Sky Sentinels

Two UT satellites are beaming down bad news about climate change.


Since 2002, two satellites nicknamed Tom and Jerry have been chasing each other through low Earth orbit at speeds exceeding 17,000 miles per hour. And while Tom will occasionally make up ground, the satellites never get closer than 136 miles apart, meaning that Jerry is never in any real danger.

This high-flying cat-and-mouse game is a groundbreaking science experiment called GRACE (Gravity Recovery And Climate Experiment). GRACE’s mission, conceived by scientists at UT’s Center for Space Research and carried out dutifully by Tom and Jerry, is to measure minuscule perturbations in Earth’s gravity field in order to weigh every bit of water, ice, and land that they fly over.

And over the past 13 years, the GRACE satellites have beamed down very bad news about global climate change, particularly the hastened retreat of the world’s ice sheets. GRACE data indicates, for example, that between 2003-14 the West Antarctica ice sheet shed 92 gigatons, or 92 billion tons, of ice per year. If stacked on the island of Manhattan, that amount of ice would stretch more than a mile high. According to GRACE, the amount of ice Greenland lost over that same period was more than three times higher.

“Nobody had ever seen data like this,” says CSR Director Michael Watkins. “Before GRACE, there was very poor knowledge of the net change in Greenland and Antarctica, but we can now get a handle on how much ice is melting.”

Onboard each GRACE device sits a suite of instruments, including a microwave range-finding system, a GPS device, and star-tracking equipment. As the satellites circle the planet, local increases in Earth’s gravity field tug at the first satellite, making it speed up and separate from the second. As the satellites progress over the object, the first one slows back down while the second one speeds up. The amount that the satellites move apart and come back together—measured by onboard instruments to a precision of one micrometer—indicates the mass below. By stringing these measurements together, the GRACE satellites produce a detailed map of Earth’s mass every 30 days.

GRACE’s precise measurements have been used for more than keeping tabs on the planet’s ice sheets. According to Watkins, GRACE’s measurements have “revolutionized our understanding of large-scale processes on Earth,” allowing scientists to measure drops in underground aquifer levels during times of drought and better understand earthquake dynamics.

The mission has also quantified how much of the rise in global sea levels comes from melting glaciers and how much comes from the expansion of water due to ocean warming. “You can’t tell … except GRACE can tell because it measures the mass of the oceans,” says UT geophysicist Clark Wilson. GRACE data has suggested that about a third of the rise is due to thermal expansion, with two-thirds coming from meltwater.

In 1995, UT space engineer Byron Tapley and his colleagues sold NASA on the GRACE project and brought in collaborators from the German Aerospace Center to build the satellites. To further cut costs, the Germans arranged for the satellites to be launched from the Baikonur Cosmodrome in Russia aboard a repurposed Soviet intercontinental ballistic missile. “We were concerned that Congress might find out that we were spending NASA dollars to buy Russian ICBMs,” says Wilson, a former NASA official who reviewed the proposal. “But they never found out, I guess.”

Originally slated for a five-year mission, GRACE continues to churn out data 13 years later. With the help of supercomputers at the Texas Advanced Computing Center, this data have been analyzed by researchers from around the world and published in more than 1,900 scientific articles. And in 2017, the second of three planned generations of GRACE satellites will begin their own decade-long mission to illustrate the rapidly changing Earth in vivid, eye-popping detail.

Photo by NASA/JPL-Caltech.


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