A meteoroid ( / ˈ m iː t i ə r ɔɪ d / MEE -tee-ə-royd ) is a small rocky or metallic body in outer space . Meteoroids are distinguished as objects significantly smaller than asteroids , ranging in size from grains to objects up to a meter wide. Objects smaller than meteoroids are classified as micrometeoroids or space dust . Many are fragments from comets or asteroids, whereas others are collision impact debris ejected from bodies such as the Moon or Mars .
176-546: A meteor or shooting star is the visible passage of a meteoroid, comet, or asteroid entering Earth's atmosphere . At a speed typically in excess of 20 km/s (72,000 km/h; 45,000 mph), aerodynamic heating of that object produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. Meteors typically become visible when they are about 100 km (62 mi) above sea level. A series of many meteors appearing seconds or minutes apart and appearing to originate from
352-545: A 19-species model. An important aspect of modelling non-equilibrium real gas effects is radiative heat flux. If a vehicle is entering an atmosphere at very high speed (hyperbolic trajectory, lunar return) and has a large nose radius then radiative heat flux can dominate TPS heating. Radiative heat flux during entry into an air or carbon dioxide atmosphere typically comes from asymmetric diatomic molecules; e.g., cyanogen (CN), carbon monoxide , nitric oxide (NO), single ionized molecular nitrogen etc. These molecules are formed by
528-471: A Gibbs free energy equilibrium program, the iterative process from the originally specified molecular composition to the final calculated equilibrium composition is essentially random and not time accurate. With a non-equilibrium program, the computation process is time accurate and follows a solution path dictated by chemical and reaction rate formulas. The five species model has 17 chemical formulas (34 when counting reverse formulas). The Lighthill-Freeman model
704-499: A challenge. The experimental measurement of radiative heat flux (typically done with shock tubes) along with theoretical calculation through the unsteady Schrödinger equation are among the more esoteric aspects of aerospace engineering. Most of the aerospace research work related to understanding radiative heat flux was done in the 1960s, but largely discontinued after conclusion of the Apollo Program. Radiative heat flux in air
880-459: A complete sphere or a spherical section forebody with a converging conical afterbody. The aerodynamics of a sphere or spherical section are easy to model analytically using Newtonian impact theory. Likewise, the spherical section's heat flux can be accurately modeled with the Fay–Riddell equation . The static stability of a spherical section is assured if the vehicle's center of mass is upstream from
1056-435: A converging conical afterbody. It flew a lifting entry with a hypersonic trim angle of attack of −27° (0° is blunt-end first) to yield an average L/D (lift-to-drag ratio) of 0.368. The resultant lift achieved a measure of cross-range control by offsetting the vehicle's center of mass from its axis of symmetry, allowing the lift force to be directed left or right by rolling the capsule on its longitudinal axis . Other examples of
1232-461: A cosmic origin. After reviewing historical records, Heinrich Wilhelm Matthias Olbers predicted the storm's return in 1867, drawing other astronomers' attention to the phenomenon. Hubert A. Newton 's more thorough historical work led to a refined prediction of 1866, which proved correct. With Giovanni Schiaparelli 's success in connecting the Leonids (as they are called) with comet Tempel-Tuttle ,
1408-425: A decisive role on local surface temperatures . Parts of many craters, particularly the bottoms of many polar craters, are permanently shadowed, these " craters of eternal darkness " have extremely low temperatures. The Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F) and just 26 K (−247 °C; −413 °F) close to
1584-736: A deviation for any object causing a meteor. Objects smaller than meteoroids are classified as micrometeoroids and interplanetary dust . The Minor Planet Center does not use the term "meteoroid". Almost all meteoroids contain extraterrestrial nickel and iron. They have three main classifications: iron, stone, and stony-iron. Some stone meteoroids contain grain-like inclusions known as chondrules and are called chondrites . Stony meteoroids without these features are called " achondrites ", which are typically formed from extraterrestrial igneous activity; they contain little or no extraterrestrial iron. The composition of meteoroids can be inferred as they pass through Earth's atmosphere from their trajectories and
1760-434: A few kilometers wide), shallower, and more irregularly shaped than impact craters. They also lack the upturned rims characteristic of impact craters. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side maria. There are also some regions of pyroclastic deposits , scoria cones and non-basaltic domes made of particularly high viscosity lava. Almost all maria are on
1936-455: A gas in equilibrium with fixed pressure and temperature can be determined through the Gibbs free energy method . Gibbs free energy is simply the total enthalpy of the gas minus its total entropy times temperature. A chemical equilibrium program normally does not require chemical formulas or reaction-rate equations. The program works by preserving the original elemental abundances specified for
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#17327718397192112-572: A gas that are important to aeronautical engineers who design heat shields: Almost all aeronautical engineers are taught the perfect (ideal) gas model during their undergraduate education. Most of the important perfect gas equations along with their corresponding tables and graphs are shown in NACA Report 1135. Excerpts from NACA Report 1135 often appear in the appendices of thermodynamics textbooks and are familiar to most aeronautical engineers who design supersonic aircraft. The perfect gas theory
2288-503: A global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating. Early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. This early dynamo field apparently expired by about one billion years ago, after the lunar core had crystallized. Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through
2464-574: A heat shield designer must use a real gas model . An entry vehicle's pitching moment can be significantly influenced by real-gas effects. Both the Apollo command module and the Space Shuttle were designed using incorrect pitching moments determined through inaccurate real-gas modelling. The Apollo-CM's trim-angle angle of attack was higher than originally estimated, resulting in a narrower lunar return entry corridor. The actual aerodynamic center of
2640-718: A hospitable environment for future astronauts, protecting them from extreme temperatures, solar radiation, and micrometeorites. However, challenges include accessibility and risks of avalanches and cave-ins. This discovery offers potential for future lunar bases or emergency shelters. The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains called maria (singular mare ; Latin for "seas", as they were once believed to be filled with water) are vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. The majority of these lava deposits erupted or flowed into
2816-533: A landscape featuring craters of all ages. The Moon was volcanically active until 1.2 billion years ago, which laid down the prominent lunar maria . Most of the mare basalts erupted during the Imbrian period , 3.3–3.7 billion years ago, though some are as young as 1.2 billion years and some as old as 4.2 billion years. There are differing explanations for the eruption of mare basalts, particularly their uneven occurrence which mainly appear on
2992-416: A meteoroid would be between 100 μm and 10 m (33 ft) across. In 2010, following the discovery of asteroids below 10 m in size, Rubin and Grossman proposed a revision of the previous definition of meteoroid to objects between 10 μm (0.00039 in) and one meter (3 ft 3 in) in diameter in order to maintain the distinction. According to Rubin and Grossman, the minimum size of an asteroid
3168-656: A mock-up of the ablative material to be analyzed within a hypersonic wind tunnel. Testing of ablative materials occurs at the Ames Arc Jet Complex. Many spacecraft thermal protection systems have been tested in this facility, including the Apollo, space shuttle, and Orion heat shield materials. Carbon phenolic was originally developed as a rocket nozzle throat material (used in the Space Shuttle Solid Rocket Booster ) and for reentry-vehicle nose tips. The thermal conductivity of
3344-580: A more rigid definition. It defines a fireball as a meteor that would have a magnitude of −3 or brighter if seen at zenith . This definition corrects for the greater distance between an observer and a meteor near the horizon. For example, a meteor of magnitude −1 at 5 degrees above the horizon would be classified as a fireball because, if the observer had been directly below the meteor, it would have appeared as magnitude −6. Fireballs reaching apparent magnitude −14 or brighter are called bolides . The IAU has no official definition of "bolide", and generally considers
