无人飞船 (wúrén fēichuán): This term refers to unmanned spacecraft that are sent into space without any crew on board.
失重 (shīzhòng): This term describes the condition of weightlessness experienced by objects or individuals when they are in freefall or orbiting around a celestial body.
载人航天 (zài rén hángtiān): This term refers to manned spaceflight, where astronauts travel into space aboard a spacecraft.