Why was the Endeavour launch postponed?

With the space shuttle Endeavour set to lift-off on its final mission, Friday was supposed to be a big day. President Obama and his family would be there to witness the historic moment, as well as recovering representative Gabrielle Giffords, whose husband, Mark Kelly, is commanding the mission. But now, due to engineering snafus, the launching has been delayed. Here, a brief instant guide:

What happened?

The shuttle was due to launch at 3:47 p.m. EST today, but the launch has been postponed at least 72 hours because the heaters on one of its three Auxiliary Power Units (APUs) malfunctioned. "It's unfortunate for Mark Kelly and his crew," says Mike Leinbach, shuttle launch director. "But today the orbiter is not ready to fly, and we will not fly until we're ready. It's the nature of our business."

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What do the Auxiliary Power Units do?

They're part of the shuttle's hydraulic system, and they power its brakes, landing gear, and flaps. All three APUs must be fully functioning for the shuttle to safely land. The heaters keep the system at a constant 45 degrees to keep it from freezing and keep the machinery well lubed.

When was the problem noticed?

The malfunction came to light about 3 hours before the scheduled launch time, as the astronauts were preparing to board the shuttle.

How complicated is this problem to fix?

Before engineers can assess the problem, the shuttle's external fuel tanks, which were filled this morning, must now be drained. It's speculated that the issue might be with the electrical system that powers the heater, but engineers won't know for sure until they go into the orbiter to check out the situation.

How often are space launches scrubbed?

Pretty often. The shuttle program started in 1981, and since then, just 40 percent of the shuttles have had lift-off on their first try. And only 30 percent have succeeded on the second attempt. The Endeavour itself was originally scheduled to launch on April 19, but the date had to be pushed back because it was too close to a Russian spaceship's launch.

Sources: ABC News, Houston Chronicle, Los Angeles Times