Translated from Alexander’s original German blog text by the Horizons team.

We had a visit from Earth! The SpaceX Dragon CRS-15 resupply vehicle arrived Monday 1 July bringing provisions, new scientific experiments and a whole lot more work for the crew on board the International Space Station (ISS).

ESA astronaut Alexander Gerst installing a new video management unit in the Fluid Science Laboratory. Credit: ESA/NASA

Thanks to this exciting arrival, the past few weeks have been even busier than usual. Ricky and Drew captured the capsule with the robotic arm, which is always a tricky manoeuvre and requires full concentration. Then we began the unloading.

Some materials and equipment had to be cooled immediately. And because the Dragon was also supposed to bring a whole series of completed experiments back to Earth, we had to hurry to get them done in time. As a result, we have spent more than 104 hours a week on research – just short of the ISS team record!

In the Columbus module I was responsible for commissioning the MagVector experiment, after a conversion that allows researchers to investigate how magnetic fields deflect cosmic radiation. I also installed a new camera in the Fluid Science Laboratory – a facility used to investigate the dynamics of foams and granules in weightlessness.

In the US module, we’ve been cultivating algae and Arabidopsis plants, carrying out research into cancer cells and even mixing “space concrete”. Here in weightlessness, where no disturbing convection fluxes occur in the material, we’re able to produce particularly pure cement and concrete. The benefits of this are twofold. On the one hand, it helps us better assess the properties of these very important building materials and improve their production on Earth. On the other, we gain experience that could be very useful for us when it comes to constructing a future Moon base.

NASA astronaut Serena Auñón-Chancellor mixes concrete. Credit: ESA/NASA-A.Gerst

On top of all this research, we also had time for a very special exercise. It’s one I had been looking forward to for a long time: Serena and I were to attempt to navigate by the stars from the ISS Cupola using a sextant.

As a huge fan of exploration, the idea that we would be using a sextant was extremely cool for me. Functioning like a protractor, it’s a tool that allows you to determine your position based on the orientation of celestial bodies. This is not only possible on the ground, but also up in space.

Navigating by sextant aboard the ISS may seem antiquated at first glance. But optical angle measurement was vital in helping the Apollo 13 astronauts return to Earth after an explosion on board their spacecraft in 1970. And NASA and ESA want to equip us with this almost 300-year-old invention for future missions to the Moon or Mars for that very reason – in case of emergency.

But, before they do so, it needs to prove its worth. So, from the Cupola, Serena and I tried to measure the angles between certain stars.

We quickly realized that the exercise planners, who know almost every star in the night sky inside out, were more concerned about technical questions – how the light comes in or where to attach the foot straps in order to assume a stable observation position. What they hadn’t considered, however, was that the most important thing when using a sextant on board the ISS is having an exact star map for the planned observation time.

You can see so many stars through the windows of the ISS (especially on a moonless night) that you can hardly find your way around. The constellations no longer stand out, and the section of the “sky” is relatively small, making it rare to be able to see the entire constellation. Moreover, our Space Station rotates around itself at four degrees per minute, so you can only see each star for a few minutes at a time before it either disappears behind the Earth’s horizon or is covered by part of the ISS structure. We managed quite well, but we still have some work to do to determine the best way forward.

ESA astronaut Alexander Gerst uses a sextant on board the International Space Station. Credit: ESA/NASA

This is often the case in space travel; in practice, completely different problems become important than those that have been carefully considered beforehand. To close this gap, you need astronauts.

The sextant exercise is also a good example of how we as astronauts deal with the risks of our missions. We rely on many different safety grids, and develop a plan B, C, D and E for as many scenarios as possible.

For example, recently we had a power failure on board the ISS, during which one of eight solar array power channels collapsed early in the morning following the unfavourable impact of a cosmic elementary particle. A second channel had been deactivated earlier due to the low sun angle. And, because the current load was then transferred to the other circuits, a third channel also collapsed before the system was able to recover.

Fortunately, no vital systems – such as the cooling system of the station – were affected. They are always fed by several independent solar array wings. But the onboard alarm woke us pretty swiftly from our sleeping cabins. We couldn’t coordinate with the ground control team because we were in a dead spot. So first we had to tackle the problem on our own. In pyjamas.

It’s at this point that we are reminded: the ISS is a highly complex system that is floating alone in the hostile cosmos. Even a power failure like this could rapidly result in serious consequences if there were not so many redundancies in place to ensure safety.

You can imagine our Space Station as a mobile that hangs over a baby’s crib, with many colourful figures on it, in perfect balance. If you cut part of the mobile off, it will affect the rest and put it in danger of falling off balance. The art is to build the mobile in such a way that error chains do not spread so far, and that the largest part of the structure always remains stable.

In this respect, the ISS is very cleverly designed. It can re-balance itself within seconds by cutting off power supply to non-vital areas. But to get all the important components – such as life support, cooling, navigation and communication systems – back up and running, astronauts must intervene.

NASA astronaut Ricky Arnold. Credit: ESA/NASA-A.Gerst

As astronauts, we train for a long time to keep calm in such critical situations and not lose sight of the system as a whole. It’s important to adapt to the special, exposed environment up here not just physically, but also mentally. From parachute jumping to deep diving underwater or a month-long research stay in extreme polar cold, our bodies are remarkably well-equipped to handle different environments and situations. It’s the mind that matters the most.

What really fascinates me about these kinds of environments is not the danger, the isolation or the adrenaline. On the contrary, I don’t like any of those three things very much. What I do like, is the possibility of conquering, controlling and mastering such a hostile environment and using it for our scientific benefit. And then ending up feeling at home in a place that’s so far from it.

That’s what drives me on such expeditions – and what makes up for the risk or suffering up here in space.