Space capsules with animals on board preceded manned space flights, with the first experiments on mammals in spacecraft taking place as early as 1948, leading, in time, to the flight of the two most famous animal space passengers: Belka and Strelka.

The dogs were chosen from among many animals after much consideration; biologists had long used them for tests, and knew their physiology in detail. Additionally, dogs were the easiest animals to train.

The first dog space crews were formed of stray mongrels: tough yet grateful, they knew what the struggle for survival was all about, and were quick to make friends with people.

The dogs were tested and trained at the Research Institute of Aviation Medicine in a red-brick building of the abandoned former Mauritania hotel just behind Dynamo Stadium north of central Moscow. Small animals weighing 6-7 kilos (13-15 lbs) were selected were selected for the first missions, because spaceships could not carry heavy payloads.

The first eligible passengers, aged two to six, had exemplary health and immunity to diseases and harsh environments, and had benign and patient dispositions. Females were preferred because their hygienic suits were easier to make.

Potential publicity mattered no less than scientific expediency, and so healthy, light-colored dogs, with clever looking faces were selected so that they would look good when televised or photographed for cover stories.

The training for short rocket flights and longer satellite expeditions started with the space suits. The dogs got accustomed to the protective and hygienic suits. Then, they learned to eat from an automated feeding system that used a conveyor belt to deliver food boxes on a schedule. The most difficult part was training the dogs to get used to confinement for up to three weeks, which was done using isolated cubicles.

The dogs also had to exercise, use the centrifuge, and be trained for the pod ejection process. The training finished with comprehensive tests, during which the dogs stayed in a sealed capsule for many days and were exposed to simulated adversities they could encounter during a space flight.

The first dog crew was launched at the Kapustin Yar space center on July 22, 1951. All told, there were 29 flights with dogs to the stratosphere at a height of 100-150 km (60-90 miles) between July 1951 and September 1962. Eight of them ended tragically due to hull breaches, parachute failures or life-support system failures.

The first returnable space vehicle with a comprehensive life-support system was built early in 1960, but the first flight ended in a crash.

The second, triumphant launch was made at the Baikonur space center at 3:44 pm on August 19, 1960, to study the space ray effect on animals and test air, food and water supply and waste disposal systems. The satellite weighed 4,600 kilos (more than 10,000 lbs), not including the carrier rocket, and consisted of a tight landing section and equipment bay.

Compressed gas containers for trajectory adjustment, jet engines, gauges, aerials, temperature regulators and solar batteries that turned toward the Sun automatically were all attached to the outside.

The two canine passengers - Belka and Strelka (whose names meant "Squirrel" and "Arrow") - wore their own space suits, one red and the other green. There were a dozen caged mice, insects, plants, fungi, microbe cultures, corn, wheat grains, peas and onions with them in the ejection pod, whose instruments recorded their physical state throughout the flight. More animals - 28 mice and two rats - were traveling in the landing section outside the capsule.

The equipment bay was doomed to burn in the dense atmosphere during reentry, while the ejection pod and landing section had separate parachutes to reduce the speed to 6-8 and 10 meters/second (20-26 and 33 feet/second), respectively.

The landing section had heat-resistant windows and tight rapid-opening hatches. The pod was ejected through a hatch at a height of about 7-8 km (4-5 miles) above the ground, as triggered by the barometer gauges.

The landing section returned to the appointed spot on August 20, 1960. All the animals were safe and sound. The world's first cosmonauts spent 25 hours in space, circling the Earth 17 times and bringing home valuable information on the impact of space flight on animal physiology, genes and cells.

The dogs became big stars. They faced a press conference the day after landing, and appeared on television a few days later. Footage of their somersaults in weightlessness was also shown - Strelka rigid in apprehension, and Belka rolling and tossing with joyful barks.

The dogs became household names, and sent children into ecstasy as they toured schools and kindergartens. Belka and Strelka did not make any further space flights, but lived at the Space Research Institute in honored retirement to an advanced age.

Pushok, one of Strelka's many puppies, was Jackie Kennedy's pet.