Nasa's unmanned Mars Reconnaissance Orbiter, which has traveled around 300 million miles since leaving Earth in August, was due to enter its most delicate phase on Friday. It will try to ease into orbit around the red planet, which has defeated two-thirds of all man-made craft sent there.
   
Fuk Li, Mars programme manager at Nasa's Jet Propulsion Laboratory in Pasadena, referring to the dangerous manoeuvre, said: "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." 

If the spacecraft reaches its planned orbit, which will take another seven months, it could collect a remarkable amount of data during its two-year mission that could help Nasa determine where to land rovers and even scout out sites for a human landing site on Mars.
    
The vast distance of Mars from Earth and the wild unpredictability of its atmosphere have been fatal to previous missions. Nasa has only a 65% success rate in getting space probes to orbit the planet, as opposed to about 80% in landing spacecrafts on its surface.
   
Difficult manoeuvre

"We're very excited about the arrival of the Mars Reconnaissance Orbiter in a couple of days"

Fuk Li, Mars programme manager, Nasa

The hard part is getting the orbiter  - which is as tall as two-storey building and will be 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 swings its main thrusters forward and fires them for about 27 minutes. About 21 minutes into that  burn, flight engineers will lose contact with the orbiter while it passes behind Mars.
   
If all goes well, the craft will enter an elliptical orbit. It must then 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. When that is accomplished, scientific operations can begin.
   
Richard Zurek, a project scientist, said: "What we're really looking for is that sweet spot where we can go down with other instruments and look for evidence of life."

For further information, visit: http://mars.jpl.nasa.gov/mro/