Answer: Fruit Flies
The use of animals in aeronautical tests has a long and colorful history. The first recorded instance of anyone using animals to test the effects of altitude on organisms was in 1783; the Montgolfier Brothers, inventors responsible for the first human-manned hot air balloon flight, sent up a hot air balloon with a payload of one rooster, sheep, and duck.
The logic behind their choices was that a sheep was believed to be reasonably close in physiology to a human, the duck could serve as a control to determine any effect the balloon itself had (as the duck was used to flying at high altitudes), and the rooster as a secondary control because it was capable of flight but not at such elevations. The flight was a success and both the balloon and animals enjoyed an eight-minute flight across the French countryside.
Some two and a half centuries later in the 1940s, both the US and Germany began conducting experiments with high-altitude weather balloons, sending everything from mice to dogs to monkeys up to heights of 144,000 feet. It wasn’t until 1947, however, that any terrestrial creatures were actually launched into low earth orbit. The first creatures to visit space were a colony of fruit flies launched aboard a US V2 rocket on February 20, 1947. The rocket reached an altitude of 68 miles before ejecting the payload; the payload was successfully recovered.
From that point forward a variety of animals were launched into space for stays anywhere from a few minutes to long enough to orbit the globe. Monkeys, dogs, mice, even bullfrogs, and tortoises were all launched into space. The first successful deep-space journey, in fact, was completed by a tortoise. A Horsfields Tortoise was launched into space by the Soviet Union in 1968; the ship it was aboard successfully circled the moon and returned to Earth after a week-long journey.
Although the majority of space programs have moved away from using larger mammals, insects, fish, and small mammals are still routinely taken into space to study the effects of microgravity, radiation, and other effects of time spent in space.
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