Results from Space Station experiment

May 07, 2003

FIRST results have been announced from one of the most promising experiments on the International Space Station. The project studies a type of matter called a "dusty plasma", which contains particles big enough to be seen and tracked with video cameras. The plasma structures created, some never seen before on Earth, will help scientists to test their ideas about conventional solids, whose atoms are too small to watch.

Plasmas, known as the fourth state of matter after solids, liquids and gases, consist of clouds of charged particles. Generally these particles are single ions or electrons, but in dusty plasmas the particles are much bigger. Scientists working on the "PKE-Nefedov" experiment on board the ISS make their dusty plasma by stripping all the electrons off argon atoms, leaving the positive nuclei floating in a turbulent sea of electrons. Tiny plastic spheres with a diameter of 7 micrometres are then injected into the mixture. These mop up the electrons and so become negatively charged. The spheres can then be tracked by video camera during various experiments, for example changing the temperature or pushing the particles about with beams of laser light.

Whereas plasmas are usually chaotic clouds of particles, dusty plasmas initially surprised researchers by organising into crystal-like structures (see Diagram). The first such "plasma crystal" was discovered in 1994 and prompted an explosion of research. Although the consistency of the crystals is something like a viscous fluid, their internal structures closely resemble the atomic lattices seen in conventional solids. This raises the possibility of testing the basic science of solids with a precision not possible until now. For example, you could track the plastic particles as a plasma crystal "melts", or as a sound wave passes through it.

However, the hundred or so teams investigating plasma crystals on Earth have a problem. Although they can study two-dimensional versions, in three-dimensional crystals gravity pulls the more massive particles out of the mixture, distorting the structure. So since February 2001, astronauts have been studying plasma crystals in microgravity conditions on the ISS.

Project head Gregor Morfill of the Max-Planck Institute for Extraterrestrial Physics in Garching, Germany, and his colleagues now report that the astronauts succeeded in creating three-dimensional structures that perfectly resembled many of the structures commonly seen in solids, such as the face-centred cubic packing of atoms in silver and gold, and the hexagonal close packing of magnesium (New Journal of Physics, vol 5, p 33). They were particularly excited to see body-centred cubic, a structure never seen before in plasma crystals on Earth. One of the cosmonauts was so intrigued, "he decided to do extra experiments in his private sleep time", recalls Morfill. This first paper is a hint of the flood of results to come. "By the time the programme is complete, there will probably be a total of 30 papers," Morfill says.

Experimental work on the ISS is now very restricted: the crew has been reduced from three to two following the Columbia shuttle disaster. But the plasma studies may benefit, because new experiments cannot be carried up to the ISS while the shuttles are grounded. So the crew have been given more time for work on plasma crystals. Following the success of the project, the European and German Space Agencies propose building a dedicated plasma physics lab for the ISS, for launch in 2008.
Jenny Hogan

New Scientist issue: 10 May 2003


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