Lunar Orbiter 5
Lunar Orbiter 5, the last of the Lunar Orbiter series, was designed to take additional Apollo and Surveyor landing site photography and to take broad survey images of unphotographed parts of the Moon's far side. It was equipped to collect selenodetic, radiation intensity, micrometeoroid impact data and was used to evaluate the Manned Space Flight Network tracking stations and Apollo Orbit Determination Program; the spacecraft was placed in a cislunar trajectory and on August 5, 1967 was injected into an elliptical near polar lunar orbit 194.5 by 6,023 kilometres with an inclination of 85 degrees and a period of 8 hours 30 minutes. On August 7 the perilune was lowered to 100 kilometers, on August 9 the orbit was lowered to a 99-by-1,499-kilometer, 3 hour 11 minute period; the spacecraft acquired photographic data from August 6 to 18, 1967, readout occurred until August 27, 1967. A total of 633 high resolution and 211 medium resolution frames at resolution down to 2 meters were acquired, bringing the cumulative photographic coverage by the five Lunar Orbiter craft to 99% of the Moon's surface.
Accurate data were acquired from all other experiments throughout the mission. The spacecraft was tracked until it struck the lunar surface on command at 2.79 degrees S latitude, 83 degrees W longitude on January 31, 1968. Features on the near side of the Moon that were photographic targets included Petavius, Messier, Copernicus, Vitello, Mons Gruithuisen Gamma, Aristarchus, Vallis Schroteri, Marius Hills, Montes Apenninus, Rimae Plato, Sinus Aestuum, Rimae Sulpicius Gallus, Rimae Calippus, Censorinus and the future landing site of Apollo 11. Lunar Orbiter Image Recovery Project Exploration of the Moon Lunar Orbiter 1 Lunar Orbiter 2 Lunar Orbiter 3 Lunar Orbiter 4 DESTINATION MOON: A history of the Lunar Orbiter Program 1976 Lunar Orbiter Photo Gallery - Mission 5 at the Lunar and Planetary Institute
Hughes Aircraft Company
The Hughes Aircraft Company was a major American aerospace and defense contractor founded in 1932 by Howard Hughes in Glendale, California as a division of Hughes Tool Company. The company was known for producing, among other products, the Hughes H-4 Hercules Spruce Goose aircraft, the atmospheric entry probe carried by the Galileo spacecraft, the AIM-4 Falcon guided missile. Hughes Aircraft was acquired by General Motors from the Howard Hughes Medical Institute in 1985 and was put under the umbrella of Hughes Electronics, now known as DirecTV, until GM sold its assets to Raytheon in 1997. During World War II the company built several prototype aircraft at Hughes Airport; these included the famous Hughes H-4 Hercules, better known by the public's nickname for it, the Spruce Goose, the H-1 racer, D-2, the XF-11. However the plant's hangars at Hughes Airport, location of present-day Playa Vista in the Westside of Los Angeles, were used as a branch plant for the construction of other companies' designs.
At the start of the war Hughes Aircraft had only four full-time employees—by the end the number was 80,000. During the war, the company was awarded contracts to build B-25 struts, centrifugal cannons, machine gun feed chutes. Hughes Aircraft was one of many aerospace and defense companies which flourished in Southern California during and after World War II and was at one time the largest employer in the area. Yet, employment had dropped to 800 by 1947. By the summer of 1947 certain politicians had become concerned about Hughes' alleged mismanagement of the Spruce Goose and the XF-11 photo reconnaissance plane project, they formed a special committee to investigate Hughes which culminated in a much-followed Senate investigation, one of the first to be televised to the public. Despite a critical committee report, Hughes was cleared; the company expanded into the booming electronics field employing 3,300 Ph. D.s. Hughes hired Ira Eaker, Harold L. George, Tex Thornton to run the company. By 1953, the company employed 17,000 had a $600,000,000 in government contracts.
