Which satellite's launch is marked as the beginning of the space age?

The launch of Sputnik I is recognized as the event that marked the beginning of the space age because it was the first artificial satellite to be sent into orbit around the Earth. Launched by the Soviet Union on October 4, 1957, Sputnik I sparked significant interest and competition in space exploration, leading to the space race between the United States and the Soviet Union. Its successful launch not only demonstrated the capabilities of space technology but also raised global awareness of the potential for space exploration and satellite communication.

The other options represent significant milestones in their own right but do not mark the initial entry into space exploration. Explorer I, for example, was the first satellite launched by the United States in 1958 and provided important scientific data about cosmic rays. Galileo was a space probe launched in 1989 to study Jupiter and its moons, while Voyager I, launched in 1977, is known for its exploration of the outer solar system and interstellar space. Each of these missions contributed greatly to our understanding of space but came after the significant moment that Sputnik I represented in initiating the era of space flight.