A SpaceX Falcon 9 rocket will launch later today carrying a new batch of satellites into orbit for communications company Iridium. After takeoff, SpaceX will try to land the Falcon 9’s first stage — the 14-story-high core of the rocket that contains the main engines and most of the fuel — on an autonomous drone ship in the Pacific Ocean. If successful, it’ll be the 13th time SpaceX has successfully landed one of these rocket stages, and the eighth time it’s performed the feat at sea.

This is the second launch SpaceX is attempting in one weekend — the quickest turnaround ever. (Launches are weeks or months apart, usually.) On Friday, SpaceX sent a Bulgarian communications satellite into space from NASA's Kennedy Space Center in Florida, in what the private spaceflight company called a “weekend doubleheader.” That flight was supposed to occur last weekend, but it was pushed back so that a valve could be replaced in the fairing, which is the cone at the top of the rocket that holds the satellite. On Sunday, Elon Musk said that the rocket has also been equipped with “significantly upgraded” fins made of titanium, which replaced aluminum fins that were prone to catching fire during reentry.

Flying with larger & significantly upgraded hypersonic grid fins. Single piece cast & cut titanium. Can take reentry heat with no shielding. https://t.co/SmyCCQRt2F — Elon Musk (@elonmusk) June 25, 2017

Today’s launch is part of Iridium’s plan to create a large constellation of 66 telecommunications satellites in lower-Earth orbit called Iridium NEXT. It will deliver mobile voice and data coverage all over the planet, including over the ocean and at the poles. The first 10 satellites were launched aboard a Falcon 9 rocket in January; today’s launch will bring the number of satellites in space to 20. When fully completed, the constellation will also be used to provide a minute-by-minute tracking system for airplanes.

Iridium has six more launches scheduled with SpaceX over the following 12 months, CEO Matt Desch said during a call with reporters on June 19th. “Our network activity in space is really going to get a bit frenetic here in the coming months,” Desch said. As these new satellites go up, they will slowly replace the old-generation satellites — some of which have been in orbit since the 1990s.

All Iridium NEXT satellites will be launched on new Falcon 9 rockets — at least for now. Desch said he’s open to the possibility of launching on already-flown Falcon 9s, if the reused rockets can speed up the schedule. “Schedule for me is very, very important,” Desch said. “Cost and risk are aligned,” he added. “I believe the risk is low right now but it’s not zero, because it’s a new thing.”

SpaceX completed its first landing of a Falcon 9 first stage in December 2015, and has completed 12 so far, including one on Friday. Two of these recycled rockets have been successfully launched and landed so far. Friday’s rocket that carried the Bulgarian satellite to space was actually the same Falcon 9 that put the first 10 Iridium NEXT satellites in orbit in January.

Today’s Falcon 9 is scheduled to launch from Vandenberg Air Force Base in California at 4:25PM ET, with an instantaneous launch window. The backup date is Monday, June 26th at around the same time. Check back this afternoon to watch the mission live. The live broadcast begins 20 minutes before launch.