3520-454: A nose radius of 1 meter, i.e., time of travel is about 18 microseconds. This is roughly the time required for shock-wave-initiated chemical dissociation to approach chemical equilibrium in a shock layer for a 7.8 km/s entry into air during peak heat flux. Consequently, as air approaches the entry vehicle's stagnation point, the air effectively reaches chemical equilibrium thus enabling an equilibrium model to be usable. For this case, most of
3696-402: A nose radius of 2.34 cm, a forward-frustum half-angle of 10.4°, an inter-frustum radius of 14.6 cm, aft-frustum half-angle of 6°, and an axial length of 2.079 meters. No accurate diagram or picture of AMaRV has ever appeared in the open literature. However, a schematic sketch of an AMaRV-like vehicle along with trajectory plots showing hairpin turns has been published. AMaRV's attitude
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#17327718397193872-461: A particular TPS material is usually proportional to the material's density. Carbon phenolic is a very effective ablative material, but also has high density which is undesirable. The NASA Galileo Probe used carbon phenolic for its TPS material. If the heat flux experienced by an entry vehicle is insufficient to cause pyrolysis then the TPS material's conductivity could allow heat flux conduction into
4048-421: A perfect gas model, the ratio of specific heats (also called isentropic exponent , adiabatic index , gamma , or kappa ) is assumed to be constant along with the gas constant . For a real gas, the ratio of specific heats can wildly oscillate as a function of temperature. Under a perfect gas model there is an elegant set of equations for determining thermodynamic state along a constant entropy stream line called
4224-499: A planet, such as Earth, and streams of debris from a comet or other source. The passage of Earth through cosmic debris from comets and other sources is a recurring event in many cases. Comets can produce debris by water vapor drag, as demonstrated by Fred Whipple in 1951, and by breakup. Each time a comet swings by the Sun in its orbit , some of its ice vaporizes and a certain amount of meteoroids are shed. The meteoroids spread out along
4400-533: A quarter that of ice, to nickel-iron rich dense rocks. The study of meteorites also gives insights into the composition of non-ephemeral meteoroids. Most meteoroids come from the asteroid belt , having been perturbed by the gravitational influences of planets, but others are particles from comets , giving rise to meteor showers . Some meteoroids are fragments from bodies such as Mars or the Moon , that have been thrown into space by an impact. Meteoroids travel around
4576-479: A radius of about 500 kilometres (310 mi). This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago. Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine , clinopyroxene , and orthopyroxene ; after about three-quarters of
4752-458: A satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit . Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits. On November 1, 2023, scientists reported that, according to computer simulations, remnants of Theia could still be present inside
4928-572: A second. Meteors were not known to be an astronomical phenomenon until early in the nineteenth century. Before that, they were seen in the West as an atmospheric phenomenon, like lightning, and were not connected with strange stories of rocks falling from the sky. In 1807, Yale University chemistry professor Benjamin Silliman investigated a meteorite that fell in Weston, Connecticut . Silliman believed
5104-412: A size scale larger than 10 cm (3.9 in) to several meters meteor visibility is due to the atmospheric ram pressure (not friction) that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporised meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about
5280-428: A specific destination on the surface at zero velocity while keeping stresses on the spacecraft and any passengers within acceptable limits. This may be accomplished by propulsive or aerodynamic (vehicle characteristics or parachute ) means, or by some combination. There are several basic shapes used in designing entry vehicles: The simplest axisymmetric shape is the sphere or spherical section. This can either be
5456-406: A specific stream of space debris . A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the meteoroids producing the meteors have been calculated. The atmospheric velocities of meteors result from the movement of Earth around the Sun at about 30 km/s (67,000 mph), the orbital speeds of meteoroids, and
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5632-407: A sphere-cone shape and were the first American example of a non-munition entry vehicle ( Discoverer-I , launched on 28 February 1959). The sphere-cone was later used for space exploration missions to other celestial bodies or for return from open space; e.g., Stardust probe. Unlike with military RVs, the advantage of the blunt body's lower TPS mass remained with space exploration entry vehicles like
5808-521: A stream of vaporized metal making it very visible to radar . These defects made the Mk-2 overly susceptible to anti-ballistic missile (ABM) systems. Consequently, an alternative sphere-cone RV to the Mk-2 was developed by General Electric. This new RV was the Mk-6 which used a non-metallic ablative TPS, a nylon phenolic. This new TPS was so effective as a reentry heat shield that significantly reduced bluntness
5984-475: A study of Ina , a tiny depression in Lacus Felicitatis , found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old. Moonquakes and releases of gas indicate continued lunar activity. Evidence of recent lunar volcanism has been identified at 70 irregular mare patches , some less than 50 million years old. This raises
6160-502: A texture resembling snow and a scent resembling spent gunpowder . The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–15 m (33–49 ft) in the highlands and 4–5 m (13–16 ft) in the maria. Beneath the finely comminuted regolith layer is the megaregolith , a layer of highly fractured bedrock many kilometers thick. These extreme conditions are considered to make it unlikely for spacecraft to harbor bacterial spores at
6336-400: A total solar eclipse . From Earth about 59% of the lunar surface is visible over time due to cyclical shifts in perspective ( libration ), making parts of the far side of the Moon visible. The Moon has been an important source of inspiration and knowledge for humans, having been crucial to cosmography , mythology, religion , art, time keeping , natural science , and spaceflight . In 1959,
6512-411: A trailing vortex behind the entry vehicle. Correctly modelling the flow in the wake of an entry vehicle is very difficult. Thermal protection shield (TPS) heating in the vehicle's afterbody is usually not very high, but the geometry and unsteadiness of the vehicle's wake can significantly influence aerodynamics (pitching moment) and particularly dynamic stability. A thermal protection system , or TPS,
6688-439: A trans-Atlantic flight, 200 times more than on Earth's surface. For further comparison radiation on a flight to Mars is about 1.84 millisieverts per day and on Mars on average 0.64 millisieverts per day, with some locations on Mars possibly having levels as low as 0.342 millisieverts per day. The Moon's axial tilt with respect to the ecliptic is only 1.5427°, much less than the 23.44° of Earth. Because of this small tilt,
6864-531: Is another entry vehicle geometry and was used with the X-23 PRIME (Precision Recovery Including Maneuvering Entry) vehicle. Objects entering an atmosphere from space at high velocities relative to the atmosphere will cause very high levels of heating . Atmospheric entry heating comes principally from two sources: As velocity increases, both convective and radiative heating increase, but at different rates. At very high speeds, radiative heating will dominate
7040-489: Is around 3 × 10 atm (0.3 nPa ); it varies with the lunar day. Its sources include outgassing and sputtering , a product of the bombardment of lunar soil by solar wind ions. Elements that have been detected include sodium and potassium , produced by sputtering (also found in the atmospheres of Mercury and Io ); helium-4 and neon from the solar wind; and argon-40 , radon-222 , and polonium-210 , outgassed after their creation by radioactive decay within
7216-532: Is asymmetric, being more dense near the boundary between the Moon's dayside and nightside. Ionizing radiation from cosmic rays , the Sun and the resulting neutron radiation produce radiation levels on average of 1.369 millisieverts per day during lunar daytime , which is about 2.6 times more than on the International Space Station with 0.53 millisieverts per day at about 400 km above Earth in orbit, 5–10 times more than during
