Space Shuttle Atlantis is expected to launch at 11:15 local time today following a successfull re-fuelling operation that saw the Shuttle's external tank's (ET) engine cut-off (ECO) sensors perform correctly.

Yesterday Atlantis's launch was cancelled due to the number three ECO sensor, which monitors hydrogen fuel levels, showed an incorrect reading. ECO sensors help ensure Shuttle main engines shut down at the correct time, while there is still some propellant left, so the engines can't run dry and damage themselves. Atlantis's mission, STS-115, is to install the International Space Station's (ISS) P3/P4 trusses to provide more solar arrays and therefore more power for the station.

Atlantis's 8 September launch was cancelled because under countdown rules, developed due to ECO sensor problems with Shuttle Discovery's propellant tank in 2005, pad 39B workers drain the ET of fuel if a sensor is malfunctioning and then re-fuels for a following day lift off.  If one sensor still shows a dry reading still, the launch countdown would go ahead but further sensor problems would have meant the Shuttle being rolled back to the vehicle assembly building and the failed sensors removed and replaced. Despite the resolution of the ECO sensor issue Atlantis still has a problem with its number one fuel cell, which is one of three cells providing onboard Orbiter power. The cell's coolant pump has had electrical problems but the NASA mission management team decided it could launch with two functioning fuel cells.

If NASA had had to remove Atlantis's ET's ECO sensors a September launch would have been abandoned and re-scheduled for the next daylight window, which starts on 26 October. Because NASA wants to image the ascending Shuttle to monitor ET foam debris loss it currently reqiures Shuttle to launch in daylight. The daylight requirement affects the launch window times and dates.

Although tropical storms and other weather issues have delayed the Shuttle launch, originally scheduled for 27 August, NASA does not expect weather to be a problem today. The US space agency's Freedom Star and Liberty Star recovery ships are now in position in the Atlantic Ocean. The ships retrieve the Shuttle's solid rocket boosters (SRB) back to KSC after they separate from the Orbiter during ascent. The SRBs will then be refurbished and reused for other shuttle launches.

NASA has been able to launch today because of Russian agreement. Russia's Federal Space Agency (FSA) agreed that the arrival of its Soyuz TMA-9 spacecraft, carrying the first female space tourist Anousheh Ansari and two ISS expedition 14 crew members, on 20 September would be possible despite Shuttle only having left the space station hours before.

With a 9 September launch Atlantis's 11-day mission means it departs on 20 September. Soyuz TMA-9 is launched on 18 September and arrives two days later. The Soyuz TMA-8 spacecraft is already docked at the ISS and has been since its 1 April arrival. It will return to Earth in late September with Ansari and two of the current ISS expedition 13 crew members. TMA-9 will remain docked until next March, acting as the ISS's emergency return vehicle.

If today's launch is cancelled for any reason NASA could attempt to launch in late September if they chose to relax the daylight launch rules.