Supply run to the International Space Station

Robot-truck delivery

The International Space Station is dependent on regular supply runs from Earth.  With the Space Shuttle gone, that task falls entirely to robotic vehicles.  This beautiful image shows the Automated Transfer Vehicle-3 (ATV-3), built under a European Space Agency (ESA) program, approaching the ISS.  'Aboard is some seven metric tons of materiel, mostly propellant for the ISS's thrusters, which maintain the orbit (for a vehicle the size of the ISS, even the rare atmopsheric atoms and molecules hundreds of km above the Earth will slow it down and lower the orbit).  Also aboard is 300kg of water, one of the limiting factors of any human space endeavor.  (Onboard water recycling systems are an advancing art, receiving their most extensive testing yet on the ISS, but still a long way from recovering 100% of the water used for drinking, hygiene, lab experiments, hydroponics, etc.)    
COMMENT: This kind of upkeep is horrendously expensive, and the ISS even more so, but I think history will look back on the ISS as worthwhile.  For all our simulations and computer-aided design, the only way to perfect the building and maintaining of large space structures is to DO IT.  The knowledge gained will be vital... unless  of course we decide to say on our own planet forever..  In the long run, that is not wise.  (Ask the ivory-billed woodpecker, if you can find one, how smart it is to specialize in one kind of habitat.)