German astronaut Alexander Gerst brought one central message back from the International Space Station (ISS): the appreciation for our common planet and the need to work together across continents, nations and borders to protect our fragile environment.

Gerst spoke at a press conference at the European Astronaut Centre (EAC) in Cologne on Saturday after having returned from the ISS two days earlier along with his colleagues, Sergey Prokopyev of Russia and Serena Aunon-Chancellor of the US.

"Astro Alex," as he is known on Twitter, said that his return made clear to him how many sensations humans can only enjoy on Earth: the feeling of the wind that greeted him in the snow-covered desert of Kazakhstan, the rain that hit his face when leaving the plane in Cologne, or the smell of the forest while doing his first exercises to recover after more than half a year in zero gravity. "Even smelling the grass shows us what a special and important place Earth is for us humans," he stressed. 

Space exploration: Not just done by astronauts 

Gerst pointed out that the work of the astronauts and cosmonauts aboard the ISS is often the most visible aspect of space exploration, but is in reality a tiny fraction of a huge collective international effort that makes such research possible. "Without all the planners, instructors, the people at ground control and the thousands of those who have built the modules of the ISS, all of this would not be possible," he said. "It is always the result of the cooperation of a large team. No single country would have been in the position to build such a space station." 

Read more: How the Soyuz rocket compares with the rest

The future of the ISS and beyond 

Johann-Dietrich Wörner, director general of the European Space Agency, recalled that the ISS came about as the "the fusion of two ideas: peace and freedom," because "Peace" was the name of the former Russian space station "Mir" while "freedom" was supposed to become the name of a once-planned but never realized US space station. The US space agency NASA eventually preferred to put its research and money into building the ISS, rather than going a separate way. 

Wörner also said he does not see the ISS outliving its usefulness anytime soon. "Everybody is talking about the year 2024, but I really don't see the end of the ISS yet." He was optimistic, however, that the ESA council of ministers and other international space agencies would soon agree to build a new "Gateway" space station to orbit the moon. 

For 2019, he said that the next ESA astronaut, Luca Parmitano of Italy, would return to the ISS aboard the mission "Beyond." The idea was that that mission would look "beyond the horizon" — a play on the name of Gerst's second space mission, "Horizons." 

Frank de Winne, a former Dutch astronaut and head of the astronaut training program at the EAC, added that other exciting projects for the coming years include a robotic mission to Mars, in which a lander would extract ground samples and return those to Earth. 

Read more: Europe's newest astronaut: 'The trainers really push you to your limit'

Failures and solutions 

Gerst, who served as ISS commander for the last six months on the "Horizons" mission, had to face some serious challenges during his stint. First, a sloppily covered tiny hole in a part of the Soyuz spaceship that was docked to the ISS resulted in a loss of cabin pressure. The astronauts were able to fix the hole quickly, but the investigation into how it occurred has not yet been finalized.

Probably more difficult for the commander: A flight of two replacement crew members to the ISS failed after takeoff and the Soyuz capsule had to make an emergency landing. 

That incident resulted in a profound change to the time schedule for maintenance work, research projects and scheduled extra-vehicular activities, including the modernization of technical components, such as batteries.

"We had to work a bit of overtime," Gerst recalled, "and we had to postpone some of the maintenance work. But we managed to make sure the ISS remained in a good working order and overall we almost managed to get all of our planned research projects done in time."

Asked about whether he intended to return to space anytime soon, Gerst pointed out that he is still an astronaut and would be happy to go on another mission, but that it is not on him to decide. Also, there are now other young trained astronauts, like Matthias Maurer, who also would like to have their chance. He would help to support them in preparing for their space flights.