Unmanned resupply spacecraft are a special kind of robotic spacecraft that operate autonomously without a human crew, designed to support space station operation. This is different from space probes, whose missions are to conduct scientific investigations.
Resupply spacecraft have been used since 1978 and have serviced Salyut 6, Salyut 7, Mir and the International Space Station.
Spacecraft in development
The American private space technology company that has made the most progress towards creating a spacecraft to deliver supplies to the International Space Station is also saying they will be the cheapest to operate. SpaceX will use their Falcon 9 rocket to launch the Dragon capsule into low Earth orbit. Using this combination SpaceX says they will be able to launch supplies or seven astronauts to the International Space Station for an average price of 57 million dollars per launch. The United States government currently sends astronauts to the International Space Station using the Russian space agency at a cost of 63 million dollars a seat. The Falcon 9 rocket can also launch supplies into low Earth orbit for approximately $2,500 per pound ($5,512/kg). SpaceX is also finishing the developmental stage on their Falcon Heavy rocket that is scheduled to make its first flight in 2014. The Falcon Heavy could launch supplies into low Earth orbit for a cost of $1,000 per pound ($2,205/kg). Those prices are less than half of the price that the United States Government spends now using the Russians and the Chinese. SpaceX will be able to save the United States government millions if not billions of dollars using their rockets and capsules.