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LOS ANGELES — The 2001 Mars Odyssey was nearing the Red Planet on Monday, poised to fire its main engine for the first and only time to slow the robotic spacecraft and allow it to settle into orbit after a six-month trip from Earth. 

If the satellite is captured into orbit Tuesday night, it will mark NASA’s first successful mission to Mars since the loss of two spacecraft, Climate Orbiter and Polar Lander, in 1999. 

During the maneuver, scheduled to begin at 7:26 p.m., Odyssey’s engine will burn through 579 pounds of propellant in just under 20 minutes. The burn should leave the satellite orbiting Mars every 20 hours or so on an elliptical path. 

Entering orbit could be the riskiest move the unmanned probe will make during a $297 million mission to map the makeup of the Martian surface: two of the last three orbiters the National Aeronautics and Space Administration sent to Mars failed, both just before or upon arrival. 

Controllers plan to direct Odyssey to dip into the fringes of Mars’ atmosphere in a technique called aerobraking to gradually lower and circularize the orbit. NASA’s Mars Global Surveyor, already at work over the Red Planet, used the same process to reach the altitude from which it has made highly detailed images of the surface since arriving in 1997. 

One of the Odyssey probe’s three instruments was designed to image Mars in the infrared to probe the distribution of minerals on the planet’s surface. Another is intended to measure gamma rays coming from the surface to pinpoint specific elements, including hydrogen, most likely in the form of buried deposits of water ice. 

Wielding those tools as would a prospector, Odyssey will assay Mars, eventually building up what will be the first inventory of the planet’s global makeup. 

A third instrument — which suffered glitches after launch — was designed to assess the radiation risks that future human missions to Mars may encounter.