Discovery before dawn on Thursday. Photo: NASA

Launch of the shuttle Discovery on a space station assembly mission has been delayed to Monday evening at the earliest to replace a sluggish valve in the orbiter's main propulsion system. Concern about a separate issue involving suspect bolts in the shuttle's external tank has not yet been resolved.

But assuming the bolt issue can, in fact, be put to rest as expected, engineers are retargeting launch for 8:05 p.m. Monday. NASA only has until Wednesday to get Discovery off the ground before standing down for an already scheduled Atlas rocket launch.

While NASA normally will not make more than two back-to-back launch attempts, senior agency managers said this evening they are not ruling out the possibility of making three attempts in a row next week if bad weather or other problems cause additional delays.

Work to replace the sluggish POGO vibration-supression valve in the shuttle's aft compartment is a relatively straight forward procedure. Engineers will open up Discovery's engine compartment overnight, remove foam insulation, remove the valve and install a replacement.

A mass spectrometer test will then be carried out to make sure the new valve and its connections to the propulsion system are leak free.

Normally, a more time-consuming helium-signature leak test would be carried out after such a repair job to verify the integrity of the entire propulsion system. While engineers say such a test is not required in this case, a formal waiver will have to be signed to clear Discovery for launch Monday.

The mission flight plan below has been updated to reflect a Monday launch as has the ascent timeline. This status report will be updated as more information becomes available.

NASA plans a 10 a.m. EDT news conference to discuss the valve replacement.