20 years of plasma research in space
Physicists have been conducting experiments with complex plasmas on the International Space Station for 20 years. The crew members on board form a perfect experimental team with the researchers on Earth. The success of the previous plasma crystal laboratories is so great that a successor is already being considered.
The Plasma Crystal Laboratory PK-4.
Photo: Max Planck Institute for Extraterrestrial Physics
For 20 years they have been a reliable source of new insights into physics: the plasma crystal experiments on board the International Space Station. Basic knowledge for the textbooks of the future is the main goal of this research. Various applications can be derived from the knowledge gained, in particular in the fields of medicine, environmental protection, space travel as well as semiconductor and microchip technologies. By means of technology transfers, plasma research also opens up new fields of application, based for example on the development of miniaturized laboratory systems suitable for space travel.
The first ISS crew had plasma research on their agenda and on March 3, 2001, the go-ahead was given for the first long-term tests under weightlessness. The current crew will now carry out the latest series of experiments at the end of March, under the leadership of the experienced research team at the German Aerospace Center (DLR) in Oberpfaffenhofen. “For research on complex plasmas, weightlessness offers the only possibility to examine the entire, scientifically interesting parameter space. It is predestined for the ISS,” says group leader Dr. Hubertus Thomas from the DLR Institute of Materials Physics in Space.
Plasma is ionized gas and is used technically in a variety of ways, for example in fluorescent tubes or plasma televisions. Plasma is very rare on earth; in its natural form it occurs, for example, as lightning. In space, on the other hand, 99 percent of visible matter is in the plasma state. These include stars, including the sun, or the ionosphere of planets. If the electrically charged gas also contains dust particles or other microparticles, so-called “complex plasmas” are created, which can form crystalline structures.
In addition to the sophisticated technology and hardware, the “executors” on board the ISS are also essential for the success of the test series. The ESA astronaut Thomas Reiter has so far been the only German to have held this role. As part of the Astrolab mission, he operated the PK-3 Plus plasma crystal laboratory in August and October 2006: “PK-3 Plus was a really interactive experiment. After commissioning, I had direct radio communication with the scientists on the ground during many test series. The description of my observations allowed them to pass on modifications to the various test parameters, which I then set in the plasma crystal laboratory. It was fascinating – despite the great distance to the ground station, you were part of a research team. The collaboration was not only extremely interesting, it was also huge The PK 3 Plus was also an example of the fact that basic research can also have unexpected applications in everyday life on earth, “recalls Reiter, who was the first European to complete a long-term mission on the ISS with Astrolab and is now an ESA coordinator international agencies and consultants to the Director General is active.
As experimenters, the astronauts see, think and act. For the scientists on the ground, they can react to unexpected situations or respond to new findings. Cosmonaut Juri Baturin proved a particularly lucky hand during the series of experiments in May 2001: the plasma could not be ignited in the laboratory chamber. However, the cosmonaut continued the experiment, shaking microparticles into the chamber’s neutral, rather than charged, gas. To the astonishment of the scientists, the particles were both positively and negatively charged and, due to the strong electrical attraction, formed a large agglomerate several millimeters in diameter and other “lumps” in a fraction of a second.
On the basis of this observation, the previous mystery of planet formation could be solved, how the first phase of agglomeration of particles with a size of micrometers takes place. This also shows how close the research topic is to natural dusty plasmas that occur in our solar system, for example in the rings of Saturn or on the moon. “Dust is one of the biggest problems on the moon! The fundamental findings of plasma research on the ISS are particularly important for the upcoming lunar missions in order to better understand the properties of lunar dust and to be able to deal with them better,” explains Thomas. The dust in the solar plasma is charged, can even float and has a strong adhesive effect. Since moon dust is sharp-edged, this leads to increased wear of surfaces and instruments and represents a health risk for the astronauts.
With over 100 scientific publications, the plasma crystal experiments are among the most successful research projects on the ISS. The findings from this have expanded and revised the teaching knowledge of physics several times. The team around Thomas was also able to prove that a complex plasma is a new state of soft matter. In weightlessness, the charged microparticles spread freely in space and form ordered three-dimensional crystal structures, so-called “plasma crystals”. Its discovery in 1994 fundamentally changed the doctrine of physics, as plasma was previously considered the most disordered state of matter.
The experiments on board the ISS make physical processes visible at the atomic level. The movement of individual “atoms” and their interactions can be tracked as if in slow motion. In the last 20 years, the scientists gained unique insights into the formation of large crystal structures and long chains, the propagation of waves, shear currents and the flow properties of complex plasmas. With the investigations on the model system, plasma research helps to better understand the dynamic processes and phenomena and to expand the basic knowledge in physics.
The fascination for space always resonates: “Sometimes you see the ISS fly over in the sky and when I imagine that our laboratory is there and a cosmonaut is doing a plasma crystal experiment there, then I find it fascinating. We have not only our laboratory in the basement, but also on the most extreme outpost of mankind – that is still something very special even after 20 years, “says Thomas. From March 22 to 29, 2021, the next plasma crystal experiments will again take place at an altitude of around 400 kilometers.
The first plasma crystal laboratory “PKE-Nefedov” was in use from 2001 to 2005, followed by “PK-3 Plus” for a further seven years. The “PK-4” laboratory has been in operation since 2014 and, like the previous projects, is a German / European-Russian success story. PK-4 is a cooperation between the European space agency ESA and the Russian space agency ROSKOSMOS, with the scientific leadership of the group “Complex Plasmas” of the DLR Institute for Materials Physics in Space (formerly at the Max Planck Institute for Extraterrestrial Physics) and the Russian Academy of Sciences (Joint Institute for High Temperatures).
The experiments are controlled from the CADMOS control center in Toulouse, France, and most recently from the DLR German Space Control Center in Oberpfaffenhofen. According to the researchers, PK-4 has impressively shown the great potential that research with complex plasmas on the International Space Station still has, even after two decades. This is also seen internationally in this way. Therefore, the German Space Agency at DLR is currently discussing the possibilities for a follow-up experiment for PK-4 with the name “COMPACT” together with NASA, ESA, ROSKOSMOS and the world’s leading research teams.