The SUV-sized pallet of old batteries is the most massive object the station has ever jettisoned. The International Space Station got a little lighter last week. The orbiting lab discarded a 2.9-ton (2.6 metric tons) pallet of used batteries on Thursday morning (March 11) — the most massive object it has ever jettisoned, NASA spokesperson Leah Cheshier told Gizmodo . The space junk is expected to fall back to Earth in two to four years, agency officials wrote in an update last week. That update also stated that the pallet will burn up "harmlessly in the atmosphere," but not everyone is convinced that's the case. The pallet is safely moving away from the station and will orbit Earth for two to four years before burning up harmlessly in the atmosphere. The batteries were removed during previous spacewalks and replaced with newer lithium-ion batteries to continue powering the station’s systems. Gizmodo reported the batteries weighed 2.9-tons, the most massive object it has ever jettisoned. Space debris (also known as space junk, space pollution, space waste, space trash, or space garbage) is a term for defunct human-made objects in space—principally in Earth orbit—which no longer serve a useful function. These include derelict spacecraft—nonfunctional spacecraft and abandoned launch vehicle stages—mission-related debris, and particularly numerous in Earth orbit, fragmentation debris from the breakup of derelict rocket bodies and spacecraft. In addition to derelict human-built objects left in orbit, other examples of space debris include fragments from their disintegration, erosion and collisions, or even paint flecks, solidified liquids expelled from spacecraft, and unburned particles from solid rocket motors. Space debris represents a risk to spacecraft.