The Equinox in orbit of a planet with a parthogenic atmosphere
A parthogenic atmoshphere was a layer of gas surrounding some planets, capable of obstructing sensor functions.
In 2376, Captain Ransom ordered his ship, the USS Equinox, into orbit of a planet in the Delta Quadrant with a parthogenic atmosphere, in an attempt to evade detection by the crew of the USS Voyager. Despite this, Captain Janeway was able to locate the Equinox by studying Ransom's service record, finding that he tended to hide when being pursued. Janeway subsequently ordered Commander Chakotay to start scanning for environments where the Equinox could conceal itself, and located the planet. (VOY: "Equinox, Part II")