Yuri Vasilievich Kondratyuk (real name Aleksandr Ignatyevich Shargei, Russian: Алекса́ндр Игна́тьевич Шарге́й, Ukrainian: Олександр Гнатович Шаргей) (21 June 1897 – February 1942) was a Ukrainian and Soviet engineer and mathematician. He was a pioneer of astronautics and spaceflight, a theoretician and a visionary who, in the early 20th century, developed the first known lunar orbit rendezvous (LOR), a key concept for landing and return spaceflight from Earth to the Moon. The LOR was later used for the plotting of the first actual human spaceflight to the Moon. Many other aspects of spaceflight and space exploration are covered in his works.