Nasa's spacecraft (Phoenix) is due to land in the far north end of Mars in less than 3 hours. Landing on Mars is very challenging and out of the 11 attempts since 1971 only 5 have succeeded.
Phoenix started its journey towards Mars on the 4th of August 2007 after a flawless countdown. The lander will reach the surface of Mars after 10 months of travelling with a distance of 423m miles from earth.
It will stay on Mars for the next 3 Months searching for signs of life by completing a series of sophisticated scientific analysis.
Phoenix is equipped with a robotic arm to dig through the top soil layer to the water ice (thought to be buried below the surface ) and to bring samples of soil and water to the lander platform of which scientific analysis will be carried out on later.
You can follow the mission overview on Nasa's website.