Brian Todd O'Leary (January 27, 1940 – July 28, 2011)[1] was an American scientist, author, and NASA astronaut candidate.

In 1977, he worked on Asteroidal Resources Group, NASA Ames Summer Study on Space Settlements as team leader.

[7] Because of his professional specialty and youth, O'Leary was selected as an astronaut in conjunction with a possible NASA human mission to Mars then envisaged for the 1980s contingent on post-Apollo funding.

[10] After O'Leary's resignation from NASA, Carl Sagan invited him to lecture at Cornell University in 1968, where he stayed until 1971 as a research associate (1968–1969) and assistant professor (1969–1971) of astronomy.

[20] O'Leary worked for U.S. presidential candidates Jesse Jackson, Dennis Kucinich, George McGovern, and Walter Mondale.

[47][48] With artist Meredith Miller, his third wife and widow, he co-founded the Montesueños Eco-Retreat in Vilcabamba, Ecuador in 2008, which is devoted to "peace, sustainability, the arts and new science".

After surviving his second heart attack (precipitated by an ayahuasca ceremony)[50] in 2010, he died of intestinal cancer on July 28, 2011, soon after diagnosis, at his home in Vilcabamba.