5:20pm ET Saturday update: The clouds broke apart enough for the Falcon 9 rocket to launch on Saturday evening from Kennedy Space Center, and, after delivering its payload into low Earth orbit, the booster made a landing back in Florida. Today marks the 11th successful first-stage recovery performed by SpaceX.

Meanwhile, the used Dragon spacecraft reached a good orbit, and it will now spend the better part of three days catching up to the International Space Station before it is grabbed by a robotic arm and unloaded by astronauts. It will stay on orbit for about a month before returning to Earth. All indications are that the spacecraft is in good condition.

Original post: Thunderstorms on Thursday scuttled an attempt by SpaceX to make an historic re-flight of its cargo Dragon spacecraft. SpaceX will now try again Saturday. Because of the orbital dynamics required to reach the International Space Station, today's attempt has an instantaneous launch window, at 5:07pm ET (10:07pm BST).

That may pose a problem because today's weather conditions at Kennedy Space Center are again iffy. Officially, there remains a 60-percent chance of favorable weather at launch time. However, by early Saturday afternoon, cumulus clouds had begun to develop near the launch site, which presages the formation of afternoon showers and thunderstorms—as is common in Florida during the summer.

This particular Dragon first flew to the International Space Station in September 2014 when it delivered nearly 2.5 tons of cargo. That was SpaceX's fourth supply mission to the orbiting laboratory. Saturday's is the company's 11th, and it will carry three tons of supplies to the station. The spacecraft has undergone significant refurbishment and has a new heat shield. Even so, flying the vehicle again represents a major milestone for SpaceX, which has made reusability a hallmark of its efforts to lower the cost of spaceflight.

In six decades of spaceflight, only NASA's five space shuttles, two X-37B space planes flown by the US Air Force, and a single Soviet VA spacecraft have made two or more orbital flights. No private company has ever achieved this feat.

The launch video below will begin about 20 minutes before the anticipated liftoff time. Just under eight minutes after the launch, the first stage of Falcon 9 will attempt to land at SpaceX’s Landing Zone 1, near the launch pad.