Sometime in the next few years-no one knows exactly when-three NASA satellites, each as heavy as an elephant, will go dark.
They are already drifting and losing height little by little. They've been staring at the planet for more than two decades, much longer than anyone expected, helping us predict the weather, control 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, final fall to Earth.
It's a moment scientists fear.
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When the three orbiters - Terra, Aqua and Aura - are powered down, much of the data they collected will end up with them, and newer satellites won't be able to pick up all the slack. Researchers will either have to rely on alternative sources that may not meet their exact needs, or find solutions to ensure their data can survive.
With some of the data these satellites collect, the situation is even worse: no other instrument will continue to collect this data. Within a few years, the fine features they reveal about our world will become much vaguer.
"The loss of this irreplaceable data is nothing short of 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 we are affected by it, and how we impact it, we seem to be disastrously asleep at the wheel."
The main area we lose sight of is the stratosphere, the all-important home of the ozone layer.
In the cold, thin air of the stratosphere, ozone molecules are constantly being formed and destroyed, tossed back and forth as they interact with other gases. Some of these gases have a natural origin; others are there because of us.
An instrument on Aura, the microwave limbometer, gives us the best view of this seething chemical drama, says Ross J. Salawitch, an atmospheric scientist at the University of Maryland. Once Aura is gone, our vision will dim significantly, he said.
Recently, microwave limb gauge data has proven its value in unexpected ways, Salawitch said. It showed how much damage was done to the ozone layer by the devastating bushfires in Australia in late 2019 and early 2020, and by the undersea volcanic eruption near Tonga in 2022. It showed how much ozone-depleting pollution ended up in the stratosphere above Eastern Europe. Asia due to the summer monsoon in the region.
If it didn't go offline so quickly, the siren could also help unravel a major mystery, Salawitch said. "The thickness of the ozone layer over populated areas in the Northern Hemisphere has changed little over the past decade," he said. "It should recover. And that's not it."
Jack Kaye, associate director of research at NASA's Earth Science Division, acknowledged researchers' concerns about the end of the echo sounder. But he argued that other sources, including instruments on newer satellites, on the International Space Station and here on Earth, would still give "a pretty good picture of what the atmosphere is doing."
Financial realities are forcing NASA to make "tough decisions," Kaye said. "Would it be great if everything lasted forever? Yes," he said. But part of NASA's mission is also to provide scientists with new tools that help them see our world in new ways, he said. "It's not the same, but you know, if everything can't be the same, then 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 may think they understand how something evolves. But only by monitoring it continuously, in an unchanging way, over a long period of time can they have confidence in what is going on.
Even a brief break in the records can cause problems. Suppose an ice shelf in Greenland collapses. Unless you measure sea level rise before, during and after the collapse, you can never be sure whether a sudden change was caused by the collapse, says William B. Gail, former president of the American Meteorological Society. "You might suspect it, but you don't have any quantitative data," he said.
Last year, NASA asked scientists for ideas on 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 raised concerns about a wide range of data from the satellites. Information about the particles in wildfire smoke, desert dust and volcanic plumes. Cloud thickness measurements. Detailed maps of the world's forests, grasslands, wetlands and crops.
Even if there are alternative sources for this information, the scientists wrote, they may be less frequent, have lower resolution, or be limited to certain times of day, all factors that determine how useful the data is.
Liz Moyer examines Earth's atmosphere up close: by flying instruments through it, with jets that fly much higher than most planes can. "I got into it because it's exciting and hard to get there," said Moyer, a lecturer at the University of Chicago. "It's hard to build instruments that work there, hard to make measurements, hard to get planes to go there."
It will be even harder when Aura is gone, she said.
Airplanes can sample the chemistry of the atmosphere directly, but to understand the big picture, scientists still need to combine aircraft measurements with satellite measurements, Moyer said. "Without the satellites, we are taking snapshots without context," she said.
Much of Moyer's research focuses on the thin, icy clouds that form 15 to 20 kilometers above the ground, in one of the most mysterious layers of the atmosphere. These clouds help warm the planet, and scientists are still trying to figure out how human-induced climate change is affecting it.
"It looks like we're just going to stop observing that part of the atmosphere, right at a time when it's changing," Moyer said.
The end of Terra and Aqua will affect how we monitor another key driver of our climate: how much solar radiation the planet receives, absorbs, and reflects back to space. The balance between these quantities - or rather the imbalance - determines how much the earth warms or cools. And to understand this, scientists are relying on the instruments of NASA's Clouds and the Earth's Radiant Energy System, or CERES.
There are currently four satellites flying with CERES instruments: Terra, Aqua, plus two newer ones that are also nearing their end. Yet only one replacement is in the works. His life expectancy? Five years.
"Within the next decade, we'll go from four missions to one, and the remaining one will have passed its peak," said Norman G. Loeb, the NASA scientist leading CERES. "For me, that's really sobering."
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 monitoring our planet. The future may lie with smaller, lighter instruments, instruments that can be put into orbit more cheaply and more manoeuvrably than Terra, Aqua and Aura were at the time.
The National Oceanic and Atmospheric Administration is developing such a fleet for monitoring weather and climate. 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, Loeb said, they must start flying before current orbiters go dark.
"You need a good, long period of overlap to understand the differences and work out the kinks," he said. "If that's not the case, it will be very difficult to have confidence in these measurements if we haven't had the chance to prove them against current measurements."
In a way, it's a credit to NASA that Terra, Aqua and Aura have lasted as long as they have, scientists said. "Thanks to a mix of great engineering and a tremendous amount of luck, we've had it for 20 years now," said Waleed Abdalati, a former NASA chief scientist now at the University of Colorado Boulder.
"We became a bit addicted to these satellites. We are victims of our own success," Abdalati said. "Eventually," he added, "luck runs out."
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