Bad weather has forced Nasa to delay by 24 hours the landing of its space shuttle Atlantis.
The landing at Cape Canaveral had been set for 1855 BST (1355 EDT) but thick clouds and nearby rain showers forced the attempt to be scrubbed.
The weather did not improve, and Nasa had to call off a second opportunity to land at 2030 BST (1530 EDT); Atlantis' return is now scheduled for Friday.
The shuttle has enough fuel and supplies to stay in space until Sunday.
On Friday, the shuttle will have two windows of opportunity to land at Kennedy Space Center, Florida. The first of these is at 1818 BST (1318 EDT)
If rains persist, it will have two more chances to touch down at the Edwards Air Force Base in California.
Atlantis spent most of its 13-day mission docked at the International Space Station (ISS).
Its crew installed a 17-tonne metal truss that included solar panels to generate additional electricity for the half-finished complex.
The construction work clears the way for Europe's Columbus module to be despatched to the station later this year.
The laboratory is Europe's key contribution to the $100bn (£50bn) space station project.
Computer woes
Before clearing the shuttle for its return, mission managers held a last-minute meeting to clear up remaining technical issues.
Material known as gap filler appeared to be sticking out of a wing and debris was found floating nearby.
Engineers had wanted to make sure the gap filler could withstand the heat and aerodynamics of re-entry and to recheck data on the thermal blanket. Mission managers have said the debris may have been ice.
During one of four spacewalks performed by the Atlantis crew, astronaut Danny Olivas patched up a torn thermal blanket that protects an area near the shuttle's tail from heat.
Nasa managers now believe they underestimated how much heat the underlying layers had experienced during launch on 8 June.
'No risk'
"When they modelled it, they made a mistake," deputy shuttle programme manager John Shannon said of the original analysis.
"Still, the engineering and safety teams believe there's absolutely no risk at all during re-entry."
Heat shield problems have been a major concern for Nasa since space shuttle Columbia broke up while returning to Earth in 2003, killing all seven crew members.
A suitcase-sized chunk of foam punctured a hole in its wing during launch, allowing superheated gases to get inside the orbiter during re-entry.
Before undocking from the ISS on Tuesday, mission controllers on Earth declared themselves happy that the big computer glitch that hit the orbiting platform during the week had been resolved - if not entirely understood.
The malfunction shut down the rocket-steering system the station needs to correct its alignment in space so that solar wing panels can track the Sun for power.
For a while, Atlantis was required to use its thrusters to maintain the platform's orientation.
Atlantis dropped off a new ISS crew member - US astronaut Clay Anderson.
He replaces Sunita Williams, whose more than six months in space set a record for the longest single spaceflight by a woman.
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