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7392-465: Is at 400,000 feet (122 km), the main heating during controlled entry takes place at altitudes of 65 to 35 kilometres (213,000 to 115,000 ft), peaking at 58 kilometres (190,000 ft). At typical reentry temperatures, the air in the shock layer is both ionized and dissociated . This chemical dissociation necessitates various physical models to describe the shock layer's thermal and chemical properties. There are four basic physical models of
7568-506: Is based upon a single ordinary differential equation and one algebraic equation. The five species model is based upon 5 ordinary differential equations and 17 algebraic equations. Because the 5 ordinary differential equations are tightly coupled, the system is numerically "stiff" and difficult to solve. The five species model is only usable for entry from low Earth orbit where entry velocity is approximately 7.8 km/s (28,000 km/h; 17,000 mph). For lunar return entry of 11 km/s,
7744-486: Is called meteoric dust or just meteor dust. Meteor dust particles can persist in the atmosphere for up to several months. These particles might affect climate, both by scattering electromagnetic radiation and by catalyzing chemical reactions in the upper atmosphere. Meteoroids or their fragments achieve dark flight after deceleration to terminal velocity . Dark flight starts when they decelerate to about 2–4 km/s (4,500–8,900 mph). Larger fragments fall further down
7920-401: Is covered in lunar dust and marked by mountains , impact craters , their ejecta , ray-like streaks , rilles and, mostly on the near side of the Moon, by dark maria ("seas"), which are plains of cooled lava . These maria were formed when molten lava flowed into ancient impact basins. The Moon is, except when passing through Earth's shadow during a lunar eclipse , always illuminated by
8096-448: Is elegant and extremely useful for designing aircraft but assumes that the gas is chemically inert. From the standpoint of aircraft design, air can be assumed to be inert for temperatures less than 550 K (277 °C; 530 °F) at one atmosphere pressure. The perfect gas theory begins to break down at 550 K and is not usable at temperatures greater than 2,000 K (1,730 °C; 3,140 °F). For temperatures greater than 2,000 K,
8272-561: Is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles). While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. Models that have the Moon acquiring a significant amount of the proto-earth are more difficult to reconcile with geochemical data for the isotopes of zirconium, oxygen, silicon, and other elements. A study published in 2022, using high-resolution simulations (up to 10 particles), found that giant impacts can immediately place
8448-555: Is given by what can be discovered from Earth-bound telescopes, so the distinction between meteoroid and asteroid is fuzzy. Some of the smallest asteroids discovered (based on absolute magnitude H ) are 2008 TS 26 with H = 33.2 and 2011 CQ 1 with H = 32.1 both with an estimated size of one m (3 ft 3 in). In April 2017, the IAU adopted an official revision of its definition, limiting size to between 30 μm (0.0012 in) and one meter in diameter, but allowing for
8624-466: Is lower than other high-heat-flux-ablative materials, such as conventional carbon phenolics. PICA was patented by NASA Ames Research Center in the 1990s and was the primary TPS material for the Stardust aeroshell. The Stardust sample-return capsule was the fastest man-made object ever to reenter Earth's atmosphere, at 28,000 mph (ca. 12.5 km/s) at 135 km altitude. This was faster than
8800-440: Is on average about 1.9 km (1.2 mi) higher than that of the near side. The discovery of fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. Similar shrinkage features exist on Mercury . Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon does not have tectonic plates, its tectonic activity
8976-528: Is pre-bonded to the aeroshell's structure thus enabling construction of a large heat shield. Phenolic-impregnated carbon ablator (PICA), a carbon fiber preform impregnated in phenolic resin , is a modern TPS material and has the advantages of low density (much lighter than carbon phenolic) coupled with efficient ablative ability at high heat flux. It is a good choice for ablative applications such as high-peak-heating conditions found on sample-return missions or lunar-return missions. PICA's thermal conductivity
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#17327718397199152-477: Is simply Moon , with a capital M. The noun moon is derived from Old English mōna , which (like all its Germanic cognates) stems from Proto-Germanic *mēnōn , which in turn comes from Proto-Indo-European *mēnsis 'month' (from earlier *mēnōt , genitive *mēneses ) which may be related to the verb 'measure' (of time). Occasionally, the name Luna / ˈ l uː n ə / is used in scientific writing and especially in science fiction to distinguish
9328-574: Is slow and cracks develop as it loses heat. Scientists have confirmed the presence of a cave on the Moon near the Sea of Tranquillity , not far from the 1969 Apollo 11 landing site. The cave, identified as an entry point to a collapsed lava tube, is roughly 45 meters wide and up to 80 m long. This discovery marks the first confirmed entry point to a lunar cave. The analysis was based on photos taken in 2010 by NASA's Lunar Reconnaissance Orbiter . The cave's stable temperature of around 17 °C could provide
9504-446: Is slowly reduced such that chemical reactions can continue then the gas can remain in equilibrium. However, it is possible for gas pressure to be so suddenly reduced that almost all chemical reactions stop. For that situation the gas is considered frozen. The distinction between equilibrium and frozen is important because it is possible for a gas such as air to have significantly different properties (speed-of-sound, viscosity etc.) for
9680-457: Is the barrier that protects a spacecraft during the searing heat of atmospheric reentry. Multiple approaches for the thermal protection of spacecraft are in use, among them ablative heat shields, passive cooling, and active cooling of spacecraft surfaces. In general they can be divided into two categories: ablative TPS and reusable TPS. Ablative TPS are required when space craft reach a relatively low altitude before slowing down. Spacecraft like
9856-476: Is the lowest point on the surface of the Moon. The highest elevations of the Moon's surface are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin. Other large impact basins such as Imbrium , Serenitatis , Crisium , Smythii , and Orientale possess regionally low elevations and elevated rims. The far side of the lunar surface
10032-447: Is the movement of an object from outer space into and through the gases of an atmosphere of a planet , dwarf planet , or natural satellite . There are two main types of atmospheric entry: uncontrolled entry , such as the entry of astronomical objects , space debris , or bolides ; and controlled entry (or reentry ) of a spacecraft capable of being navigated or following a predetermined course. Technologies and procedures allowing
10208-453: Is typically better than that of a spherical section. The vehicle enters sphere-first. With a sufficiently small half-angle and properly placed center of mass, a sphere-cone can provide aerodynamic stability from Keplerian entry to surface impact. (The half-angle is the angle between the cone's axis of rotational symmetry and its outer surface, and thus half the angle made by the cone's surface edges.) The original American sphere-cone aeroshell
10384-490: The Columbia was upstream from the calculated value due to real-gas effects. On Columbia ' s maiden flight ( STS-1 ), astronauts John Young and Robert Crippen had some anxious moments during reentry when there was concern about losing control of the vehicle. An equilibrium real-gas model assumes that a gas is chemically reactive, but also assumes all chemical reactions have had time to complete and all components of
10560-642: The G77 Fortran compiler. A non-equilibrium real gas model is the most accurate model of a shock layer's gas physics, but is more difficult to solve than an equilibrium model. The simplest non-equilibrium model is the Lighthill-Freeman model developed in 1958. The Lighthill-Freeman model initially assumes a gas made up of a single diatomic species susceptible to only one chemical formula and its reverse; e.g., N 2 = N + N and N + N = N 2 (dissociation and recombination). Because of its simplicity,
10736-592: The Galileo Probe with a half-angle of 45° or the Viking aeroshell with a half-angle of 70°. Space exploration sphere-cone entry vehicles have landed on the surface or entered the atmospheres of Mars , Venus , Jupiter , and Titan . The biconic is a sphere-cone with an additional frustum attached. The biconic offers a significantly improved L/D ratio. A biconic designed for Mars aerocapture typically has an L/D of approximately 1.0 compared to an L/D of 0.368 for
