Replica of Sputnik 1
|Mission type||Technology demonstration|
|Operator||Soviet space program|
|Harvard designation||1957 Alpha 2|
|Mission duration||Final: 21 days|
Ministry of Radiotechnical Industry
|Launch mass||83.6 kg (184 lb)|
|Dimensions||58 cm (23 in) diameter|
|Start of mission|
|Launch date||4 October 1957, 19:28:34UTC|
|Launch site||Baikonur 1/5|
|End of mission|
|Last contact||26 October 1957|
|Decay date||4 January 1958|
|Semi-major axis||6,955 km (4,322 mi)|
|Perigee||215 km (134 mi)|
|Apogee||939 km (583 mi)|
|Epoch||4 October 1957, 15:12:00 UTC|
Sputnik 1 (//; Russian: Спутник-1 [ˈsputnʲɪk] "Satellite-1", or ПС-1 ["PS-1", i.e., Russian: Простейший Спутник-1, Prosteyshiy Sputnik-1, "Elementary Satellite 1"]) was the first artificial Earth satellite. The Soviet Union launched it into an elliptical low Earth orbit on 4 October 1957. It was a 58 cm (23 in) diameter polished metal sphere, with four external radio antennae to broadcast radio pulses. It was visible all around the Earth and its radio pulses were detectable. This surprise success precipitated the American Sputnik crisis and triggered the Space Race, a part of the larger Cold War. The launch ushered in new political, military, technological, and scientific developments.
Tracking and studying Sputnik 1 from Earth provided scientists with valuable information, even though the satellite itself wasn't equipped with sensors. The density of the upper atmosphere could be deduced from its drag on the orbit, and the propagation of its radio signals gave information about the ionosphere.
Sputnik 1 was launched during the International Geophysical Year from Site No.1/5, at the 5th Tyuratam range, in Kazakh SSR (now known as the Baikonur Cosmodrome). The satellite travelled at about 29,000 kilometres per hour (18,000 mph; 8,100 m/s), taking 96.2 minutes to complete each orbit. It transmitted on 20.005 and 40.002 MHz, which were monitored by amateur radio operators throughout the world. The signals continued for 21 days until the transmitter batteries ran out on 26 October 1957. Sputnik burned up on 4 January 1958 while reentering Earth's atmosphere, after travelling about 70 million km (43 million mi) and spending three months in orbit.
On 17 December 1954, chief Soviet rocket scientist Sergei Korolev addressed Dimitri Ustinov and proposed a developmental plan for an artificial satellite. Korolev forwarded a report by Mikhail Tikhonravov with an overview of similar projects abroad. Tikhonravov had emphasized that the launch of an orbital satellite was an inevitable stage in the development of rocket technology.
On 29 July 1955, U.S. President Dwight D. Eisenhower announced through his press secretary that the United States would launch an artificial satellite during the International Geophysical Year (IGY). A week later, on 8 August, the Politburo of the Communist Party of the Soviet Union approved the proposal to create an artificial satellite. On 30 August Vasily Ryabikov – the head of the State Commission on R-7 rocket test launches – held a meeting where Korolev presented calculation data for a spaceflight trajectory to the Moon. They decided to develop a three-stage version of the R-7 rocket for satellite launches.
On 30 January 1956 the Council of Ministers approved practical work on an artificial Earth-orbiting satellite. This satellite, named Object D, was planned to be completed in 1957–58; it would have a mass of 1,000 to 1,400 kg (2,200 to 3,100 lb) and would carry 200 to 300 kg (440 to 660 lb) of scientific instruments. The first test launch of "Object D" was scheduled for 1957. Work on the satellite was to be divided between institutions as follows:
Preliminary design work was completed by July 1956 and the scientific tasks to be carried out by the satellite were defined. These included measuring the density of the atmosphere and its ion composition, the solar wind, magnetic fields, and cosmic rays. These data would be valuable in the creation of future artificial satellites. A system of ground stations was to be developed to collect data transmitted by the satellite, observe the satellite's orbit, and transmit commands to the satellite. Because of the limited time frame, observations were planned for only 7 to 10 days and orbit calculations were not expected to be extremely accurate.
