In late August, as Hurricane Harvey began smashing into the Texas coast, a flood of data began pouring in along with the catastrophic quantities of rainwater. It wasn’t from the nonstop news coverage on CNN and elsewhere; it was from the transmissions that lay behind it, in the pulses of information coming down from space. The National Oceanic and Atmospheric Administration’s geostationary and polar-orbiting satellites, crucial tools for monitoring big storms in the Gulf of Mexico, were capturing cloud formations, surface temperatures and barometric pressures, which were then fed into computer models tracking the storm’s strength and intensity. At the same time, NASA was using a group of satellites to keep tabs on soil moisture, flood patterns and power failures all over East Texas. In various ways, this torrent of data was being collected continuously from hundreds (or even thousands) of miles overhead, through radar instruments and spectroradiometer sensors and exquisitely calibrated imaging cameras. The machines being used aren’t household names — they go by acronyms like GOES-13, Modis and SMAP — but they demonstrate why the popular view of Earth as a big blue planet with only the Moon as its companion could do with some revising. We are also surrounded by a constellation of satellites spinning elliptical webs of environmental observation, day and night.

This array of American satellites, comprising dozens of NOAA and NASA missions, is the product of some 40 years of experimentation and investment on the part of the federal government. They’re joined in their orbit by weather and climate satellites from scientific agencies in Europe and Asia, along with a host of satellite-borne sensors from both the private sector and the military, that measure everything from air pollution to land development to agriculture. Without question, we’re living at the start of a dark era of warming climates. But we’re also living in a golden age of environmental data, in which our technology in space can deliver surprising measurements with profound implications. After a big hurricane like Harvey or Irma dumps extraordinary amounts of water on a region, for instance, a pair of NASA satellites known as Grace, which is short for Gravity Recovery and Climate Experiment, is able to assess how much water floods in and how it dissipates as the storm recedes. This has been merely one of their functions. Grace’s two spacecraft have been circling Earth every 90 minutes for the past 15 years at an altitude of 300 miles or so. On a dark, clear night, you can sometimes look up and see them for a brief moment: two bright, blurry dots, rushing by at a velocity of about 17,200 miles per hour. They chase each other around the sky, one pursuing the other like cat and mouse (hence their nicknames, Tom and Jerry). By monitoring how their positions in space are affected by gravity, the scientists at NASA can draw a number of conclusions about what’s happening on Earth, especially to our freshwater resources.

Yet Grace also illustrates how tenuous the golden age of data really is. The two craft, which were launched in 2002, were originally expected to orbit the planet for five years. They are now dying, and in fact the batteries on one of the satellites are so depleted that it periodically goes to sleep. Since 2010, NASA has been planning and building replacements, and if all goes well, they will be in orbit early next year. But if Grace goes dark or perishes before then, there will be a break in NASA’s continuous observation of Earth’s gravity field and water dynamics. Climate researchers will be confronted with what’s known as a “data gap,” which can leave them at a loss for drawing scientific conclusions about environmental trends.

This sort of gap has threatened to become a more common problem in recent months. The federal government’s entire climate-science enterprise, much of it linked to NASA’s satellite research, is under duress. The most prominent of the Trump administration’s related proposals was a request to cut 31 percent of the Environmental Protection Agency’s budget; later, pages featuring global-warming data were removed from E.P.A. websites. But the proposed reductions for NOAA and NASA were similarly drastic. The White House asked that NASA drop four climate-related missions in its Earth-sciences division, which accounts for about 10 percent of NASA’s $19 billion annual budget. It also unveiled a plan to cut 18 percent from NOAA’s annual spending on satellites, which would force a huge reduction in the agency’s climate work.

At this point, an ax has been sharpened but not yet swung. In the coming weeks, as the 2018 budget is debated in Congress, the two chambers will try to work out a deal. If the House gets its way — it has mostly endorsed the White House plan and calls for reducing NASA’s Earth-sciences budget by $217 million, while the Senate has proposed restoring the cuts — missions will most likely be scuttled and holes will open in the data-collection records. Even if our science agencies avoid the worst, however, the Trump administration’s intent to slash funding on technology that helps make planetary surveys possible — much of it obscure to the public — signals an embattled future for this type of research.

