Past missions to the Red Planet have shown that water once flowed across its surface. For example, in 1999 the Mars Global Surveyor took pictures of gullies that indicated sources of liquid water at or near the planet surface, while in 2002 the Mars Odyssey Orbiter detected large amounts of hydrogen, implying vast water ice deposits in the upper three metres of Mars' soil.

Despite these successes, it remains a mystery whether water existed for long enough on Mars to support life. The MRO is designed to find out when water existed on the Martian surface and where that water is now. To do so, it will fly in a low-altitude orbit over the next three years and collect data using a powerful suite of instruments.

A telescopic camera - the most powerful ever flown on a planetary exploration mission and capable of resolving objects as small as a dinner table - will take extreme close-up shots, while a spectrometer will map and analyze minerals. A radar instrument will look for subsurface water and rock, and others will trace the distribution of dust and water in the atmosphere, as well as monitor the Martian weather.

The mission should be able to establish whether the underground ice discovered by the Mars Odyssey Orbiter is the top layer of a deep ice deposit or merely a shallow layer linked to the seasonal cycle of water vapour in the Martian atmosphere. As an added bonus, the MRO will also look for safe and scientifically important landing sites for future exploration.

Over the next six months, the MRO will transform its current elongated elliptical orbit into a smaller circular orbit suitable for making its scientific measurements. It will do this by slowing itself down through a series of carefully calculated dips into the Martian atmosphere. Meanwhile, astronomers around the world will be waiting impatiently.