By the end of 1956 it became clear that the complexity of the ambitious design meant that 'Object D' could not be launched in time because of difficulties creating scientific instruments and the low specific impulse produced by the completed R-7 engines (304 sec instead of the planned 309 to 310 sec). Consequently, the government rescheduled the launch for April 1958. Object D would later fly as Sputnik 3.
Fearing the U.S. would launch a satellite before the USSR, OKB-1 suggested the creation and launch of a satellite in April–May 1957, before the IGY began in July 1957. The new satellite would be simple, light (100 kg or 220 lb), and easy to construct, forgoing the complex, heavy scientific equipment in favour of a simple radio transmitter. On 15 February 1957 the Council of Ministers of the USSR approved this simple satellite, designated 'Object PS'. This version allowed the satellite to be tracked visually by Earth-based observers, and it could transmit tracking signals to ground-based receiving stations. The launch of two satellites, PS-1 and PS-2, with two R-7 rockets (8K71) was approved, provided that the R-7 completed at least two successful test flights.
The R-7 Semyorka was initially designed as an ICBM by OKB-1. The decision to build it was made by the Central Committee of the Communist Party of the Soviet Union and the Council of Ministers of the USSR on 20 May 1954. The R-7 was also known by its GRAU (later GURVO) designation 8K71. At the time, the R-7 was known to NATO sources as the T-3 or M-104, and Type A. A special reconnaissance commission selected Tyuratam for the construction of a rocket proving ground (the 5th Tyuratam range, usually referred to as "NIIP-5", or "GIK-5" in the post-Soviet time). The selection was approved on 12 February 1955 by the Council of Ministers of the USSR, but the site would not be completed until 1958. Actual work on the construction of the site began on 20 July by military building units. On 14 June 1956 Sergei Korolev decided to adapt the R-7 rocket to the 'Object D', that would later be replaced by the much lighter 'Object PS'.
The first launch of an R-7 rocket (8K71 No.5L) occurred on 15 May 1957. A fire began in the Blok D strap-on almost immediately at liftoff, but the booster continued flying until T+98 seconds when the strap-on broke away and the vehicle crashed some 400 km downrange. Three attempts to launch the second rocket (8K71 No.6) were made on 10–11 June, but an assembly defect prevented launch. The unsuccessful launch of the third R-7 rocket (8K71 No.7) took place on 12 July. An electrical short caused the vernier engines to put the missile into an uncontrolled roll which resulted in all of the strap-ons separating 33 seconds into the launch. The R-7 crashed about 7 km from the pad.
The launch of the fourth rocket (8K71 No.8), on 21 August at 15:25 Moscow Time, was successful. The rocket's core boosted the dummy warhead to the target altitude and velocity, reentered the atmosphere, and broke apart at a height of 10 km (6.2 mi) after traveling 6,000 km. On 27 August TASS issued a statement on the successful launch of a long-distance multistage ICBM. The launch of the fifth R-7 rocket (8K71 No.9), on 7 September was also successful, but the dummy was also destroyed on atmospheric re-entry, and hence needed a redesign to completely fulfill its military purpose. The rocket, however, was deemed suitable for satellite launches, and Korolev was able to convince the State Commission to allow the use of the next R-7 to launch PS-1, allowing the delay in the rocket's military exploitation to launch the PS-1 and PS-2 satellites.
On 22 September a modified R-7 rocket, named Sputnik and indexed as 8K71PS, arrived at the proving ground and preparations for the launch of PS-1 began. Compared to the military R-7 test vehicles, the mass of 8K71PS was reduced from 280 tonnes to 272 tonnes; its length with PS-1 was 29.167 metres (95 ft 8.3 in) and the thrust at lift off was 3.90 MN (880,000 lbf). These weight reductions were accomplished by deleting the inertial guidance system, several telemetry measurements, and assorted hardware designed to support a warhead.
PS-1 was not designed to be controlled, it could only be observed. Initial data at the launch site would be collected at six separate observatories and telegraphed to NII-4. Located back in Moscow (at Bolshevo), NII-4 was a scientific research arm of the Ministry of Defence dedicated to missile development. The six observatories, designated IP-1 through IP-6, were clustered around the launch site, with the closest (IP-1) situated at a distance of 1 km (0.62 mi) from the launch pad.
