The two Gravity Recovery and Climate Experiment Follow-On spacecraft will continue the GRACE mission's objective of measuring gravitational anomalies. The work has been ongoing for over a decade now, and these two new satellites are replacements for older ones that served a similar objective.
SpaceX, the rocket company founded by Elon Musk, will use its Falcon 9 rocket to launch seven satellites on Tuesday: two state-of-the-art NASA satellites and five commercial communications satellites.
The pair will fly 137 miles (220 kilometers) apart, or about the distance from Los Angeles to San Diego. The result is a monthly map of the Earth's gravity field and gives clues as to how mass (normally water) is moving around the planet.
There was some recovery in the amount of mass - water in the ground - due to heavy rains just as the original GRACE mission was ending, but despite the gap in measurements, the new mission will allow scientists to see how much water stayed in the ground and how much ran off into the ocean, Webb said.
The satellites use a monthly reference point, because unless there is an quake or other unusual event, only water has the capacity to change that fast. When it rains a lot in a certain region, the volume of the aquifers mounts. But the satellites meant to upgrade the Global Positioning System, which is widely used for military and civilian applications, are already years behind schedule.
The GRACE-FO satellites were assembled by Airbus Defence and Space in Germany.
They operate with a precision equivalent to a change of 0.4 inches (one centimeter) in water height across areas of about 211 miles (340 kilometers) in diameter.
Iridium and the GRACE-FO team originally planned to launch today's satellites on separate Dnepr missions, but had to find another ride when Russian Federation halted missions on the converted intercontinental ballistic missile. Five Iridium NEXT satellites were launched as part of the company's campaign to replace the world's largest commercial satellite network. SpaceX has signed up for a total of eight Iridium NEXT launches with the aim of putting 75 new satellites into orbit. The Earth-facing satellite will be pushed down into a lower orbit that is faster on average, while the space-facing satellite will be pushed up into a higher orbit that is slower on average.
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.
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