Very low earth orbit satellites … known as V-L-E-Os … have become important in a variety of surveillance and imaging applications. Trouble is, they're *so* low, they don't stay up very long without propulsion to keep them up. Eventually they run out of fuel. That's a problem the Defense Advanced Research Projects Agency is trying to solve with a program known as Otter. For details, DARPA program manager Sarah Popkin joins me in studio.
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