A second, nationwide observation complex was established to track the satellite after its separation from the rocket. Called the Command-Measurement Complex, it consisted of the coordination center in NII-4 and seven distant stations situated along the line of the satellite's ground track. These tracking stations were located at Tyuratam; Sary-Shagan; Yeniseysk; Klyuchi; Yelizovo; Makat in Guryev Oblast; and Ishkup in Krasnoyarsk Krai. Stations were equipped with radar, optical instruments, and communications systems. Data from stations were transmitted by telegraphs into NII-4 where ballistics specialists calculated orbital parameters. The complex became an early prototype of the Soviet Mission Control Center.
The observatories used a trajectory measurement system called "Tral," developed by OKB MEI, by which they received and monitored data from transponders mounted on the R-7 rocket's core stage. The data was useful even after the satellite's separation from the second stage of the rocket; Sputnik's location was calculated from the data on the second stage's location which followed Sputnik at a known distance. Tracking of the booster during launch had to be accomplished through purely passive means such as visual coverage and radar detection. R-7 test launches demonstrated that the tracking cameras were only good up to an altitude of 200 km (120 mi) but radar could track it for almost 400 km (250 mi).
Outside the Soviet Union, the satellite was tracked by amateur radio operators in many countries. The US government followed it from the Central Radio Propagation Laboratory of the National Bureau of Standards, and the booster rocket was located and tracked by the Lovell Telescope at the Jodrell Bank Observatory. Canada's Newbrook Observatory was the first facility in North America to photograph Sputnik 1.
The chief constructor of Sputnik 1 at OKB-1 was Mikhail S. Khomyakov. The satellite was a 585-millimetre (23.0 in) diameter sphere, assembled from two hemispheres that were hermetically sealed with o-rings and connected by 36 bolts. It had a mass of 83.6 kilograms (184 lb). The hemispheres were 2 mm thick, and were covered with a highly polished 1 mm-thick heat shield made of aluminium-magnesium-titanium AMG6T alloy ("AMG" is an abbreviation for "aluminium-magnesium" and "T" stands for "titanium"; the alloy is 6% magnesium and 0.2% titanium). The satellite carried two pairs of antennas designed by the Antenna Laboratory of OKB-1 led by Mikhail V. Krayushkin. Each antenna was made up of two whip-like parts: 2.4 and 2.9 meters (7.9 and 9.5 ft) in length, and had an almost spherical radiation pattern, so that the satellite beeps were transmitted with equal power in all directions, making reception of the transmitted signal independent of the satellite's rotation.
The power supply, with a mass of 51 kg (112 lb), was in the shape of an octagonal nut with the radio transmitter in its hole. It consisted of three silver-zinc batteries, developed at the All-Union Research Institute of Current Sources (VNIIT) under the leadership of Nikolai S. Lidorenko. Two of these batteries powered the radio transmitter and one powered the temperature regulation system. The batteries had an expected lifetime of two weeks, and operated for 22 days. The power supply was turned on automatically at the moment of the satellite's separation from the second stage of the rocket.
The satellite had a one-watt, 3.5 kg (7.7 lb) radio transmitting unit inside, developed by Vyacheslav I. Lappo from NII-885, that worked on two frequencies, 20.005 and 40.002 MHz. Signals on the first frequency were transmitted in 0.3 sec pulses (under normal temperature and pressure conditions on-board), with pauses of the same duration filled by pulses on the second frequency. Analysis of the radio signals was used to gather information about the electron density of the ionosphere. Temperature and pressure were encoded in the duration of radio beeps. A temperature regulation system contained a fan, a dual thermal switch, and a control thermal switch. If the temperature inside the satellite exceeded 36 °C (97 °F) the fan was turned on and when it fell below 20 °C (68 °F) the fan was turned off by the dual thermal switch. If the temperature exceeded 50 °C (122 °F) or fell below 0 °C (32 °F), another control thermal switch was activated, changing the duration of the radio signal pulses. Sputnik 1 was filled with dry nitrogen, pressurized to 1.3 atm. The satellite had a barometric switch, activated if the pressure inside the satellite fell below 130 kPa, which would have indicated failure of the pressure vessel or puncture by a meteor, and would have changed the duration of radio signal impulse.
While attached to the rocket, Sputnik 1 was protected by a cone-shaped payload fairing, with a height of 80 cm (31.5 in). The fairing separated from both Sputnik and the spent R-7 second stage at the same time as the satellite was ejected. Tests of the satellite were conducted at OKB-1 under the leadership of Oleg G. Ivanovsky.
