Explorer 1

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This article is about the U.S. satellite. For the Iranian Explorer-1 rocket, see Kavoshgar-1.
Explorer 1
Explorer1.jpg
A model of the Explorer 1 built with a clear acrylic shell, showing the interior structure and components.
Mission type Earth science
Operator Army Ballistic Missile Agency
Harvard designation 1958 Alpha 1
SATCAT № 4
Mission duration 111 days
Spacecraft properties
Manufacturer Jet Propulsion Laboratory
Launch mass 13.97 kilograms (30.8 lb)
Power 60 watts
Start of mission
Launch date February 1, 1958, 03:48 (1958-02-01UTC03:48Z) UTC[1]
Rocket Juno I RS-29
Launch site Cape Canaveral LC-26A
End of mission
Last contact May 23, 1958 (1958-05-24)
Decay date March 31, 1970
Orbital parameters
Reference system Geocentric
Regime MEO
Semi-major axis 7,832.2 kilometers (4,866.7 mi)
Eccentricity 0.13985
Perigee 358 kilometers (222 mi)
Apogee 2,550 kilometers (1,580 mi)
Inclination 33.24°
Period 114.8 minutes
RAAN 334.6171 degrees
Argument of perigee 311.5310 degrees
Mean anomaly 48.3249 degrees
Mean motion 16.275
Epoch 31 March 1970, 00:50:24 UTC[citation needed]
Revolution number 58402

Explorer 1 was the very first successful satellite launched by the United States. Its primary instruments were a Geiger counter and a micrometeroid detector, together with provisions for the radio telemetry of their data to Earth.

The flight of Explorer 1 closely followed the first two satellites ever launched into space, the Soviet Union's Sputnik 1 and Sputnik 2, which had been successfully placed on Earth orbit the previous year. Together these satellites comprise the crucial first moves Man had taken into outer space, a feat widely considered to be as important as the first creatures to climb out of the sea and live on the land.

The launch of the Sputniks, with the Explorer 1 reply, is also considered the beginning of the Cold War Space Race between the two nations. The historic launch of the Soviet Sputniks caused something of a panic among the Americans, both publicly and privately. Because the US Vanguard rocket had exploded on the launch pad, Explorer 1 became the US's first successful response to the Sputniks.

Explorer 1 was given Satellite Catalog Number 4, and the Harvard designation 1958 Alpha 1,[2] the forerunner to the modern International Designator. It was also launched under the aegis of the US participation in the International Geophysical Year.

This satellite, along with its sister satellites Explorer 3 and Explorer 4, spun out of control in outer space, though all three still were able to return useful data. The reason for the loss of control was a design flaw that became a well-known subject of study in spacecraft dynamics. Less well known, the satellite also did not make the calculated and intended orbit, instead flying far higher than the engineers had designed for. The reason for this mysterious excursion is a subject of controversy.

Explorer 1 was launched on January 31, 1958 at 22:48 Eastern Time (equal to February 1, 03:48 UTC) atop the first Juno booster from LC-26 at the Cape Canaveral Missile Annex, Florida. It is also famous for being the first spacecraft to detect the previously-unknown Van Allen radiation belt,[3] returning data until its batteries were exhausted after nearly four months. It remained in orbit until 1970, and has been followed by more than 90 military and scientific spacecraft grouped together in the Explorer series.

Background

The U.S. Earth satellite program began in 1954 as a joint U.S. Army and U.S. Navy proposal, called Project Orbiter, to put a scientific satellite into orbit during the International Geophysical Year. The proposal, using a military Redstone missile, was rejected in 1955 by the Eisenhower administration in favor of the Navy's Project Vanguard, using a booster produced for civilian space launches.[4] Following the launch of the Soviet satellite Sputnik 1 on October 4, 1957, the initial Project Orbiter program was revived as the Explorer program to catch up with the Soviet Union.[5]

Explorer 1 was designed and built by the Jet Propulsion Laboratory (JPL), while a Jupiter-C rocket was modified by the Army Ballistic Missile Agency (ABMA) to accommodate a satellite payload; the resulting rocket known as the Juno I. The Jupiter-C design used for the launch had already been flight-tested in nose cone reentry tests for the Jupiter IRBM, and was modified into Juno I. Working closely together, ABMA and JPL completed the job of modifying the Jupiter-C and building Explorer 1 in 84 days. However, before work was completed, the Soviet Union launched a second satellite, Sputnik 2, on November 3, 1957. The U.S. Navy's attempt to put the first U.S. satellite into orbit failed with the launch of the Vanguard TV3 on December 6, 1957.[6]

Spacecraft design

File:Explorer1 preparations.jpg
Explorer 1 is mated to its booster at LC-26

Explorer 1 was designed and built by the California Institute of Technology's JPL under the direction of Dr. William H. Pickering. It was the second satellite to carry a mission payload (Sputnik 2 was the first).

