The US is dispatching a six-wheeled rover the size of a car, called Perseverance, to collect rock samples that will be brought back to Earth for analysis in about a decade. This summer’s third and final mission to Mars, after the United Arab Emirates’ Hope orbiter and China’s Quest for Heavenly Truth orbiter-rover combo, begins with a launch scheduled for Thursday morning from Cape Canaveral in Florida. Each spacecraft will travel more than 300 million miles before reaching the red planet next February. It takes six to seven months, at the minimum, for a spacecraft to loop out beyond Earth’s orbit and sync up with Mars’ more distant orbit around the sun. Scientists want to know what Mars was like billions of years ago when it had rivers, lakes and oceans that may have allowed simple, tiny organisms to flourish before the planet morphed into the barren, wintry desert world it is today. The three nearly simultaneous launches are no coincidence. The timing is dictated by the opening of a one-month window in which Mars and Earth are in ideal alignment on the same side of the sun, which minimizes travel time and fuel use. Such a window opens only once every 26 months. Mars has long exerted a powerful hold on the imagination, but has proved to be the graveyard for numerous missions. Only the U.S. has successfully put a spacecraft on Mars, doing so eight times, beginning with the twin Vikings in 1976. Two NASA landers are now operating there, InSight and Curiosity. Six other spacecraft are exploring the planet from orbit: three American, two European and one from India. The United Arab Emirates and China are looking to join the elite club. Perseverance’s mission is seen by NASA as a comparatively low-risk way of testing out some of the technology that will be needed to send humans to the red planet and bring them home safely.