Since NASA’s Terra satellite launched in 1999, it has seen a world utterly transformed. Surface temperatures have risen half a degree. Sea levels have climbed 80 millimeters higher. Plants have expanded across an area as big as the Amazon rainforest. Through it all, Terra and two other satellites—Aqua, launched in 2002, and Aura, in 2004—served as the foremost sentinels of a changing planet, running far past their expected 6-year missions.

Now, all three satellites are approaching their end. They must save their remaining fuel to dodge space junk and slow down for a final plunge into Earth’s atmosphere. Too little is left to maintain the precise pole-crossing orbits that allow them to swoop past lower latitudes at exactly the same time every day. In the next month, NASA is expected to decide whether to terminate the missions, which cost $85 million a year to run, and invest that money in its next-generation Earth System Observatory satellite program, which will replace some of the trio’s capabilities by the late 2020s.

But killing them off now would be a mistake, the missions’ scientists argue. The satellites are healthy enough to survive until the middle of this decade and provide measurements no other spacecraft can match. Their drifting orbits, far from hampering them, will allow them to see Earth from new vantages, at other times of day. “These are almost like new missions,” says Norman Loeb, a physical scientist at NASA’s Langley Research Center and principal investigator of Ceres, an instrument on Terra and Aqua that measures Earth’s energy imbalance.

NASA is worried about the cost. If all three missions are extended, “it would require delays in the Earth System Observatory,” NASA said in a statement to Science. But the scientists are now lobbying NASA to let them make their case next year at a “senior review,” a meeting convened every 3 years to decide the fate of ongoing missions.

First conceived in the 1980s, the bus-size Terra and Aqua were each outfitted with an array of instruments, including the pioneering Moderate Resolution Imaging Spectroradiometer (MODIS), capable of parsing reflected light from vegetation, clouds, and sea ice in 36 different frequency channels. Other instruments, such as the infrared sounder on Aqua, capture temperature, water vapor, and dozens more variables globally each day. Aura, meanwhile, makes a close study of atmospheric chemistry. Because the instruments were mounted on the same spacecraft and tightly calibrated, they caught trends that otherwise would have been invisible. “These will be sorely missed observations,” said Waleed Abdalati, a remote sensing scientist at the University of Colorado, Boulder, and former NASA chief scientist.

Before Aqua, for example, clouds were thought to cover half the planet at any time; now it’s known to be nearly 70%. Terra has shown that plants cover 5% more of the planet than 20 years ago, a “greening” aided by rising carbon dioxide levels and warming. Aura has captured the intrusion of water vapor into the stratosphere from volcanic eruptions, where it adds to global warming and damages protective ozone. And the Ceres instruments have helped researchers discover that the deficit of energy escaping Earth has doubled since 2005, because of rising greenhouse gases and a decline in Sun-reflecting pollution hazes. “We’re the only ones in the world doing this,” Loeb says. “We’re it.”

New polar-orbiting weather satellites from the National Oceanic and Atmospheric Administration (NOAA) carry similar instruments, but they only have two-thirds as many channels as MODIS, says Miguel Román, chief climate scientist at Leidos, a large Washington, D.C.–based technology contractor, and principal investigator of MODIS. “At a fundamental level, you’re trading a Swiss Army knife with 36 features down to 22,” he says. The NOAA instruments are also designed to prioritize weather forecasting, not climate science, which few satellites are tailored for. “The public in general assumes that everyone is monitoring the climate,” says João Teixeira, an atmospheric scientist at NASA’s Jet Propulsion Laboratory and principal investigator of Aqua’s infrared sounder.

Keeping the satellites alive would also ensure overlap with observations that will begin in early 2024 on the International Space Station. A new instrument on the station, the Climate Absolute Radiance and Refractivity Observatory (CLARREO) Pathfinder, will measure Earth’s reflected light 10 times more precisely than past sensors. If the Ceres and MODIS measurements are calibrated with CLARREO Pathfinder’s, long-term trends in Earth’s energy balance will be easier to identify, says Kurtis Thome, Terra’s project scientist at NASA’s Goddard Space Flight Center (GSFC). “We’ll have a long enough record to start and feel confident that if we see a trend, it’s a real trend.”

If the Aqua mission is continued, it will drift out of its current orbit, which crosses the equator more than a dozen times a day at 1:30 p.m. local time, and end up in one that crosses at 3:50 p.m. local time, says Claire Parkinson, Aqua project scientist at GSFC, who has led the mission for 29 years. Terra will drift to an earlier morning pass, from 10:30 a.m. to 9 a.m., and Aura will drift 50 minutes later, from 1:45 p.m. to 2:35 p.m.

The time changes will enable critical new investigations, researchers say. By observing earlier in the morning or later in the afternoon, the satellites will capture long shadows, which can reveal the 3D structure of surface features or clouds. Crossing the Arctic Ocean earlier and later in the day will reveal how it exchanges heat with the atmosphere at previously unobserved times. And Aqua will be primed to explore the severe storms and wildfires that tend to peak in late afternoon. “We’re getting a new fire satellite by maintaining orbital drift,” Román says. “They will get so much bang for their buck.”

Even if NASA decides to end the missions, they have already secured their legacy. The researchers who conceived them originally wanted three sets of satellites, each lasting 6 years, to capture 18 years of change. Only one set was launched, but their longevity ensured the wish was fulfilled, Thome says. “We got it. We got the data we wanted to get.”

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