This story is set in a series about American human space flight.
The space – with its voluminous spacesuits and microgravity – does not seem to be a good place to exercise. However, astronauts have found time to play through the years.
The most famous was that astronaut Alan Shepard hid a collapsible golf club in his space suit when he, as commander of Apollo 14 in 1971, stepped to the surface of the moon. Shepard dropped two golf balls into the moon dust and then swung away with his improvised 6-iron.
More recently, astronauts have organized relay races, as well as tennis and badminton competitions at the International Space Station (ISS).
However, it is not all fun and games in space. There is the very serious case of keeping astronauts fit and healthy in a microgravity environment. Do not forget that astronauts stay in space for a long time. In recent years, Scott Kelly and his Russian counterpart, Mikhail Kornienko, have spent almost a full year (340 days) on the ISS.
By living in a weightless environment, muscles can weaken and bones become more brittle. Weightlessness is also heavy for the heart, because the heart does not have to work so hard with little gravity to pump blood to all parts of the body.
Some people compared long-term in space to lie in bed for months. The body becomes weak without moving and without regular exercise.
How can astronauts practice in space and stay in shape? It is a difficult question, because almost all exercises – weightlifting, running, jumping – make the body stronger because the body fights against gravity.
I asked Sunita "Suni" (pronounced Sunny) Williams for some answers. She has been an astronaut for 20 years and has spent more than 300 days in space. Williams has also logged on spacewalks for more than 50 hours. She is training for a six-month mission at the ISS planned for next year.
Williams said it helps to be in good shape before you leave. An astronaut must be fit at a certain level so that she can do the work in space. Spacewalks are particularly exhausting according to Williams. Working for several hours outside the ISS can "feel like a whole day of skiing".
Williams, who is 53, started to take shape as a girl who grew up in Needham, Massachusetts. She was a competition swimmer who also walked, played cross-country and played football. Williams is still running and even has her passion for running in space.
The astronauts on the ISS (there are usually six) have to train for two hours every day. They spend about an hour practicing their legs and their hearts by pedaling on an exercise bike and running on a treadmill.
The automated treadmill is not as you would see in a local gym. The astronauts must wear a special harness and straps to prevent them from falling off the machine.
Williams used the treadmill to participate in the Boston Marathon in 2007. She had qualified for the famous 26.2-mile event but would be in space during the real race. Her sister contacted race organizers and asked if Williams was allowed to run anyway. . . on the ISS treadmill.
The organizers of the race said okay. So Williams jumped on the treadmill at the start of the marathon and drove all over the distance. She even had an official race number.
The astronauts also "lift weights" to keep their upper body in shape. Of course they do not really lift weights, because everything in space is weightless. Instead, they use a resistance trainer that the astronaut uses to pull, pump and squat against the force of a vacuum.
The advice of Williams to children who think that all this microgravity fuss sounds like fun?
First: "Do not take your health for granted" and "stay in shape." Astronauts must be in good physical condition to withstand long space flights and work on Mars. "We do not send people to Mars to be confectioneries," she said.
Williams encourages children to get used to thinking "out of the box". Everything about this weightless world requires people to think in new and creative ways.
Like when NASA engineers and astronauts keep asking themselves: how do you stay fit in space?