In 1948 Hughes created a new division of the Aerospace Group. Two Hughes engineers, Simon Ramo and Dean Wooldridge, had new ideas on the packaging of electronics to make complete fire control systems, their MA-1 system combined signals from the aircraft's radar with a digital computer to automatically guide the interceptor aircraft into the proper position for firing missiles. At the same time other teams were working with the newly formed US Air Force on air-to-air missiles, delivering the AIM-4 Falcon known as the F-98; the MA-1/Falcon package, with several upgrades, was the primary interceptor weapon system of the USAF for many years, lasting into the 1980s. Ramo and Wooldridge, having failed to reach an agreement with Howard Hughes regarding management problems, resigned in September 1953 and founded the Ramo-Wooldridge Corporation to join Thompson Products to form the Thompson-Ramo-Wooldridge based in Canoga Park, with Hughes leasing space for nuclear research programs (present day West Hills.
The company became TRW in another aerospace company and a major competitor to Hughes Aircraft. In 1951 Hughes Aircraft Co. built a missile plant in Arizona. The construction of this plant, wrote David Leighton, in the Arizona Daily Star newspaper, was due to "Howard Hughes’ long-held fear that his plant in Culver City, was vulnerable to enemy attack because it was on the Pacific Coast." By the end of that year, the U. S. Air Force had purchased the property but allowed the company to continue to run day to day operations of the site; this Tucson plant is still in operation under the ownership of Raytheon Co. Howard Hughes donated Hughes Aircraft to the newly formed Howard Hughes Medical Institute in 1953 as a way of avoiding taxes on its huge income; the next year, L. A. "Pat" Hyland was hired as general manager of Hughes Aircraft. Under Hyland's guidance, the Aerospace Group continued to diversify and become massively profitable, became a primary focus of the company; the company developed radar systems, electro-optical systems, the first working laser, aircraft computer systems, missile systems, ion-propulsion engines, many other advanced technologies.
The'Electronic Properties Information Center' of the United States was hosted at the Hughes Culver City library in the 1970s. EPIC published the multi-volume Handbook of Electronic Materials as public documents. Nobel Laureates Richard Feynman and Murray Gell-Mann had Hughes connections: Feynman would hold weekly seminars at Hughes Research Laboratories. Greg Jarvis and Ronald McNair, two of the astronauts on the last flight of the Space Shuttle Challenger, were Hughes alumni. Hughes Aircraft Ground Systems Group was located in California; the facility was 3 million square feet and included manufacturing, offices, a Munson road test course. It designed developed and produced the Air Defense Systems that replaced the Semi Automatic Defense Ground Environment in the United States with the Joint Surveillance System AN/FYQ-93 including NORAD with Joint Tactical Information Distribution System and provided defense systems and air traffic control systems around the world; these systems are massive and at its peak Ground Systems Group employed 15,000 people and generated revenue in excess of $1 billion per year.
They were the largest revenue producer and with its massive systems engineering division coordinated the inclusion of
Soyuz 1
Soyuz 1 was a manned spaceflight of the Soviet space program. Launched into orbit on 23 April 1967 carrying cosmonaut Colonel Vladimir Komarov, Soyuz 1 was the first crewed flight of the Soyuz spacecraft; the flight was plagued with technical issues, Komarov was killed when the descent module crashed into the ground due to a parachute failure. This was the first in-flight fatality in the history of spaceflight; the original mission plan was complex, involving a rendezvous with Soyuz 2 and an exchange of crew members before returning to Earth. However, the launch of Soyuz 2 was called off due to thunderstorms. Mass: 6,450 kg Perigee: 197 km Apogee: 223 km Inclination: 50.8° Period: 88.7 minutes Soyuz 1 was the first manned flight of the first-generation Soyuz 7K-OK spacecraft and Soyuz rocket, designed as part of the Soviet lunar program. It was the first Soviet manned spaceflight in over two years, the first Soviet manned flight following the death of the Chief Designer of the space program Sergei Korolev.