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#173277183971910912-468: The Greek βολίς ( bolis ) which can mean a missile or to flash . If the magnitude of a bolide reaches −17 or brighter it is known as a superbolide . A relatively small percentage of fireballs hit Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs . Such an event happened in broad daylight over North America in 1972 . Another rare phenomenon is a meteor procession , where
11088-508: The Phoenicids can make atmospheric entry at as slow as about 11 km/s. On January 17, 2013, at 05:21 PST, a one-meter-sized comet from the Oort cloud entered Earth atmosphere over California and Nevada . The object had a retrograde orbit with perihelion at 0.98 ± 0.03 AU . It approached from the direction of the constellation Virgo (which was in the south about 50° above
11264-480: The Soyuz ), or unbounded (e.g., meteors ) trajectories. Various advanced technologies have been developed to enable atmospheric reentry and flight at extreme velocities. An alternative method of controlled atmospheric entry is buoyancy which is suitable for planetary entry where thick atmospheres, strong gravity, or both factors complicate high-velocity hyperbolic entry, such as the atmospheres of Venus , Titan and
11440-610: The Stardust probe. Crewed space vehicles must be slowed to subsonic speeds before parachutes or air brakes may be deployed. Such vehicles have high kinetic energies, and atmospheric dissipation is the only way of expending this, as it is highly impractical to use retrorockets for the entire reentry procedure. Ballistic warheads and expendable vehicles do not require slowing at reentry, and in fact, are made streamlined so as to maintain their speed. Furthermore, slow-speed returns to Earth from near-space such as high-altitude parachute jumps from balloons do not require heat shielding because
11616-611: The V-2 , stabilization and aerodynamic stress were important issues (many V-2s broke apart during reentry), but heating was not a serious problem. Medium-range missiles like the Soviet R-5 , with a 1,200-kilometer (650-nautical-mile) range, required ceramic composite heat shielding on separable reentry vehicles (it was no longer possible for the entire rocket structure to survive reentry). The first ICBMs , with ranges of 8,000 to 12,000 km (4,300 to 6,500 nmi), were only possible with
11792-408: The giant planets . The concept of the ablative heat shield was described as early as 1920 by Robert Goddard : "In the case of meteors, which enter the atmosphere with speeds as high as 30 miles (48 km) per second, the interior of the meteors remains cold, and the erosion is due, to a large extent, to chipping or cracking of the suddenly heated surface. For this reason, if the outer surface of
11968-437: The gravity well of Earth. Meteors become visible between about 75 to 120 km (250,000 to 390,000 ft) above Earth. They usually disintegrate at altitudes of 50 to 95 km (160,000 to 310,000 ft). Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with Earth. Most meteors are, however, observed at night, when darkness allows fainter objects to be recognized. For bodies with
12144-419: The isentropic chain . For a real gas, the isentropic chain is unusable and a Mollier diagram would be used instead for manual calculation. However, graphical solution with a Mollier diagram is now considered obsolete with modern heat shield designers using computer programs based upon a digital lookup table (another form of Mollier diagram) or a chemistry based thermodynamics program. The chemical composition of
12320-403: The strewn field . The visible light produced by a meteor may take on various hues, depending on the chemical composition of the meteoroid, and the speed of its movement through the atmosphere. As layers of the meteoroid abrade and ionize, the colour of the light emitted may change according to the layering of minerals. Colours of meteors depend on the relative influence of the metallic content of
12496-548: The 70° sphere-cone entry vehicles sent by NASA to Mars other than the Mars Science Laboratory (MSL). SLA-561V begins significant ablation at a heat flux of approximately 110 W/cm , but will fail for heat fluxes greater than 300 W/cm . The MSL aeroshell TPS is currently designed to withstand a peak heat flux of 234 W/cm . The peak heat flux experienced by the Viking 1 aeroshell which landed on Mars
12672-634: The Apollo mission capsules and 70% faster than the Shuttle. PICA was critical for the viability of the Stardust mission, which returned to Earth in 2006. Stardust's heat shield (0.81 m base diameter) was made of one monolithic piece sized to withstand a nominal peak heating rate of 1.2 kW/cm . A PICA heat shield was also used for the Mars Science Laboratory entry into the Martian atmosphere . An improved and easier to produce version called PICA-X
12848-787: The Apollo-CM. The higher L/D makes a biconic shape better suited for transporting people to Mars due to the lower peak deceleration. Arguably, the most significant biconic ever flown was the Advanced Maneuverable Reentry Vehicle (AMaRV). Four AMaRVs were made by the McDonnell Douglas Corp. and represented a significant leap in RV sophistication. Three AMaRVs were launched by Minuteman-1 ICBMs on 20 December 1979, 8 October 1980 and 4 October 1981. AMaRV had an entry mass of approximately 470 kg,
13024-676: The DC-X also served as the basis for an unsuccessful proposal for what eventually became the Lockheed Martin X-33 . Non- axisymmetric shapes have been used for crewed entry vehicles. One example is the winged orbit vehicle that uses a delta wing for maneuvering during descent much like a conventional glider. This approach has been used by the American Space Shuttle and the Soviet Buran . The lifting body
13200-504: The Earth and the material accreted and formed the Moon just beyond the Earth's Roche limit of ~ 2.56 R 🜨 . Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of
13376-452: The Earth under the influence of Earth's gravity , and are slowed by friction upon encountering Earth's atmosphere. Meteors are also often travelling quite fast relative to the Earth simply because their own orbital path is different from that of the Earth before they encounter Earth's gravity well . Most objects enter at hypersonic speeds due to their sub-orbital (e.g., intercontinental ballistic missile reentry vehicles), orbital (e.g.,
13552-520: The Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon. Cynthia / ˈ s ɪ n θ i ə / is another poetic name, though rare, for the Moon personified as a goddess, while Selene / s ə ˈ l iː n iː / (literally 'Moon') is the Greek goddess of the Moon. The English adjective pertaining to the Moon is lunar , derived from the Latin word for
13728-581: The Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the United States from coast to coast ). Within the Solar System , it is the largest and most massive satellite in relation to its parent planet , the fifth largest and most massive moon overall, and larger and more massive than all known dwarf planets . Its surface gravity is about one sixth of Earth's, about half of that of Mars , and
13904-429: The Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to restore hydrostatic equilibrium at its current orbital distance. The Moon is by size and mass the fifth largest natural satellite of
14080-469: The Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, although this is debated. The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system. The newly formed Moon would have had its own magma ocean ; its depth
14256-530: The Earth. The newly formed Moon settled into a much closer Earth orbit than it has today. Each body therefore appeared much larger in the sky of the other, eclipses were more frequent, and tidal effects were stronger. Due to tidal acceleration , the Moon's orbit around Earth has become significantly larger, with a longer period. Following formation, the Moon has cooled and most of its atmosphere has been stripped. The lunar surface has since been shaped by large impact events and many small ones, forming
14432-533: The Lighthill-Freeman model is a useful pedagogical tool, but is too simple for modelling non-equilibrium air. Air is typically assumed to have a mole fraction composition of 0.7812 molecular nitrogen, 0.2095 molecular oxygen and 0.0093 argon. The simplest real gas model for air is the five species model , which is based upon N 2 , O 2 , NO, N, and O. The five species model assumes no ionization and ignores trace species like carbon dioxide. When running
14608-504: The Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising up to 100 kilometers above the surface. Dust counts made by LADEE 's Lunar Dust EXperiment (LDEX) found particle counts peaked during the Geminid , Quadrantid , Northern Taurid , and Omicron Centaurid meteor showers , when the Earth, and Moon pass through comet debris. The lunar dust cloud
14784-458: The Moon derived from the impactor, rather than the proto-Earth. However, models from 2007 and later suggest a larger fraction of the Moon derived from the proto-Earth. Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of
