The failed Russian Mars probe, Phobos-Grunt, continued its descent to Earth on Saturday, losing hundreds of metres in height every hour. By 23:00 Saturday (GMT), the 13-tonne spacecraft was circling the planet at a mean altitude of 147km. It is likely to impact the thicker parts of the atmosphere and burn up sometime late on Sunday, according to orbital tracking experts. The Russian space agency says little of the probe will survive to the surface. It calculates no more than 200kg in maybe 20-30 fragments. Precisely where on the Earth\'s surface - and when - this material could impact is impossible to say, however. There are huge uncertainties in forecasting the final moments of a re-entry. \"The major uncertainty for prediction is the atmospheric density the spacecraft encounters in orbit, but it\'s also due to the orientation of the vehicle as it comes in,\" explained Prof Richard Crowther, the UK Space Agency\'s chief engineer. \"It can very quickly tumble and if pieces break off - that all changes the trajectory and where debris might impact.\" This is the third high-profile spacecraft re-entry in four months, following the return in September of the American UARS satellite and the German Rosat telescope in October. Both fell over the ocean. With so much of the Earth\'s surface covered by water, there is every chance Phobos-Grunt will do the same. (More information on the spacecraft\'s orbit can be found here)