Komarov was launched on Soyuz 1 despite failures of the previous unmanned tests of the 7K-OK, Cosmos 133 and Cosmos 140. A third attempted test flight was a launch failure; the escape system pulled the spacecraft to safety. Prior to launch, Soyuz 1 engineers are said to have reported 203 design faults to party leaders, but their concerns "were overruled by political pressures for a series of space feats to mark the anniversary of Lenin's birthday." It is not clear how much of this pressure resulted from wanting to continue beating the United States in the Space Race and to have Soviets first on the Moon, or to take advantage of the recent setbacks in the U. S. space program with the Apollo 1 disaster. Yuri Gagarin was the backup pilot for Soyuz 1, was aware of the design problems and the pressures from the Politburo to proceed with the flight, he attempted to "bump" Komarov from the mission, knowing that the Soviet leadership would not risk a national hero on the flight. At the same time, Komarov refused to pass on the mission though he believed it to be doomed.
He explained. Mission planners intended to launch a second Soyuz flight the next day carrying cosmonauts Valery Bykovsky, Yevgeny Khrunov, Aleksei Yeliseyev, with Khrunov and Yeliseyev scheduled to do an EVA over to Soyuz 1. Soyuz 1 was launched on 23 April 1967 at 00:32 UTC from Baikonur Cosmodrome carrying Komarov, the first Soviet cosmonaut to fly in space twice. Problems began shortly after launch when one solar panel failed to unfold, leading to a shortage of power for the spacecraft's systems. Further problems with the orientation detectors complicated maneuvering the craft. By orbit 13, the automatic stabilization system was dead, the manual system was only effective; the crew of Soyuz 2 modified their mission goals, preparing themselves for a launch that would include fixing the solar panel of Soyuz 1. However, that night, thunderstorms at Baikonur Cosmodrome in Kazakhstan affected the booster's electrical system, causing the mission to be called off; as a result of Komarov's report during the 13th orbit, the flight director decided to abort the mission.
After 18 orbits, Soyuz 1 reentered the Earth's atmosphere. Despite the technical difficulties up to that point, Komarov might still have landed safely. To slow the descent, first the drogue parachute was deployed, followed by the main parachute. However, due to a defect, the main parachute did not unfold. Komarov activated the manually deployed reserve chute, but it became tangled with the drogue chute, which did not release as intended; as a result, the Soyuz reentry module fell to Earth in Orenburg Oblast entirely unimpeded, at about 40 m/s. A rescue helicopter spotted the descent module lying on its side with the parachute spread across the ground; the retrorockets started firing which concerned the rescuers since they were supposed to activate a few moments prior to touchdown. By the time they landed and approached, the descent module was in flames with black smoke filling the air and streams of molten metal dripping from the exterior; the entire base of the capsule burned through. By this point, it was obvious that Komarov had not survived, but there was no code signal for a cosmonaut's death, so the rescuers fired a signal flare calling for medical assistance.
Another group of rescuers in an aircraft arrived and attempted to extinguish the blazing spacecraft with portable fire extinguishers. This proved insufficient and they instead began using shovels to throw dirt onto it; the descent module completely disintegrated, leaving only a pile of debris topped by the entry hatch. When the fire at last ended, the rescuers were able to dig through the rubble to find Komarov's remains strapped into the center couch. Doctors pronounced the cause of death to be from multiple blunt-force injuries; the body was transported to Moscow for an official autopsy in a military hospital where the cause of death was verified to match the field doctors' conclusions. The Soyuz 1 crash site coordinates are 51.3615°N 59.5622°E / 51.3615. This is about 275 km east-southeast of Orenburg. There is a memorial monument at the site in the form of a black column with a bust of Komarov at the top, in a small park on the roadside. Eight years after Komarov's death, a story began circulating that Komarov cursed the engineers and flight staff, spoke to his wife as he descended, these transmi
Alpha particle
Alpha particles called alpha ray or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are produced in the process of alpha decay, but may be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α; the symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are sometimes written as He2+ or 42He2+ indicating a helium ion with a +2 charge. If the ion gains electrons from its environment, the alpha particle becomes a normal helium atom 42He. Alpha particles, like helium nuclei, have a net spin of zero. Due to the mechanism of their production in standard alpha radioactive decay, alpha particles have a kinetic energy of about 5 MeV, a velocity in the vicinity of 5% the speed of light, they are a ionizing form of particle radiation, have low penetration depth. They can be stopped by the skin. However, so-called long range alpha particles from ternary fission are three times as energetic, penetrate three times as far.