The control system of the Sputnik rocket was adjusted to an intended orbit of 223 by 1,450 km (139 by 901 mi), with an orbital period of 101.5 min. The trajectory had been calculated earlier by Georgi Grechko, using the USSR Academy of Sciences' mainframe computer.
The Sputnik rocket was launched on 4 October 1957 at 19:28:34 UTC (5 October at the launch site) from Site No.1 at NIIP-5. Telemetry indicated that the strap-ons separated 116 seconds into the flight and the core stage engine shut down 295.4 seconds into the flight. At shut down, the 7.5 tonne core stage with PS-1 attached had attained an altitude of 223 km (139 mi) above sea level, a velocity of 7,780 m/s (25,500 ft/s) and velocity vector inclination to the local horizon of 0 degrees 24 minutes. This resulted in an initial orbit of 223 kilometres (139 mi) by 950 kilometres (590 mi), with an apogee approximately 500 kilometres (310 mi) lower than intended, and an inclination of 65.1 degrees and a period of 96.2 minutes.
The launch came very close to failure—a postflight examination of telemetry data found that the Blok G strap-on had not attained full power at ignition and the resulting imbalanced thrust caused the booster to pitch over about 2° six seconds after liftoff. Two seconds later, the flight control system tried to compensate by rapidly moving the vernier engines and stabilizer fins. The Blok G strap-on finally reached 100% thrust only one second before the pitch angle would have been sufficient enough to trigger an automatic shutdown command, which would have terminated the launch and sent the R-7 and Sputnik 1 crashing to the ground in a fireball only a short distance from the pad.
A fuel regulator in the booster also failed around 16 seconds into launch, which resulted in excessive RP-1 consumption for most of powered flight and engine thrust 4% above nominal. Core stage cutoff was intended for T+296 seconds, but the premature propellant depletion caused thrust termination to occur one second earlier when a sensor detected overspeed of the empty RP-1 turbopump. There were 375 kilograms (827 lb) of LOX remaining at cutoff.
At 19.9 seconds after engine cut-off, PS-1 separated from the second stage and the satellite's transmitter was activated. These signals were detected at the IP-1 station by Junior Engineer-Lieutenant V.G. Borisov, where reception of Sputnik 1's "beep-beep-beep" tones confirmed the satellite's successful deployment. Reception lasted for two minutes, until PS-1 fell below the horizon. The Tral telemetry system on the R-7 core stage continued to transmit and was detected on its second orbit.
The designers, engineers and technicians who developed the rocket and satellite watched the launch from the range. After the launch they drove to the mobile radio station to listen for signals from the satellite. They waited about 90 minutes to ensure that the satellite had made one orbit and was transmitting, before Korolev called Soviet premier Nikita Khrushchev.
On the first orbit the Telegraph Agency of the Soviet Union (TASS) transmitted: "As result of great, intense work of scientific institutes and design bureaus the first artificial Earth satellite has been built". The R-7 core stage, with a mass of 7.5 tonnes and a length of 26 meters, also reached Earth orbit and was visible from the ground at night as a first magnitude object following the satellite. Deployable reflective panels were placed on the booster in order to increase its visibility for tracking. The satellite itself, a small, highly polished sphere, was barely visible at sixth magnitude, and thus more difficult to follow optically. A third object, the payload fairing, also achieved orbit.
The core stage of the R-7 remained in orbit for two months until 2 December 1957, while Sputnik 1 orbited until 4 January 1958, having completed 1,440 orbits of the Earth.
Our movies and television programs in the fifties were full of the idea of going into space. What came as a surprise was that it was the Soviet Union that launched the first satellite. It is hard to recall the atmosphere of the time.
The Soviets provided details of Sputnik 1 before the launch but few outside the Soviet Union noticed. After reviewing information publicly available before the launch, the science writer Willy Ley wrote in 1958:
If somebody tells me that he has the rockets to shoot — which we know from other sources, anyway — and tells me what he will shoot, how he will shoot it, and in general says virtually everything except for the precise date — well, what should I feel like if I'm surprised when the man shoots?"
Teams of visual observers at 150 stations in the United States and other countries were alerted during the night to watch for the Soviet sphere at dawn and during the evening twilight. They had been organized in Project Moonwatch to sight the satellite through binoculars or telescopes as it passed overhead. The USSR asked radio amateurs and commercial stations to record the sound of the satellite on magnetic tape.
