MOSCOW — Russia on Thursday marked the 50th anniversary of the space flight of two dogs who became the first living creatures to circle the Earth and come back alive.

The Aug. 19, 1960 mission by mixed-breeds Belka and Strelka was a key step in preparations for the flight of Yuri Gagarin, who on April 12, 1961 became the first human in space.

“Their task was to test the spacecraft which carried Gagarin into space,” Russian cosmonaut Alexander Lazutkin said on Rossiya television.

Belka and Strelka followed Laika, the first dog to orbit Earth in a non-returnable capsule who died of overheating soon after her launch on Nov. 3, 1957.

By 1960, Soviet space engineers gained enough experience to design a returnable spacecraft capable of carrying a human into orbit, but they needed to run an extensive program of animal tests first. Only stray mutts were picked up for such flights — doctors believed they were able to adapt quicker to harsh conditions — and they were all very small so they could fit into the tiny capsules.

Many of the dogs died during tests, including Bars and Lisichka, the two launched just three weeks before Belka and Strelka, who were killed when their rocket exploded seconds after launch.

Boris Chertok, a top engineer in the Soviet space program at the time, said that despite that launch failure he had a feeling that the next dog flight will be a success.

“Belka and Strelka were so active and joyful that we didn’t doubt their successful return,” he wrote in his memoirs.

Belka (Squirrel) and Strelka (Little Arrow) were accompanied by mice, rats, flies and some plants and fungi. The spacecraft landed successfully a day after making 17 orbits.

“These dogs acted like real pros,” said Vladimir Tsvetov, an engineer who took part in the mission, said on Rossiya state television.

Soviet official reports claimed that the dogs felt well throughout the flight, but a participant in the program recalled later that it wasn’t completely trouble-free. Doctor Vladimir Yazdovsky, who prepared the experiment, said that Belka was very nervous during the flight.

“She was very restless, tossing about and trying to get rid of the belts fixing her and barking,” Yazdovsky wrote in his book chronicling the story of Soviet space medicine. However, post-flight medical checkups showed that both dogs were in fine condition without any adverse effects from the flight.

The successful mission showcased the Soviet lead in space exploration and turned the two photogenic dogs into global celebrities.

Strelka later had six puppies, one of which, Pushinka (Fluffy), was sent by Soviet leader Nikita Khrushchev to President John F. Kennedy’s daughter Caroline.

Earlier this year, the dogs’ story came to the screen in Russia’s first 3D computer-animated movie, “Belka and Strelka: Star Dogs.”