As noted, the helium nuclei that form 10–12% of cosmic rays are usually of much higher energy than those produced by nuclear decay processes, are thus capable of being penetrating and able to traverse the human body and many meters of dense solid shielding, depending on their energy. To a lesser extent, this is true of high-energy helium nuclei produced by particle accelerators; when alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest, due to the high relative biological effectiveness of alpha radiation to cause biological damage. Alpha radiation is an average of about 20 times more dangerous, in experiments with inhaled alpha emitters, up to 1000 times more dangerous than an equivalent activity of beta emitting or gamma emitting radioisotopes; some science authors use alpha particles as interchangeable terms. The nomenclature is not well defined, thus not all high-velocity helium nuclei are considered by all authors to be alpha particles.
As with beta and gamma particles/rays, the name used for the particle carries some mild connotations about its production process and energy, but these are not rigorously applied. Thus, alpha particles may be loosely used as a term when referring to stellar helium nuclei reactions, when they occur as components of cosmic rays. A higher energy version of alphas than produced in alpha decay is a common product of an uncommon nuclear fission result called ternary fission. However, helium nuclei produced by particle accelerators are less to be referred to as "alpha particles"; the best-known source of alpha particles is alpha decay of heavier atoms. When an atom emits an alpha particle in alpha decay, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle; the atomic number of the atom goes down by two, as a result of the loss of two protons – the atom becomes a new element. Examples of this sort of nuclear transmutation are when uranium becomes thorium, or radium becomes radon gas, due to alpha decay.
Alpha particles are emitted by all of the larger radioactive nuclei such as uranium, thorium and radium, as well as the transuranic elements. Unlike other types of decay, alpha decay as a process must have a minimum-size atomic nucleus that can support it; the smallest nuclei that have to date been found to be capable of alpha emission are beryllium-8 and the lightest nuclides of tellurium, with mass numbers between 104 and 109. The process of alpha decay sometimes leaves the nucleus in an excited state, wherein the emission of a gamma ray removes the excess energy. In contrast to beta decay, the fundamental interactions responsible for alpha decay are a balance between the electromagnetic force and nuclear force. Alpha decay results from the Coulomb repulsion between the alpha particle and the rest of the nucleus, which both have a positive electric charge, but, kept in check by the nuclear force. In classical physics, alpha particles do not have enough energy to escape the potential well from the strong force inside the nucleus.
However, the quantum tunnelling effect allows alphas to escape though they do not have enough energy to overcome the nuclear force. This is allowed by the wave nature of matter, which allows the alpha particle to spend some of its time in a region so far from the nucleus that the potential from the repulsive electromagnetic force has compensated for the attraction of the nuclear force. From this point, alpha particles can escape, in quantum mechanics, after a certain time, they do so. Energetic alpha particles deriving from a nuclear process are produced in the rare nuclear fission process of ternary fission. In this process, three charged particles are produced from the event instead of the normal two, with the smallest of the charged particles most being an alpha particle; such alpha particles are termed "long range alphas" since at their typical energy of 16 MeV, they are at far higher energy than is produced by alpha decay. Ternary fission happens in both neutron-induced fission (the nuclear reacti
Surveyor 1
Surveyor 1 was the first lunar soft-lander in the unmanned Surveyor program of the National Aeronautics and Space Administration. This lunar soft-lander gathered data about the lunar surface that would be needed for the manned Apollo Moon landings that began in 1969; the successful soft landing of Surveyor 1 on the Ocean of Storms was the first by an American space probe on any extraterrestrial body, occurring on the first attempt and just four months after the first Moon landing by the Soviet Union's Luna 9 probe. Surveyor 1 was launched May 30, 1966, from the Cape Canaveral Air Force Station at Cape Canaveral, it landed on the Moon on June 2, 1966. Surveyor 1 transmitted 11,237 still photos of the lunar surface to the Earth by using a television camera and a sophisticated radio-telemetry system; the Surveyor program was managed by the Jet Propulsion Laboratory, in Los Angeles County, but the Surveyor space probe was designed by Gary Mizuhara of EOS and built by the Hughes Aircraft Company in El Segundo, California.
