"If it gets to what they think is a critical level, they'll bring the astronauts home," NASA spokesman Andre Valentine said.

Mission managers were due to meet at noon in Houston (1800 GMT), when they will decide on the fate of the mission, said Valentine.

Two Freon-filled loops cool equipment on the space shuttle Columbia. Both loops must be working properly for the mission to continue, said Valentine.

"Flight rules require that if one fails or (if pressure) falls below the critical level," the mission must be scrapped, Valentine said.

Columbia lifted off Friday at dawn, carrying seven astronauts on an 11-day mission to update the Hubble space telescope's camera and its failing electrical system.

The system does not cool the astronauts and the astronauts are in no danger, Valentine said. The cooling loop is not leaking Freon, but rather shows low pressure, he said.

The loop may even be working properly, but a faulty reading can still be reason to scrap a mission. Valentine said there is speculation that a piece of debris inside the loop could be the culprit.

If the mission proceeds, astronauts are scheduled to conduct five space walks to complete the work of replacing solar panels on the Hubble and the install a new camera which will increase the resolution of the telescope's images ten fold.

Astronauts will also replace Hubble's generator, which is showing signs of weakness, during a six-hour space walk. The repair is complex since the Hubble's electrical supply will need to be cut.

After performing other upgrades, shuttle is scheduled to boost the Hubble to a higher orbit, giving it a longer life in space.

The crew's seven US astronauts are under the command of Scott Altman. This is the 27th mission for Columbia, which is due to return March 11. Columbia's mechanical instruments were updated and replaced with electronic screens before this trip.

Columbia is one of four space shuttles that together have flown 107 missions.

SPACE.WIRE