From there, the station's 56-foot (17-meter) mechanical arm can safely reach out, capture the craft, and attach it to a docking port.
Flight engineer Nicole Stott, who will be operating Canadarm2, will then have just 99 seconds to snatch the spaceship safely before it gets out of range. Otherwise, she'll have to abort, and the HTV will need to be remotely nudged back to the capture location for a second try.
JAXA's Miyake compared the maneuver to connecting two Formula One racecars driving parallel to each other at high speed—"except here we are dealing with vehicles in low Earth orbit, and with speeds of over 17,000 kilometers [10,563 miles] per hour."
The tricky maneuver is worth it, mission managers say, because the 35,000-pound (15,875 kilogram) HTV can deliver both pressurized and unpressurized cargo.
Progress and the ATVs must be fully pressurized—an expensive and complicated requirement—since only astronauts can unload the craft.
Once docked to the station, the HTV will still be in reach of the robotic arm, which can then unload unpressurized components and attach them to the outside of the station.
The HTV also has a much wider hatch than that on either the Progress craft or the ATVs, allowing for delivery of much larger components.
"While Progress's hatch is about as wide as your shoulder, HTV's hatch is big enough to slide a refrigerator through it," said Kirk Shireman, NASA's deputy program manager for the space station.
"This gives the HTV a clear advantage over Russian and European cargo ships by allowing us to bring up large experimental racks and exposed pallets and batteries that [need to be] attached to the outside of the station."
Japan and Europe each have plans to send about one cargo ship a year to the space station until its slated demise in 2015.
"This international group of vehicles will take over much of the equipment transportation to the ISS after the retirement of the U.S. space shuttle fleet," Shireman said.
In addition, two U.S. companies, SpaceX and Orbital Sciences, have won NASA contracts to develop unmanned space ferries, with demonstration flights slated to start in 2010.
Having more unmanned vehicles should make access to low Earth orbit cheaper, which might have an impact on the actual retirement date for the International Space Station.
But there is a flip side to relying too heavily on commercially built craft, Shireman cautioned.
"If, for some reason, these vehicles don't come about once the shuttle is mothballed, this would put a severe limitation on the [amount of material that can be taken] to the space station and thereby limit its effectiveness."
"There is no question," he said, "the ISS's future is really wrapped up in these unmanned cargo vehicles."
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