By Jonathan Amos
Science reporter, BBC News
Nasa is now looking at Friday for a third attempt at a launch
The latest mission to the International Space Station (ISS) by the US shuttle Discovery has been postponed for a second time in two days.
Wednesday's launch was called off several hours before lift-off after because of faulty fuel valve.
It came after stormy weather over Cape Canaveral halted Tuesday's launch attempt, confounding the forecasters who had expected benign conditions.
Friday is now the new target date for the orbiter to lift off.
US space agency (Nasa) managers at the Kennedy Space Center in Florida say the third attempt will occur no earlier than 0022 local time (0422 GMT).
Launch officials halted the latest countdown midway through fuelling of the shuttle's giant external tank when they received an indication that a valve in the main propulsion system was not operating as expected.
The seven astronauts had not yet boarded the shuttle for the scheduled early Wednesday morning flight to the international space station.
Discovery's mission will be the 30th flight dedicated to station maintenance.
Nasa plans an additional six sorties to the orbiting platform before retiring its re-useable spaceship fleet at the end of next year or early in 2011.
Christer Fuglesang is part of the mission's strong European focus
The latest mission has a strong European focus.
The lab equipment was made in Europe, which is represented in Discovery's crew by Swede Christer Fuglesang.
Mr Fuglesang - who is with the European Space Agency (Esa) - will conduct two of the three spacewalks planned during Discovery's stay at the ISS.
On one of these walks, the Swede will move cabling on the exterior of the station in readiness for the arrival next year of a connecting unit, called Node 3 or "Tranquility", and a huge window referred to as the Cupola.
The two modules will be Europe's final large-scale contributions to the assembly of the ISS.
Discovery's payload bay is taken up with the Italian-built Multi-Purpose Logistics Module (MPLM), which acts as a giant packing box on shuttle logistics missions.
For this flight, the MPLM contains almost seven tonnes of cargo. This includes two vital European donations - a new freezer to store samples and a furnace for baking materials.
The former is a Melfi (Minus Eighty Laboratory Freezer for ISS), which can store biological samples.
Increased science means more freezer space is needed on the ISS
"This is the second such freezer," explained Martin Zell, Esa's head of ISS Utilisation.
"This first one is already up there since three years and working extremely well.
"It's the main freezer element on the station and can operate between plus-4C, at the upper temperature, down to minus-80 degrees; and even in different temperatures in its four cold volumes, or compartments," he told BBC News.
The additional Melfi will facilitate the increased science workload taking place on the station now that its resident crew has been raised from three to six.
All manner of biological samples will be stored in the new facility, including blood taken from the astronauts.
This is routinely drawn for study, to further scientists' understanding of the impacts of long-duration spaceflight on the human body.
The other notable European cargo item is the Materials Science Laboratory.
This contains a safe furnace (up to 1,400C) in which astronauts can first melt and then solidify a range of samples, such as metal alloys.
The MSL is the first dedicated materials science facility for the ISS
The weightless conditions on the station mean the fine-scale structures in these cooling samples will grow in a different way from how they would at the surface of the Earth.
Scientists expect these experiments to provide novel information that can be applied to everyday industrial manufacturing processes.
With MSL and the Melfi units, Europe is providing both the coldest and the hottest conditions for science on the station.
As well as preparing the platform for the arrival of Tranquility and the Cupola, the mission's spacewalks will replace experiments that currently live on the outside of Esa's Columbus laboratory.
They will also exchange one of the tanks for storing ammonia, which is used to move excess heat from inside the station to the radiators located outside.
Discovery will also drop off US astronaut Nicole Stott for a three-month stay on the ISS, and pick up colleague Tim Kopra for the ride home. Kopra has been living on the platform for the past five weeks.