"Three, two, one, liftoff!" said a SpaceX commentator as the Falcon 9 rocket launched from Vandenberg Air Force Base in California at 12:47 pm Pacific time (1947 GMT).
The $521 million payload is called the Gravity Recovery and Climate Experiment Follow-on (GRACE-FO).
It 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 pair will fly 137 miles (220 kilometers) apart, or about the distance from Los Angeles to San Diego.
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