The flag of Mars is a tricolour used to represent the planet Mars, its exploration, and its eventual transformation into a more Earth-like world. It is not official in a legal or any other sense, as the Outer Space Treaty forbids the "national appropriation" of celestial bodies by states.
The design of the flag was originally conceived by NASA planetary scientist Pascal Lee during the 1998 Haughton-Mars Project arctic expedition to Devon Island, Canada. In addition to symbolizing liberty as a tricolour flag, the design evokes a vision of the "future history" of Mars in which the planet is transformed from red, to green, and then blue like the Earth. The red bar, which lies closest to the mast, symbolizes Mars as it is today. The green and blue symbolize stages in the possible terraforming of Mars, should humanity ever have the ability and will to undertake such a task; the ethics of terraforming remaining a matter of debate. Kim Stanley Robinson's popular science-fiction trilogy Red Mars, Green Mars, and Blue Mars also provided inspiration for the flag. Beyond any human-induced terraforming, the color sequence also depicts the environmental change Mars might experience on astronomical timescales as the Sun evolves into a red giant and Mars becomes warmer. In the Pantone Color Matching System (PCMS), the red is the "Red Clay" (18-1454), the green is the "Mint Green" (17-6333), and the blue is the "Imperial Blue" (19-4245).
The flag was first displayed and flown at the Haughton-Mars Project Research Station (HMPRS) in summer 1998, and then on the Mars Society's Flashline Mars Arctic Research Station (FMARS) in summer 2000. The flag is widely used by the Mars Society and The Planetary Society. It is displayed in several locations on the Mars Society's Mars Desert Research Station campus in Utah. A version of the Flag of Mars sewn by Maggie Zubrin specifically to be flown in space, was carried aboard the space shuttle Discovery by astronaut John M. Grunsfeld on STS-103 in 1999. The flag is also used as a sew on patch on concept spacesuits used in Mars mission simulations.