1969-06-21 USA Explorer 41

An Explorer 41 launch cover from "Orbit Covers", postmarked at Vandenberg Air Force Base with a machine cancel on June 21, 1969.

1967-07-19 USA Explorer 35

An Explorer 35 launch cover from "SpaceCraft Swanson", postmarked at Cape Canaveral with a machine cancel on July 19, 1967.

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.

An Explorer 35 launch cover from "Orbit Covers", postmarked at Cape Canaveral with a machine cancel on July 19, 1967.

1966-07-01 USA Explorer 33 (IMP)

An Explorer 33 launch cover from "Orbit Covers", postmarked at Cape Canaveral with a hand cancel on July 1, 1966.

Explorer 33 (IMP-D) was a NASA scientific spacecraft launched in 1966, originally intended to become the first U.S. spacecraft to enter lunar orbit. Part of NASA’s Interplanetary Monitoring Platform (IMP) program, it was designed to study the space environment between Earth and the Moon, including the solar wind, cosmic rays, magnetic fields, and radiation—information important for understanding the conditions astronauts would encounter during future lunar missions. Although it failed to achieve lunar orbit, Explorer 33 successfully gathered valuable data on the Earth–Moon environment, contributing to NASA’s preparation for the Apollo missions.

1965-05-29 USA Explorer 28 (IMP)

An Explorer 28 launch cover from "SpaceCraft Swanson", postmarked at Cape Canaveral with a hand cancel on May 29, 1965, signed by Paul Butler, the Project Manager for Explorer 28.

Explorer 28 (IMP-C) was a NASA Interplanetary Monitoring Platform launched on May 29, 1965. It was particularly relevant to NASA's developing lunar program because understanding the radiation environment and solar activity in cislunar space was important for future human missions. NASA's stated objectives for the IMP-C mission specifically included studying the radiation environment of cislunar space and developing solar-flare prediction capabilities for Apollo. Explorer 28 operated successfully for almost two years, transmitting scientific data until May 1967, with the spacecraft eventually reentering Earth's atmosphere in July 1968.

1962-10-03 USA Explorer 14

An Explorer 14 launch cover from "Sarzin", postmarked at Cape Canaveral with a hand cancel on October 3, 1962.

Explorer 14, launched on October 3, 1962, was an early NASA scientific satellite that advanced understanding of Earth's radiation belts and space environment. 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.

Understanding the radiation environment around Earth was particularly important as NASA moved toward increasingly ambitious crewed missions. Its measurements helped scientists understand the radiation environment that spacecraft and astronauts would encounter beyond the lower atmosphere.

(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 P-14, was launched from Cape Canaveral aboard a Thor-Delta rocket to investigate Earth's magnetic field and the surrounding solar-wind plasma. Placed into a highly elliptical orbit extending more than 112,000 miles from Earth, the spacecraft traveled deep into the magnetosphere and provided the first direct observations of the boundary where Earth's magnetic field interacts with the solar wind.

An Explorer 10 launch cover, postmarked at Patrick Air Force Base with a machine cancel on March 25, 1961, signed by Dr. James Van Allen, the University of Iowa space scientist whose pioneering research established the existence of Earth's radiation belts.

Despite transmitting useful data for only about 52 hours, Explorer 10 revealed the complex structure of Earth's magnetosphere, including its elongated magnetotail on the side facing away from the Sun. The mission was an important early step in the development of magnetospheric physics and in understanding the space environment that future astronauts would encounter.

(Reference from Explorer 10)

1959-08-07 USA Explorer 6

An Explorer 6 launch cover from "Goldcraft", postmarked at Port Canaveral with a hand cancel 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.

An Explorer 6 launch cover postmarked at Patrick Air Force Base with a machine cancel on August 7, 1959.

(Reference from Explorer 6)

1958-07-26 USA Explorer 4

An Explorer 4 launch cover from "Goldcraft", postmarked at Patrick Air Force Base with a machine cancel 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.

An Explorer 4 launch cover from "Sarzin", postmarked at Port Canaveral with a hand cancel 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

An Explorer 3 launch cover from "H. Flick," postmarked at Patrick Air Force Base with a machine cancel 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.

An Explorer 3 launch cover from "Goldcraft", postmarked at Patrick Air Force Base with a machine cancel on March 26, 1958.

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

An Explorer 3 launch cover from "Sarzin", postmarked at Port Canaveral with a hand cancel on March 26, 1958.

(Reference from Explorer 3)

1958-01-31 USA Explorer 1

An Explorer 1 launch cover from "H. Flick", postmarked at Patrick Air Force Base with a machine cancel 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.

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.

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

An Explorer 1 launch cover from "Goldcraft", postmarked at Patrick Air Force Base with a machine cancel 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.

An Explorer 1 launch cover from "Sarzin", postmarked at Port Canaveral with a hand cancel on January 31, 1958.

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

An Explorer 1 re-entry commemorative cover from "The Manned Spaceflight Cover Society", commemorating the "Death of Explorer I" after more than 12 years in orbit, postmarked in Houston with a hand cancel on March 31, 1970, the day Explorer 1 re-entered and burned up over the South Pacific.