a) Explorer 1

Explorer 1, launched in 1958, marked a significant milestone in the U.S. space program as the first American-made satellite to reach space. This achievement came amidst the Cold War and the Space Race between the United States and the Soviet Union. Explorer 1’s successful launch was a response to the earlier launch of Sputnik by the Soviet Union, demonstrating America’s capabilities in space exploration. The satellite carried scientific instruments, including a cosmic ray detector, which led to the discovery of the Van Allen radiation belt. Explorer 1’s mission not only signaled the United States’ entry into the space age but also laid the groundwork for future space exploration and scientific discovery.