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Celebrating America’s First Spacecraft

January 31, 2008

Explorer 1 was the first satellite launched by the United States when it was sent into space on January 31, 1958. Following the launch of the Soviet Union’s Sputnik 1 on October 4, 1957, the U.S. Army Ballistic Missile Agency was directed to launch a satellite using its Jupiter C rocket developed under the direction of Dr. Wernher von Braun.

The Jet Propulsion Laboratory received the assignment to design, build and operate the artificial satellite that would serve as the rocket’s payload. JPL completed this job in less than three months.

The primary science instrument on Explorer 1 was a cosmic ray detector designed to measure the radiation environment in Earth orbit. Once in space this experiment, provided by Dr. James Van Allen of the State University of Iowa, revealed a much lower cosmic ray count than expected.

Van Allen theorized that the instrument may have been saturated by very strong radiation from a belt of charged particles trapped in space by Earth’s magnetic field. The existence of these radiation belts was confirmed by another U.S. satellite launched two months later, and they became known as the Van Allen Belts in honor of their discoverer.

Explorer 1 revolved around Earth in a looping orbit that took it as close as 354 kilometers (220 miles) to Earth and as far as 2,515 kilometers (1,563 miles). It made one orbit every 114.8 minutes, or a total of 12.54 orbits per day. The satellite itself was 203 centimeters (80 inches) long and 15.9 centimeters (6.25 inches) in diameter.

Explorer 1 made its final transmission on May 23, 1958. It entered Earth’s atmosphere and burned up on March 31, 1970, after more than 58,000 orbits. The satellite weighed 14 kilograms (30.8 pounds).

A launch attempt of a similar satellite, Explorer 2, was made on March 5, 1958, but the fourth stage of the Jupiter-C rocket failed to ignite. Explorer 3 was successfully launched on March 26, 1958, and operated until June 16 of that year. Explorer 4 was launched July 26, 1958, and operated until October 6 of that year. Launch of Explorer 5 on August 24, 1958, failed when the rocket’s booster collided with its second stage after separation, causing the firing angle of the upper stage to be incorrect.

The craft’s mission

Explorer 1 carried instrumentation for the study of cosmic rays, micrometeorites, and for monitoring of the satellite’s temperature.

The instrumentation package, developed by a team at the University of Iowa under the direction of professor James A. Van Allen, was mounted inside the rocket’s body.

A single Geiger-Mueller detector was used for the detection of cosmic rays.

Micrometeorite detection was accomplished using both a wire grid and an acoustic detector. It was to detect hits from tiny meteorites.

Data from the instruments were transmitted continuously, but acquisition was limited to those times when the spacecraft passed over appropriately equipped receiving stations.

Its place in history

Van Allen’s realization that charged solar particles are trapped by the Earth’s magnetic field in concentric rings around the planet was the first major scientific discovery of the Space Age. They are now called the Van Allen radiation belts.

The mission revolutionized scientific understanding of the Earth and the solar system and created an entirely new field of research, called magnetospheric physics.

Later missions in both the Explorer and Pioneer series were to expand on the knowledge and extent of the zones of radiation and were the foundation of modern magnetospheric studies.

The impetus

The launch of Sputnik 1 by the USSR in late 1957 had shocked the American people, introducing the typical citizen to the Space Age in a crisis setting.

Not only had the Soviet Union been first in orbit, but Sputnik 1 weighed nearly 200 pounds, compared to the intended 3.5 pounds for the first satellite to be launched in Project Vanguard. In the Cold War environment of the late 1950s, this disparity of capability carried menacing implications.

The event created an illusion of a technological gap and provided the impetus for increased spending for aerospace endeavors, technical and scientific educational programs. It also led to the creation of NASA in 1958.

What happened to previous U.S. attempts to launch a rocket into space?

On Dec. 6, 1957, the Navy’s Vanguard, the first U.S. attempt to launch an Earth-orbiting satellite, rose a few feet above the ground, but then fell back to Earth and burst into flames.

On March 17, 1958, the Navy successfully launched Vanguard 1. It remains in orbit today as the oldest human-made object in space.

What were some of the next milestones in space?

A launch attempt of a second satellite, Explorer 2, was made on March 5, 1958, but the fourth stage of the Jupiter-C rocket failed to ignite.

Explorer 3 was successfully launched March 26, 1958, and operated until June 16 of that year.

Explorer 4 was launched July 26, 1958, and operated until Oct. 6 of that year. Launch of Explorer 5 on Aug. 24, 1958, failed when the rocket’s booster collided with its second stage after separation, causing the firing angle of the upper stage to be incorrect.

Where c an I see Explorer 1 now?

Full-scale replicas of Explorer 1 are in the Smithsonian National Air and Space Museum collection in Washington.

News researchers Jakon Hays, Kimberly R. Kent and Maureen Watts contributed to this report.

Sources: The Washington Post, NASA

How big was the satellite?

Spacecraft dimensions (not including rocket)

Length: 80 inches

Width: 6.25 inches in diameter

Weight: 30.8 pounds How big was the rocket?

Height: 68.6 feet – a four-stage rocket.

Weight (in pounds) loaded:

Overall (takeoff) 64,000

Stage 162,700

Stage 21,020

Stage 3280

Stage 480 Is it still up there?

The signal ended when batteries died on May 23, 1958.

The satellite burned up during re-entry over the Pacific Ocean on March 31, 1970. How high was it?

Altitudes ranged from 224 to 1,575 miles. How often did it circle Earth?

Approximately once every 115 minutes. Where was it launched?

Explorer 1 was launched at 10:48 p.m. on Jan. 31, 1958, from Cape Canaveral, Fla.

On the Net:

NASA