How a pair of satellites will ‘weigh’ water on Earth

WASHINGTON: The reason we know today just how much ice is melting in Greenland and Antarctica is because of a pair of satellites, launched in 2002 by Nasa and the German Research Centre for Geosciences (GFZ). Now, they are set to be replaced by a more modern duo. A SpaceX Falcon 9 rocket is scheduled to blast off at 3:47 pm (19:47 GMT) on Tuesday from Vandenberg Air Force Base in California, hoisting into orbit the spacecraft known as GRACE-FO, a follow-on to the prior, 15-year mission known as the Gravity Recovery and Climate Experiment (GRACE).

Two satellites, each the size of a car, will circle the Earth at a distance of 220 km from each other. They will be flying about 490 km above the Earth for the next five years. According to the laws of physics, the slightest variation in mass on Earth modifies the pull of gravity on satellites. When the lead satellite passes over a mountain, it will get slightly farther from its twin for a few instants because of the extra mass in this area and a slightly stronger pull of gravity.

These slight variations in distance will be constantly recorded by the spacecraft, because each shift signals a change in mass on the planet underneath. The satellites use a monthly reference point, because unless there is an earthquake or other unusual event, only water has the capacity to change that fast. Water always has mass, whether it is in the form of liquid, solid or gas. When ice melts, the oceans’ mass rises. When it rains a lot in a certain region, the volume of the aquifers mounts. The satellites will pick this up, and the data will show that the mass in a certain area was higher than it was in the prior month, or year. — AFP