After the planned 30 November landing of the Space Shuttle Endeavour following its 17-day mission to the International Space Station, the orbiter fleet will have only 10 flights left, while the ISS will have been in orbit for 10 years and 10 days.
The Space Shuttle was conceived as a space station resupply vehicle. It will end its life in 2010, while the ISS is likely to continue for another decade. The station, originally to have been completed by 2002 and deorbited in 2016 with a total mission price tag of $100 billion, will have taken eight years longer to finish in its 400km (248 miles) low-Earth orbit than planned.
Originating in a 1980s initiative of President Ronald Reagan, Space Station Freedom, as it was called, was repeatedly redesigned because of cost issues. President Bill Clinton brought the Russians on board in the 1990s and it became the international space station, with Canada, Japan and a number of the member states of the European Space Agency, such as France, Germany and Italy, joining the project.
Further design alterations followed since its first module, Russia's Zarya, was launched on 20 November 1998. The first Shuttle ISS docking was Discovery's STS-96 mission in May 1999. There have been changing on-orbit configurations to ISS during its assembly, due to NASA's Shuttle difficulties and partner nations' budgetary issues.
The delay in ISS assembly and the retirement of the Shuttle fleet are both linked to the 1 February 2003 Columbia re-entry disaster. When the Shuttle retirement decision was taken, during the George Bush administration, it was intended to run the fleet until the ISS was completed, which NASA administrator Michael Griffin then determined would be in 2010.
But now that partner nations want to carry on using the station until 2020, to ensure that as much science as possible can be conducted aboard the world's largest-ever spacecraft, the Shuttle retirement date of 2010 poses issues for NASA.
Remaining scheduled flights for the Shuttle fleet are the Hubble Space Telescope servicing mission, STS-125, now expected no earlier than May next year, and nine ISS trips that end with the August 2010 mission, STS-134.
That mission will see Atlantis take the international Alpha Magnetic Spectrometer (AMS) experiment to the orbiting outpost. However, president-elect Barack Obama could cancel the AMS mission under the NASA Authorisation Act 2008, which would make Endeavour's STS-133 ISS resupply flight the final journey.
Included in the seven remaining ISS missions will be the delivery of the fourth and final set of solar arrays, along with the related starboard truss segment. Also scheduled is the installation of the last elements of Japan's Kibo laboratory and the final major habitable module, Node 3, and its cupola viewing port, to be put in place by 2010. The cupola can have command and control workstations to assist with extra vehicular activities.
Space Shuttle Discovery is to deliver Russia's Mini Research Module (MRM) 1 in April 2010, to be installed at the Zarya module's nadir port. There are also three resupply and experiment payload logistics flights.
However, ISS assembly does not end with the Shuttle. Russia has announced its intentions to add more modules and its own Samara Space Center Soyuz launcher will send the MRM2 to the station in August next year. The MRM2 will be fitted to the Zvezda service module's zenith port.
The completion of Kibo and MRM installation will follow the increase of the ISS crew to six from May. From then on the crew of six will be maintained, with the rotation of crews of three using Energia Soyuz spacecraft launches, four times a year, until NASA begins to operate its six-passenger Orion crew exploration vehicle, expected from 2015.
Without the Shuttle, ISS resupply will be performed by Russia's Energia Progress cargo vehicle, the Japan Aerospace Exploration Agency's HII-B Transfer Vehicle (HTV), whose maiden flight is due in mid-2009, and the flight-proven European Space Agency Automated Transfer Vehicle (ATV).
NASA is relying on its commercial cargo programme for the resupply services for which it is responsible, declining to place more orders for Progress, HTV or ATV. The lack of any demonstrated US resupply system has led to calls for Shuttle operations to be extended until there is a viable service.
NASA's Shuttle extension study has various options for orbiters to operate up to 2015. Such an extension would have a day-to-day delay impact on Orion's development, according to the Congressional Budget Office.
To date there have been 27 Space Shuttle missions to the ISS, which now has a mass of over 300,200kg (660,000lb) after the launch of 18 major elements, not all by Space Shuttles, from 20 November 1998 to 3 June this year.
Critics of the ISS say it does not have as much scientific research conducted on board to date as had originally been intended, but the station's proponents point out that the spacecraft is itself an experiment. The largest space vehicle ever constructed, lessons learned from its assembly and operation will contribute to future long-term manned missions, including to Mars.
NASA has touted the ISS as a testbed for the proposed lunar outpost's technologies and all the ISS nations have described the station as an example of what international co-operation can achieve.