CAPE CANAVERAL, Fla. — NASA mission managers have delayed space shuttle Discovery’s launch to the International Space Station by at least a day because of an electrical glitch in a backup computer circuit in one of the orbiter’s main engines.

The National Aeronautics and Space Administration’s Mission Management Team met late Tuesday to discuss the problem and possible solutions and decided to take an extra day to examine what might be causing the problem. Sources said the drop in voltage in the backup computer when the system was tested Tuesday morning was very small and within engineering specifications.

But it was unexpected, prompting extensive troubleshooting.

Mike Moses, chairman of the shuttle’s Mission Management Team, said he wanted his engineers and technicians to figure out was if the change in voltage has happened before and was not an uncommon event.

“”Maybe it’s been there before but never been noticed before,”” he said.

Moses said not flying until the team was satisfied it was safe to fly was part of the agency’s approach to launching the shuttle ever since the Columbia accident in 2003.

“”We don’t fly with unknown risk and right now this is risk is still unknown to us,”” he told a press conference. “”We are going to take another day to know it better and make this a known risk that we do understand.””

If no major repairs are needed, or if engineers can get comfortable launching Discovery as is, commander Steven Lindsey and his five crewmates could be cleared for launch Thursday, weather permitting.

But forecasters are predicting a 70 percent chance of low clouds and rain from an approaching frontal system Thursday that could block a launch attempt. The forecast improves to 70 percent “”go”” on Friday.

The current launch window closes Sunday, with a possible extension to Monday. The next shuttle launch window this year opens Dec. 1 and closes Dec. 5.

This is destined to be Discovery’s final flight before being decommissioned and handed over to a museum.

“”Discovery is not going out easy,”” said NASA Shuttle Launch Director Mike Leinbach. “”She is giving us a little bit of trouble but that’s fine. She’ll fly perfectly when she does.””

Moses and Leinbach both said they thought studying the problem and meeting again on Wednesday to make a final decision was the correct course of action because they did not want to unnecessarily start swapping out circuit breakers if they didn’t have to.

“”This is part of the business. You fly if you are ready and you don’t if you are not, and we are not ready to go now,”” Leinback said.