News reports at the time pointed out that "anyone possessing a short wave receiver can hear the new Russian earth satellite as it hurtles over this area of the globe".[this quote needs a citation] Directions, provided by the American Radio Relay League were to "Tune in 20 megacycles sharply, by the time signals, given on that frequency. Then tune to slightly higher frequencies. The 'beep, beep' sound of the satellite can be heard each time it rounds the globe." The first recording of Sputnik 1's signal was made by RCA engineers near Riverhead, Long Island. They then drove the tape recording into Manhattan for broadcast to the public over NBC radio. However, as Sputnik rose higher over the East Coast, its signal was picked up by ham station W2AEE, the ham radio station of Columbia University. Students working in the university's FM station, WKCR, made a tape of this, and were the first to rebroadcast the Sputnik signal to the American public (or whoever could receive the FM station).
At first the Soviet Union agreed to use equipment "compatible" with that of the United States, but later announced the lower frequencies. The White House declined to comment on military aspects of the launch, but said "it did not come as a surprise." On 5 October the Naval Research Laboratory announced it had recorded four crossings of Sputnik 1 over the United States. The USAF Cambridge Research Center collaborated with Bendix-Friez, Westinghouse Broadcasting Co., Smithsonian Astrophysical Observatory, and MIT, to obtain a motion picture of Sputnik's rocket body crossing the pre-dawn sky of Baltimore, broadcast on 12 October by WBZ-TV in Boston. U.S. President Eisenhower obtained photographs of the Soviet facilities from Lockheed U-2 flights conducted since 1956.
The success of Sputnik 1 seemed to have changed minds around the world regarding a shift in power to the Soviets.
In Britain the media and population initially reacted with a mixture of fear for the future, but also amazement about humankind's progress. Many newspapers and magazines heralded the arrival of the Space Age. However, when the Soviet Union launched a second craft containing the dog Laika, the media narrative returned to one of anti-communism and many people sent protests to the Russian embassy and the RSPCA.
Sputnik 1 was not immediately used for Soviet propaganda. The Soviets had kept quiet about their earlier accomplishments in rocketry, fearing that it would lead to secrets being revealed and failures being exploited by the West. When the Soviets began using Sputnik in their propaganda, they emphasized pride in the achievement of Soviet technology, arguing that it demonstrated the Soviets' superiority over the West. People were encouraged to listen to Sputnik's signals on the radio and to look out for Sputnik in the night sky. While Sputnik itself had been highly polished, its small size made it barely visible to the naked eye. What most watchers actually saw was the much more visible 26 meter core stage of the R-7. Shortly after the launch of PS-1, Khrushchev pressed Korolev to launch another satellite in time for the 40th anniversary of the October Revolution on 7 November 1957.
The launch of Sputnik 1 surprised the American public and shattered the perception, furthered by American propaganda, of the United States as the technological superpower and the Soviet Union as a backward country. Privately, however, the CIA and President Eisenhower were aware of progress being made by the Soviets on Sputnik from secret spy plane imagery. Together with the Jet Propulsion Laboratory (JPL), the Army Ballistic Missile Agency built Explorer 1, and launched it on 31 January 1958. Before work was completed, however, the Soviet Union launched a second satellite, Sputnik 2, on 3 November 1957. Meanwhile, the televised failure of Vanguard TV3 on 6 December 1957 deepened American dismay over the country's position in the Space Race. The Americans took a more aggressive stance in the emerging space race, resulting in an emphasis on science and technological research and reforms in many areas from the military to education systems. The federal government began investing in science, engineering and mathematics at all levels of education. An advanced research group was assembled for military purposes. These research groups developed weapons such as ICBMs and missile defense systems, as well as spy satellites for the U.S.
