This happens when NASA loses sight of Earth

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At some point in the next few years (no one knows exactly when) three NASA satellites, each as heavy as an elephant, will shut down.

They are already drifting, losing height little by little. They’ve been watching the planet for more than two decades, much longer than anyone expected, helping us forecast the weather, manage wildfires, monitor oil spills, and more. But age is catching up with them and soon they will send their last transmissions and begin their slow and final fall to Earth.

It’s a moment scientists fear.

When the three orbiters (Terra, Aqua and Aura) go dark, much of the data they have been collecting will end up with them, and the newer satellites will not take over. Researchers will have to rely on alternative sources that may not meet their exact needs or seek alternative solutions to allow their records to continue.

With some of the data these satellites collect, the situation is even worse: no other instruments will continue to collect it. In a few years, the subtle features they reveal about our world will become much more confusing.

“Losing this irreplaceable data is simply tragic,” said Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology. “Just when the planet most needs us to focus on understanding how it affects us and how we are affecting it, we seem to be disastrously asleep at the wheel.”

The main area we are losing sight of is the stratosphere, the important home of the ozone layer.

In the cold, thin air of the stratosphere, ozone molecules are constantly formed and destroyed, shed and swept up as they interact with other gases. Some of these gases have natural origins; others are there thanks to us.

An instrument on Aura, the microwave probe, gives us our best line of sight into this seething chemical drama, said Ross J. Salawitch, an atmospheric scientist at the University of Maryland. Once Aura disappears, our vision will dim considerably, he said.

Recently, microwave probe data have proven their value in unexpected ways, Dr. Salawitch said. It showed how much damage was done to the ozone by the devastating wildfires in Australia in late 2019 and early 2020, and the underwater volcanic eruption near Tonga in 2022. It helped show how much ozone-depleting pollution was rising into the stratosphere in the East. Asia due to the region’s summer monsoon.

If it were not disconnected so soon, the probe could also help unravel a big mystery, Dr. Salawitch said. “The thickness of the ozone layer over populated regions of the Northern Hemisphere has barely changed over the past decade,” he said. “He should be recovering. And it is not.”

Jack Kaye, associate director of research at NASA’s Earth Sciences Division, acknowledged researchers’ concerns about the end of the probe. But he argued that other sources, including instruments on newer satellites, on the International Space Station and here on Earth, would still provide “a pretty good window into what the atmosphere is doing.”

Financial realities are forcing NASA to make “difficult decisions,” Dr. Kaye said. “Would it be great if everything lasted forever? Yes,” he said. But part of NASA’s mission is also to offer scientists new tools, to help them look at our world in new ways, he said. “It’s not the same, but, you know, if everything can’t be the same, do the best you can,” he said.

For scientists studying our changing planet, the difference between the same data and almost the same data can be enormous. They might think they understand how something is evolving. But only if you monitor it continuously, invariably, over a long period of time, can you have confidence in what is happening.

Even a brief break in records can create problems. Let’s say an ice shelf collapses in Greenland. Unless you measure sea level rise before, during and after, you will never be sure that the collapse caused a sudden change, said William B. Gail, former president of the American Meteorological Society. “It can be assumed, but a quantitative record is not available,” he said.

Last year, NASA polled scientists for ideas about how the end of Terra, Aqua and Aura would affect their work. More than 180 of them responded to the call.

In their letters, which The New York Times obtained through a Freedom of Information Act request, the researchers expressed concerns about a wide range of satellite data. Information about particles in wildfire smoke, desert dust, and volcanic plumes. Measurements of cloud thickness. Fine-scale maps of the world’s forests, grasslands, wetlands and crops.

Even if there are alternative sources for this information, the scientists wrote, they could be less frequent, lower resolution, or limited to certain times of day, all factors that determine the usefulness of the data.

Liz Moyer takes a closer approach to studying Earth’s atmosphere: flying instruments through it, in jets that travel much higher than most airplanes can reach. “I got into this because it’s exciting and it’s hard to get there,” said Dr. Moyer, who teaches at the University of Chicago. “It’s difficult to build instruments that work there, to make measurements, to get airplanes to get there.”

It will be even more difficult once Aura disappears, he said.

Planes can sample the atmosphere’s chemistry directly, but to understand the bigger picture, scientists still need to combine plane measurements with satellite readings, Dr. Moyer said. “Without satellites, we’re taking snapshots without context,” he said.

Much of Dr. Moyer’s research focuses on the thin icy clouds that form nine to 12 miles above the ground, in one of the most mysterious layers of the atmosphere. These clouds are helping to warm the planet and scientists are still trying to figure out how human-induced climate change is influencing them.

“It looks like we’re going to stop observing that part of the atmosphere, and exactly at the moment when it’s changing,” Dr. Moyer said.

The end of Terra and Aqua will affect how we monitor another important factor of our climate: how much solar radiation the planet receives, absorbs, and bounces back into space. The balance between these quantities (or, actually, the imbalance) determines how much the Earth warms or cools. And to understand it, scientists rely on instruments from NASA’s Cloud and Earth Radiant Energy System, or CERES.

At the moment, four satellites fly with CERES instruments: Terra, Aqua and two newer ones that are also coming to an end. However, only a replacement is in the works. Your life expectancy? Five years.

“Within the next 10 years, we will go from four missions to one, and the remaining one will be past its prime,” said Norman G. Loeb, the NASA scientist who directs CERES. “To me, that’s really humbling.”

Today, with the rise of the private space industry and the proliferation of satellites around Earth, NASA and other agencies are exploring a different approach to keeping an eye on our planet. The future may lie in smaller and lighter instruments, which could be put into orbit more economically and agilely than Terra, Aqua and Aura in their day.

The National Oceanic and Atmospheric Administration is developing such a fleet to monitor weather and climate. Dr. Loeb and others at NASA are working on a lightweight instrument to continue their measurements of Earth’s energy balance.

But for such technologies to be useful, Dr. Loeb said, they have to start flying before the current orbiters go dark.

“It takes a good, long period of overlap to understand the differences and resolve the issues,” he said. “If not, it will be really difficult to trust these measurements, if we haven’t had the opportunity to test them with current measurements.”

In some ways, it’s a credit to NASA that Terra, Aqua and Aura lasted as long as they did, the scientists said. “Thanks to a combination of excellent engineering and a lot of luck, we’ve had them for 20 years,” said Waleed Abdalati, former chief scientist at NASA now at the University of Colorado Boulder.

“We got hooked on these satellites. We are victims of our own success,” said Dr. Abdalati. “Eventually,” he added, “luck runs out.”