NASA's 10 Greatest Achievements

Immediately ­after the news of S­putnik's success, the Jet Propulsion Laboratory (JPL), soon to be the NASA Jet Propulsion Laboratory, began designing the satellite that would follow Sputnik into space. It took three months for the JPL to finish Explorer 1.

The satellite rode into space onboard a rocket, and its mission was to study the cosmic rays in Earth's orbit. Explorer 1 measured 80 inches (203 cm) long and 6.25 inches (15.9 cm) in diameter, and weighed 30.8 pounds (14 kg). The satellite circled the planet 12 and a half times a day, its altitude fluctuating from 1,563 miles (2,515 km) to 220 miles (354 km) above Earth as it measured the cosmic radiation in its environment [source: NASA].

But something happened even before it entered orbit, revealing a discovery that would change our understanding of Earth's atmosphere. Once Explorer 1 made it into space, its instruments began collecting information on the cosmic rays there. The first data scientists interpreted from Explorer 1 showed cosmic-ray activity that was significantly lower than they expected. One of those scientists, Dr. James Van Allen, hypothesized that the cause of the anomaly was essentially an interference with the satellite's cosmic-ray detector. He believed Explorer 1 had passed through a radiation belt that had bombarded the detector with X-rays to the point that it was too full to collect much more when it was in orbit.

Another satellite, sent into orbit two months later, delivered data that backed up Van Allen's theory, and the Van Allen radiation belts surrounding Earth entered the science books. Explorer 1 dipped into Earth's atmosphere and burned up in March 1970, after orbiting Earth 56,000 times.

A satellite launched 20 years later revealed an understanding of our world that went far beyond science books. The payload on this satellite would deliver high-resolution photos of space into our homes.