The effects are hard to predict. Most of our climatic and meteorological information shares a common origin, even when it seems to come from the Weather Channel or CNN. “The monitoring of the atmosphere, of the surface of the Earth, of what’s going on in the ocean and under the ice — all of that is overwhelmingly funded by the federal government,” John Holdren, President Obama’s chief science adviser and a Harvard professor of environmental science and policy, told me recently. While some of this information is made familiar to us — the unending stream of satellite images on TV as Hurricane Harvey flooded Houston — some of it is experimental and unknown outside scientific circles. That sort of data comes from missions like Grace, funded in the belief that a risky idea might turn up something valuable for human understanding or even, in the case of climate change, for human survival.

Every satellite has its own story, and Grace’s begins in the summer of 1969 at a NASA-sponsored conference in Williamstown, Mass. The scientists convened there to discuss how a variety of new technological tools and sensors might allow them to gain a better understanding of our planet. Several recommendations emerged from the gathering, among them the suggestion that the agency launch satellites to measure sea levels, monitor changes in Earth’s crust and analyze the planet’s gravity.

The reason for wanting precise gravity measurements was practical as well as scientific. The gravitational pull on an object can be greater where Earth’s mass is denser — above mountain ranges like the Rockies or Alps, say, or over vast ice sheets like Antarctica’s. The resulting variations in gravity have subtle but important effects on the paths of ballistic missiles, for example, which the Defense Department cares deeply about. “You want to be able to predict where they’re going to fly and exactly where they’re going,” Mike Watkins, the director of NASA’s Jet Propulsion Laboratory and the original project scientist on Grace, told me. Starting in the 1970s, Watkins said, oceanographers were also trying to map the surface of the ocean, an effort complicated by the fact that the gravitational effects of features far beneath the surface — like deep trenches, submerged mountain ranges and lost continents that slid under Earth’s crust billions of years ago — are constantly distorting the sea level above.

By the time of the Williamstown conference, satellite observations of Earth, which were beginning to be referred to as “remote sensing,” seemed enormously promising. Researchers and mapmakers had been collecting aerial images since the 1840s, usually from balloons, but remote sensing advanced drastically using cameras on aircraft during World Wars I and II, and then later still on U-2 planes that began conducting detailed military surveys in the late 1950s. In the 1960s, NASA and other government agencies started launching Earth-orbiting satellites to study weather patterns, ocean circulation and agriculture. By the late 1970s, some of them, with names like Landsat and Seasat, carried sophisticated microwave and laser instruments and imaging equipment.

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.

The “E” in Grace may stand for “experiment,” but the project produced useful data fairly quickly. Measuring the Greenland and Antarctic ice sheets was a case in point. For most of the 20th century, one of the essential questions bedeviling glaciologists was how sea levels and coastlines could be affected by the fluctuations in the size of these ice caps. As a 1985 NASA report put it, “Despite 25 years of intensive fieldwork in Greenland and Antarctica, and the expenditure of billions of dollars, we are still unable to answer the most fundamental glaciological question: Are the polar ice sheets growing or shrinking?” In the 1990s, researchers tried various methods on the ground to record the height of the Greenland ice sheet from year to year to determine its loss or gain. But Grace promised a different, and previously impossible, kind of calculation.

In 2006, Isabella Velicogna and John Wahr, both at the University of Colorado at Boulder, published two studies that interpreted the first few years of Grace data; their initial paper was about Antarctica’s loss of ice, while the second was about Greenland’s, which appeared to be losing at least 100 billion tons per year. Some scientists were awe-struck. “I remember reading their first paper, and I literally couldn’t believe it,” said Berrien Moore, a dean at the University of Oklahoma who has worked on NASA missions on and off for several decades. “A quantitative measure of a mass change from year to year? It was just unheard-of.” Velicogna, now a professor at the University of California at Irvine and a J.P.L. scientist, told me that the Grace measurements didn’t suddenly make field studies of individual glaciers less important — her data couldn’t tell scientists why the ice sheet was losing mass. But they allowed her to systematically account for drastic losses in places so far-flung that they were almost impossible for human beings to reach, like parts of Greenland and West Antarctica. What’s more, the measurements enabled glaciologists to look at the decline of massive mountain glaciers, like those in Central Asia, which are a critical resource for regional water supplies. As Velicogna noted, those glaciers “could mean the difference between life and death in those places.” They can also lead to profound geopolitical conflicts. Grace soon indicated that many were shrinking.

