When the rocket's second stage has completed its ascent to the injection orbit altitude, it will pitch down (its nose points down) 30 degrees and roll so that one of the twin GRACE-FO satellites is facing down, toward Earth, and the other is facing up, toward space.
The first stage of the SpaceX Falcon 9 rocket was used previously for a launch in January. SpaceX's Falcon 9 rocket will tote two satellites that are part of NASA's Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, as well as five more Iridium Next communications satellites, when it takes off from Vandenberg Air Force Base in California at 3:47 p.m. EDT (1947 GMT).
Although SpaceX will not seek to recover the first stage, it will attempt to recover one half of the rocket's payload fairing, using a system of on-board thrusters and parachutes to steer the fairing-half back to the Pacific Ocean toward a boat named Mr. Steven.
"The GRACE-FO satellites will be deployed approximately eleven minutes and thirty seconds after launch, followed by the deployment of five Iridium NEXT satellites beginning about an hour after launch", SpaceX says, making it sound oh so easy. This boat will seek to "catch" the fairing before it falls into the ocean.
Meanwhile the SpaceX second-stage booster will fire until 11 minutes and 33 seconds into the flight, when NASA's GRACE-FO spacecraft will be deployed.
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The two satellites will replace the previous GRACE mission, which operated between 2002 and 2017. Then the lower, leading satellite will be raised into the same orbit as the higher, trailing satellite. This launch makes the Iridium NEXT satellite constellation a conglomeration of 75 satellites (nine of which are spares). It is an effort to observe the movement of water and other mass around the planet by precisely tracking the changing pull of gravity. If needed, an additional launch opportunity is available on Wednesday, May 23.
"GRACE was really a revolutionary mission for us understanding the water cycle and how the climate behaves and the trends that are taking place", said Frank Webb, GRACE-FO project scientist at NASA's Jet Propulsion Laboratory.
If you want to watch the launch live you can do so via the YouTube window embedded above. A microwave ranging system will measure minute changes in that distance created by variations in the Earth's gravitational field.
The GRACE-FO spacecraft are equipped with laser retroreflectors that allow ground stations to accurately measure their orbits, further improving the accuracy of the data. GRACE-Fo is a joint project between NASA and Germany's GFZ German Research Center for Geosciences.