Van Flandern had a career as a professional scientist but was noted as an outspoken proponent of certain fringe views in astronomy, physics, and extraterrestrial life.

While there, he helped start the Cleveland branch of Operation Moonwatch, an amateur science program initiated by the Smithsonian Astrophysical Observatory to track satellites.

[17] Following his separation from the USNO, Van Flandern started a business organizing eclipse-viewing expeditions and promoting his non-mainstream views in a newsletter and website.

[20] During the mid-1970s, Van Flandern believed that lunar observations gave evidence of variation in Newton's gravitational constant (G), consistent with a speculative idea that had been put forward by Paul Dirac.

[3] Following claims by David Dunham in 1978 to have detected satellites for some asteroids (notably 532 Herculina) by examining the light patterns during stellar occultations,[27] Van Flandern and others began to report similar observations.

[3] Van Flandern described in his 1993 book Dark Matter, Missing Planets, New Comets[29] how he had become increasingly dissatisfied with the mainstream views of science by the early 1980s.

Van Flandern supported Georges-Louis Le Sage's theory of gravitation, according to which gravity is the result of a flux of invisible "ultra-mundane corpuscles" impinging on all objects from all directions at superluminal speeds.

He gave public lectures in which he claimed that these particles could be used as a limitless source of free energy and to provide superluminal propulsion for spacecraft.

[31][32] In 1998 Van Flandern wrote a paper[33] asserting that astronomical observations imply that gravity propagates at least twenty billion times faster than light, or even infinitely fast.