Similar changes began to be revealed in the world’s hidden aquifers. Jay Famiglietti, a hydrologist at J.P.L. who focuses on tracking changes in groundwater — water stored in underground aquifers around the world — worked as a professor at the University of Texas at Austin in the late 1990s. Back then, a typical way to study aquifers was to monitor wells in the field. When Famiglietti was invited to meetings in Austin to hear about what Watkins and Tapley were planning, he told me: “I didn’t believe it would work. They were all talking about how we’re going to be able to see groundwater. I thought, these guys are nuts.” As the data began coming in, however, Famiglietti found that Grace could measure groundwater with astounding effectiveness. He came up with a nickname for Grace — “the scale in the sky” — and began tracking California’s water supplies during what eventually became a decade of unrelenting drought.

One of Grace’s shortcomings is its limited resolution: It can map increases and losses in only large aquifers. Still, Famiglietti told me that from 2011 to 2015, California lost so much water every year — trillions of gallons — that it showed up clearly in the Grace measurements. About two-thirds of the losses appeared to be groundwater. He also noted that the satellites were able to capture a freshwater predicament bigger than California’s. Before Grace, Famiglietti said, most of our knowledge about underground water reserves was a hodgepodge. “What Grace added was a regional, global understanding — like, holy crap, this is happening all over the world.” In 2015, Famiglietti’s team used Grace to determine that more than half of the world’s largest aquifers were “past sustainability tipping points.” They were being depleted significantly faster than they were being replenished. The Arabian Aquifer System, on which 60 million people rely, appeared severely overstressed; so did water reserves in northwestern India and northern Africa.

Beyond showing declines in the world’s ice sheets and aquifers, Grace clarified the factors influencing the rise in sea levels. In the early 1990s, NASA began putting a succession of Earth satellites into orbit, the most recent being NOAA’s Jason-3, that use a tool called a radar altimeter to measure the ocean’s surface, which has been rising by an eighth of an inch per year since 1993. But in a warming world, sea levels increase for two distinct reasons. The first is that the ocean expands as it gains heat: It just gets bigger. The second is that ice sheets and glaciers melt and break into the ocean. “With an altimetric measure like Jason,” Felix Landerer, a J.P.L. Earth scientist, told me, “you know the height change of the ocean, but you don’t know really what’s causing it.” How much is from heat expansion, in other words, and how much is because of the displacement caused by more ice? Grace, however, constantly weighs the oceans, which makes it possible to determine how much water is pouring into them from melting ice and other sources.

Landerer showed me a video he had just made from Grace data. It showed losses on the Greenland ice sheet from 2002 to 2016. A map of Greenland was white, and the areas with the biggest declines grew yellow and then dark brown and then black with each passing year. Since about 2008, the ice loss has totaled nearly 300 billion tons annually. To watch the progression was to see the entire southeast and southwest coasts of Greenland become increasingly dark over the course of a decade. The ice sheet looked like a piece of paper burning from the edges.

The goal of remote sensing, as Landerer’s map demonstrates, is not merely to measure unmeasured aspects of the planet. It’s to measure them nonstop, ideally for decades on end, so that long-term trends can be identified in a notoriously chaotic and variable natural world. This is why recent calls in Washington to defund some satellite missions rattled so many of the NASA scientists I spoke with. The end of a mission means the data stream will stop or pause, and a gap in the data makes it more difficult to infer whether the melting of an ice sheet or the loss of water from an aquifer is accelerating. Just as crucially, a gap can interrupt a long series of observations that allow researchers to understand future events. “From the perspective of Earth scientists,” Watkins told me, “you need to understand the system so that you can then model that system physically and make forecasts with some skill.” In the case of Grace, such observations could help predict the future of Greenland’s ice, as well as California’s water.

The administration’s exit from the Paris accords this year represents a public retreat from diplomatic engagement on climate change. But its budget priorities — seeking to minimize the need, as well as the means, for gathering climate information — suggest the start of a quieter but arguably more consequential shift: undercutting the very data and evidence that has helped bring urgency to the issue. This strategy has hardly been covert (“We’re not spending money on that anymore,” Mick Mulvaney, President Trump’s budget director, said about climate change), yet the implications are almost certainly more significant than they appear, especially if they become law in a new federal budget. While there have always been policy debates about allocating taxpayer dollars to environmental research, according to Holdren, the fundamental reason we collect the data is because it has long been considered an apolitical “public good,” with a variety of benefits for the nation’s economy, public health and safety. Even the curiosity-driven forays at NASA — undertaken more in pursuit of scientific understanding than a desire to improve, say, storm or drought forecasting — seem to be gaining value in an era of disruptive climate-related events. “About a decade ago, NASA could have gone out and counted easily the users of all its data,” Bill Gail, an executive at the Global Weather Corporation, a forecasting-services company based in Boulder, Colo., told me. But now it’s used by a multitude of local planners, fishermen, agribusiness companies and shippers who want longer-term insights — even if they avoid calling it climate-change forecasting. Sometimes, Gail said, they prefer to call it “long-range weather prediction.”