14960-495: The Moon for longer than just one lunar orbit. The topography of the Moon has been measured with laser altimetry and stereo image analysis . Its most extensive topographic feature is the giant far-side South Pole–Aitken basin , some 2,240 km (1,390 mi) in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System . At 13 km (8.1 mi) deep, its floor
15136-466: The Moon formed around 50 million years after the origin of the Solar System . Historically, several formation mechanisms have been proposed, but none satisfactorily explains the features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force would require too great an initial rotation rate of Earth. Gravitational capture of a pre-formed Moon depends on an unfeasibly extended atmosphere of Earth to dissipate
15312-455: The Moon have synchronized the Moon's orbital period ( lunar month ) with its rotation period ( lunar day ) at 29.5 Earth days, causing the same side of the Moon to always face Earth. The Moon's gravitational pull—and, to a lesser extent, the Sun 's—are the main drivers of Earth's tides . In geophysical terms , the Moon is a planetary-mass object or satellite planet . Its mass is 1.2% that of
15488-455: The Moon is a crescent\decrescent, [REDACTED] \ [REDACTED] , for example in M ☾ 'lunar mass' (also M L ). The lunar geological periods are named after their characteristic features, from most impact craters outside the dark mare , to the mare and later craters, and finally the young, still bright and therefore readily visible craters with ray systems like Copernicus or Tycho . Isotope dating of lunar samples suggests
15664-508: The Moon's solar illumination varies much less with season than on Earth and it allows for the existence of some peaks of eternal light at the Moon's north pole , at the rim of the crater Peary . The surface is exposed to drastic temperature differences ranging from 120 °C to −171 °C depending on the solar irradiance . Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow, making topographical details play
15840-403: The Moon, lūna . Selenian / s ə l iː n i ə n / is an adjective used to describe the Moon as a world, rather than as a celestial object, but its use is rare. It is derived from σελήνη selēnē , the Greek word for the Moon, and its cognate selenic was originally a rare synonym but now nearly always refers to the chemical element selenium . The element name selenium and
16016-494: The Solar System relative to their primary planets. The Moon's diameter is about 3,500 km, more than a quarter of Earth's, with the face of the Moon comparable to the width of either Mainland Australia , Europe or the Contiguous United States (which excludes Alaska , etc.). The whole surface area of the Moon is about 38 million square kilometers, comparable to North and South America combined,
16192-467: The Solar System, categorizable as one of its planetary-mass moons , making it a satellite planet under the geophysical definitions of the term . It is smaller than Mercury and considerably larger than the largest dwarf planet of the Solar System, Pluto . While the minor-planet moon Charon of the Pluto-Charon system is larger relative to Pluto, the Moon is the largest natural satellite of
16368-618: The Solar System. On such stony bodies as the Moon or Mars that have little or no atmosphere, they leave enduring craters. Meteoroid collisions with solid Solar System objects, including the Moon, Mercury , Callisto , Ganymede , and most small moons and asteroids , create impact craters, which are the dominant geographic features of many of those objects. On other planets and moons with active surface geological processes, such as Earth, Venus , Mars , Europa , Io , and Titan , visible impact craters may become eroded , buried, or transformed by tectonics over time. In early literature, before
16544-444: The Sun in a variety of orbits and at various velocities. The fastest move at about 42 km/s (94,000 mph) through space in the vicinity of Earth's orbit. This is escape velocity from the Sun, equal to the square root of two times Earth's speed, and is the upper speed limit of objects in the vicinity of Earth, unless they come from interstellar space. Earth travels at about 29.6 km/s (66,000 mph), so when meteoroids meet
16720-410: The Sun, but from Earth the visible illumination shifts during its orbit, producing the lunar phases . The Moon is the brightest celestial object in Earth's night sky . This is mainly due to its large angular diameter , while the reflectance of the lunar surface is comparable to that of asphalt . The apparent size is nearly the same as that of the Sun, allowing it to cover the Sun completely during
16896-488: The TPS bondline material thus leading to TPS failure. Consequently, for entry trajectories causing lower heat flux, carbon phenolic is sometimes inappropriate and lower-density TPS materials such as the following examples can be better design choices: SLA in SLA-561V stands for super light-weight ablator . SLA-561V is a proprietary ablative made by Lockheed Martin that has been used as the primary TPS material on all of
17072-442: The apparatus were to consist of layers of a very infusible hard substance with layers of a poor heat conductor between, the surface would not be eroded to any considerable extent, especially as the velocity of the apparatus would not be nearly so great as that of the average meteor." Practical development of reentry systems began as the range, and reentry velocity of ballistic missiles increased. For early short-range missiles, like
17248-634: The atmosphere as a meteor and has impacted the ground. In 1961, the International Astronomical Union (IAU) defined a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom". In 1995, Beech and Steel, writing in the Quarterly Journal of the Royal Astronomical Society , proposed a new definition where
17424-416: The atmosphere head-on (which only occurs when meteors are in a retrograde orbit such as the Leonids , which are associated with the retrograde comet 55P/Tempel–Tuttle ) the combined speed may reach about 71 km/s (160,000 mph) (see Specific energy#Astrodynamics ). Meteoroids moving through Earth's orbital space average about 20 km/s (45,000 mph), but due to Earth's gravity meteors such as
17600-399: The atmosphere, and thus ionization trails can be found in the upper atmosphere more or less continuously. When radio waves are bounced off these trails, it is called meteor burst communications . Meteor radars can measure atmospheric density and winds by measuring the decay rate and Doppler shift of a meteor trail. Most meteoroids burn up when they enter the atmosphere. The left-over debris
17776-422: The atmospheric entry returns to the same body that the vehicle had launched from, the event is referred to as reentry (almost always referring to Earth entry). The fundamental design objective in atmospheric entry of a spacecraft is to dissipate the energy of a spacecraft that is traveling at hypersonic speed as it enters an atmosphere such that equipment, cargo, and any passengers are slowed and land near
17952-469: The case of the Galileo probe's entry into Jupiter's atmosphere, the shock layer was mostly in equilibrium during peak heat flux due to the very high pressures experienced (this is counterintuitive given the free stream velocity was 39 km/s during peak heat flux). Determining the thermodynamic state of the stagnation point is more difficult under an equilibrium gas model than a perfect gas model. Under
18128-402: The center of curvature (dynamic stability is more problematic). Pure spheres have no lift. However, by flying at an angle of attack , a spherical section has modest aerodynamic lift thus providing some cross-range capability and widening its entry corridor. In the late 1950s and early 1960s, high-speed computers were not yet available and computational fluid dynamics was still embryonic. Because
18304-413: The combined American landmass having an area (excluding all islands) of 37.7 million square kilometers. The Moon's mass is 1/81 of Earth's, being the second densest among the planetary moons, and having the second highest surface gravity , after Io , at 0.1654 g and an escape velocity of 2.38 km/s ( 8 600 km/h; 5 300 mph) . The Moon is a differentiated body that
18480-598: The controlled atmospheric entry, descent, and landing of spacecraft are collectively termed as EDL . Objects entering an atmosphere experience atmospheric drag , which puts mechanical stress on the object, and aerodynamic heating —caused mostly by compression of the air in front of the object, but also by drag. These forces can cause loss of mass ( ablation ) or even complete disintegration of smaller objects, and objects with lower compressive strength can explode. Reentry has been achieved with speeds ranging from 7.8 km/s for low Earth orbit to around 12.5 km/s for
