Sometime in the next few years — no one knows exactly when — three NASA satellites, each one as heavy as an elephant, will go dark.

Already they are drifting, losing height bit by bit. They have been gazing down at the planet for over two decades, far longer than anyone expected, helping us forecast the weather, manage wildfires, monitor oil spills and more. But age is catching up to them, and soon they will send their last transmissions and begin their slow, final fall to Earth.

It’s a moment scientists are dreading.

When the three orbiters — Terra, Aqua and Aura — are powered down, much of the data they’ve been collecting will end with them, and newer satellites won’t pick up all of the slack. Researchers will either have to rely on alternate sources that might not meet their exact needs or seek workarounds to allow their records to continue.

With some of the data these satellites gather, the situation is even worse: No other instruments will keep collecting it. In a few short years, the fine features they reveal about our world will become much fuzzier.

The main area we’re losing eyes on is the stratosphere, the all-important home of the ozone layer.

Across the stratosphere’s cold, thin air, ozone molecules are constantly being formed and destroyed, tossed and swept, as they interact with other gases. Some of these gases have natural origins; others are there because of us.

An instrument on Aura, the microwave limb sounder, 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 is gone, our vision will dim considerably, he said.

Recently, data from the microwave limb sounder has been proving its worth in unexpected ways, Salawitch said. It showed how much damage was done to ozone by the devastating wildfires in Australia in late 2019 and early 2020, and by the undersea volcanic eruption near Tonga in 2022. It helped show how much ozone-depleting pollution was getting lofted into the stratosphere over East Asia by the region’s summer monsoon.

If it weren’t going offline so soon, the sounder might also help unravel a big mystery, Salawitch said. “The thickness of the ozone layer over populated regions in the Northern Hemisphere has hardly changed over the last decade,” he said. “It should be recovering. And it’s not.”

Jack Kaye, the associate director of research at NASA’s Earth Science Division, acknowledged researchers’ concerns about the end of the sounder. 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.”

The end of Terra and Aqua will affect the way we monitor another important driver of our climate: how much solar radiation the planet receives, absorbs and bounces back to space. The balance between these amounts — or, really, the imbalance — determines how much Earth warms or cools. And to understand it, scientists rely on the instruments of NASA’s Clouds and the Earth’s Radiant Energy System, or CERES.

Right now, four satellites are flying with CERES instruments: Terra, Aqua, plus two newer ones that are also nearing their end. Yet only one replacement is in the works. Its life expectancy? Five years.