In the event of a drastic scaling back of our Earth-sciences efforts, is it possible that other countries — China, Japan, India, the nations of Europe — would step up their satellite investments? “They’re a valuable complement,” Waleed Abdalati, a professor at the University of Colorado at Boulder and former chief scientist at NASA, told me. “But to really understand these processes requires more than any one nation can do, and the U.S. has really been the leader. And to say that other nations can pick up the slack is not really accurate.” Stopgap solutions don’t look so promising, either. While Gov. Jerry Brown of California has vowed that his state would send up its own climate-research satellites, the logistics and financial costs — climate satellites now usually require half a billion dollars and a decade to plan, build and launch — would prove formidable, even if backed by California voters. Private-sector satellite companies have in recent years been expanding the business of collecting and selling Earth observational data, but it’s very unlikely that such firms (or a group of tech philanthropists) could adequately replace NASA’s work. “These are projects that are too expensive or require a large and diverse group of collaborators that can only be assembled as an international project,” said Rush Holt, a former Democratic congressman who is now the head of the American Association for the Advancement of Science. “Or this is work that has to be sustained for a longer period of time than any board of directors from a private company would consider, because it’s not clear enough that it would produce a return on investment in anyone’s lifetime.” That explains why the government’s involvement in basic research, going back at least to the late 1940s, was premised on the idea that it fills a role that would not be filled otherwise. As one scientist put it to me, If the government thinks a project is too long-term to make it worth funding, no other organization is going to pay for it either.

Grace, as one of those long missions, has enjoyed some good fortune: Its durability has exceeded expectations, and thanks to international partnerships it has almost certainly dodged contemporary political disruptions. What’s less fortunate, though, is Grace’s current death spiral. Over the past year, a soft-spoken engineer at J.P.L. named Rob Gaston has been focused on extending Grace’s life as long as possible. When I visited him at his office in the spring, he explained that the batteries were already failing and that the fuel, which made adjustments in orientation possible, was nearly exhausted. Another problem was its distance from Earth. “You don’t think about it,” he said, “but there’s actually atmosphere at that altitude, and even though it’s very thin, it does create some friction on a satellite. And it does cause the orbit to degrade.” Even if Grace’s depleted batteries allow it to function for the next few months, its physical demise is approaching; it will begin when the satellites fall to 186 miles in altitude. At the beginning of September, Gaston updated me: Grace was at 201 miles and dropping about 250 feet per day.

For almost a decade, the fear has been that Grace will die before a replacement mission could sustain its data stream. Tapley, the lead investigator, recalled that in 2010 his team settled on replicating Grace precisely rather than building a more sophisticated and expensive improvement. “We decided we want to get the follow-on as quick as we can,” he told me. “We don’t want to break the timeline. We’ll use a cookie-cutter approach. And let’s see if we can get the Germans to partner with us again.” In fact, the proposed model wasn’t called Grace-2 — that was the souped-up version — but Grace-FO, for Grace Follow-On. The team received a green light from NASA, and after striking a deal with several German science agencies, the design and contracting began in 2012. This time, though, the process would not be “better, faster cheaper” — not when it would require about $450 million from NASA and roughly $100 million from Germany.

It’s possible that some policy edict could curtail the planned satellite mission — the recent Trump budget proposal, for instance, made the unusual request of turning off the Earth sensors on an orbiting spacecraft, Dscovr, to save $1.2 million. To Tapley’s great relief, though, German engineers will maintain Grace’s operations (and the European Space Agency is now contributing financial support). He pointed out to me that his goal going back to the 1960s has been to create an instrument to measure Earth’s gravity as precisely, and for as long, as it could be measured; for him today, this means thinking beyond Grace-FO and ahead to the more advanced Grace-2, a project now being discussed. He did not think he would see it through, because he is 84, but he thought he could help it begin. “I am worried,” he told me. “Not so much worried about Grace-FO, because the satellites are built, and it isn’t NASA dollars that are launching it — it’s German.” And it would be a surprising thing if it failed to proceed, he said. “But I’m very worried about the future, both for NASA as a whole and the Earth sciences in particular.”

