The collective, known as the Inter-Agency Space Debris Coordination Committee (IADC), are trying to forecast the most likely time and place for the laboratory's re-entry. And one hour still means almost one revolution around the Earth," said Holger Krag, the head of Esa's space debris office. Tiangong is certainly on the large size for uncontrolled re-entry objects but it is far from being the biggest, historically. The US space agency's Skylab was almost 80 tonnes in mass when it came back partially uncontrolled in 1979. Nasa's Columbia shuttle would also have to be classed as an uncontrolled re-entry.