The Surveyor series of space probes was designed to carry out the first soft landings on the Moon by any American spacecraft. No instrumentation was carried for scientific experiments by Surveyor 1, but considerable scientific data were collected by its television camera and returned to Earth via the Deep Space Network from 1966 to 1967; these spacecraft carried two television cameras — one for its approach, not used in this case, one for taking still pictures of the lunar surface. Over 100 engineering sensors were on board each Surveyor, their television systems transmitted pictures of the spacecraft footpad and surrounding lunar terrain and surface materials. These spacecraft acquired data on the radar reflectivity of the lunar surface, the load-bearing strength of the lunar surface, the temperatures for use in the analysis of the lunar surface temperatures. Surveyor 1 was launched May 30, 1966 and sent directly into a trajectory to the Moon without any parking orbit, its retrorockets were turned off at a height of about 3.4 meters above the lunar surface.
Surveyor 1 fell to the surface from this height, it landed on the lunar surface on June 2, 1966, on the Oceanus Procellarum. This location was at 2.474°S 43.339°W / -2.474. The duration of the spaceflight of Surveyor 1 was 30 minutes. Surveyor I's lunar launch weight was about 995.2 kilograms, its landing weight was about 294.3 kilograms. Surveyor 1 transmitted video data from the Moon beginning shortly after its landing through July 14, 1966, but with a period of no operations during the two-week long lunar night of June 14, 1966 through July 7, 1966; because the Moon always presents the same face to Earth, "line-of-sight" radio communications with Surveyor 1 required only changes in ground stations as the Earth rotated. However, since it was solar-powered, Surveyor 1 had no electricity with which to do anything at all during the two weeks of the lunar nights; the return of engineering information from Surveyor 1 continued through January 7, 1967, with several interruptions during the lunar nights.
The landing of Surveyor 1 was carried live on some television networks, the success of the first Surveyor landing was considered surprising after the failure of a number of the Ranger spacecraft en route to the Moon. Justin Rennilson of Jet Propulsion Laboratory, stated, "We figured the probability of success at around 10 to 15 percent." Among hundreds of other challenges, an uninterrupted communication link for navigation and control was critical to success. The TV camera consisted of a vidicon tube, a zoom lens operated at either end of its range resulting in 25 millimeter and 100 millimeter focal-lengths, resulting in optical fields of view of 25.3 or 6.43 degrees, a shutter, several optical filters, iris-system mounted along an axis inclined 16 degrees from the central axis of Surveyor 1. The camera was mounted under a mirror that could be moved in elevation; the rotation of the mirror in the azimuth direction, while providing azimuth coverage capability results in an image rotation proportional to the angular azimuth position of the mirror.
This is because the image plane and scanning raster of the vidicon are stationary with respect to the mirror azimuth axis. The mirror drive mechanism consisting of stepper motors provided a step size of 2.48° ±0.1° in elevation and 3.0° ±0.1° in azimuth. This calibrated stepping reference allowed the creation of large composite mosaics of the lunar surface and using the data read-back from the iris and focus positioning of the lens permitted some photogrammetric measurements of various lunar features; the TV camera's operation was dependent on the receipt of the proper radio commands from the Earth. Frame-by-frame coverage of the lunar surface was obtained over 360 degrees in azimuth and from +40 degrees above the plane normal to the camera's axis to -65 degrees below this plane. Both 600-line and 200-line modes of operation were used; the 200-line mode transmitted over an omnidirectional antenna for the first 14 photos and scanned one frame every 61.8 seconds. The remaining transmissions were of 600-line pictures over a directional antenna, each frame was scanned every 3.6 seconds.
