Today’s the day for SpaceX’s launch of Iridium’s NEXT communications satellites and a pair of twin birds from NASA that will monitor the fresh water on the surface of the Earth. You can watch the launch right here:

Liftoff is scheduled for 12:47 PM Pacific Time, so SpaceX’s live stream should fire up about 15 minutes ahead of that; NASA will also have its own updates, since it has skin in the game.

This is an unusual launch — the rocket will be making some complicated maneuvers 300 miles up to make sure NASA’s satellites are deployed right, then it travels the rest of the way to the targeted orbit for the communication satellites.

A few minutes after liftoff, the Falcon 9 first stage (incidentally, the one that launched the lost Zuma satellite in January) will detach and burn up. This will be its last mission — it’s not one of the “Block 5” rockets with all the durability improvements, so it would take a lot of money and time to fly again, and the risk of failure would grow considerably every time. So this it’s swan song. Rocket, we salute you.

The second stage will take the payload to about 300 miles up, at which point it will cut off, and about 11 minutes after liftoff the rocket will dip its nose and spin a bit to get the GRACE-FO satellites into position. 10 minutes after they deploy, it will make a second burn and take itself up to nearly 500 miles altitude (this will take about half an hour), where Iridium’s satellites will be let out, ending the mission.

If for some reason things are delayed, the next launch opportunity is tomorrow at the same time.