1969-06-21 USA Explorer 41

1967-07-19 USA Explorer 35

One spacecraft not so closely tied to the Apollo program was Explorer 35, launched on 10 July 1967, and sent 12 days later into a highly elliptical orbit around the moon. Explorer 35 was also known as a lunar-anchored interplanetary monitoring platform, or IMP. It found the moon had been volcanically active. No evidence of trapped radiation belts around the moon that is similar to the Van Allen belt surrounding the earth.

1966-07-01 USA Explorer 33 (IMP)

1965-05-29 USA Explorer 28

1962-10-03 USA Explorer 14

A Sarzin Explorer 14 launch cover, postmarked at Cape Canaveral on October 3, 1962.

Explorer 14, launched on October 3, 1962, aimed to study solar wind and radiation phenomena. Its instruments included a cosmic ray detector, an ion electron detector, a solar cell measurer, a proton analyzer, and a trapped particle radiation experiment. The spacecraft also featured an experiment to measure how radiation affected solar power cells, and its own solar cells were protected by a 60mm glass cover.

(Reference from Discovering the Cosmos with Small Spacecraft: The American Explorer Program By Brian Harvey)

1961-03-25 USA Explorer 10

Explorer 10 (also known as Explorer X or P14) was an American Earth-orbital satellite that studied Earth's magnetic fields and surrounding plasma. Launched on March 25, 1961, it was an early mission in the Explorer program and the first satellite to detect the "shock wave" created by a solar flare. Its primary goal was to examine magnetic fields and plasma as the spacecraft traversed Earth's magnetosphere and entered cislunar space.

Explorer 10 "SpaceCraft" illustrated cachet cover signed by Dr. James Van Allen, an American space scientist at the University of Iowa, played a key role in establishing the field of magnetospheric research in space.

Due to the limited lifespan of its batteries, the spacecraft was able to transmit useful data in real time for just 52 hours during the ascent of its first orbit, after which all transmissions ceased several hours later.

(Reference from Explorer 10)

1959-08-07 USA Explorer 6

Explorer 6 "Goldcraft" launch cover postmarked at Port Canaveral on August 7, 1959.

Explorer 6 was a NASA satellite launched on August 7, 1959, at 14:24:20 GMT. This small, spherical satellite was designed to investigate trapped radiation of different energies, galactic cosmic rays, geomagnetism, radio propagation in the upper atmosphere, and the flux of micrometeorites. Additionally, it tested a scanning device intended for capturing images of the Earth's cloud cover. On August 14, 1959, Explorer 6 captured the first photographs of Earth from a satellite.

Explorer 6 launch cover (unknown cachet maker) with Patrick Air Force Base postmark.

(Reference from Explorer 6)

1958-07-26 USA Explorer 4

Explorer 4 "Goldcraft" launch cover postmarked at Patrick Air Force Base (PAFB) on July 26, 1958.

Explorer 4 was launched on July 26, 1958. The Department of Defense's Advanced Research Projects Agency (ARPA) had originally intended to launch two satellites to study the Van Allen radiation belts and the impact of nuclear explosions on these belts, as well as the Earth's magnetosphere more broadly. However, Explorer 4 was the only satellite successfully launched, as the other satellite, Explorer 5, experienced a launch failure.

Explorer 4 "Sarzin" launch cover postmarked at Port Canaveral on July 26, 1958.

Explorer 4 was a cylindrical satellite equipped to conduct the first detailed measurements of charged particles (protons and electrons) trapped in the Earth's radiation belts. This experiment aimed to build upon the initial measurements of the trapped radiation belts identified by Explorer 1 and Explorer 3, as well as to record data on artificially injected electrons resulting from three high-altitude Argus nuclear detonations.

The mission was kept secret from the public for six months. The satellite's telemetry was analyzed in relation to three Operation Argus nuclear weapons tests conducted at high altitudes. Explorer 4 was operational and in orbit during the three Project Argus launches, which took place between August 27 and September 6, 1958. One of the mission's objectives was to observe the effects of these high-altitude atomic bomb detonations on the surrounding space environment.

(Reference from Explorer 4)

1958-03-26 USA Explorer 3

Explorer 3 launch cover from "H. Flick," postmarked at Patrick Air Force Base on March 26, 1958.

Explorer 3, a U.S. satellite launched on March 26, 1958, was the second successful mission in the Explorer program, nearly identical to Explorer 1 in both design and objectives. Launched by the U.S. Army (Ordnance) as part of the International Geophysical Year (IGY), the satellite entered an eccentric orbit. Its mission was to continue experiments initiated by Explorer 1, carrying instruments including a cosmic ray counter (Geiger-Müller tube) and a micrometeorite detector (wire grid array and acoustic detector). The Explorer missions' discovery of the Van Allen radiation belt was one of the most significant achievements of the IGY.

Photo: An illustration depicting the orbital trajectory of Explorer 3 as it circled the Earth.

Explorer 3 "Goldcraft" launch cover, postmarked at Patrick Air Force Base on March 26, 1958.

Photo: An illustration depicting the stages of the Explorer 3 launch.

Explorer 3 "Sarzin" launch cover, postmarked at Port Canaveral on March 26, 1958.

(Reference from Explorer 3)

1958-01-31 USA Explorer 1

Explorer 1launch cover from "H. Flick" postmarked at Patrick Air Force Base on January 31, 1958.

The United States joined the Space Race with the launch of Explorer 1 on January 31, 1958, as part of its program for the International Geophysical Year and in reaction to the Soviet Union's launch of Sputnik 1.

Photo: Dr. William H. Pickering, Director of the Jet Propulsion Laboratory from 1954 to 1976. Under his guidance, Pickering's team launched Explorer I aboard a Jupiter-C rocket from Cape Canaveral on January 31, 1958, less than four months after the Soviet Union's launch of Sputnik. During his tenure, JPL achieved numerous successes in space missions, including Explorer, Pioneer, Mariner, Rangers 7-9, Surveyor, and Viking.

Explorer 1, recognized as the first U.S. satellite, is famous for its discovery of the Van Allen radiation belts, which are radiation zones surrounding Earth. The presence of these belts was confirmed through the Explorer 1 and Explorer 3 missions in early 1958, led by Dr. James Van Allen at the University of Iowa. Explorer 1 carried a micrometeorite detector and a cosmic ray experiment designed by Van Allen and his graduate students, with the deployment of the sensor package supervised by Ernst Stuhlinger, who also had expertise in cosmic rays. The data collected from Explorer 1 and Explorer 3 (launched on March 26, 1958) enabled the Iowa team to make the first space-age scientific discovery: the identification of a doughnut-shaped region of charged particle radiation trapped by Earth's magnetic field.

Photo: Professor Ernest Ray (left) and James Van Allen (right) analyze the orbit of Explorer 1.

Explorer 1 "Goldcraft" launch cover postmarked at Patrick Air Force Base on January 31, 1958.

Photo: Dr. Wernher Von Braun addresses a news conference, explaining how the Jupiter-C missile successfully launched Explorer 1 into orbit. The Jupiter-C was developed by the U.S. Army Ballistic Missile Agency (ABMA) under von Braun's leadership.

Photo: An image showing the Jupiter-C missile on the launch pad.

Explorer 1 "Sarzin" launch cover postmarked at Port Canaveral on January 31, 1958.

Photo: An illustration showing the various stages of the flight of Explorer 1.

(Reference from Explorer 1)