NASA's Messenger spacecraft is poised to go into orbit around Mercury tonight, after a six-year, five billion-mile journey.
At 8.45pm EDT, the probe will its largest thruster - pointed very close to the direction of travel - for nearly 14 minutes. Other thrusters will fire for a minute more, slowing the spacecraft by 1,929 mph.
The process is so fuel-intensive that it will account for nearly a third of the propellant that the spacecraft carried at launch.
"This is a milestone event for our small, but highly experienced, operations team, marking the end of six and one half years of successfully shepherding the spacecraft through six planetary flybys, five major propulsive maneuvers, and sixteen trajectory-correction maneuvers, all while simultaneously preparing for orbit injection and primary mission operations," says Messenger systems engineer Eric Finnegan.
Tonight's manoeuver will leave Messenger in a 12-hour orbit around Mercury, taking it as close as 124 miles from the planet. It will provide the first new spacecraft data from Mercury since the Mariner 10 mission -over 30 years ago.
Messenger's seven sets of instruments will map its surface and establish whether there's any water ice at the poles. The probe will also examine Mercury's magnetic field and atmosphere. It will stay in orbit for a year, allowing it to orbit 730 times.
"The cruise phase of the Messenger mission has reached the end game," says principal investigator Sean Solomon. "Orbit insertion is the last hurdle to a new game level, operation of the first spacecraft in orbit about the solar system's innermost planet."