18656-418: The convective heat fluxes, as radiative heating is proportional to the eighth power of velocity, while convective heating is proportional to the third power of velocity. Radiative heating thus predominates early in atmospheric entry, while convection predominates in the later phases. During certain intensity of ionization, a radio-blackout with the spacecraft is produced. While NASA's Earth entry interface
18832-519: The cosmic origin of meteors was firmly established. Still, they remain an atmospheric phenomenon and retain their name "meteor" from the Greek word for "atmospheric". A fireball is a brighter-than-usual meteor that also becomes visible when about 100 km from sea level. The International Astronomical Union (IAU) defines a fireball as "a meteor brighter than any of the planets" ( apparent magnitude −4 or greater). The International Meteor Organization (an amateur organization that studies meteors) has
19008-433: The crust and mantle. The absence of such neutral species (atoms or molecules) as oxygen , nitrogen , carbon , hydrogen and magnesium , which are present in the regolith , is not understood. Water vapor has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith. These gases either return into
19184-447: The dense mare basaltic lava flows that fill those basins. The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism. The Moon has an external magnetic field of less than 0.2 nanoteslas , or less than one hundred thousandth that of Earth . The Moon does not have
19360-514: The depressions associated with impact basins , though the Moon's largest expanse of basalt flooding, Oceanus Procellarum , does not correspond to an obvious impact basin. Different episodes of lava flows in maria can often be recognized by variations in surface albedo and distinct flow margins. As the maria formed, cooling and contraction of the basaltic lava created wrinkle ridges in some areas. These low, sinuous ridges can extend for hundreds of kilometers and often outline buried structures within
19536-587: The development of modern ablative heat shields and blunt-shaped vehicles. In the United States, this technology was pioneered by H. Julian Allen and A. J. Eggers Jr. of the National Advisory Committee for Aeronautics (NACA) at Ames Research Center . In 1951, they made the counterintuitive discovery that a blunt shape (high drag) made the most effective heat shield. From simple engineering principles, Allen and Eggers showed that
19712-468: The energy of the passing Moon. A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon. None of these hypotheses can account for the high angular momentum of the Earth–Moon system. The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars -sized body (named Theia ) with the proto-Earth . The oblique impact blasted material into orbit about
19888-511: The entire impactor may be vaporized, leaving no meteorites. Geologists use the term, "bolide", in a different sense from astronomers to indicate a very large impactor . For example, the USGS uses the term to mean a generic large crater-forming projectile in a manner "to imply that we do not know the precise nature of the impacting body ... whether it is a rocky or metallic asteroid, or an icy comet for example". Meteoroids also hit other bodies in
20064-468: The entire orbit of the comet to form a meteoroid stream, also known as a "dust trail" (as opposed to a comet's "dust tail" caused by the very small particles that are quickly blown away by solar radiation pressure). The frequency of fireball sightings increases by about 10–30% during the weeks of vernal equinox . Even meteorite falls are more common during the northern hemisphere's spring season. Although this phenomenon has been known for quite some time,
20240-422: The expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the antipodes of the giant impact basins. The Moon has an atmosphere so tenuous as to be nearly vacuum , with a total mass of less than 10 tonnes (9.8 long tons; 11 short tons). The surface pressure of this small mass
20416-1182: The exposed ones. Conversely, mare lava has obscured many impact melt sheets and pools. Impact melts are formed when intense shock pressures from collisions vaporize and melt zones around the impact site. Where still exposed, impact melt can be distinguished from mare lava by its distribution, albedo, and texture. Sinuous rilles , found in and around maria, are likely extinct lava channels or collapsed lava tubes . They typically originate from volcanic vents , meandering and sometimes branching as they progress. The largest examples, such as Schroter's Valley and Rima Hadley , are significantly longer, wider, and deeper than terrestrial lava channels, sometimes featuring bends and sharp turns that again, are uncommon on Earth. Mare volcanism has altered impact craters in various ways, including filling them to varying degrees, and raising and fracturing their floors from uplift of mare material beneath their interiors. Examples of such craters include Taruntius and Gassendi . Some craters, such as Hyginus , are of wholly volcanic origin, forming as calderas or collapse pits . Such craters are relatively rare, and tend to be smaller (typically
20592-510: The first human-made objects to leave Earth and reach another body arrived at the Moon, with the flyby of the Soviet Union 's Luna 1 and the intentional impact of Luna 2 . In 1966, the Moon became the first extraterrestrial body with a soft landing by Luna 9 and a orbital insertion by Luna 10 were achieved . On July 20, 1969, humans for the first time landed on the Moon and any extraterrestrial body, at Mare Tranquillitatis with
20768-427: The flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth. The crust is on average about 50 kilometres (31 mi) thick. The Moon is the second-densest satellite in the Solar System, after Io . However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less, around 20% of
20944-447: The gas and varying the different molecular combinations of the elements through numerical iteration until the lowest possible Gibbs free energy is calculated (a Newton–Raphson method is the usual numerical scheme). The data base for a Gibbs free energy program comes from spectroscopic data used in defining partition functions . Among the best equilibrium codes in existence is the program Chemical Equilibrium with Applications (CEA) which
21120-432: The gas have the same temperature (this is called thermodynamic equilibrium ). When air is processed by a shock wave, it is superheated by compression and chemically dissociates through many different reactions. Direct friction upon the reentry object is not the main cause of shock-layer heating. It is caused mainly from isentropic heating of the air molecules within the compression wave. Friction based entropy increases of
21296-514: The generation of these sounds may partially explain them. For example, scientists at NASA suggested that the turbulent ionized wake of a meteor interacts with Earth's magnetic field , generating pulses of radio waves . As the trail dissipates, megawatts of electromagnetic power could be released, with a peak in the power spectrum at audio frequencies . Physical vibrations induced by the electromagnetic impulses would then be heard if they are powerful enough to make grasses, plants, eyeglass frames,
21472-648: The gravitational acceleration of an object starting at relative rest from within the atmosphere itself (or not far above it) cannot create enough velocity to cause significant atmospheric heating. For Earth, atmospheric entry occurs by convention at the Kármán line at an altitude of 100 km (62 miles; 54 nautical miles) above the surface, while at Venus atmospheric entry occurs at 250 km (160 mi; 130 nmi) and at Mars atmospheric entry at about 80 km (50 mi; 43 nmi). Uncontrolled objects reach high velocities while accelerating through space toward
21648-489: The hearer's own body (see microwave auditory effect ), and other conductive materials vibrate. This proposed mechanism, although proven plausible by laboratory work, remains unsupported by corresponding measurements in the field. Sound recordings made under controlled conditions in Mongolia in 1998 support the contention that the sounds are real. (Also see Bolide .) A meteor shower is the result of an interaction between
21824-408: The heat load experienced by an entry vehicle was inversely proportional to the drag coefficient ; i.e., the greater the drag, the less the heat load. If the reentry vehicle is made blunt, air cannot "get out of the way" quickly enough, and acts as an air cushion to push the shock wave and heated shock layer forward (away from the vehicle). Since most of the hot gases are no longer in direct contact with
22000-399: The heat of entry and force of impact. A noted 4-metre (13 ft) asteroid , 2008 TC 3 , was observed in space on a collision course with Earth on 6 October 2008 and entered Earth's atmosphere the next day, striking a remote area of northern Sudan. It was the first time that a meteoroid had been observed in space and tracked prior to impacting Earth. NASA has produced a map showing
