Several press reports say U.S. intelligence agencies have not detected reprocessing at North Korea's Yongbyon reactor. How they would detect it, if reprocessing were going on, is not entirely clear, but some methods can be inferred. Photo reconnaissance satellites in geosynchronous orbit (in other words, positioned over the same spot continuously) could observe activity around the plant—trucks bringing materials in or taking them out. Lower-orbiting satellites, with more elaborate sensors, could pick up thermal signatures (reprocessing would give off lots of heat). Various sensors could also detect steam, which would be a product of reprocessing; and multispectral sensors might be able to analyze the contents of that steam. Princeton University physicist and arms-control specialist Frank Von Hippel, as well as intelligence analyst Jeffrey Richelson, say sensors might also be able to detect Krypton-85, a chemical released when fuel rods are dipped in acid during the reprocessing. It is unclear at what distances Krypton-85 can be detected—probably not far enough to be picked up by satellites, but electronically equipped aircraft, even flying just outside North Korean borders, might have the range to detect slight traces of the chemical in the air (which is all they need to do, since Krypton-85 is not a natural gas). Or secret sensors might have been installed inside North Korean territory.