Each 200-line picture required 20 seconds for a complete video transmission and it used a radio bandwidth of about 1.2 kilohertz. Each 600-line pictu
Surveyor 7
Surveyor 7 was the seventh and last lunar lander of the American unmanned Surveyor program sent to explore the surface of the Moon. A total of 21,091 pictures were transmitted to Earth. Surveyor 7 was the fifth and final spacecraft of the Surveyor series to achieve a lunar soft landing; the objectives for this mission were to perform a lunar soft landing. This spacecraft was similar in design to the previous Surveyors, but it carried more scientific equipment including a television camera with polarizing filters, a surface sampler, bar magnets on two footpads, two horseshoe magnets on the surface scoop, auxiliary mirrors. Of the auxiliary mirrors, three were used to observe areas below the spacecraft, one to provide stereoscopic views of the surface sampler area, seven to show lunar material deposited on the spacecraft; the spacecraft landed on the lunar surface on January 10, 1968, on the outer rim of the crater Tycho. Operations of the spacecraft began shortly after the soft landing and were terminated on January 26, 1968, 80 hours after sunset.
On January 20, while the craft was still in daylight, the TV camera saw two laser beams aimed at it from the night side of the crescent Earth, one from Kitt Peak National Observatory, Tucson and the other at Table Mountain at Wrightwood, California. Operations on the second lunar day occurred from February 12 to 21, 1968; the mission objectives were satisfied by the spacecraft operations. Battery damage was suffered during the first lunar night and transmission contact was subsequently sporadic. Contact with Surveyor 7 was lost on February 21, 1968, it was planned to be visited by the cancelled Apollo 20 mission, however Skylab and subsequent budget cuts stopped this from happening. Surveyor 7 was the first probe to detect the faint glow on the lunar horizon after dark, now thought to be light reflected from electrostatically levitated Moon dust; the TV camera consisted of a vidicon tube, 25 and 100 mm focal length lenses, polarizing filters, iris mounted nearly vertically and surmounted by a mirror that could be adjusted by stepping motors to move in both azimuth and elevations.
The polarizing filters served as analyzers for the detection of measurements of the linearly polarized component of light scattered from the lunar surface. The frame by frame coverage of the lunar surface provided a 360 deg azimuth view and an elevation view from +90 deg above the plane normal to the camera A axis to -60 deg below this same plane. Both 600 line and 200 line modes of operation were used; the 200 line mode transmitted over an omnidirectional antenna and scanned one frame each 61.8 seconds. A complete video transmission of each 200 line picture required 20 seconds and utilized a bandwidth of 1.2 kHz. Most transmissions consisted of 600 line pictures; the frames were scanned each 3.6 seconds. Each frame required nominally one second to be read from the vidicon and utilized a 220 kHz bandwidth for transmission; the dynamic range and sensitivity of this camera were less than those on the Surveyor 6 camera. Resolution and quality were excellent; the television images were displayed on a slow scan monitor coated with a long persistency phosphor.
The persistency was selected to optimally match the nominal maximum frame rate. One frame of TV identification was received for each incoming TV frame and was displayed in real time at a rate compatible with that of the incoming image; these data were recorded on 70 mm film. The camera transmitted 20,961 pictures during the first lunar day, January 10 to January 22, 1968. From February 12 to February 14, the camera was operated in the 200 line mode because of loss of horizontal sweep in the 600 line mode. During the second lunar day, 45 pictures were transmitted before loss of power caused suspension of camera operation; the alpha-scattering surface analyzer was designed to measure directly the abundances of the major elements of the lunar surface. The instrumentation consisted of an alpha source collimated to irradiate a 10 mm diameter opening in the bottom of the instrument where the sample was located and two parallel but independent charged particle detector systems. One system, containing two sensors, detected the energy spectra of the alpha particles scattered from the lunar surface, the other, containing four sensors, detected energy spectra of the protons produced via reaction in the surface material.