22176-515: The horizon at the time), and collided head-on with Earth's atmosphere at 72 ± 6 km/s (161,000 ± 13,000 mph) vaporising more than 100 km (330,000 ft) above ground over a period of several seconds. When meteoroids intersect with Earth's atmosphere at night, they are likely to become visible as meteors . If meteoroids survive the entry through the atmosphere and reach Earth's surface, they are called meteorites . Meteorites are transformed in structure and chemistry by
22352-444: The hot shock layer gas away from the heat shield's outer wall (creating a cooler boundary layer ). The boundary layer comes from blowing of gaseous reaction products from the heat shield material and provides protection against all forms of heat flux. The overall process of reducing the heat flux experienced by the heat shield's outer wall by way of a boundary layer is called blockage . Ablation occurs at two levels in an ablative TPS:
22528-471: The lander Eagle of the United States ' Apollo 11 mission. Five more crews were sent between then and 1972, each with two men landing on the surface. The longest stay was 75 hours by the Apollo 17 crew. Since then, exploration of the Moon has continued robotically, and crewed missions are being planned to return beginning in the late 2020s. The usual English proper name for Earth's natural satellite
22704-658: The light spectra of the resulting meteor. Their effects on radio signals also give information, especially useful for daytime meteors, which are otherwise very difficult to observe. From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see meteor showers ) often associated with a parent comet , others apparently sporadic. Debris from meteoroid streams may eventually be scattered into other orbits. The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about
22880-421: The magma ocean had crystallized, lower-density plagioclase minerals could form and float into a crust atop. The final liquids to crystallize would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly anorthosite . The Moon rock samples of
23056-521: The mantle could be responsible for prolonged activities on the far side in the Orientale basin. The lighter-colored regions of the Moon are called terrae , or more commonly highlands , because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean. In contrast to Earth, no major lunar mountains are believed to have formed as
23232-406: The mare. Another result of maria formation is the creation of concentric depressions along the edges, known as arcuate rilles . These features occur as the mare basalts sink inward under their own weight, causing the edges to fracture and separate. In addition to the visible maria, the Moon has mare deposits covered by ejecta from impacts. Called cryptomares, these hidden mares are likely older than
23408-635: The meteor breaks up into several fireballs traveling nearly parallel to the surface of Earth. A steadily growing number of fireballs are recorded at the American Meteor Society every year. There are probably more than 500,000 fireballs a year, but most go unnoticed because most occur over the ocean and half occur during daytime. A European Fireball Network and a NASA All-sky Fireball Network detect and track many fireballs. The entry of meteoroids into Earth's atmosphere produces three main effects: ionization of atmospheric molecules, dust that
23584-472: The meteor had a cosmic origin, but meteors did not attract much attention from astronomers until the spectacular meteor storm of November 1833. People all across the eastern United States saw thousands of meteors, radiating from a single point in the sky. Careful observers noticed that the radiant , as the point is called, moved with the stars, staying in the constellation Leo. The astronomer Denison Olmsted extensively studied this storm, concluding that it had
23760-413: The meteoroid sheds, and the sound of passage. During the entry of a meteoroid or asteroid into the upper atmosphere , an ionization trail is created, where the air molecules are ionized by the passage of the meteor. Such ionization trails can last up to 45 minutes at a time. Small, sand-grain sized meteoroids are entering the atmosphere constantly, essentially every few seconds in any given region of
23936-563: The meteoroid versus the superheated air plasma, which its passage engenders: The sound generated by a meteor in the upper atmosphere, such as a sonic boom , typically arrives many seconds after the visual light from a meteor disappears. Occasionally, as with the Leonid meteor shower of 2001, "crackling", "swishing", or "hissing" sounds have been reported, occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth's auroras . Theories on
24112-524: The mid-2010s. It was first flight tested on the Crew Dragon spacecraft in 2019 during the flight demonstration mission , in April 2019, and put into regular service on that spacecraft in 2020. Moon The Moon is Earth 's only natural satellite . It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and
24288-438: The molecules within the wave also account for some heating. The distance from the shock wave to the stagnation point on the entry vehicle's leading edge is called shock wave stand off . An approximate rule of thumb for shock wave standoff distance is 0.14 times the nose radius. One can estimate the time of travel for a gas molecule from the shock wave to the stagnation point by assuming a free stream velocity of 7.8 km/s and
24464-413: The most notable asteroid collisions with Earth and its atmosphere from 1994 to 2013 from data gathered by U.S. government sensors (see below). A meteor , known colloquially as a shooting star or falling star , is the visible passage of a glowing meteoroid , micrometeoroid , comet or asteroid through Earth's atmosphere, after being heated to incandescence by collisions with air molecules in
24640-524: The near side of the Moon, and cover 31% of the surface of the near side compared with 2% of the far side. This is likely due to a concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. Most of the Moon's mare basalts erupted during the Imbrian period , 3.3–3.7 billion years ago, though some being as young as 1.2 billion years and as old as 4.2 billion years. In 2006,
24816-440: The near-side. Causes of the distribution of the lunar highlands on the far side are also not well understood. Topological measurements show the near side crust is thinner than the far side. One possible scenario then is that large impacts on the near side may have made it easier for lava to flow onto the surface. The Moon is a very slightly scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing
24992-602: The original meteorite, created or modified by an impact of a meteorite is called impactite . Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow ( → ) may be read as "within" or "part of". Atmospheric entry Atmospheric entry (sometimes listed as V impact or V entry )
25168-481: The outer surface of the TPS material chars, melts, and sublimes , while the bulk of the TPS material undergoes pyrolysis and expels product gases. The gas produced by pyrolysis is what drives blowing and causes blockage of convective and catalytic heat flux. Pyrolysis can be measured in real time using thermogravimetric analysis , so that the ablative performance can be evaluated. Ablation can also provide blockage against radiative heat flux by introducing carbon into
25344-432: The possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements. Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell, inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in
25520-587: The prefix seleno- (as in selenography , the study of the physical features of the Moon) come from this Greek word. Artemis , the Greek goddess of the wilderness and the hunt, came to also be identified as the goddess of the Moon ( Selene ) and was sometimes called Cynthia , after her birthplace on Mount Cynthus . Her Roman equivalent is Diana . The names Luna, Cynthia, and Selene are reflected in technical terms for lunar orbits such as apolune , pericynthion and selenocentric . The astronomical symbol for
25696-436: The radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyzes of the Moon's time-variable rotation suggest that it is at least partly molten. The pressure at the lunar core is estimated to be 5 GPa (49,000 atm). On average the Moon's surface gravity is 1.62 m/s ( 0.1654 g ; 5.318 ft/s ), about half of
25872-474: The reason behind the anomaly is not fully understood by scientists. Some researchers attribute this to an intrinsic variation in the meteoroid population along Earth's orbit, with a peak in big fireball-producing debris around spring and early summer. Others have pointed out that during this period the ecliptic is (in the northern hemisphere) high in the sky in the late afternoon and early evening. This means that fireball radiants with an asteroidal source are high in
26048-471: The regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field. Studies of Moon magma samples retrieved by the Apollo missions demonstrate that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions,
