Explorer 1

By Gregory P. Kennedy

The United States Army launched the first American satellite, Explorer-1, on January 31, 1958. A four-stage Jupiter C rocket propelled the satellite into orbit. Explorer 1, which was 80 inches long, 6 inches in diameter, and weighed 30.7 pounds with the attached fourth stage of the booster. It carried instruments to measure radiation in space. Explorer was part of the American response to the successful launches of two Sputnik satellites by the Soviet Union. On October 4, 1957, Sputnik 1 became the first manmade object to orbit the earth. The Soviets followed less than a month later with Sputnik 2, which carried the dog Laika.

Wernher von Braun led the team that created the Jupiter C, which he based on the Army Redstone ballistic missile. During World War II, von Braun was technical director for the German V-2 missile program. When the war ended, von Braun and his senior personnel surrendered to the United States. They were taken to Fort Bliss, Texas, where they helped the US Army and the General Electric Company launch V-2s from the White Sands Proving Ground. As the V-2 program wound down in 1950, von Braun and his team moved to Redstone Arsenal. Located in northern Alabama, Redstone Arsenal had been a manufacturing center for chemical munitions during World War II. Late in 1949, the Army decided to centralize its missile programs at the arsenal.

In July 1950, less than a month after Communist North Korea invaded the Republic of Korea, the Army Chief of Ordnance asked the Ordnance Guided Missile Center at Redstone Arsenal to design a missile with a 500-mile range. The Army subsequently redefined its needs, and called for a more modest 200-mile range missile that could be launched by mobile field crews. Von Braun named the missile after its development center -- Redstone.

Following its first launch on August 20, 1953, Redstone established a reputation for reliability, as 35 of the first 38 flights were totally successful. The Rocketdyne A-6 engine that propelled the missile burned liquid oxygen and alcohol to produce a thrust of 75,000 pounds. Like the V-2, Redstone had air vanes on the tips of the fins and jet vanes in the engine exhaust for steering. Significant advances over its predecessor included an inertial guidance system and detachable warhead.

In 1954, Frederick C. Durant III, a Director for the American Rocket Society and President of the International Astronautical Federation, helped arrange a meeting between von Braun and Commander George Hoover of the Office of Naval Research to discuss how to launch a satellite. Other participants included Alexander Satin, David Young, Dr. Fred L. Whipple, and Dr. S. Fred Singer. The group agreed that a Redstone topped with a cluster of Loki solid-fuel rockets developed by the Jet Propulsion Laboratory (JPL) could place a scientific payload in a 200-mile high orbit. This became known as Project Orbiter. At the time, the Navy also sponsored a satellite proposal, Project Vanguard.

In 1955, Orbiter and Vanguard were both presented to the White House for consideration for the upcoming International Geophysical Year. President Eisenhower did not wish to have the first American satellite launched by a military missile and opted for Vanguard. One factor in his decision was his desire to establish the concept of freedom of space under international law, which would allow satellites to overfly other nations without diplomatic protests. Such a policy would permit the United States to freely launch reconnaissance satellites to observe the Soviet Union. Eisenhower felt a scientific satellite launched by a non-missile booster would be less provocative in establishing a precedent for the freedom of space. The launch of Sputnik-1, however, established the precedent that Eisenhower wanted.

By the mid-1950s, von Braun and the group at Redstone Arsenal were busy working on the Jupiter intermediate range ballistic missile. Because Jupiter would fly higher and farther than Redstone, the missile warhead needed protection from reentry heating. Rather than go to the expense of using full scale Jupiter rockets for nose cone reentry trials, von Braun suggested a more economical test vehicle. He proposed using a Redstone topped with solid rocket motors to push a model of the Jupiter nose through the atmosphere. He named the test bed Jupiter-C.

The Jupiter-C had three stages; an elongated Redstone topped by two concentric groups of scaled down Sergeant (another Army tactical missile) solid fuel rocket motors. The second stage comprised a ring of 11 motors; three motors nested inside the ring made up the third stage. Recognizing how close the three-stage Jupiter-C could come to orbital speed, President Eisenhower ordered Major General John Medaris, commander of the Army Ballistic Missile Agency, to travel to Cape Canaveral on September 20, 1956, and personally verify there would be no "accidental" satellites launched. Jupiter-C #27 had only three stages and lobbed its nosecone 3,100 miles down range.

Meanwhile, Vanguard experienced a frustrating series of technical delays and problems that were not atypical for any new program. Then came Sputnik-1, followed by Sputnik-2. On November 8, 1957, the Secretary of Defense gave von Braun the order to launch a satellite. Von Braun put his team at the Redstone Arsenal in high gear. At first he said he could launch a satellite in 60 days, but superiors gave him 90 days to provide additional time to make sure the flight succeeded.

Vanguard was ready for its first orbital attempt on December 6, 1957. A few seconds after ignition, the first stage engine faltered and the entire vehicle toppled over in a fierce conflagration. The six-inch diameter test satellite, scorched and battered, lay on the ground near the pad. Newspaper reporters referred to the failed attempt as "Flopnik." The prestige of the United States rested on von Braun and the U. S. Army.

Just 84 days after receiving the go-ahead, von Braun had an orbital vehicle ready. The only difference between it and the last Jupiter-C flown was the addition of a single scaled-down Sergeant as a fourth stage topped with an 11-pound instrument package. Dr. James Van Allen from the State University of Iowa designed and built the instrument package which included Geiger counters to measure radiation in space, internal and exterior temperature sensors, a micrometeorite impact microphone and micrometeorite erosion gauges. A pair of transmitters broadcast data from the instruments. Nickel-cadmium batteries powered the transmitters. The instrument package, which weighed 18.35 pounds, and fourth stage were assembled at JPL. The instrument package and attached fourth stage motor were called Explorer-1. This was the 29th Jupiter-C, which had been the backup for vehicle #27. At 10:48 PM, EST, January 31, 1958, Explorer-1 blasted off from Cape Canaveral, Florida. For the next hour, everyone waited until tracking stations in California reported hearing Explorer-1. It was confirmed -- the United States had successfully orbited a satellite!

The Jupiter-C placed Explorer-1 into an orbit with an apogee of 1,575 miles and a perigee of 224 miles. As Explorer orbited the earth once every 115 minutes, scientists on the ground noticed the Geiger counters in the instrument package stopped working periodically then came back on again. It wasn't until the launch of Explorer-3 that they understood what was happening. In addition to the Geiger counters, Explorer-3 carried a data recorder. When the information stored on the recorder was broadcast to the ground and studied, the scientists realized the Geiger counters stopped working because they became saturated as the satellite passed into a pair of radiation "belts" that encircle the Earth. These became known as the "Van Allen Radiation Belts."

The high power transmitter on Explorer-1 broadcast for 31 days; the batteries powering the low power transmitter lasted 105 days. Its transmitters silent, Explorer-1 continued to orbit until March 31, 1970, when its orbit decayed and it reentered the atmosphere and burned up.