Editing Explorer 2

{{Short description|1958 American failed space mission which attempted to place a satellite into orbit}}
{{about|the U.S. Army exploratory satellite|other uses|Explorer Two (disambiguation)}}
{{Use American English|date=February 2021}}
{{Use dmy dates|date=February 2021}}
{{Infobox spaceflight
| name                = Explorer 2
| names_list          = Explorer II
| image               = Juno-1 explorer-2.jpg
| image_caption       = Launch of Explorer 2 satellite
| image_size          = 300px

| mission_type        = [[Earth science]]
| operator            = [[Jet Propulsion Laboratory|JPL]] / [[Army Ballistic Missile Agency]]
| Harvard_designation = 
| COSPAR_ID           = 
| SATCAT              = 
| mission_duration    = Failed to orbit<br/>120 days (planned)

| spacecraft          = Explorer II
| spacecraft_type     = Science Explorer
| spacecraft_bus      = [[Explorer 1]]
| manufacturer        = [[Jet Propulsion Laboratory]]
| launch_mass         = {{cvt|14.22|kg}}
| dimensions          = {{cvt|203|cm}} length<br/>{{cvt|15.2|cm}} diameter
| power               = 60 [[watt]]s

| launch_date         = 5 March 1958, 18:27:57 [[Greenwich Mean Time|GMT]]<ref name="JSR1"/>
| launch_rocket       = [[Juno I]] (RS-26)
| launch_site         = [[Eastern Range|Atlantic Missile Range]], [[Cape Canaveral Launch Complex 26|LC-26A]]
| launch_contractor   = [[Army Ballistic Missile Agency]]

| entered_service     = 
| last_contact        = 
| destroyed           = Failed to orbit
| decay_date          = 

| orbit_reference     = [[Geocentric orbit]] (planned) <ref name="Trajectory">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=1958-001A|title=Trajectory: Explorer-1 1958-001A|publisher=NASA|date=14 May 2020|access-date=13 February 2021}} {{PD-notice}}</ref>
| orbit_regime        = [[Medium Earth orbit]]
| orbit_periapsis     = {{cvt|358|km}}
| orbit_apoapsis      = {{cvt|2550|km}}
| orbit_inclination   = 32.24°
| orbit_period        = 114.80 minutes
| apsis               = gee

| instruments         = Geiger counter<br/>Micrometeorite Detector<br/>Satellite Drag Atmospheric Density<br/>Resistance Thermometers<br/>Thermometers

| programme           = '''Explorer program'''
| previous_mission    = [[Explorer 1]]
| next_mission        = [[Explorer 3]]
}}

'''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), [[Cape Canaveral Launch Complex 26|LC-26A]] in Florida on 5 March 1958 at 18:27:57 GMT by a [[Juno I]] launch vehicle.<ref name="jsr">{{cite web|url=http://www.planet4589.org/space/lvdb/launch/Redstone|title=Launch Vehicle Database - Redstone|publisher=Jonathan's Space Report|access-date=13 February 2021|archive-date=10 October 2007|archive-url=https://web.archive.org/web/20071010035912/http://www.planet4589.org/space/lvdb/launch/Redstone|url-status=dead}}</ref> 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]].

== Background ==
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 [[PGM-19 Jupiter|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.

== Spacecraft ==
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 {{cvt|203|cm}} long, {{cvt|15.2|cm}} diameter cylinder and nosecone that comprised the fourth stage of the Jupiter-C launch vehicle. With a mass of {{cvt|14.22|kg}}, it was about {{cvt|0.25|kg}} heavier than Explorer 1. The spacecraft body was made of stainless AISI-410 steel, {{cvt|0.058|cm}} 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.<ref name="Display">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EXPLR2|title=Display: Explorer 2 EXPLR2|publisher=NASA|date=14 May 2020 |access-date=13 February 2021}} {{PD-notice}}</ref>

The base of the cylinder held the Sergeant solid-fuel rocket motor. The sub-carrier oscillators and [[Mercury battery|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.<ref name="Display"/>

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 {{cvt|0.23|kg}}, {{cvt|5.7|cm}} 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.<ref name="Display"/>

Explorer 2 was equipped with a [[Geiger counter]] for the purposes of detecting [[cosmic ray]]s. 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 [[micrometeoroid|micrometeorite]] detection.

