Space travel | Gardening in space

An experiment on the ISS in 2015 examined growth in weightlessness. Photo: Nasa/Wikicommons Even if you don't have a particularly green thumb: you can at least rely on earthly gravity when gardening. This is how water flows into the ground by slowly seeping down through cavities or cracks. Plant roots absorb water and dissolved minerals from the moist soil.

However, when gravity is significantly lower or absent, as is the case on a space station or the moon, things quickly become complicated for plants and gardeners. Without gravity, plants not only lack the orientation in which direction the roots have to grow. There are also difficulties with supply, as liquids behave completely differently in weightlessness. They form spherical accumulations, float or distribute themselves unevenly in the substrate and thus also on the roots, so that they can quickly dry out or suffocate without controlled active intervention.

Since the first successful flowering of the inconspicuous, herbaceous thale cress in 1982 on board the Soviet space station "Salyut 7", a colorful "mixed salad" has been grown on space stations. For example, romaine lettuce, tomatoes and peppers were successfully harvested and tasted. However, a self-sufficient supply of fresh plant products for space travelers is still a long way off.

In order to better understand the behavior of liquids under low gravity, scientists can simulate a weightless state for a short time in various experiments: For example, almost five seconds of weightlessness can be achieved at the 120-meter-high “Bremen Drop Tower”.

However, these time periods are not sufficient to accurately examine the behavior of liquids under microgravity, i.e. significantly reduced gravity. As part of the “New Shepard 29” (NS-29) mission of the US space company Blue Origin, the weightless time has now been increased significantly. After separation from the launch vehicle, an unmanned capsule was set into rapid rotation (a good eleven revolutions per minute) by its control nozzles. This created an artificial centrifugal acceleration of around 1.76 meters per square second for just over two minutes, which is only slightly more than the gravity of the moon.

Due to the lack of erosion processes, lunar soil is sharp-edged, powdery and water-repellent.

To analyze how liquids behave under this condition, scientists used a housing with 18 different liquid-filled test chambers in the NS-29 capsule, supplemented by an LED backlight and a camera. The chambers were filled with three different liquids: demineralized water, a 30 percent glycerol solution and a 0.8 percent saline solution.

The test chambers also had different wall geometries (vertical and at angles of 15 and 20 degrees). What was examined was how the liquids behave along the surfaces of the individual chambers and how their “meniscus” forms – the curvature of the surface of a liquid, which is due to the interaction between the liquid and the wall surface.

The images showed that the meniscus was always more curved, in contrast to the flatter profiles under normal Earth gravity. At lower gravity, surface tension becomes the dominant force, and the intermolecular forces cause the liquid surface to "pull" inward and higher up the container walls, resulting in a significantly rounder, highly curved profile.

The results of the experiment could also help improve the flow properties of liquids in the lunar soil if something were to be grown there in the future. Due to the lack of erosion processes, this is sharp-edged, powdery and water-repellent and therefore hardly suitable as a plant substrate. A possible solution could be the use of glycerin, which showed particularly good wetting properties in experimental comparisons and could thus improve the moisture balance of fine cavities.

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