2001 Mars Odyssey is a robotic spacecraft orbiting the planet Mars. The project was developed by NASA, and contracted out to Lockheed Martin, with an expected cost for the entire mission of US$297 million. Its mission is to use spectrometers and a thermal imager to detect evidence of past or present water and ice, as well as study the planet's geology and radiation environment. The data Odyssey obtains is intended to help answer the question of whether life once existed on Mars and create a risk-assessment of the radiation that future astronauts on Mars might experience. It also acts as a relay for communications between the Curiosity rover, and previously the Mars Exploration Rovers and Phoenix lander, to Earth. The mission was named as a tribute to Arthur C. Clarke, evoking the name of his and Stanley Kubrick's 1968 film 2001: A Space Odyssey.

Mars is the fourth planet from the Sun. The surface of Mars is orange-red because it is covered in iron(III) oxide dust, giving it the nickname "the Red Planet". Mars is among the brightest objects in Earth's sky, and its high-contrast albedo features have made it a common subject for telescope viewing. It is classified as a terrestrial planet and is the second smallest of the Solar System's planets with a diameter of 6,779 km (4,212 mi). In terms of orbital motion, a Martian solar day (sol) is equal to 24.5 hours, and a Martian solar year is equal to 1.88 Earth years. Mars has two natural satellites that are small and irregular in shape: Phobos and Deimos.