 Juno I rocket with Explorer 3 satellite | |
| Names | Explorer III 1958 Gamma |
|---|---|
| Mission type | Earth science |
| Operator | JPL /U.S. Army Ordnance |
| Harvard designation | 1958 Gamma |
| COSPAR ID | 1958-003A |
| SATCATno. | 00006 |
| Mission duration | 93 days (achieved) 120 days (planned) |
| Spacecraft properties | |
| Spacecraft | Explorer III |
| Spacecraft type | Science Explorer |
| Bus | Explorer 1 |
| Manufacturer | Jet Propulsion Laboratory |
| Launch mass | 14.1 kg (31 lb) |
| Payload mass | 8.4 kg (19 lb) |
| Dimensions | 203 cm (80 in) length 15.2 cm (6.0 in) diameter |
| Power | 60watts |
| Start of mission | |
| Launch date | 26 March 1958, 17:38:01GMT |
| Rocket | Juno I (RS-24) |
| Launch site | Atlantic Missile Range,LC-5 |
| Contractor | Army Ballistic Missile Agency |
| Entered service | 26 March 1958 |
| End of mission | |
| Last contact | 28 June 1958[1] |
| Decay date | 28 June 1958[1] |
| Orbital parameters | |
| Reference system | Geocentric orbit[1] |
| Regime | Medium Earth orbit |
| Perigee altitude | 186 km (116 mi) |
| Apogee altitude | 2,799 km (1,739 mi) |
| Inclination | 33.38° |
| Period | 115.70 minutes |
| Instruments | |
| Cosmic Ray counter Micrometeorite detector | |
Explorer Program | |
Explorer 3 (Harvard designation1958 Gamma) was an American artificial satellite launched intomedium Earth orbit in 1958. It was the second successful launch in theExplorer program, and was nearly identical to the first U.S. satelliteExplorer 1 in its design and mission.
Explorer 3 was the third satellite in the Explorer small satellite series, which started withExplorer 1, America's first artificial satellite.[2]: 288 The Explorer program was a direct successor to theArmy Ballistic Missile Agency (ABMA)'sProject Orbiter, initiated in November 1954 to use a slightly modifiedRedstone missile[3] combined with solid-propellant rocket cluster upper stage to put a satellite into orbit.[2]: 17–18, 43
In 1955, the "Stewart Committee", under the chairmanship ofHomer J. Stewart ofJet Propulsion Laboratory (JPL), chose aNaval Research Laboratory (NRL) satellite plan using a rocket based on itsViking rocket (Project Vanguard) for theInternational Geophysical Year, which would start 1 July 1957.[2]: 43, 51–56 Nevertheless, ABMA hoped Redstone-Orbiter could still be used as a backup orbital system. Reentry tests that year conducted with the newly developed, Redstone-basedJupiter-C, further strengthened ABMA confidence in their vehicle as an orbital launcher.
Following the launch of the Soviet satelliteSputnik 1 on 4 October 1957, Project Orbiter was revived,[4] with two shots authorized as a back-up to Vanguard in early November.[2]: 238 The failure of America'sfirst attempted Vanguard launch on 6 December 1957,[5] cleared the way for an "Explorer" (as the crash program was dubbed) to be the first American satellite.[2]: 74, 199–200, 212–213
Working closely together, ABMA and JPL completed the job of modifying the Jupiter-C to the Juno 1 and building Explorer 1 in 84 days.[4] An experiment developed for Vanguard by George Ludwig,[2]: 238 comprising an Anton 314 omnidirectional Geiger tube detector for measuring the flux of high energy charged protons and electrons, was adapted for Explorer 1.[6] Because of the high spin rate of the Explorer 1 rocket, the experiment's tape recorder had to be omitted, which meant that data could only be collected when the satellite was in sight and range of a ground station.[2]: 238
Explorer 1 took off 31 January 1958, becoming America's first satellite. Its Geiger tube worked properly, but acted contrary to expectations. As the satellite ascended in its orbit, the radiation count increased, then abruptly dropped to zero. When the satellite was descending, the tube abruptly began detecting charged particles again. As data could only be received about 15% of the time, it was yet impossible to determine the phenomenon Explorer had detected.[2]: 241–242
The objective of this spacecraft was a continuation of experiments started with Explorer 1. Thepayload consisted of amicrometeorite detector (a wire grid array and acoustic detector) and the samecosmic ray counter (aGeiger-Müller tube) experiment included on Explorer 1, but this time with an on-boardtape recorder to provide a completeradiation history for each orbit, Ludwig having had time to accommodate for thespin-stabilization of the satellite.[1]
Its total weight was 14.1 kg (31 lb), of which 8.4 kg (19 lb) was instrumentation. The instrument section at the front end of the satellite and the empty scaled-downfourth-stage rocket casing orbited as a single unit, to be spun around its long axis at 750revolutions per minute. Data from these instruments would be transmitted to the ground by a 60milliwatt transmitter operating on 108.03MHz and a 10 milliwatt transmitter operating on 108.00 MHz.[1]
Transmittingantennas consisted of twofiberglass slot antennas in the body of the satellite itself. The four flexible whip antennas of Explorer 1 were removed from the design.[7] The external skin of the instrument section was painted in alternate strips of white and dark green to provide passive temperature control of the satellite. The proportions of the light and dark strips were determined by studies of shadow-sunlight intervals based on firing time,trajectory, orbit, andorbital inclination.[1]
Electrical power was provided by Mallory type RM Mercury batteries that made up approximately 40% of the payload weight. These provided power that operated the high power transmitter for 31 days and the low-power transmitter for 105 days. Because of the limited space available and the requirements for low weight, the Explorer 3 instrumentation was designed and built with simplicity and high reliability in mind.[1]
Explorer 3 was launched at 17:38:03GMT on 26 March 1958 fromCape Canaveral Launch Complex 5[8] via the same type of modified Jupiter-C (Juno 1) as the prior two Explorers. A guidance system orbit placed the satellite into an orbit with a higherapogee and lowerperigee than planned: 1,735 mi (2,792 km) and 125 mi (201 km), respectively. The ensuingorbital decay made for a comparatively short lifespan: initial estimates placed it at 4.6 months.[9] In fact, the satellite reentered the atmosphere on 28 June 1958, after just 93 days of operation;[1] by the week before reentry, Explorer 3's apogee had dropped to 375 mi (604 km) and the perigee to 99 mi (159 km).[10]
Explorer 3 largely confirmed the findings of Explorer 1, with the same zero count returned above an altitude of around 1,000 km (620 mi) to 1,200 km (750 mi). Because of the consistent results, Van Allen hypothesized that the satellites' equipment might have been saturated by unexpectedly high radiation concentrations, trapped in a belt of charged particles by the Earth's magnetic field.Explorer 4, equipped with a lead-shielded counter, flew in July and confirmed the existence of the radiation fields subsequently known as theVan Allen Belts.[2]: 242–243
On 7 May 1958, micrometeorites associated with theEta Aquariids meteor shower ruptured two of Explorer 3's micrometeorite erosion gauges.[10]
A replica of the spacecraft is currently located in theSmithsonian Institution'sNational Air and Space Museum, Milestones of Flight Gallery.
This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain.
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