Document 12-571-3570 (also entitled NASA No. 12 571-3570) is a hoax document originally posted to the Usenet newsgroup alt.sex on November 28, 1989.
According to this document, astronauts aboard space shuttle mission STS-75 performed a variety of sex acts to determine which positions are most effective in zero gravity. The document goes on to report that of the 10 positions tested, six required the use of a belt and an inflatable tunnel, while four were contingent on hanging on. The document also discusses a video record of the 10 one-hour sessions in the lower deck of the shuttle, and notes that the subjects added their own personal footnotes to help scientists.
Document 12-571-3570 on Wikipedia.