On Friday, 4 October 1957, the Soviets had orbited the world's first artificial satellite. Anyone who doubted its existence could walk into the backyard just after sunset and see it.— Mike Gray, Angle of Attack
Initially U.S. President Eisenhower was not surprised by Sputnik 1. He had been forewarned of the R-7s capabilities by information derived from U-2 spy plane overflight photos as well as signals and telemetry intercepts. The Eisenhower administration's first response was low-key and almost dismissive. Eisenhower was even pleased that the USSR, not the U.S., would be the first to test the waters of the still-uncertain legal status of orbital satellite overflights. Eisenhower had suffered the Soviet protests and shoot-downs of Project Genetrix (Moby Dick) balloons and was concerned about the probability of a U-2 being shot down. To set a precedent for "freedom of space" before the launch of America's secret WS-117L spy satellites, the U.S. had launched Project Vanguard as its own "civilian" satellite entry for the International Geophysical Year. Eisenhower greatly underestimated the reaction of the American public, who were shocked by the launch of Sputnik and by the televised failure of the Vanguard Test Vehicle 3 launch attempt. The sense of fear was inflamed by Democratic politicians and professional cold warriors, who portrayed the United States as woefully behind. One of the many books that suddenly appeared for the lay-audience noted seven points of "impact" upon the nation: Western leadership, Western strategy and tactics, missile production, applied research, basic research, education, and democratic culture.
The U.S. soon had a number of successful satellites, including Explorer 1, Project SCORE, and Courier 1B. However, public reaction to the Sputnik crisis led to the creation of the Advanced Research Projects Agency (renamed the Defense Advanced Research Projects Agency or DARPA in 1972), NASA, and an increase in U.S. government spending on scientific research and education. Not only did the launch of Sputnik spur America to action in the space race, it also led directly to the creation of NASA through the space act bill.
Sputnik also contributed directly to a new emphasis on science and technology in American schools. With a sense of urgency, Congress enacted the 1958 National Defense Education Act, which provided low-interest loans for college tuition to students majoring in math and science. After the launch of Sputnik, a poll conducted and published by the University of Michigan showed that 26% of Americans surveyed thought that Russian sciences and engineering were superior to that of the United States. (A year later, however, that figure had dropped to 10% as the U.S. began launching its own satellites into space.)
One irony of the Sputnik event was the initially low-key response of the Soviet Union. The Communist Party newspaper Pravda only printed a few paragraphs about Sputnik 1 on 4 October. In the days following the world's startled response, the Soviets started celebrating their great accomplishment.
Sputnik also inspired a generation of engineers and scientists. Harrison Storms, the North American designer who was responsible for the X-15 rocket plane, and went on to head the effort to design the Apollo Command/Service Module and Saturn V launch vehicle's second stage was moved by the launch of Sputnik to think of space as being the next step for America. Astronauts Alan Shepard, who was the first American in space, and Deke Slayton later wrote of how the sight of Sputnik I passing overhead inspired them to their new careers. Homer Hickam's memoir Rocket Boys and the movie October Sky tell the story of how a coal miner's son, inspired by Sputnik, started building rockets in the mining town where he lived.
At least two vintage duplicates of Sputnik 1 exist, built apparently as backup units. One resides just outside Moscow in the corporate museum of Energia, the modern descendant of Korolev's design bureau, where it is on display by appointment only. Another is in the Museum of Flight in Seattle, Washington. Unlike Energia's unit, it has no internal components, but it does have casings and molded fittings inside (as well as evidence of battery wear), which suggest it was built as more than just a model. Authenticated by the Memorial Museum of Cosmonautics in Moscow, the unit was auctioned in 2001 and purchased by an anonymous private buyer who donated it to the museum. Two more Sputnik backups are said to be in the personal collections of American entrepreneurs Richard Garriot and Jay S. Walker.
In addition to the Soviets' donation to the UN, there are dozens of other full-size Sputnik replicas, more or less accurate, on display in locations around the world, including the National Air and Space Museum in Washington, D.C.; the Frontiers of Flight Museum and the Fort Worth Museum of Science and History, both in Texas; the Armstrong Air and Space Museum and the National Museum of the United States Air Force, both in Ohio; the Cosmosphere in Hutchinson, Kansas; the California Science Center in Los Angeles; the Science Museum, London; the World Museum in Liverpool; Powerhouse Museum in Sydney, Australia and outside the Russian embassy in Madrid, Spain.
Three one-third scale student-built replicas of Sputnik 1 were deployed from the Mir space station between 1997 and 1999. The first, named Sputnik 40 to commemorate the fortieth anniversary of the launch of Sputnik 1, was deployed in November 1997. Sputnik 41 was launched a year later, and Sputnik 99 was deployed in February 1999. A fourth replica was launched but never deployed, and was destroyed when Mir was deorbited.
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