Japan’s space agency has been working diligently since the 1990s on an unmanned spacecraft, called the H-II Transfer Vehicle (HTV), to resupply the International Space Station (ISS). On September 11 (September 10 in the United States), the HTV will finally lift off from Tanegashima Space Center in southern Japan.

Japan’s new spacecraft is 10 meters long and capable of carrying 4.5 metric tons of internal cargo and 1.5 tons of external cargo. It is comprised of two segments–one pressurized and one unpressurized–and can carry supplies as well as scientific experiments. The HTV, along with the Russian Progress vehicles and the European Space Agency’s Automated Transportation Vehicle, is among a handful of spacecraft that delivers supplies to the space station. Now that the station has doubled its crew capacity, the extra vehicle is much needed.

Dan Hartman, manager of integration and operations for the ISS program, said in a media briefing earlier this year that HTV will be “a major new capability to resupply the station, allowing for the launch of rack modules and external payloads.”

On its first trip, HTV will carry 2.5 metric tons internally and two scientific payloads externally: a Japanese instrument to study the effects of trace gases on Earth’s ozone layer, and a NASA experiment to study the oceans and map the ionosphere and thermosphere.

The new spacecraft is larger and has a simpler docking system than the Progress spacecraft. It will be flown just close enough to the station to allow the station’s robotic arm to pull it in, before attaching to the Earth-facing docking port on the station’s Harmony module. The crew will then start unloading the supplies and will move the experiments to the Japanese Experimental Module, called Kibo. The HTV will spend about six weeks attached to the station. Two days after release, it will re-enter Earth’s atmosphere.

The vehicle is an important step in Japanese efforts to solidify a strong role in the development and operations of the space station, which is scheduled to be completed by 2011 (and operational until at least 2016). The HTV will be another vehicle that the United States will have to rely on for sending supplies to the station once the space shuttles retire in 2010, according to the current schedule.

HTV will launch aboard an H-IIB rocket at 2:00 A.M. Japan time, 1:30 P.M. EST.