== Instruments ==
=== Geiger counter ===
An Anton 314 omnidirectional Geiger tube detector was used to measure the flux of energetic [[charged particle]]s (protons E>30 MeV and electrons E>3 MeV). The instrument consisted of a single [[Geiger–Müller tube|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 {{cvt|0.12|cm}} thickness. The instrument was mounted within the spacecraft hull, which had {{cvt|0.58|mm}} thick stainless steel walls. The counter was {{cvt|10.2|cm}} long by {{cvt|2.0|cm}} diameter and the internal wire was {{cvt|10|cm}} in length. The tube had a very small variation in counting efficiency over the range -55° to +175&nbsp;°C. It had approximately 85% counting efficiency for [[cosmic ray]]s, and about 0.3% counting efficiency for [[photon]]s 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 [[Amplifier|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&nbsp;°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.<ref name="Instrument1">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1958-001A-01|title=Experiment: Geiger counter|publisher=NASA|date=14 May 2020|access-date=13 February 2021}} {{PD-notice}}</ref>

=== Micrometeorite Detector ===
Direct measurements of [[micrometeorite]]s were made on Explorer 1 using two separate detectors: a wire grid detector and a crystal transducer. The parameters determined were the influx rates of each size interval, the impinging velocity, the composition, and the density of the micrometeorite.<ref name="Instrument2">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1958-001A-02 |title=Experiment: Micrometeorite Detector|publisher=NASA|date=14 May 2020|access-date=14 February 2021}} {{PD-notice}}</ref>

The wire grid detector consisted of 12 cards (connected in parallel) mounted in a [[fiberglass]] supporting ring which in turn was mounted on the satellite's cylindrical surface. Each card was wound with enameled 17-micron-diameter nickel alloy wire. Two layers of wire were wound on each card to ensure that a total area of {{cvt|1|xx|1|cm}} completely covered. A micrometeorite of about 10 microns would fracture the wire upon impact, destroy the electrical connection, and thus record the event.<ref name="Instrument2"/>

The acoustic detector (transducer and solid-state amplifier) was placed in acoustical contact with the middle section skin where it could respond to meteorite impacts on the spacecraft skin such that each recorded event would be a function of mass and velocity. The effective area of this section was 0.075 m<sup>2</sup>, and the average threshold sensitivity was 0.0025 g-cm/s.<ref name="Instrument2"/>

=== Satellite Drag Atmospheric Density ===
Because of its symmetrical shape, Explorer 2 was selected for use in determining upper atmospheric densities as a function of altitude, latitude, season, and [[Solar activity and climate|solar activit]]y. Density values near [[Apsis|perigee]] were deduced from sequential observations of the spacecraft position, using optical ([[Schmidt camera|Baker-Nunn camera network]]) and radio and/or radar tracking techniques.<ref name="Instrument3">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1958-001A-03|title=Experiment: Satellite Drag Atmospheric Density|publisher=NASA|date=14 May 2020 |access-date=14 February 2021}} {{PD-notice}}</ref>

=== Resistance Thermometers ===
The Explorer 2 satellite was equipped with four [[resistance thermometer]]s that made direct temperature measurements, three external and one internal. The primary purpose of the experiment was to study the efficacy of passive thermal control (in this case, insulation and exterior coatings) on the exterior and interior of a satellite, and to document the temperature of the instrumentation to study its effect on instrument operation.<ref name="Instrument4">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1958-001A-04|title=Experiment: Resistance Thermometers|publisher=NASA|date=14 May 2020 |access-date=14 February 2021}} {{PD-notice}}</ref>

==== Thermometers ====
The thermometer designated external temperature gauge no. 1 was mounted on the outer hull at the bottom of the upper (instrumentation) section of the satellite. This measured the cylinder skin temperature over a range of -50&nbsp;°C to +110&nbsp;°C, with an accuracy of 4&nbsp;°C over the range -10&nbsp;°C to +80&nbsp;°C. External temperature gauge no. 2 was mounted along the bottom of the nose cone to measure the nose cone skin temperature. It could cover a range of -50&nbsp;°C to +220&nbsp;°C. The accuracy was 16&nbsp;°C at a temperature of 50&nbsp;°C and 18&nbsp;°C at 0&nbsp;°C. External temperature gauge no. 3 was mounted at the top of the nose cone and measured the stagnation-point temperature. It covered from -50&nbsp;°C to +450&nbsp;°C with an accuracy of approximately 20&nbsp;°C.<ref name="Instrument4"/>

The internal temperature gauge was mounted in the high powered transmitter at the base of the instrumentation section. It could cover a range of -60&nbsp;°C to +110&nbsp;°C. The accuracy was 2&nbsp;°C at temperatures from 0&nbsp;°C to +30&nbsp;°C and fell off to an accuracy of 20&nbsp;°C at a temperature of 90&nbsp;°C. External temperature gauges no. 2 and no. 3 transmitted on the low-powered (10 mW, 108.00-MHz) transmitter, and the other two gauges transmitted on the high-powered (60 mW, 108.03-MHz) transmitter. Additionally, the nose cone internal temperature could be indirectly estimated by measuring the frequency of the cosmic ray channel. Calibrations of the oscillator indicate the internal nose cone temperature could be known within 12&nbsp;°C from 0 to +25&nbsp;°C, and to 6&nbsp;°C for 25 to 50&nbsp;°C.<ref name="Instrument4"/>

