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takyon [soylentnews.org] writes:

A NASA and German Aerospace Center mission [wikipedia.org] using two spacecraft to map the strength of Earth's gravitational field has come to an end [spaceflightinsider.com]:

The Gravity Recovery and Climate Experiment (GRACE [nasa.gov]) mission has come to an end after more than 15 years in Earth orbit. The twin satellites chronicled the changes of the Earth's water, ice, and land since the spacecraft were launched from the Plesetsk Cosmodrome on March 17, 2002, on a mission that was originally only slated to last some five years. More than a decade after that, GRACE was still beaming data back to Earth when a technical issued forced mission planners to close out the program.

Similar in some aspects to other missions launched, GRACE [utexas.edu] made precise measurements via the two spacecraft – GRACE-1 and GRACE-2 – that comprised the mission. For GRACE's overall scientific objectives to be achieved the two satellites both had to be fully functional. However, this past September (2017), GRACE-2 encountered a battery issue that made it clear by mid-October that the battery would not allow scientists to operate its science instruments and telemetry transmitter. It was decided to decommission GRACE-2 [nasa.gov] and, in so doing, end GRACE's scientific mission.

[...] GRACE helped detail how our home world's changing seasons move water, ice, and even land (as a result of surface water mass changes) across the planet's surface, providing researchers with a better understanding of what drives the motion of these substances. Earth's climate, earthquakes, and our own activities all play their part in shaping the face of our world and GRACE provided insights into the dynamics of this change.

GRACE was able to detect changes in Earth's gravitational field that is related to our planet's mass which is, perhaps unsurprisingly, impacted by the redistribution of water across the globe. The spacecraft judged the distance between its two components using a microwave ranging system which, according to NASA, had the ability to judge that distance "...within a fraction of the diameter of a human hair over 137 miles (220 kilometers)."

The "Follow-On" mission is scheduled to launch within the next few months [spaceflightinsider.com]. GRACE-FO will have a laser ranging system with 20 times the sensitivity of GRACE.

Two similar missions: the ESA's Gravity Field and Steady-State Ocean Circulation Explorer [wikipedia.org] (GOCE), and Gravity Recovery and Interior Laboratory [wikipedia.org] (GRAIL) which mapped the Moon.

Related: Discovery of a Massive, 198 Kilometer-Wide Crater on the Moon [soylentnews.org]
Enter the Moon Cave [soylentnews.org]

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