NASA feels it has a good shot at sending shuttle Discovery to the international space station on Sunday.

While the agency still doesn't know what caused a hydrogen gas leak that prevented the space shuttle from flying earler this week, officials say they are hopeful repairs have solved the problem.

Leaking hydrogen gas — a dangerous situation because it could ignite and cause an explosion — forced NASA to call off Wednesday's launch attempt.

The space shuttle is scheduled to blast off Sunday evening.

• Click here for more photos.

• Click here to visit FOXNews.com's Space Center.

Discovery is more than a month late for its scheduled trip to the international space station.

First, hydrogen gas valves inside the shuttle had to be double-and-triple-checked. Then on Wednesday, hydrogen gas began leaking during fueling where a vent line hooks up to the external fuel tank.

Discovery and seven astronauts are set to carry up one last set of solar wings for the space station.

A half-dozen or so workers were out at the launch pad Friday, putting in a new hydrogen vent line hookup for Discovery's external fuel tank, along with two fresh seals.

The removed pieces were being scrutinized for any signs of damage that might have led to Wednesday's leak.

One seal was nicked, but that may have nothing to do with the leak, Leinbach said. Testing at normal outside temperatures with helium — nowhere close to the frigid conditions of the super-cold hydrogen fuel — came up empty.

"We don't have any smoking guns yet," Leinbach told reporters.

The leak was just the latest delay for Discovery's space station construction mission. Earlier postponements were caused by hydrogen gas valves inside Discovery's engine compartment, and pushed the flight back by a month. The leak occurred out at the fuel tank.

Because of all the delays, NASA is up against a tight deadline for launching Discovery. Russia is sending up a fresh space station crew on March 26, and the shuttle needs to have undocked by then.

The desire to avoid a traffic jam means that Discovery and its seven astronauts must be flying by Tuesday. That gives NASA just three days to get the space shuttle up. It also means that the shuttle flight — originally a 14-day mission — will be shorter with fewer spacewalks.

If Discovery lifts off Sunday, the shuttle will be in orbit 13 days and its crew will carry out three spacewalks instead of four. A Monday or Tuesday launch will mean an even shorter flight with even fewer spacewalks.

To make way for Discovery, the Air Force indefinitely put off its launching of a military communication satellite aboard an unmanned rocket, which had been scheduled for Saturday.

The top priority for the shuttle mission is to deliver and install the final set of solar wings for the space station.

NASA also wants to carry out a crew member exchange — replacing space station resident Sandra Magnus with Japanese shuttle astronaut Koichi Wakata — and drop off and set up a machine for turning astronauts' urine into drinking water. The urine processor that's up there now is not working properly.

Good weather is forecast for Sunday's 7:43 p.m. launch attempt. Forecasters put the odds of acceptable conditions at 80 percent.