https://qz.com/1284915/watch-spacex-launch-a-rocket-mission-to-refresh-iridiums-satellites-and-measure-the-earths-water-with-grace-fo/

SpaceX will reuse a rocket booster today to continue a globe-spanning technology upgrade in space, and also launch a pair of satellites to precisely measure shifts in the earth’s mass.

The Falcon 9 rocket is expected to lift off from California’s Vandenberg Air Force Base at 3:47 pm ET (12:47 pm PT). If there are any delays ahead of the launch, there will be another opportunity tomorrow, May 23 at 3:24 pm ET (12:42 pm PT). Viewers will be able to watch the lift off and separation of the satellites, but SpaceX will not be recovering this older booster as it shifts its focus to the final version of its workhorse rocket.