For 20 years now, the plasma crystal experiments conducted on board the International Space Station (ISS) have proved a reliable source of new insights into physics. The main aim of such research is to acquire fundamental knowledge for the future. The findings from the experiments lend themselves to a range of applications, especially in the fields of medicine, environmental protection, spaceflight, as well as semiconductor and integrated circuit technologies. Plasma research also opens up new fields of application through technology transfer; the development of miniaturised laboratory systems that are suitable for space missions is one example. Plasma research has been on the agenda since the very first crew boarded the ISS; the first long-term tests in microgravity conditions were initiated on 3 March 2001. The current crew will carry out the latest round of experiments towards the end of March, under the direction of the experienced research team at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) in Oberpfaffenhofen.
"For research into complex plasmas, microgravity conditions offer a unique opportunity to study the entire range of parameters that are of scientific interest. It is practically made for the ISS," says Group Leader Hubertus Thomas of the DLR Institute of Materials Physics in Space.
Plasma is an ionised gas and is used for a variety of technological applications, including in fluorescent tubes and plasma televisions. On Earth, it is very rare; in its natural form it occurs only during lightning strikes. In space, by contrast, 99 percent of visible matter is in a plasma state. This includes stars, such as the Sun, and the ionospheres of planets. If the electrically charged gas also contains dust particles or other microparticles, this creates what are referred to as complex plasmas, which can form crystalline structures.
Astronauts are critical to success
In addition to the sophisticated technology and hardware, those who are actually executing the experiments on board the ISS are critical to their success. ESA astronaut Thomas Reiter is the only German to have held this role. Reiter operated the plasma crystal laboratory ‘PK-3 Plus’ in August and October 2006 as part of his Astrolab mission.
"'PK-3 Plus' was a really interactive experiment. Once it was commissioned, I had direct radio contact with the scientists on the ground during many of the test sequences. Upon hearing what I was seeing, they were able to determine which parameters needed to be adjusted, allowing me to make the appropriate modifications to the plasma crystal laboratory. Despite the great distance from the ground station, it was fascinating to be part of the research team. Working together in this way was immensely interesting, but also great fun. 'PK-3 Plus' was also an example of the fact that fundamental research can reveal unexpected applications for everyday life on Earth," recalls Reiter, who was the first European to complete a long-term mission on the ISS. Today he is ESA's Interagency Coordinator and Advisor to the Director General.
When conducting the experiments, astronauts do the observing, thinking and acting. They can react to unexpected situations or address new findings on behalf of the scientists on Earth. Cosmonaut Yuri Baturin had a particular stroke of luck during his series of experiments in May 2001. The plasma could not be initiated in the laboratory chamber. However, the cosmonaut continued with the experiment, shaking microparticles into the gas in the chamber, which was neutral, not charged. To the astonishment of the scientists, the particles were both positively and negatively charged. Due to the strong electrical attraction, within a fraction of a second they formed a large agglomerate measuring several millimetres across, plus other 'clumps'. Observing this phenomenon solved what had hitherto been the mystery of planetary formation, revealing how the first phase of agglomeration of particles with a size of micrometres takes place.
This also shows just how pertinent this research topic is to the natural dusty plasmas that occur in the Solar System, such as in the rings of Saturn or on the surface of the Moon. "Dust is one of the biggest problems on the Moon. The basic findings arising from plasma research on the ISS are vital for acquiring a more accurate understanding of the properties of lunar dust and being able to deal with it more effectively, which will feed into the upcoming lunar missions," says Reiter. The dust in solar plasma is charged, can float and is highly adhesive. Lunar dust grains are sharp-edged, so they cause increased wear and tear to surfaces and instruments, while posing a health risk for astronauts.
Expanding our knowledge of physics
Having resulted in over 100 scientific publications, the plasma crystal experiments are among the most successful research projects to have taken place on the ISS. Their findings have expanded upon and changed physics theory on a number of occasions. Hubertus Thomas of the DLR Institute of Materials Physics in Space and his team have also been able to prove that a complex plasma is a new state of soft matter. In microgravity conditions, the particles are able to spread out freely in space and form ordered, three-dimensional crystal structures, referred to as plasma crystals. Their discovery in 1994 fundamentally changed physics, as plasma had previously been considered to be the most disordered state of matter.
The experiments on board the ISS make physical processes visible at an atomic level. The movement of individual 'atoms' and the way in which they interact can be tracked as though in slow motion. Over the last 20 years, scientists have gained unique insights into the formation of large crystal structures and long chains, the propagation of waves, shear flows and the flow properties of complex plasmas. Through their investigations into the model system, the plasma researchers are contributing towards a better understanding of the dynamic processes and phenomena and expanding our fundamental knowledge of physics.
Space remains as compelling as ever: "Sometimes I see the ISS fly over in the sky, and it is fascinating to think that our laboratory is up there, with a cosmonaut working on a plasma crystal experiment at that very moment. Not only do we have our laboratory in the basement, but we also have one at humanity’s furthest outpost, and there is still something very special about that, even 20 years on," says Hubertus Thomas. The next series of plasma crystal experiments will take place from 22 to 29 March 2021 at an altitude of approximately 400 kilometres.
International cooperation in space
The first plasma crystal laboratory, 'PKE-Nefedov', was in use from 2001 to 2005, followed by 'PK-3 Plus' for another seven years. The 'PK-4' laboratory has been in operation since 2014. Like its predecessors, it is a successful example of collaboration between the European Space Agency (ESA) and its Russian counterpart, Roscosmos, with scientific direction from the Complex Plasmas group at the DLR Institute of Materials Physics in Space (formerly at the Max Planck Institute of Extraterrestrial Physics; MPE) and the Russian Academy of Sciences Joint Institute for High Temperatures (JIHT). The experiments are controlled from the CADMOS control centre in Toulouse, France, and, most recently, from the DLR-run German Space Operations Center in Oberpfaffenhofen. The experimental hardware was developed in-house by the group during their time at MPE, and by OHB System AG (formerly Kayser Threde). Additional funding for the project in Germany was provided by the Max Planck Society and the German Space Agency at DLR, which has supported plasma research on board the ISS from the very beginning.
Once again, 'PK-4' impressively demonstrates the enormous potential of complex plasma research on board the International Space Station, even after two decades. This is also apparent at an international level. The German Space Agency at DLR is currently in talks with NASA, ESA, Roscosmos and world-leading scientists on the possibilities for a follow-up experiment to 'PK-4' called 'COMPACT'. "This new experimental facility could continue the success story of research into complex plasmas and add another exciting chapter," says Thomas Driebe, Programme Director for Physics and Materials Research at the German Space Agency at DLR.