Each detector assembly was connected to a pulse height analyzer. A digital electronics package, located in a compartment on the spacecraft, continuously telemetered signals to earth whenever the experiment was operating; the spectra contained quantitative information on all major elements in the samples except for hydrogen and lithium. The experiment provided 46 hours of data accumulated from three lunar surface sample measurements; these measurements were of a portion of undisturbed local lunar surface, a lunar rock, an extensively trenched area of the lunar surface. Data were obtained during the first and second lunar days, January 12 to 23, 1968, February 13 to 21, 1968; the alpha backscattering instrument failed to deploy properly. Mission controllers used the surface soil sampler claw to push the alpha backscattering instrument into the proper position to conduct its experiments; the soil mechanics surface sampler was designed to p
Moon
The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, the largest among planetary satellites relative to the size of the planet that it orbits; the Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known. The Moon is thought to have formed not long after Earth; the most accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia. The Moon is in synchronous rotation with Earth, thus always shows the same side to Earth, the near side; the near side is marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. After the Sun, the Moon is the second-brightest visible celestial object in Earth's sky, its surface is dark, although compared to the night sky it appears bright, with a reflectance just higher than that of worn asphalt.
Its gravitational influence produces the ocean tides, body tides, the slight lengthening of the day. The Moon's average orbital distance is 1.28 light-seconds. This is about thirty times the diameter of Earth; the Moon's apparent size in the sky is the same as that of the Sun, since the star is about 400 times the lunar distance and diameter. Therefore, the Moon covers the Sun nearly during a total solar eclipse; this matching of apparent visual size will not continue in the far future because the Moon's distance from Earth is increasing. The Moon was first reached in September 1959 by an unmanned spacecraft; the United States' NASA Apollo program achieved the only manned lunar missions to date, beginning with the first manned orbital mission by Apollo 8 in 1968, six manned landings between 1969 and 1972, with the first being Apollo 11. These missions returned lunar rocks which have been used to develop a geological understanding of the Moon's origin, internal structure, the Moon's history. Since the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft.
Both the Moon's natural prominence in the earthly sky and its regular cycle of phases as seen from Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in language, lunar calendar systems and mythology; the usual English proper name for Earth's natural satellite is "the Moon", which in nonscientific texts is not capitalized. The noun moon is derived from Old English mōna, which stems from Proto-Germanic *mēnô, which comes from Proto-Indo-European *mḗh₁n̥s "moon", "month", which comes from the Proto-Indo-European root *meh₁- "to measure", the month being the ancient unit of time measured by the Moon; the name "Luna" is used. In literature science fiction, "Luna" is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of Earth's moon; the modern English adjective pertaining to the Moon is lunar, derived from the Latin word for the Moon, luna. The adjective selenic is so used to refer to the Moon that this meaning is not recorded in most major dictionaries.
It is derived from the Ancient Greek word for the Moon, σελήνη, from, however derived the prefix "seleno-", as in selenography, the study of the physical features of the Moon, as well as the element name selenium. Both the Greek goddess Selene and the Roman goddess Diana were alternatively called Cynthia; the names Luna and Selene are reflected in terminology for lunar orbits in words such as apolune and selenocentric. The name Diana comes from the Proto-Indo-European *diw-yo, "heavenly", which comes from the PIE root *dyeu- "to shine," which in many derivatives means "sky and god" and is the origin of Latin dies, "day"; the Moon formed 4.51 billion years ago, some 60 million years after the origin of the Solar System. Several forming mechanisms have been proposed, including the fission of the Moon from Earth's crust through centrifugal force, the gravitational capture of a pre-formed Moon, the co-formation of Earth and the Moon together in the primordial accretion disk; these hypotheses cannot account for the high angular momentum of the Earth–Moon system.
The prevailing hypothesis is that the Earth–Moon system formed after an impact of a Mars-sized body with the proto-Earth. The impact blasted material into Earth's orbit and the material accreted and formed the Moon; the Moon's far side has a crust, 30 mi thicker than that of the near side. This is thought to be; this hypothesis, although not perfect best explains the evidence. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona, the giant impact hypothesis emerged as the most consensual theory. Before the conference, there were parti