The total weight of the satellite was 13.37 kilograms (30.80 lb), of which 8.3 kg (18.3 lb) were instrumentation. In comparison, the first Soviet satellite Sputnik 1 weighed 83.6 kg (184 lb). The instrument section at the front end of the satellite and the empty scaled-down fourth-stage rocket casing orbited as a single unit, spinning around its long axis at 750 revolutions per minute.

Data from the scientific instruments was transmitted to the ground by two antennas. A 60 milliwatt transmitter fed a dipole antenna consisting of two fiberglass slot antennas in the body of the satellite operating on 108.03 MHz, and four flexible whips forming a turnstile antenna were fed by a 10 milliwatt transmitter operating on 108.00 MHz.[7][8]

Because of the limited space available and the requirements for low weight, the payload instrumentation was designed and built with simplicity and high reliability in mind, using transistor electronics, consisting of both germanium and silicon devices. This was a very early time frame in the development of transistor technology, and was the first documented use of transistors in the U.S. Earth satellite program.[9] A total of 29 transistors were used in Explorer 1, plus additional ones in the Army's micrometeorite amplifier. Electrical power was provided by mercury chemical batteries that made up approximately 40 percent of the payload weight.

The external skin of the instrument section was sandblasted stainless steel with white stripes. Several other color schemes had been tested, resulting in backup articles, models, and photographs showing different configurations, including alternate white and green striping and blue stripes alternating with copper. The final coloration was determined by studies of shadow–sunlight intervals based on firing time, trajectory, orbit, and inclination.

File:Explorer1 sketch.jpg
Explorer 1 schematic

Science payload

Universal Newsreel about the satellite

The Explorer 1 payload consisted of the Iowa Cosmic Ray Instrument without a tape data recorder which was not modified in time to make it onto the spacecraft. The real-time data received on the ground was therefore very sparse and puzzling showing normal counting rates and no counts at all. The later Explorer 3 mission, which included a tape data recorder in the payload, provided the additional data for confirmation of the earlier Explorer 1 data.

The scientific instrumentation of Explorer 1 was designed and built under the direction of Dr. James Van Allen of the University of Iowa containing:[7]

  • Anton 314 omnidirectional Geiger-Müller tube, designed by Dr. George Ludwig of Iowa's Cosmic Ray Laboratory, to detect cosmic rays. It could detect protons with E > 30 MeV and electrons with E > 3 MeV. Most of the time the instrument was saturated;[10]
  • Five temperature sensors (one internal, three external and one on the nose cone);
  • Acoustic detector (crystal transducer and solid-state amplifier) to detect micrometeorite (cosmic dust) impacts. It responded to micrometeorite impacts on the spacecraft skin in such way that each impact would be a function of mass and velocity. Its effective area was 0.075 m2 and the average threshold sensitivity was 2.5×10−3 g cm/s;[11][12]
  • Wire grid detector, also to detect micrometeorite impacts. It consisted of 12 parallel connected cards mounted in a fiberglass supporting ring. Each card was wound with two layers of enameled nickel alloy wire with a diameter of 17 µm (21 µm with the enamel insulation included) in such way that a total area of 1 cm by 1 cm was completely covered. If a micrometeorite of about 10 µm impacted, it would fracture the wire, destroy the electrical connection, and thus record the event.[11][12]

Launch vehicle

The Juno 1 was essentially a Jupiter-C with a fourth solid-motor stage on top. The Jupiter-C in turn was a three-stage, modified Redstone rocket. It had 2.3 meters more tankage length than a Redstone, increasing the burn time to 155-157 seconds. It also had two solid-motor stages mounted on top of the main nosecone. It had been used to test scaled down, suborbital, ballistic missile re-entry vehicles, but was designed from the start with the idea of adding a fourth stage to launch a small payload into orbit. Each of the solid stages had burn times of 6 seconds.

