Following close on the heels of the launch and installation of the European-built Harmony and Columbus modules, the successful delivery of the first part of the Japan Aerospace Exploration Agency's (JAXA) Kibo laboratory to the International Space Station marks another milestone in expanding the station's capabilty.

But this expansion of ISS capabilities also presents a future logistical challenge for JAXA and its space station partners, even with the imminent arrival of the European Space Agency's first Automated Transfer Vehicle, Jules Verne, on 3 April.

Kibo consists of three parts and the first to arrive is the Experiment Logistics Module Pressurised Section (ELM-PS), which is for stowage.

On 15 March, flight day five of Space Shuttle Discovery's mission STS-123 to deliver the Canadian Space Agency's special purpose dextrous manipulator and the ELM-PS, the hatch to the module was opened.

The other two parts are the Exposed Facility and the main section, the Japanese Experiment Module Pressurized Module (JEM-PM),

The JEM-PM, and Kibo's remote manipulator system, will be launched aboard Shuttle Discovery on its 25 May STS-124 flight and the Exposed Facility will arrive with Endeavour's STS-127 mission, likely to be in 2009.

Kibo's ELM-PS is temporarily docked at the Harmony module's zenith port but will be moved when the entire three-part Kibo structure is attached to its port side. Columbus is on the starboard side.

Kibo's initial research by the ISS' current crew of three will be in the areas of crystal growth, cell biology and fluid physics. The JAXA facility will be used along with the station's US Destiny and European Columbus labs by the station's expanded crew of six from May 2009.

This increase to a crew of six, 16 months before the expected retirement of NASA's Space Shuttle, presents a logistics challenge once the Shuttle is no longer flying.

The orbiter's 10,000kg (22,000lb) cargo capability, using the Italian built Multi-Purpose Logistics Module, has to be replaced.

That will require the successful operation of the ongoing Russian Progress vehicles, whose deliveries will remain at four a year the annual arrival of one 20,000kg ESA ATV with its 7,600kg cargo capacity and the use of JAXA's other large-scale contribution to ISS, its 16,500kg H-II Transfer Vehicle (HTV) that will deliver 6,000kg of supplies once a year.

On 9 March ESA's first of five ATVs, named Jules Verne, was launched and has overcome a propulsion system glitch and completed an in-orbit test of its collision avoidance manoeuvre required as the ATV docks automatically with ISS.

The ATV is currently positioned 20km (12.4miles) behind and below the station waiting for the Space Shuttle to depart.

JAXA's first HTV is planned to arrive at ISS next year. However its logistics capability is not as advanced in development as the ATV and its Ariane 5 Evolution Storable (ES) booster. HTV's launcher, the H-IIB, has never flown.

The two-stage H-IIB is based on JAXA's H-IIA rocket that has been launched 14 times although the sixth effort failed.

Launched from the same facility as the H-IIA - the Tanegashima Space Center's Yoshinobu launch pad - the two differ because the H-IIB has a redesigned first-stage that is 1.2m (3.9ft) wider, 1m longer, has two LE-7A engines instead of one and four solid rocket boosters attached, instead of two.

The H-IIb will also use a special fairing. In comparison, the ATV's Ariane 5 ES booster only differs from the ECA version by a strengthened upper stage and an upgraded engine for a multiple restart capability, which was tested once on a commercial flight in 2007.

Preparations for the delivery of the second part of Kibo, the JEM-PM, began with the August 2007 mission STS-118, when brackets were fitted to the station's S1 truss to allow the the 15m-long, 222kg orbiter boom sensor system (OBSS) to be left there.

Normally, OBSS is used with the orbiter's remote manipulator system to inspect its thermal protection system, but because the JEM-PM is so large the orbiter cannot be launched with it and OBSS on board.

So, NASA is leaving the OBSS at station at the end of STS-123. Instead of the orbiter's heat shield inspection, carried out by all missions since the Shuttle Columbia disaster, taking place post-ISS docking, which is day 15 for STS-123, it is now a pre-departure day 12 activity.

While JAXA's contribution to the ISS is already presenting challenges for Shuttle missions, the real test of Japan's partnership is the operation of its HTV, which since the decision to retire Shuttle, has become a mission-critical element and whose failure could severely diminish the station's ability to carry out its primary mission, scientific exploitation.