SpaceX will once again attempt its fifth rocket launch of the year this evening, after scrubbing yesterday's attempt. CEO Elon Musk tweeted that a "glitch in the motion of an upper stage engine actuator" was the reason for the delay. "Probably not a flight risk, but still worth investigating," he added.

The company will use its flagship Falcon 9 rocket to deliver the THAICOM-8 telecommunications satellite to geostationary transfer orbit (GTO). GTO is a high-elliptical orbit more than 20,000 miles above the Earth, and a common parking spot for communications satellites. Liftoff is scheduled for 5:39PM ET, and you can watch the live stream above. The weather at the time of launch appears to be 70 percent favorable.

But the fun stuff is scheduled to happen after SpaceX does its job. Once the Falcon 9 is well on its way, the rocket's 14-story first stage — which contains the rocket's engines and main fuel tank — will separate and head back to Earth. SpaceX will once again attempt to land this stage on an autonomous drone ship in the Atlantic Ocean.

After a spate of failed attempts that started in January 2015, Elon Musk's spaceflight company seems to have gotten the hang of landing its rockets. SpaceX has successfully executed two drone ship landings in the past two months, and in December the company stuck the landing on its first attempt on solid ground. (A second ground attempt is scheduled for July.)

SpaceX is typically careful not to inflate expectations of a successful landing of GTO missions. Since geostationary transfer orbit is a very high-elliptical orbit, the Falcon 9 first stage burns up more fuel than normal. That leaves less fuel for the controlled engine burns that lower the rocket onto the ship. The first stage is also subjected to "extreme velocities and re-entry heating," making landings more difficult. That said, SpaceX's most recent mission on May 6th was also a GTO satellite delivery, and that rocket stuck the landing.

The reason for all these landing attempts is that Musk is trying to make SpaceX's rockets reusable. Rockets have been discarded after each use for as long as we've been launching them, so making it possible to reuse them could drastically reduce the cost of going to space. SpaceX has yet to relaunch one of the three landed rockets, but hopes to do so some time this summer. Until then, those rockets are stuck in an increasingly crowded hangar at Cape Canaveral.

Update 7:26PM ET, May 26th: The launch has been postponed at least 24 hours because of a "glitch in the motion of an upper stage engine actuator," according to SpaceX CEO Elon Musk.

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