Have you ever heard of satelloons?
From about 1956 until 1964, US aeronautics engineers and rocket scientists at the Langley Research Center developed a series of spherical satellite balloons called, awesomely enough, satelloons. Dubbed Project Echo, the 100-foot diameter aluminumized balloons were one of the inaugural projects for NASA, which was established in 1958.
Link -via Dark Roasted Blend
(but, not just Roswell - it would be a good cop-out for ALL sightings! heh heh).
(http://en.wikipedia.org/wiki/List_of_Doctor_Who_vehicles#Void_ship)