WASHINGTON — Debris from a large Chinese rocket is expected to fall back to Earth in an uncontrolled re-entry this weekend. The main segment from the Long March-5b vehicle was used to launch the first module of China"s new space station last month. At 18 tons it is one of the largest items in decades to have an undirected dive into the atmosphere. The United States on Thursday said it was watching the path of the object but currently had no plans to shoot it down. "We"re hopeful that it will land in a place where it won"t harm anyone," US Defense Secretary Lloyd Austin said. "Hopefully in the ocean, or someplace like that." Various space debris modeling experts are pointing to late Saturday or early Sunday (GMT) as the likely moment of re-entry. However, such projections are always highly uncertain. Space debris has crashed into Earth on a number of occasions, including last year. The good news is that debris plunging toward Earth generally poses very little threat to personal safety. As Jonathan McDowell, an astrophysicist at the Astrophysics Center at Harvard University, told CNN: "This is not the end of days." Most pieces will burn up in the Earth"s atmosphere before having a chance to make an impact on the surface. But parts of larger objects, like rockets, can survive reentry and potentially reach populated areas. Last year, one of the largest pieces of uncontrolled space debris ever passed directly over Los Angeles and Central Park in New York City before landing in the Atlantic Ocean. Weighing in at nearly 20 tons, the debris — an empty core stage from a Chinese rocket — was the largest piece of space junk to fall uncontrolled back to Earth since 1991 and the fourth biggest ever. The only larger pieces were from NASA"s Skylab space station in 1979, Skylab"s rocket stage in 1975 and the Soviet Union"s Salyut 7 space station in 1991. The space shuttle Columbia from 2003 could be added to that list since NASA lost control of it on its descent back to Earth. This doesn"t happen more often because space agencies around the world have generally tried to avoid leaving big objects in orbit that have the potential to reenter Earth"s atmosphere and that they cannot control. "Norms have been established," McDowell said. "There"s no international law or rule — nothing specific — but the practice of countries around the world has been: "Yeah, for the bigger rockets, let"s not leave our trash in orbit in this way."" The Chinese rocket set to enter Earth"s atmosphere this weekend, however, is designed in a way that "leaves these big stages in low orbit," McDowell said. "It"s really not best practice compared to what other space agencies do. They go to quite fair lengths to avoid doing this." — Agencies