NASA's 10 Greatest Achievements

Explorer 1, the First U.S. Satellite
Launch of Jupiter-C/Explorer 1 at Cape Canaveral, Florida on Jan. 31, 1958. After the Russian Sputnik was launched in October 1957, the launching of an American satellite assumed much greater importance.  NASA

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 less than three months for the JPL to finish Explorer 1 [source: Dick].

The satellite rode into space onboard a rocket, and it carried equipment designed to help scientists study the cosmic rays in Earth's orbit. Explorer 1 measured 80 inches (203 centimeters) long and 6.25 inches (15.9 centimeters) in diameter, and weighed 30 pounds (14 kilograms). The satellite circled the planet 12 and a half times a day, its altitude fluctuating from 1,563 miles (2,515 kilometers) to 220 miles (354 kilometers) above Earth as it measured the cosmic radiation in its environment [source: Loff].

Our understanding of Earth's atmosphere was forever changed by this little object. Once Explorer 1 made it into space, it began collecting information on the cosmic rays there. Some of the readings transmitted from Explorer 1 showed cosmic ray activity that was significantly lower than scientists expected. Physicist 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 hitherto-unknown radiation belt that had oversaturated the on-board instruments with charged particles. [source: Moore & Arnold].

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 58,000 times.

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

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