"We're very excited about the arrival of the Mars Reconnaissance Orbiter in a couple of days, but my heart rate is going up for a different reason," said Fuk Li, Mars program manager at NASA's Jet Propulsion Laboratory in Pasadena, referring to the dangerous maneuver.

If the spacecraft succeeds in reaching its planned orbit, which will take another seven months, it could collect a remarkable amount of data in its two-year mission that could help NASA determine where to land rovers and even make early plans for a human landing site on Mars.

"What we're really looking for is that sweet spot where we can go down with other instruments and look for evidence of life," said project scientist Richard Zurek.

The vast distance of Mars from Earth and the wild unpredictability of its atmosphere have been fatal to previous missions there. NASA has only a 65 percent success rate in getting space probes to orbit the planet, as opposed to some 80 percent success in landing spacecrafts on its surface.

The hard part is getting the robot orbiter - which is as tall as two-story building and cruising at about 11,000 miles per hour - to slow down enough to be captured by the planet's gravity.

That will happen at midday on Friday, when the orbiter slams on the brakes by swinging its main thrusters forward and firing them for about 27 minutes. Some 21 minutes into that thruster burn, flight engineers will lose contact with the orbiter while it passes behind Mars.

If the orbiter threads that needle and goes into an elliptical orbit, it must spend the next six months using the drag of the planet's atmosphere to reel itself in from an elongated 35-hour loop to a nearly circular two-hour orbit. Once that is accomplished, scientific operations can begin.