After a nearly 5-billion-mile journey that took more than six years, a NASA probe is finally ready to make history tonight (March 17) and become the first spacecraft ever to enter into orbit around the planet Mercury.

At 8:45 p.m. EDT (0045 GMT Friday), NASA's Messenger spacecraft will fire its main thruster for about 15 minutes in what scientists call an orbital insertion burn. If all goes well, the maneuver should slow the probe down enough for it to be caught by Mercury's gravity and enter orbit.