After hurtling hundreds of millions of miles through space since last summer, three robotic explorers are ready to hit the brakes at Mars.

The stakes—and anxiety—are sky high.

The United Arab Emirates' orbiter reaches Mars on Tuesday, followed less than 24 hours later by China's orbiter-rover combo. NASA's rover, the cosmic caboose, will arrive on the scene a week later, on Feb. 18, to collect rocks for return to Earth—a key step in determining whether life ever existed at Mars.

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