Top officials at Russian space agency Roscosmos, cosmonauts and regular Russians on Friday came to pay their last respects to Boris Chertok, a rocket engineer and key architect of the Soviet space program, who died on December 14. He was 99.

State-controlled rocket manufacturer RKK Energiya, where Chertok worked as a top consultant, said he died in Moscow of pneumonia, just three months before his 100th birthday.

A brilliant engineer, Chertok was an integral member of the team responsible for the Soviet Union’s early successes in space. For many years he played a key role in engineering Soviet-era space programs and was closely involved in preparing the first human flight to space by Yuri Gagarin in 1961.

Chertok was at the center of the Cold War space race during the 1950s and 1960s, working as a deputy to Sergei Korolev, the legendary head of the Soviet space program.