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 s