But Nasa officials say they'll decide whether the mission has to be abandoned on Saturday.
The hitch is in the shuttle's system which keeps the massive engines at the right temperature.
Their mission was supposed to last 11 days and was to carry out minor repairs to the Hubble Telescope.
The astronauts on board are scheduled to do five spacewalks to replace old equipment.
The telescope's been in orbit sending back amazing images of distant stars and planets for 12 years.