I asked Tapley what we wouldn’t know if Grace had never existed. “An awful lot,” he said. We wouldn’t have the same sense of how steadily the ice sheets are melting — “and we would have no clue how much of the sea-level rise is temperature-related and how much is coming from land ice.” We wouldn’t be able to interpret the losses of various mountain glaciers and the changes to aquifers in Texas and California. Above all, we would lack an enhanced sense of how Earth works, how its masses of water flow from land to ocean in ways never witnessed or understood before. All this has taken money, but also time. Indeed, it has taken Tapley’s entire life.

In early February I went to see the replacement satellites at an aerospace facility on the outskirts of Munich. The Grace-FO twins — each about 10 feet long and 3 feet high — were positioned next to each other, horizontally, under bright lights in a sealed white room the size of a high school gymnasium. Supporting racks held each spacecraft at shoulder height; each had its side panels open, exposing metallic innards: snaking tubes and wires covered in foil. The engineers hovering around gave the place the feel of a surgical theater. Frank Webb, Grace-FO’s project scientist, who works out of J.P.L., happened to be there. We were joined by Peter Gath, the project manager for the German team. Everyone wore long white coats and covers over shoes and hair. As we entered the clean room, Gath warned me, “Don’t touch anything.”

You might think it would be easy to recreate a satellite you’ve already made before. But an engineering team tasked with designing and building a new spacecraft can’t easily replicate something that was constructed in a different technological era. Mike Gross, now the deputy project manager for Grace-FO, suggested it would be akin to building an iPhone 1 — but doing so in 2017. “You wouldn’t want to take parts for an iPhone 6 or 7 and try to rebuild an iPhone 1,” he said. “You’d want all the parts from an iPhone 1 to build it.” But the exact parts that went into making the iPhone 1 no longer exist. Nor do the parts that made up the original Grace. Old contractors are gone; old materials have been altered and improved.

The J.P.L. and German teams both began their work by consulting old blueprints. The new satellite models were to be assembled at an Airbus factory in Germany, with the manufacturing of parts and instruments contracted out to as many of the original suppliers as possible, in Denmark, France, Italy, Spain and the United States. The “cookie-cutter approach” that Tapley described made sense from a science perspective, because the most important thing about Grace-FO is to get the kind of data that Grace is now collecting. If you build a different spacecraft — one that’s bigger, or shaped a hairbreadth differently, or made from different materials — it could introduce all sorts of errors and complications. In short, the new spacecraft must function precisely as Grace does and look just like Grace. “But that’s where the rub came in,” explained Phil Morton, the project manager. The new satellite could not actually be Grace.

At the time of my visit, the satellites were essentially finished. Over the course of several months, they would be exposed to vacuum tests that simulated the airlessness and low temperatures of space and to vibration and acoustic tests that mimic the shaking and caterwaul of an actual launch. If all went according to plan, before the end of the year they would return to California and be installed within a SpaceX Falcon 9 rocket that would take off from Vandenberg Air Force Base, northwest of Los Angeles. The launch date, Webb said, would most likely be in early 2018.

Listening to Webb walk me through the interior, part by part, the complexity and engineering seemed considerable. Communication links so the twins can talk with each other and with Earth. Accelerometers to measure forces felt by the satellites. Star cameras to point at the sky for orientation. There were computers, horn antennas and tanks for the nitrogen thrusters that would enable the spacecraft to swap places every few years to minimize wear and tear. There were heating pads to ensure that the interior would stay at a comfortable 68 degrees, even as the satellite’s outside panels chill in Earth’s shadow (to minus 75 degrees) and heat up in the sun’s glare (to 250 degrees). The list went on. “The original Grace satellites are not nearly as dense in electronics as these are,” Webb remarked, almost apologetically, adding that each craft had gained a few hundred pounds since the last version, largely because they now included a new, experimental laser system. The weight of each original satellite was about 1,000 pounds. “Now we’re at roughly 600 kilos,” Gath added, or just over 1,300 pounds.

The extra weight would not be a problem, he assured me, because the engineers had found ways to accommodate it. And before we left the room in Munich, Gath also let me know that while the satellites had been built separately, “they will stay together from now on.” I found this reassuring. It was as though the two spacecraft were the kind of close companions that would need each other’s support to endure a difficult set of circumstances, which in a sense they will. Whatever the larger fate of our climate research, if you assume a safe deployment for Grace-FO, the twins might circle Earth together for two billion miles or so, measuring the subtle gravitational tug of our water much as their predecessors have, until their orbits drop and drop and then drop so much that both burn up and vanish. And at that far-off point — assuming that there is still the will to spend the money and keep taking the measure of the planet — the whole process would begin again.