NASA will make a fifth try at launching space shuttle Endeavour this evening, after storms twice delayed attempts over the weekend. The shuttle's mission to the International Space Station (ISS) was originally slated to begin in June but was twice postponed that month due to leaks in a venting system that carries hydrogen gas away from the launch pad.

After failing to get the shuttle Endeavour off the launch pad this week, NASA had better luck with its mission to map the moon.

An Atlas V rocket lifted off from Florida's Cape Canaveral Air Force Station carrying the Lunar Reconnaissance Orbiter (LRO) and Lunar Crater Observation and Sensing Satellite (LCROSS). The spacecraft are expected to reach lunar orbit Tuesday morning, and NASA has been tweeting about mission as it unfolds.

Within hours of blastoff, NASA established communication with the LRO to power up the systems needed to control the spacecraft, which will circle the moon at an orbit of 31 miles (50 kilometers) for at least one year to study the lunar surface and scope out potential landing sites for a possible manned moon mission in 2020.

Another hydrogen leak will keep the space shuttle Endeavour's crew on the ground for another month while NASA investigates the problem. The space agency scrubbed today's scheduled launch early this morning when a gaseous hydrogen leak was detected at the same location where a similar leak halted the shuttle's original June 13 launch.

"We're going to step back and figure out what the problem is and go fix it," Deputy Space Shuttle Program Manager LeRoy Cain said in a statement. Even if all goes well, NASA expects the shuttle to launch no earlier than July 11.

NASA engineers found the leak at the Ground Umbilical Carrier Plate (GUCP), which is the vent to the launch pad and the flare stack (or elevated chimney) where the vented hydrogen is burned off. (For pictures of what the GUCP looks like, visit GalaxyWire.net.) The venting system (pdf), located outside the shuttle's external fuel tank, is designed to carry excess hydrogen safely away from the launch pad as the Endeavor blasts off. During launch, the external fuel tank supplies the shuttle's main engines with liquid oxygen and hydrogen propellants.

Space shuttle Discovery reached orbit 200 miles above earth tonight at 7:51 local time, after taking off from Kennedy Space Center in Cape Canaveral, Florida at 7:43:44.

The shuttle will drop off the final pieces of the International Space Station's (ISS) solar arrays, and parts for its urine recycling system that would expand the ISS’s capacity from three to six crew members.

The launch was pushed back twice, first on February 4 when NASA said it needed more time to make sure that the valves controlling the flow of hydrogen gas into the external fuel tank do not pose a hazard. Engineers discovered that one of those valves had been damaged when another shuttle, Endeavour, lifted off in November.

Tonight's scheduled launch of the space shuttle Endeavour, weather permitting, may be visible to viewers along the East Coast of the U.S.—also weather permitting.

If the skies are clear, the shuttle's engines should burn bright enough to show up during its initial climb. Joe Rao of the Hayden Planetarium in New York City blogs: "For most locations, Endeavour ... should appear as a very bright, pulsating, fast-moving star, shining with a yellowish-orange glow."

Rao advises that shuttle-gazers in the Southeast U.S. look toward Cape Canaveral, midway down Florida's Atlantic coast; that those in the mid-Atlantic look southwest three to six minutes after liftoff (scheduled for 7:55 P.M. in the East); and that those in the Northeast look low along the horizon to the south or south-southwest, six to eight minutes after takeoff.