The upper-stage tub, holding all three upper stages, was spun-up before launch to 750 RPM. During first-stage flight, the vehicle was guided by a gyro-controlled autopilot controlling both air-vanes and jet vanes on the first stage by means of servos. Following a vertical launch from a simple steel table, the vehicle was programmed so that it was traveling at an angle of 40 degrees from the horizontal at burnout of the first stage, which occurred about 157 seconds after launch. At first-stage burnout, explosive bolts fired and springs separated the instrument section nosecone from the first-stage tankage. The instrument section and the spinning tub were slowly tipped to a horizontal position by means of four air jets located at the base of the instrument section. When the apogee of the suborbital flight occurred after a coasting flight of about 247 seconds, a radio signal from the ground ignited the eleven-rocket cluster of the second stage. The third and fourth stages were fired in turn to boost the satellite and fourth stage to an orbital velocity of about 18,000 mph (8 km/s).

This staging protocol does not use an optimal trajectory from the surface to orbit, which varies for each rocket and launch. It's inefficient, but simple. The cost is a tiny payload. No flight computers were used as none existed. The 2nd stage solids were lit "by hand", by a man on the ground with a stopwatch and some intuition. Then the rest of the final orbit parameters were determined by the vagaries of the three stages of solid rocket burns.

This method of orbiting a payload obviated the need for a guidance system in the upper stages. It was invented by Wernher von Braun in 1956 for his proposed Project Orbiter, which would have been like the Jupiter-C but using even smaller solid-fuel upper stages which were the only ones available at that time. His method was the simplest and most immediate method for putting a payload into orbit, but as it had no upper-stage guidance, it was not put into a precisely specified orbit.

Flight

The Juno I rocket was launched January 31, 1958, putting Explorer 1 into orbit with a perigee of 358 kilometers (222 mi) and an apogee of 2,550 kilometers (1,580 mi) having a period of 114.8 minutes.[13][14][15] At about 1:30 a.m. ET, after confirming that Explorer 1 was indeed in orbit, a news conference was held in the Great Hall at the National Academy of Sciences in Washington, DC to announce it to the world.[16]

Mercury batteries powered the high-power transmitter for 31 days and the low-power transmitter for 105 days. Explorer 1 stopped transmission of data on May 23, 1958[17] when its batteries died, but remained in orbit for more than 12 years. It reentered the atmosphere over the Pacific Ocean on March 31, 1970 after more than 58,000 orbits.

Results

William Hayward Pickering, James Van Allen, and Wernher von Braun display a full-scale model of Explorer 1 at a crowded news conference in Washington, DC after confirmation the satellite was in orbit.

Explorer 1 changed rotation axis after launch. The elongated body of the spacecraft had been designed to spin about its long (least-inertia) axis but refused to do so, and instead started precessing due to energy dissipation from flexible structural elements. Later it was understood that on general grounds, the body ends up in the spin state that minimizes the kinetic rotational energy for a fixed angular momentum (this being the maximal-inertia axis). This motivated the first further development of the Eulerian theory of rigid body dynamics after nearly 200 years—to address this kind of momentum-preserving energy dissipation.[18][19]

Sometimes the instrumentation would report the expected cosmic ray count (approximately 30 counts per second) but sometimes it would show a peculiar zero counts per second. The University of Iowa (under James Van Allen) noted that all of the zero counts per second reports were from an altitude of more than 2,000 kilometres (1,200 mi) over South America, while passes at 500 km (310 mi) would show the expected level of cosmic rays. Later, after Explorer 3, it was concluded that the original Geiger counter had been overwhelmed ("saturated") by strong radiation coming from a belt of charged particles trapped in space by the Earth's magnetic field. This belt of charged particles is now known as the Van Allen radiation belt. The discovery was considered to be one of the outstanding discoveries of the International Geophysical Year.

The acoustic micrometeorite detector detected 145 impacts of cosmic dust in 78,750 seconds. This calculates to an average impact rate of 8.0×10−3 impacts m−2 s−1 over the twelve-day period (29 impacts per hour per square meter).[20]

Legacy

Explorer 1 was the first of at least five similar satellites, all of which carried nuclear radiation detectors.

