They noticed that one of the myopic telescope's solar- power panels was badly twisted just before Swiss astronaut Claude Nicollier hauled it into the Endeavour's cargo bay at the end of a 50ft robot arm.
It complicates the 11-day repair flight - billed as a make- or-break mission - not just for the short-sighted telescope, but for Nasa. Yesterday's operation was critical, as there was only enough fuel and time for one attempt to grab the telescope.
Hubble's two 12-metre-long solar arrays, built by British Aerospace, must be replaced because they vibrate due to temperature changes as they move in and out of sunlight.
The crew will attempt to roll in the bent array at the end of their first space walk today. Jeff Hoffman and Story Musgrave will first fit two new gyroscope units. Only three of the original six gyroscopes, used to point the pounds 1bn telescope in the right direction, are working.
The hope is that the twisted array will wind into its storage cassette successfully. However, one of the crippled panel's support rods is almost flattened in one section, so it is not clear that the winding mechanism will cope. The crew aim to fit a new pair of more stable panels. Stephen Young, editor of Astronomy Now, said yesterday: 'The array on the right-hand side has buckled badly. The bent arm may fold up, but it is in pretty poor shape, so it could cause the whole array to jam.'
If this happens, the six-man, one-woman crew could jettison the faulty arm, although Nasa is keen to get both panels back so it can see how they have been affected by three years in space.
Despite this hiccup, the five planned spacewalks remain as scheduled. The astronauts will roll in the twisted array before the good one. That way, they should be able to unfurl the good panel if they have to repeat the operation. If this fails, they may take the telescope through temperature changes by putting it in and out of sunlight - hoping that the array will bend back into shape.
Lobbing the arrays overboard takes time, since it is hard to disconnect the array with the panels open. It would also require an extra space walk, restricting the crew's options if anything else crops up.
Hubble went into orbit in April 1990, but was soon found to have flaws, including a misshapen main mirror that blurred the images sent back.
The mission's principal aim is to fit corrective optics for the main mirror. If the new 'spectacles' work Hubble should be able to fulfil all its objectives - chief of which is to determine the age and size of the universe.
As it is now, it can make out objects only 4 billion light-years away, just one-third of the intended distance. For maximum success, the crew must complete all seven tasks, otherwise a second shuttle may be sent up.