The first satellites built by people were very small. The Soviet Union's Sputnik, which opened the Space Age in 1957, weighed only 84 kilograms (185 pounds). The American response, Explorer-1, weighed 14 kilograms (31 pounds). These early small satellites proved that it was possible to put equipment in orbit and use it, providing the first opportunities for scientific observation outside Earth's atmosphere. Explorer provided the data that led to the identification of the Van Allen radiation belts that surround Earth. Bell Labs's Telstar, about the size of a car tire, provided the first transatlantic television link, and Pioneer 10, weighing about 270 kilograms (595 pounds), was launched in 1972, and is the first satellite to leave the solar system. Thirty years after its launch, Pioneer is still functioning and was more than 7 billion miles from Earth.

Low launching costs and rapid development makes small satellites suitable for new applications that are not possible...