Name Date Duration Planned Orbit Actual Orbit Inclination Mass Instruments Findings
Explorer 1 February 1, 1958 (UTC) 111 days 220 x 1000 miles 358 km × 2,550 km (222 mi × 1,584 mi) 33.24 degrees 13.97 kg (30.8 lb) Geiger, micrometeorite detectors Saturated Geiger (van Allen Belt)
Explorer 2 March 5, 1958 0 days  ??? failed to orbit X 14.53 kg (32.0 lb) Geiger, micrometeorite detectors ???
Explorer 3 March 26, 1958 93 days 220 x 1000 miles 186 km × 2,799 km (116 mi × 1,739 mi) 33.38 degrees 14.1 kg (31 lb) Geiger, micrometeorite detectors van Allen belt confirm, Yucca bomb test
Explorer 4 July 26, 1958 454 days ??? 263 km × 2,213 km (163 mi × 1,375 mi) 50.3 degrees 25.5 kg (56 lb) Geiger,CsI detector Hardtack and Argus nuclear radiation
Explorer 5 August 24, 1958 0 days  ??? failed to orbit X 16.86 kg (37.2 lb) Geiger,CsI detector ??? Intended for Hardtack and Argus bombs

Explorer 5 was the last of this series of satellites. Different designs were used beginning with Explorer 6. The name became a brand for the long-running Explorer program. For example, a follow-up to the first mission, Explorer-1 [PRIME], was successfully launched aboard a Delta II rocket in late October, 2011. The PRIME was built using modern satellite construction techniques. The orbiting satellite was a backup, since the initial Explorer-1 PRIME, failed to reach orbit.[21]

An identically constructed flight backup of Explorer 1 is on display in the Smithsonian Institution's National Air and Space Museum, Milestones of Flight Gallery in Washington, DC.

Gallery

Explorer 1 statistics and orbital diagram. This had to have been made after the launch because the orbit data is different from the planned orbit. (Figures do not quite match infobox. Note that 33.28 is rounded up to 33.3 Degrees.)
Explorer 1 statistics and orbital diagram. This had to have been made after the launch because the orbit data is different from the planned orbit. (Figures do not quite match infobox. Note that 33.28 is rounded up to 33.3 Degrees.) 
Officials with Explorer 1 model at Redstone Arsenal, including Maj. Gen. John Medaris (3rd from left), Walter Haeussermann, Wernher von Braun and Ernst Stuhlinger
Officials with Explorer 1 model at Redstone Arsenal, including Maj. Gen. John Medaris (3rd from left), Walter Haeussermann, Wernher von Braun and Ernst Stuhlinger 
The Juno 1 launch vehicle, aka Jupiter-C with the 4th stage and satellite.
The Juno 1 launch vehicle, aka Jupiter-C with the 4th stage and satellite. 
"Sergeant-derived", scaled solid rocket motors. There are 11 motors arranged in a ring for the 2nd stage, three more inside the ring for the 3rd stage. The fourth stage was bolted to the Explorer satellite and attached to the top of the third stage. The entire assembly was mounted on a "tub" and spun up before launch. 
Explorer 1 mated to Juno I booster
Explorer 1 mated to Juno I booster 
Explorer 1 and Juno I booster in gantry at LC-26
Explorer 1 and Juno I booster in gantry at LC-26 
Close-up of Explorer 1 atop Juno I booster
Close-up of Explorer 1 atop Juno I booster 
Launch of Explorer 1 on January 31, 1958
Launch of Explorer 1 on January 31, 1958[22] 
Preliminary satellite tracking tests in a field near JPL
Preliminary satellite tracking tests in a field near JPL[23] 
Purported photo of von Braun, wondering what happened to his Explorer 1 satellite during the 12 minute hiatus on the first orbit. 
Trajectory calculations were simple enough to be done by hand by this group of young women.
Trajectory calculations were simple enough to be done by hand by this group of young women. 

See also

References

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  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Paul Dickson, Sputnik: The Launch of the Space Race. (Toronto: MacFarlane Walter & Ross, 2001), 190.
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  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
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  11. ↑ 11.0 11.1 Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ 12.0 12.1 Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  16. ↑ Lua error in package.lua at line 80: module 'strict' not found.
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  18. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  19. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  20. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  21. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  22. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  23. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

Template:Explorer program

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