Washington, Mar 3 (IANS): NASA said its MAVEN spacecraft performed a previously unscheduled manoeuvre this week to avoid a collision next week with Mars' moon Phobos.

This is the first collision avoidance manoeuvre that the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft has performed at Mars to steer clear of Phobos, NASA said on Thursday.

The MAVEN spacecraft has been orbiting Mars for just over two years, studying the Red Planet's upper atmosphere, ionosphere and interactions with the sun and solar wind.

On Tuesday the spacecraft carried out a rocket motor burn that boosted its velocity by 0.4 metres per second (less than one mile per hour).

Although a small correction, it was enough that MAVEN would miss the lumpy, crater-filled moon by about 2.5 minutes, the US space agency added.

The orbits of both MAVEN and Phobos are known well enough that this timing difference ensures that they will not collide.

MAVEN, with an elliptical orbit around Mars, has an orbit that crosses those of other spacecraft and the moon Phobos many times over the course of a year.

When the orbits cross, the objects have the possibility of colliding if they arrive at that intersection at the same time.

Scientists at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, sounded the alert regarding the possibility of MAVEN's collision with Phobos on March 6.

Given Phobos' size (modelled for simplicity as a 30-kilometre sphere), they had a high probability of colliding if no action were taken.

"Kudos to the JPL navigation and tracking teams for watching out for possible collisions every day of the year, and to the MAVEN spacecraft team for carrying out the manoeuvre flawlessly," said MAVEN Principal Investigator Bruce Jakosky of the University of Colorado in Boulder.