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.
This first-stage booster has flown once before, a little more than four months ago when it launched the Zuma mission for the United States government-a satellite or spacecraft that was apparently lost in space after it failed to separate from the second stage of the Falcon 9 rocket.
Live coverage of the countdown and launch of a SpaceX Falcon 9 rocket from Vandenberg Air Force Base in California with five next-generation Iridium communications satellites and two U.S. Each Block 4 core will fly just two times as the company seeks to move all of its launches onto the newer Block 5 version of the rocket, which has slightly increased performance and numerous upgrades to optimize the first stage for reusability.
While SpaceX has previously recovered and reused the Falcon 9 rocket's first stage, the company won't be recycling that piece of equipment from this launch. The mission will measure changes in how mass is redistributed within and among Earth's atmosphere, oceans, land and ice sheets, as well as within Earth itself.
The two Gravity Recovery and Climate Experiment Follow-On spacecraft will continue the GRACE mission's objective of measuring gravitational anomalies.
Because ride-sharing is caring, SpaceX will send the next batch of Iridium NEXT satellites into orbit along with a pair of NASA gravity monitoring orbiters, GRACE-FO.
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About 10 minutes after liftoff, a separation system on the second stage will deploy the GRACE-FO satellites. This boat will seek to "catch" the fairing before it falls into the ocean.
The mission picks up from GRACE, a satellite pair that launched in 2002 and tracked, among other things, precisely how much ice was lost each year in Greenland and Antarctica until 2017. The full constellation is to consist of 66 operational satellites, nine in-orbit spares and six ground spares.
Iridium used the extra lift capacity to launch three more Iridium Next satellites than it would have using the Dnepr vehicle.
"GRACE was really a revolutionary mission for us understanding the water cycle and how the climate behaves", said Frank Webb, GRACE-FO project scientist at the Jet Propulsion Laboratory, during a pre-launch briefing May 21.
NASA's share of the GRACE-FO mission cost is $430 million. "You can't manage it well until you can measure it. GRACE-FO provides a unique way to measure water in many of its phases, allowing us to manage water resources more effectively".