A few of these satellites fulfilled the recommendations laid out at Williamstown. Subsequent missions took their cue from an expanded vision for Earth observations put forward by NASA administrators in the 1980s. One of their reports presciently urged that scientists focus on how human activity was warming the planet, noting that “the burning of oil and coal is injecting carbon dioxide into the atmosphere at unprecedented and accelerating rates.” Eventually, the NASA directives of the 1980s led to a number of satellites that were deployed in the 1990s and early 2000s, like Aqua, Terra, Aura and ICESat. Some of these missions are still orbiting and considered vital to Earth observations. Byron Tapley, the director of the Center for Space Research at the University of Texas at Austin, told me that during this era he worked on various NASA satellite efforts that would have measured Earth’s gravity field, but none of them made it to launch. That changed with the proposal for Grace, which was written in large part by Tapley’s former student from Austin, Mike Watkins, who had gone to work at J.P.L. NASA gave the go-ahead in 1997. “That was the last piece of the puzzle from Williamstown,” said Tapley, who was named the principal investigator — in effect, the research leader — of the project.

The goal for Grace was to produce an unprecedentedly accurate reading of Earth’s gravity field. But early on, Watkins began to think that its two craft could also register details on what’s known as variable gravity, which mostly depends on the way water moves around the world under the influence of seasonal changes, droughts and other climate factors. Where there’s more water in one place, there’s more gravitational pull; where there’s less water, there’s less pull. A good illustration of the satellite’s promise had to do with the problem of measuring variation in the world’s great ice caps. When the first Grace satellite approached, say, the Greenland ice sheet, which weighs about three quadrillion tons, the craft would presumably respond to the subtle gravitational tug and be pulled slightly forward and away from its trailing partner. The distance between them — 137 miles or so — might increase by less than a human hair. But because the twin spacecraft were in constant contact with each other through a microwave communication link, that change could still be measured precisely. And it could be measured over and over again, month after month, year after year. If the ice on Greenland kept pouring into the ocean, scientists could convert that remote measurement into a calculation of ice loss. In this respect, Grace would be unlike so many other satellites: It wouldn’t render beautiful images of our planet from space. Its movement — or more exactly, its change in movement from one month to the next — would itself create the measurement.

In the mid-1990s, Watkins and his colleagues started to do detailed simulations. “We wondered: How much can we measure changes in the Greenland and Antarctic ice sheets? How well can we measure aquifer changes in groundwater? And we started to realize that this was the thing that was really going to break the mission wide open.” The proposal he wrote expressed confidence that they could get a measurement for the planet’s gravity field, but as Watkins recalled, it also hinted, “Here’s this other supercool thing we can do.”

Grace was authorized during an era at NASA, the late 1990s, when some science missions were approved on the condition that they satisfy an agency directive to be “faster, better, cheaper.” The joke at NASA at the time was that you get only two out of the three. What ultimately made Grace possible was a cost-sharing partnership struck between American scientists and the German Research Center for Geosciences and the German space agency. The German contribution was to pay for a launch vehicle and conduct the mission operations. “We worked with a German company that’s now part of Airbus on the design of the satellite, and J.P.L. did most of the instrumentation,” Watkins explained. By the time of the launch, the cost amounted to $97 million for NASA and about $30 million for Germany.

The German team secured a Russian rocket, and Grace was sent into space from the Plesetsk Cosmodrome, a launchpad about 500 miles north of Moscow, on March 17, 2002. The two spacecraft — looking like oversize gold bars, each about the size of a small automobile — moved into orbit and, using onboard tanks of nitrogen for acceleration and positioning, eventually achieved the necessary 137 miles of separation. The satellites travel in a circumpolar orbit, meaning that instead of tracing the Equator they fly on a path that we might consider northerly, cruising over the poles. They soon began transmitting measurements several times per day, often to a ground station in Svalbard, in the Arctic Ocean north of Norway. From there, the information was routed to the German science team, near Munich, and to the engineers at J.P.L. In those early days, as the raw data about gravity fields began coming back — data no one had ever really seen before — scientists didn’t immediately gape in wonder. Mostly they scratched their heads and tried to figure out what to make of it.

One lesson of publicly funded science is that Americans are not very good at predicting how useful it will be. It’s only later that we look back and see how the investments paid off. Some of the returns are economic; most of the crucial components of smartphones (not to mention the internet itself) began with publicly funded science, for instance. Investments in the collection of climate data fall into a similar category: They started as science projects, then gave us significant economic and social information — like insights into hurricanes and droughts. Among other things, satellite data about oceans has helped scientists create models to predict El Niño and La Niña patterns that wield considerable influence over the global climate. It has even helped predict shortfalls in Russian wheat harvests.