Explorer 2 was an American unmanned space mission within the Explorer program. Intended to be a repetition of the previous Explorer 1 mission, which placed a satellite into medium Earth orbit, the spacecraft was unable to reach orbit due to a failure in the launch vehicle during launch.

Explorer 2 was launched from Cape Canaveral Missile Test Center of the Atlantic Missile Range (AMR), LC-26A in Florida on 5 March 1958 at 18:27:57 GMT by a Juno I launch vehicle. The Juno I had its origins in the United States Army's Project Orbiter in 1954. The project was canceled in 1955 when the decision was made to proceed with Project Vanguard.

Following the launch of the Soviet Sputnik 1 on 4 October 1957, the Army Ballistic Missile Agency (ABMA) was directed to proceed with the launching of a satellite using the Juno I four-stage variant of the three-stage Jupiter-C, which had already been flight-tested in nose-cone re-entry tests for the Jupiter IRBM (intermediate-range ballistic missile). Working closely together, ABMA and Jet Propulsion Laboratory (JPL) completed the job of modifying the Jupiter-C and building Explorer 1 in 84 days.

Explorer 2 was identical to Explorer 1 except for the addition of a tape recorder designed to enable playback of data. The satellite was a 203 cm (80 in) long, 15.2 cm (6.0 in) diameter cylinder and nosecone that comprised the fourth stage of the Jupiter-C launch vehicle. With a mass of 14.22 kg (31.3 lb), it was about 0.25 kg (0.55 lb) heavier than Explorer 1. The spacecraft body was made of stainless AISI-410 steel, 0.058 cm (0.023 in) thick. The case was heat-oxidized to a gold color and eight alternate stripes of white Rokide A (flame sprayed aluminum oxide) were used for temperature control.

The base of the cylinder held the Sergeant solid-fuel rocket motor. The sub-carrier oscillators and Mallory mercury batteries for the low power transmitter were in the upper part of the nose cone. Below these was the low power (10 mW, 108.00 MHz) transmitter for the carrier and sub-carrier signals, which used the stainless steel satellite skin as a dipole antenna.

Below the nose cone was the detector deck, holding the Geiger-Mueller counter tube for the cosmic ray experiment, the command receiver, for recorder interrogations, high power playback transmitter (60 mW, 108.03 MHz) for interrogation response, cosmic ray experiment electronics, Mallory mercury batteries for the high power transmitter, and a 0.23 kg (0.51 lb), 5.7 cm (2.2 in) diameter magnetic tape recorder. An acoustic micrometeorite detector was mounted to the inside of the spacecraft cylinder near the cosmic ray device. Near the bottom of the detector deck four circularly polarized turnstile stainless steel wire whip antennas protruded radially from the side of the spacecraft, equally spaced around the axis. A gap for the high powered antenna and a heat radiation shield were between the payload and the rocket motor. The micrometeorite detectors were arranged in a ring around the cylinder near the bottom of the spacecraft. Four temperature gauges were mounted a various locations in the spacecraft.

Explorer 2 was equipped with a Geiger counter for the purposes of detecting cosmic rays. After Explorer 3, it was decided that the original Geiger counter had been overwhelmed by strong radiation coming from a belt of charged particles trapped in space by the Earth's magnetic field (see: Van Allen radiation belt). Explorer 2 was also equipped with a wire grid array and an acoustic detector for the purpose of micrometeorite detection.

An Anton 314 omnidirectional Geiger tube detector was used to measure the flux of energetic charged particles (protons E>30 MeV and electrons E>3 MeV). The instrument consisted of a single Geiger-Mueller tube, a scaling circuit to reduce the number of pulses, and a telemetry system to transmit the data to ground receiving stations. The Geiger-Mueller tube was a type 314 Anton halogen quenched counter with stainless steel (approximately 75% iron, 25% chromium) wall of approximately 0.12 cm (0.047 in) thickness. The instrument was mounted within the spacecraft hull, which had 0.58 mm (0.023 in) thick stainless steel walls. The counter was 10.2 cm (4.0 in) long by 2.0 cm (0.79 in) diameter and the internal wire was 10 cm (3.9 in) in length. The tube had a very small variation in counting efficiency over the range -55° to +175 °C. It had approximately 85% counting efficiency for cosmic rays, and about 0.3% counting efficiency for photons of energy 660 keV. The "dead time" (time to reset to record the next count) of the counters was about 100 microseconds. The counter was connected to a current amplifier, which directly fed a scaler stage, a bistable transistor multivibrator that could operate over a wide range of voltages and a temperature range of -15° to +85 °C, limited primarily by the supply batteries. The scaler resolving time was 250 microseconds. For pulse counts higher than 4000 per second, the scaler indicated a count of 4000. Results were sent to the ground through the telemetry system in real time. The experiment had no onboard data storage device, and could only send telemetry to the ground when it was passing over an Earth receiving station, so some regions had no coverage during the flight.