How Wilfried Goldacker, Sonja Schlachter and Hong Wu want to protect astronauts as they enter the Earth's atmosphere.
On February 1, 2003, seven people died at an altitude of just under 70 kilometers above Texas when the space shuttle Columbia broke apart on entering the Earth's atmosphere. A heat tile that fell off unnoticed during launch was resposible for this tragedy; the resulting hole in the wing became Columbia's Achilles' heel. Hot plasma penetrated through it on re-entry into the atmosphere, damaging the wings and subsequently the entire structure of the space shuttle within a short time. Space agencies around the world are investigating how crews and spacecraft can be better protected. In addition to heat protection, the focus is also on communication interruptions caused by the plasma generated during re-entry. One idea to mitigate this phenomenon is to redirect plasma particles in a targeted manner by magnetic fields. In cooperation with the German Aerospace Center (DLR) and the Russian IOFFE Institute, a research team from KIT has developed a magnetic system to influence the plasma in the COMBIT project and conducted successful ground tests. "Our project is primarily aimed at maintaining communication," says Wilfried Goldacker of the KIT Institute of Technical Physics.
This is because the electrically charged particles in the plasma ensure that no radio waves can penetrate from or to ground stations or satellites for a certain time. Thus, not only voice communication between the crew and the ground station breaks down for a certain period of time, but also position determination by means of GPS and any data transmission. Communication interruptions of a few seconds or minutes have been observed on various missions in the past; on early space shuttle flights, they even lasted up to half an hour. This radio blackout is dangerous: "The goal is to create a 'hole' in the plasma during Earth entry in order to maintain radio contact," explains Sonja Schlachter, who supervised the scientific work surrounding the construction of the new magnet together with her colleague Hong Wu. The method for mitigating the radio blackout is based on magneto-hydrodynamic effects. It involves using crossed electric and magnetic fields to selectively redirect the plasma flow to create clearances for radio waves in the vicinity of transmitters or antennas on the spacecraft.
"Working on a superconducting magnet for use in space is extremely exciting because we have to adapt our technologies to a complex overall system. You learn a lot of new things and you are under pressure to deliver a reliable result."
To be able to create this hole in the plasma at all, it needs a very high magnetic field. "This is not feasible with common permanent magnets. We therefore use high-temperature superconducting magnets that can generate a much larger stray field," says Schlachter, a physicist who has been working on superconducting materials for 20 years and, like her colleagues at KIT, brings the necessary experience to magnet development. It took the researchers about three years to develop a magnet system and test it in the DLR plasma tunnel in Cologne. The developed system would be located below the outer wall when used on the spacecraft. Schlachter says: "The biggest challenge was the geometry. The entire magnet system, including cooling for the superconductor, was only allowed to be ten centimeters in diameter in the experiment, had to practically fit into a special thermos and in doing so, the magnet has to be fitted in such a way that it is as close to the plasma as possible and still doesn't get warm."
The ground experiments in the plasma channel show: the radio blackout mitigation method and the magnet system basically work. The use of superconducting magnets would also be conceivable for a manned mission to Mars, where the astronauts need to be protected from immense cosmic and solar radiation exposure.The use of much larger magnets, that can shield large areas, is being discussed in international collaborations worldwide. However, it still is a long way before they can be used in manned spaceflight. Schlachter says: "Therefor, superconducting magnet systems still need to be significantly developed further in terms of design, weight, energy consumption and service life. Nevertheless, we hope to see our magnets launched into space at some point."