It looks like you're using an Ad Blocker.
Please white-list or disable AboveTopSecret.com in your ad-blocking tool.
Some features of ATS will be disabled while you continue to use an ad-blocker.
The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move.
The notion that space is considered to be a vacuum filled with an "aether", or just a cold, dark vacuum continued up until the 1950s (see below).
And Akasofu recounted that: "The view that interplanetary space is a vacuum into which the Sun intermittently emitted corpuscular streams was changed radically by Ludwig Biermann (1951, 1953) who proposed on the basis of comet tails, that the Sun continuously blows its atmosphere out in all directions at supersonic speed" (Syun-Ichi Akasofu, Exploring the Secrets of the Aurora, 2002)
In March 1987, British UFO researcher Timothy Good also wrote the Army Directorate of Counterintelligence and again received a letter confirming the existence of the IPU from a Colonel William Guild. Guild was more definitive about the existence of IPU records and that they had been turned over to the U.S. Air Force Office of Special Investigations (AFOSI), the USAF counterintelligence unit, and the Air Force's Project Blue Book:
"...the aforementioned Army unit was disestablished during the late 1950's and never reactivated. All records pertaining to this unit were surrendered to the U.S. Air Force Office of Special Investigations in conjunction with operation BLUEBOOK." 
Originally posted by Gazrok
4) How did hoaxers know enough that the unit even existed? (if it is indeed hoaxed...the document)
Originally posted by OneisOne
So in my quirky mind, Interplanetary Phenomenon Unit could have been the precursor to the Air Force's Space Weather Operations.
inter- (prefix): used to form adjectives meaning 'between or among the stated people, things or places'
an interdepartmental meeting