The dramatic, deadly impact is scheduled for 3:26 p.m. ET today, when the spacecraft runs out propellant. At that point, the force of the Sun's gravity will cause it to fall and crash into the surface of Mercury, the closest planet to the sun, NASA says.
Messenger, which has been orbiting Mercury since March 2011, will dive bomb the planet at 3.91 kilometres per second (14,000 kilometres an hour). It's expected to leave a crater 16 kilometres wide on the planet's surface.
The impact will take place on the side of Mercury facing away from Earth, giving the spacecraft some privacy during its last moments.
Messenger, launched in August 2004, is the first spacecraft ever to orbit Mercury. In the past four years, it has snapped more than 270,000 images of the planet and made measurements that have revealed many surprises including: