought impossible, this type of solitary wave, now called a soliton, is not unique to the situation Russell observed. Solitons are not even unique to water waves. Scientists have studied optical solitons, pressure solitons, magnetic solitons, and even an electrical soliton in space. Solitons keep their shape and speed after colliding, making them especially useful for applications like communicating information through optical fibers. Some have theorized that neurological solitons exist and play a role in nerve function. Although it takes just the right conditions for solitons to form, they come up often in the mathematics that describes physical situations.

Recently, scientists at the Institute for Plasma Research experimentally observed solitons like Russell’s traveling in a plasma—a flowing gas so highly energetic that electrons get dissociated from their parent atoms. The collaboration brought together two projects, one using a new tabletop device to study the flow of plasmas containing charged dust particles (“dusty plasmas”) and the other on the dynamics of space debris.

The tabletop device was constructed by Pintu Bandyopadhyay and his PhD student, Surabhi Jaiswal, and enabled them to study plasma flow by illuminating the dust in a moving dusty plasma with a laser beam. Around the same time the device was up and ready, Abhijit Sen, a co-advisor for Jaiswal and longtime collaborator of Bandyopadhyay, was working on a project related to tracking space debris and warning satellites of impending collisions.

Satellites and space debris orbiting the Earth travel through its ionosphere, where they oftentimes pick up an electric charge. Sen was curious about the effect this might have on the debris, and whether it might impact the nearby plasma. While looking into this, he learned that objects traveling in a liquid above a critical speed can excite precursor solitons—solitons that travel ahead of the object, as Russell observed in front of the ship. If this happened in plasmas as well, Sen reasoned, solitons could act as a kind of warning beacon to a satellite that it was on a collision course with a piece of debris. The search was on, and the new tabletop device was an ideal place to start.

Designing the experiment was a technical challenge, but a key relativity realization helped with the engineering. Instead of moving an object at the high speed predicted to lead to soliton formation, the team realized that the object could be stationary if the plasma flowed over it instead. The physics would be the same and the experiment more straightforward since the device was designed to study plasmas flowing in a controlled way. Solitons in liquid have been observed using this method before.

After six months of tweaking the experimental conditions, everything came together late one evening. The team saw clear experimental evidence of precursor solitons in the dusty plasma.
An experimental image of the excited solitons. The dusty plasma is flowing to the left and flows over the object whose location is indicated by the dashed line. Recall that in this experiment the "boat" is standing still, so this situation is equivalent to one where the object is traveling to the right and the plasma is not flowing.
Image Credit: Surabhi Jaiswal
Their observations and analysis were published last week in Physical Review E and indicate that the same physical model of how solitons form in Union Canal holds true for solitons in plasmas.

The group is continuing to study this phenomenon and its potential applications to space debris and other areas, and hopes that other scientists will too. The conditions for this type of soliton formation also occur in astrophysical jets, solar wind, and some fusion energy research systems, among other settings. Solitons may have important implications in these areas as well, so don’t be surprised if you see the word cropping up again in a future edition of Physics Buzz!

—Kendra Redmond

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