These days the International Space Station (ISS) is expecting another visitor: an unmanned Russian Progress cargo ship stuffed to the gills with supplies. Restocking the ISS is not your average trip to the corner store – impulse buys just don’t happen, and every ounce of material is precisely planned and weighed.

You may recall how the European supply vessel Jules Verne docked at the ISS, delivering over 10,000 pounds of much needed stuff to the residents. Last week it separated from the Zvezda module where it was attached to the station to make room for the new supply ship.

What was on its shopping list?

It may surprise you that a great portion of its load was propellant for the ISS. Although we tend to think of the Space Station as a travelling outpost quietly orbiting the Earth it needs quite a bit of rocket fuel: there are small adjustments to its speed and thus its altitude without which it would eventually crash back to Earth. Then there is the station’s general orientation, and all sorts of unpredictable maneuvers if, say, a piece of space junk gets too close and it’s just easier to gently nudge the ISS before it ends up in a fatal heads-on collision. At the speed the ISS travels even a tiny paint chip can cause some serious damage.

Next are of course the things necessary for life in outer space: food, water, oxygen and toilet paper. While much of the air and water is constantly recycled and re-used countless times after being filtered, cleaned and scrubbed the process is not a 100% closed system: there are constant small losses that have to be replenished. Although a lot of research is going on into creating self-sustaining systems for long-term space travel we are a good way away from practical applications as they pertain to the ISS. Biospheres that could sustain several people indefinitely need to be really quite large, and space is at a premium on the station.

And let’s not forget the science part of the whole kit’n’kaboodle. New experiments are brought up and installed – most of them pre-packed in modules that fit into existing compartments like building blocks – standardization has tremendous advantages.

Finally, there is the garbage. Packing materials, dried waste, experiments and hardware that are no longer needed are packed onto the empty supply ship which then takes them on an exclusive one-way trip on a fiery re-entry into Earth’s atmosphere where everything burns up and disappears.

While cargo ships have a decidedly unglamorous job they play a vital part in keeping the ISS up and running. Without them the ISS could simply not operate.

Find out more at http://www.russianspaceweb.com/progress.html