== Telemetry ==
The [[telemetry]] was broadcast continuously. There were no recorders or data storage devices on board, so temperature data could only be received in real time covering periods when Explorer 1 was over a receiving station. There were 5 receiving stations: [[Patrick Space Force Base|Patrick Air Force Base]] (Cape Canaveral), [[Earthquake Valley]] (near San Diego), San Gabriel, California, Singapore (Malaya), and Ibadan (Nigeria). All 5 could receive data from the low-power transmitter, only Patrick AFB and San Gabriel could receive from the high-power transmitter. There were typically 4 passes per day over Patrick AFB, Earthquake Valley, and San Gabriel, and 7 passes per day over Nigeria and Singapore.<ref name="Instrument4"/>

== Thermal control ==
The exterior [[temperature control]] was achieved by coating a fraction of the stainless steel satellite casing with an aluminum oxide ceramic (Rokide A). Approximately 30% of the nose cone (upper 30 cm of the satellite) and 25% of the upper {{cvt|51.4|cm}} of the cylindrical body were coated in longitudinal stripes. There was insulation between the nose cone and instrument compartment, and between the instrument compartment and rocket motor section.<ref name="Instrument4"/>

An internal temperature range of -5&nbsp;°C to +45&nbsp;°C was required for proper operation of the equipment in the satellite. The batteries would not operate below -5&nbsp;°C, but low temperatures would not damage the batteries or equipment. Permanent damage to the equipment would not occur unless the temperature rose above +80&nbsp;°C.<ref name="Instrument4"/>

== Telecommunications ==
Data were continuously transmitted using a 60 mW amplitude-modulated transmitter and a 10 mW phase-modulated transmitter, both transmitting as a frequency of 108 MHz. Data were recorded only when the spacecraft was over one of seventeen receiving stations. Both the high-power and low-power transmitters were battery powered and operated.<ref name="Telecommunications">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTelemetry.action?id=1958-001A|title=Telecommunications: Explorer 1 1958-001A|publisher=NASA|date=14 May 2020|access-date=14 February 2021}} {{PD-notice}}</ref>

== Launch vehicle ==
The launch vehicle was a Juno I, a variant of the three-stage Jupiter-C with an added fourth propulsive stage, which in this case was the Explorer 2. The first stage was an upgraded Redstone liquid-fueled launch vehicle. The second stage comprised a cluster of eleven Sergeant solid-fuel rocket motors and the third stage held three Sergeants. The booster was equipped to spin the fourth stage in increments, leading to a final rate of 750 [[Revolutions per minute|rpm]] about its long axis.<ref name="Display"/>

== Mission ==
Explorer 2 launched from the Cape Canaveral Missile Test Center of the Atlantic Missile Range (AMR), pad 26A, on 5 March 1958 at 18:27:57 GMT.<ref name="JSR1">{{cite web |url=https://planet4589.org/space/log/launchlog.txt|title=Launch Log|publisher=Jonathan's Space Report|date=21 July 2021|access-date=12 November 2021}}</ref> The flight was nominal through third stage ignition. The fourth stage failed to ignite, making attainment of orbital velocity impossible. The spacecraft reentered the atmosphere and fell into the Atlantic Ocean near [[Trinidad]], some {{cvt|3000|km}} from the launch site. The cause of the failure was believed to be due to failure of a light plastic cone, which held the igniter in place at the fourth stage nozzle, under the launch stresses. This allowed the igniter to fall out of position. The igniter support was strengthened for later flights.<ref name="Display"/><ref name="JPL">{{cite web|url=http://msl.jpl.nasa.gov/Programs/explorer.html|title=Mission and Spacecraft Library - Explorer Program|publisher=Jet Propulsion Laboratory|url-status=dead|archive-url=https://web.archive.org/web/20080127144116/http://msl.jpl.nasa.gov/Programs/explorer.html|archive-date=2008-01-27 |access-date=13 February 2021}} {{PD-notice}}</ref>

== See also ==
{{Portal|Spaceflight}}
* [[Explorer 1]]
* [[Explorer program]]

== References ==
{{Reflist|30em}}

== External links ==
{{Commons category}}
* {{youtube|kjlr6v0Psvo|Big Picture: Army Satellites - Explorer 1, Explorer 2 and Explorer 3}}

{{Explorer program}}
{{Orbital launches in 1958}}

[[Category:Explorers Program]]
[[Category:Satellite launch failures]]
[[Category:Spacecraft launched in 1958]]