3:30am ET Saturday Update: The Falcon 9 rocket launched on time early Saturday morning from Cape Canaveral Air Force Station in Florida, streaking into the nighttime sky and depositing the Cargo Dragon spacecraft into a good orbit. About 12 minutes after the launch, Dragon deployed its solar panels and set course for the International Space Station. Dragon will be captured by the station on Sunday.

Meanwhile, the rocket's first stage returned to the Of Course I Still Love You droneship, landing 28km downrange from the launch site. The nighttime landing sequence was visible through the use of an infrared camera, and looked pretty spectacular.

During the post-launch news conference for SpaceX's 17th mission to the International Space Station, the lab's Operations Integration Manager, Kenny Todd, explained the thinking behind the scrub of the Falcon 9 launch 24 hours earlier. SpaceX had said the launch was scrubbed because of an electrical issue with the droneship, which returned to port Friday, and then immediately headed back out to sea for the first stage landing.

"When we went through the countdown yesterday the SpaceX team was working hard, there was a lot going on, and I've got to applaud them because everything that was hitting the screen they were dealing with," Todd said early Saturday morning. "There was a lot of talk on the loops when you consider the weather, the wind, the issues with the drone ship, the helium leak," he said.

The helium leak Todd referenced occurred with ground-based equipment. According to SpaceX, the company was monitoring the leak in the supply to helium on-board the rocket, but probably could have worked through the issue. Engineers wouldn't have known for sure until the last minutes of the countdown, however.

Todd said knowing the agency had a back-up day Saturday for the launch made a big difference. "We knew we had today, we knew the weather was supposed to be much better today," he said. "In the end, SpaceX had to make the call. But I think one of our senior engineers whose watched an incredible number of these missions said, 'You know, sometimes the universe is talking to you, and sometimes you need to listen to it.' And the reality is, when we went through all of that yesterday it seemed like the universe was talking to us. So in the end, I thought it was an OK trade."

The NASA official also revealed that the agency plans to use this booster again. "Flight proven" Falcon 9 rockets have launched missions to the space station before, but now NASA is considering using the same rocket a third time. "Quite frankly we have a vested interest in this booster," Todd said. "The intent is for us to use it for (CRS) 18, for sure, and potentially 19. From our standpoint it made a difference."

And so that is why, for the first time in history, the launch of a rocket was scrubbed because of concerns about recovering its first stage.

Original post: Early on Friday morning, within minutes of the opening—and closing—of an instantaneous launch window, SpaceX scrubbed the launch of its Falcon 9 rocket and Cargo Dragon supply mission to the International Space Station. Now it will try again early Saturday morning.

The company said it made the decision to stand down due to an "electrical issue" on its Of Course I Still Love You droneship, positioned just offshore for the Falcon 9 rocket's first stage to land on. This is the first time SpaceX has stood down a launch attempt due to a problem related to recovering a first-stage booster. The company can probably thank NASA for being an understanding customer.

After the scrub, SpaceX also said there was a ground-based helium leak it wanted to check out before the rocket's next launch attempt. This leak was located in a "quick-disconnect" interface on the rocket's second stage, where helium is used to pressurize fuel tanks.

Weather had been a concern for Friday morning's launch attempt, but near the launch time at 3:11am ET (07:11 UTC), the rains had remained well offshore and SpaceX had proceeded with fueling the rocket.

The instantaneous launch window opens at 2:48am ET (06:48 UTC) Saturday morning. Weather is forecast to be favorable, with a 70 percent chance of "go" conditions. If the Falcon 9 rocket cannot make this launch attempt for some reason, officials said at a news conference Thursday that the Cape Canaveral launch range would enter into a week of maintenance and stand down from launches during that time.

SpaceX will use a new Falcon 9 rocket for this launch attempt, but the Cargo Dragon spacecraft previously flew to the International Space Station in August 2017. For this mission, it will ferry about 2.5 tons of supplies to the station, including more than 700kg of science experiments, as well as transporting NASA’s Orbiting Carbon Observatory 3 and Space Test Program-Houston 6 in its unpressurized trunk. As part of a contract for 20 missions in total, this will be SpaceX's 17th supply mission to the space station for NASA.

Overall, this will be the company's fifth launch of 2019, and SpaceX's 70th launch of the Falcon 9 rocket. The webcast below should begin 15 minutes before the launch window opens.