A reconnaissance satellite aka spy satellite used for military or intelligence applications. One of such large U.S. spy satellite has lost power and could hit the Earth in late February or March. The worst part is US government not sure where it will hit on earth. From the yahoo news source:
The satellite, which no longer can be controlled, could contain hazardous materials, and it is unknown where on the planet it might come down, hey said. The officials spoke on condition of anonymity because the information is classified as secret. Appropriate government agencies are monitoring the situation," said Gordon Johndroe, a spokesman for the National Security Council. "Numerous satellites over the years have come out of orbit and fallen harmlessly. We are looking at potential options to mitigate any possible damage this satellite may cause
According to wikipedia:
The KH-11, also referenced by the codenames 1010, Crystal and Kennan, also commonly known as "Key Hole", was a type of reconnaissance satellite launched by the American National Reconnaissance Office between December 1976 and 1990 and used until present. It is believed to resemble the Hubble Space Telescope in size and shape, as the satellites were shipped in similar containers. Using a powerful 2.3-meter mirror, the theoretical ground resolution with no atmospheric degradation and 50% MTF would be roughly 0.15 meter (6 inches). Operational resolution would be worse due to effects of the atmosphere. Different versions of the KH-11 vary in mass from 13,000 to 13,500 kilograms. Its length is believed to be 19.5 meters, and diameter is 3 meters or less.