Nasa says its six-tonne UARS satellite entered the earth's atmosphere over the Pacific Ocean early on Saturday. The spacecraft was expected to fall to Earth by about 0500 GMT - officials say it is not possible yet to give a precise time. The Upper Atmosphere Research Satellite (UARS) is the largest American space agency satellite to return uncontrolled into the atmosphere in about 30 years. Officials said the risk to public safety was remote. A statement on the Nasa UARS website read: "The Joint Space Operations Center at Vandenberg Air Force Base in California said the satellite penetrated the atmosphere over the Pacific Ocean. The precise re-entry time and location are not yet known with certainty." There have been some unconfirmed reports on Twitter that suggested debris might have fallen in western Canada. Most of the decommissioned spacecraft should simply have burnt up, but modelling work indicated perhaps 500kg could have survived to the surface. Any pieces of debris should have been scattered over a 800km path; but with more than 70% of the Earth's surface covered by water, many experts said the pieces were most likely to end up in the ocean. Stephen Cole, a Nasa spokesman in Washington DC, told BBC News: "You have to remember that they're very, very small pieces, even though the original satellite was large - as large as a bus. Most of that burns up in the atmosphere and just a few dozen pieces survive. They're highly damaged, and if they're in the ocean - they're gone." UARS was deployed in 1991 from the space shuttle Discovery on a mission to study the Earth's upper atmosphere. It contributed important new understanding on subjects such as the chemistry of the protective ozone layer and the cooling effect volcanoes can exert on the global climate. In the past few days, Nasa warned members of the public not to touch any pieces of the spacecraft that might survive the fall to land, urging them to contact local law enforcement authorities instead. "I've seen some things that have re-entered and they tend to have sharp edges, so there's a little concern that they might hurt themselves if they try to pick them up," said Mark Matney, an orbital debris scientist from Nasa's Johnson Space Center. Under the terms of the Outer Space Treaty of 1967, the US government retains ownership of the debris and could, if it so wished, seek to take possession of any items found on the ground. With those ownership rights also comes absolute liability if a piece of UARS is found to have damage property or injured someone. "There is something called international responsibility; they're internationally liable," explained Joanne Wheeler of law firm CMS Cameron McKenna, and an expert representative for the UK on the UN Subcommittee for the Peaceful Uses of Outer Space. "The Americans have to retain jurisdiction and control, and that pretty much can be interpreted as ownership. So they own it up there, they own it if it comes down to Earth and they're liable if it crashes into something." Tracking stations will typically witness the uncontrolled return of at least one piece of space debris every day; and on average, one intact defunct spacecraft or old rocket body will come back into the atmosphere every week. Something the size of UARS is seen perhaps once a year. Much larger objects such as space station cargo ships return from orbit several times a year, but they are equipped with thrusters capable of guiding their dive into a remote part of the Southern Ocean.