After 200 days of travel and more than 460 million kilometers (about 285 million miles) logged on its odometer, NASA’s 2001 Mars Odyssey spacecraft will fire its main engine for the first and only time October 23 and put itself into orbit around the red planet. To enter orbit, Odyssey’s propellant tanks, the size of big beachballs, must first be pressurized, plumbing lines heated, and the system primed before 262.8 kilograms (579.4 pounds) of propellant is burned in exactly the right direction for 19.7 minutes.