This is a Gemini retro rocket that provided a means of slowing down the two-man Gemini Earth-orbiting U.S. manned spacecraft to permit its reentry into Earth's atmosphere. Four of the solid-fuel rockets fired successively. The rockets could also be used to separate the spacecraft from the launch vehicle in case of a high altitude launch abort but were never used for that purpose.
The retro rocket was fitted onto the top half of the spacecraft's adapter section. The motor produced a thrust of 2,580 pounds for about five seconds. The retro system was used in all ten manned Gemini missions from 1965-1966. This motor was donated to the Smithsonian in 1973 by the McDonnell-Douglas Corp.