Nasa says it detected a gouge in shuttle Endeavour's heatshield during a routine inspection as it docked with the International Space Station (ISS).
A robot arm will be used on Sunday to inspect the 3in (7.6cm) square, spotted as the shuttle performed a backflip so images of its underside could be taken.
Nasa said ice may have struck the shuttle shortly after Wednesday's lift-off from Cape Canaveral, Florida.
If deemed dangerous, astronauts may have to repair it during a spacewalk.
Mission manager John Shannon said: "What this means, I don't know at this point."
On Sunday, the astronauts will use a robot arm and extension boom, tipped with a laser and camera, to determine the exact size and depth of the gouge, he said.
Experts will then decide whether the damage warrants repair. If it cannot be fixed, the crew would have to remain at the space station until a rescue shuttle could be launched, Mr Shannon said.
The digital images were taken as Nasa engineers needed to check whether foam insulation that broke off during Wednesday's launch had damaged the ship.
Nine pieces of foam are thought to have broken away - three of which appear to have struck the shuttle - none were believed to have been big enough to cause serious damage.
Foam damage has been a major concern for Nasa since the Columbia disaster in 2003 when a briefcase-sized chunk of foam insulation broke off during launch and pierced the shuttle's heat-protection tiles. This caused the shuttle to disintegrate on re-entry into the atmosphere, killing all seven crew.
As Endeavour prepared to dock with the ISS on Friday, the crew guided it into a backflip manoeuvre once it was about 200m (600ft) away from the station, so it could be photographed and checked for damage.
Afterwards, the shuttle re-approached the space station and made its final connection at about 1900 BST (1400 EDT).
The mission is scheduled to last for 11 or possibly up to 14 days thanks to a new piece of equipment that can tap into the power grid of the ISS to extend its mission.
'Teacher in space'
As Endeavour's crew went aboard the ISS, residents of the space station took photos of their arrival, while former primary school teacher Barbara Morgan recorded the moment on a video camera.
Ms Morgan was once the reserve behind Christa McAuliffe, who died along with six other astronauts in the 1986 Challenger shuttle tragedy.
After the incident, Nasa asked Ms Morgan to stay on as its Teacher in Space representative and pledged a shuttle flight to fulfil McAuliffe's educational agenda.
Endeavour's crew will complete at least three spacewalks
But when the agency subsequently banned civilians from flying in its spacecraft Ms Morgan had to become a fully trained astronaut to get her chance to enter space.
Ms Morgan and crew will be delivering a 1.58 tonne "truss" section to extend the space station to a length of 108m (354ft).
The astronauts will also replace a defective gyroscope, one of four keeping the ISS on an even keel, and will install a 3.3-tonne exterior stowage platform.
Endeavour is also carrying in its cargo bay a pressurised container with 2.7 tonnes of supplies, food stuffs and equipment.
Three spacewalks, lasting about six hours and 30 minutes each, will allow the two-astronaut teams to accomplish the assembly and repair tasks.
Should Nasa choose to extend the mission, astronauts could fit in a fourth spacewalk, to prepare for installation of a boom that will allow crews to inspect for damage to the heat shields of future shuttles while docked with the space station.
The mission is the second of four that Nasa plans this year to finish the $100bn (£49.1bn) space station before the shuttle fleet retires in 2010.