26224-401: The same fixed point in the sky is called a meteor shower . An estimated 25 million meteoroids, micrometeoroids and other space debris enter Earth's atmosphere each day, which results in an estimated 15,000 tonnes of that material entering the atmosphere each year. A meteorite is the remains of a meteoroid that has survived the ablation of its surface material during its passage through
26400-451: The same thermodynamic state; e.g., pressure and temperature. Frozen gas can be a significant issue in the wake behind an entry vehicle. During reentry, free stream air is compressed to high temperature and pressure by the entry vehicle's shock wave. Non-equilibrium air in the shock layer is then transported past the entry vehicle's leading side into a region of rapidly expanding flow that causes freezing. The frozen air can then be entrained into
26576-515: The seasonal variation. Research is in progress for mapping the orbits of the meteors to gain a better understanding of the phenomenon. In a range of countries networks of sky observing installations have been set up to monitor meteors. A meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and hits the ground without being destroyed. Meteorites are sometimes, but not always, found in association with hypervelocity impact craters ; during energetic collisions,
26752-410: The second highest among all Solar System moons, after Jupiter 's moon Io . The body of the Moon is differentiated and terrestrial , with no significant hydrosphere , atmosphere , or magnetic field . It formed 4.51 billion years ago, not long after Earth's formation , out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called Theia . The lunar surface
26928-430: The shock layer between the shock wave and leading edge of an entry vehicle is chemically reacting and not in a state of equilibrium. The Fay–Riddell equation , which is of extreme importance towards modeling heat flux, owes its validity to the stagnation point being in chemical equilibrium. The time required for the shock layer gas to reach equilibrium is strongly dependent upon the shock layer's pressure. For example, in
27104-447: The shock layer contains a significant amount of ionized nitrogen and oxygen. The five-species model is no longer accurate and a twelve-species model must be used instead. Atmospheric entry interface velocities on a Mars–Earth trajectory are on the order of 12 km/s (43,000 km/h; 27,000 mph). Modeling high-speed Mars atmospheric entry—which involves a carbon dioxide, nitrogen and argon atmosphere—is even more complex requiring
27280-589: The shock layer thus making it optically opaque. Radiative heat flux blockage was the primary thermal protection mechanism of the Galileo Probe TPS material (carbon phenolic). Early research on ablation technology in the USA was centered at NASA 's Ames Research Center located at Moffett Field , California. Ames Research Center was ideal, since it had numerous wind tunnels capable of generating varying wind velocities. Initial experiments typically mounted
27456-399: The shock wave dissociating ambient atmospheric gas followed by recombination within the shock layer into new molecular species. The newly formed diatomic molecules initially have a very high vibrational temperature that efficiently transforms the vibrational energy into radiant energy ; i.e., radiative heat flux. The whole process takes place in less than a millisecond which makes modelling
27632-414: The significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth. Molten terrestrial material ejected from a meteorite impact crater can cool and solidify into an object known as a tektite . These are often mistaken for meteorites. Terrestrial rock, sometimes with pieces of
27808-399: The size of a grain of sand, i.e. they are usually millimeter-sized or smaller. Meteoroid sizes can be calculated from their mass and density which, in turn, can be estimated from the observed meteor trajectory in the upper atmosphere. Meteors may occur in showers , which arise when Earth passes through a stream of debris left by a comet, or as "random" or "sporadic" meteors, not associated with
27984-417: The sky (facilitating relatively high rates) at the moment the meteoroids "catch up" with Earth, coming from behind going in the same direction as Earth. This causes relatively low relative speeds and from this low entry speeds, which facilitates survival of meteorites. It also generates high fireball rates in the early evening, increasing chances of eyewitness reports. This explains a part, but perhaps not all of
28160-408: The space shuttle are designed to slow down at high altitude so that they can use reuseable TPS. (see: Space Shuttle thermal protection system ). Thermal protection systems are tested in high enthalpy ground testing or plasma wind tunnels that reproduce the combination of high enthalpy and high stagnation pressure using Induction plasma or DC plasma. The ablative heat shield functions by lifting
28336-471: The spherical section geometry in crewed capsules are Soyuz / Zond , Gemini , and Mercury . Even these small amounts of lift allow trajectories that have very significant effects on peak g-force , reducing it from 8–9 g for a purely ballistic (slowed only by drag) trajectory to 4–5 g, as well as greatly reducing the peak reentry heat. The sphere-cone is a spherical section with a frustum or blunted cone attached. The sphere-cone's dynamic stability
28512-483: The spherical section was amenable to closed-form analysis, that geometry became the default for conservative design. Consequently, crewed capsules of that era were based upon the spherical section. Pure spherical entry vehicles were used in the early Soviet Vostok and Voskhod capsules and in Soviet Mars and Venera descent vehicles. The Apollo command module used a spherical section forebody heat shield with
28688-460: The surface gravity of Mars and about a sixth of Earth's. The Moon's gravitational field is not uniform. The details of the gravitational field have been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons , large positive gravitational anomalies associated with some of the giant impact basins, partly caused by
28864-471: The term synonymous with "fireball". Astronomers often use "bolide" to identify an exceptionally bright fireball, particularly one that explodes in a meteor air burst . They are sometimes called detonating fireballs. It may also be used to mean a fireball which creates audible sounds. In the late twentieth century, bolide has also come to mean any object that hits Earth and explodes, with no regard to its composition (asteroid or comet). The word bolide comes from
29040-496: The upper atmosphere, creating a streak of light via its rapid motion and sometimes also by shedding glowing material in its wake. Although a meteor may seem to be a few thousand feet from the Earth, meteors typically occur in the mesosphere at altitudes from 76 to 100 km (250,000 to 330,000 ft). The root word meteor comes from the Greek meteōros , meaning "high in the air". Millions of meteors occur in Earth's atmosphere daily. Most meteoroids that cause meteors are about
29216-455: The vehicle, the heat energy would stay in the shocked gas and simply move around the vehicle to later dissipate into the atmosphere. The Allen and Eggers discovery, though initially treated as a military secret, was eventually published in 1958. When atmospheric entry is part of a spacecraft landing or recovery, particularly on a planetary body other than Earth, entry is part of a phase referred to as entry, descent, and landing , or EDL. When
29392-446: The winter solstice in the north polar crater Hermite . This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto . Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened mostly gray surface layer called regolith , formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has
29568-402: Was 21 W/cm . For Viking 1 , the TPS acted as a charred thermal insulator and never experienced significant ablation. Viking 1 was the first Mars lander and based upon a very conservative design. The Viking aeroshell had a base diameter of 3.54 meters (the largest used on Mars until Mars Science Laboratory). SLA-561V is applied by packing the ablative material into a honeycomb core that
29744-418: Was controlled through a split body flap (also called a split-windward flap ) along with two yaw flaps mounted on the vehicle's sides. Hydraulic actuation was used for controlling the flaps. AMaRV was guided by a fully autonomous navigation system designed for evading anti-ballistic missile (ABM) interception. The McDonnell Douglas DC-X (also a biconic) was essentially a scaled-up version of AMaRV. AMaRV and
29920-657: Was developed by SpaceX in 2006–2010 for the Dragon space capsule . The first reentry test of a PICA-X heat shield was on the Dragon C1 mission on 8 December 2010. The PICA-X heat shield was designed, developed and fully qualified by a small team of a dozen engineers and technicians in less than four years. PICA-X is ten times less expensive to manufacture than the NASA PICA heat shield material. A second enhanced version of PICA—called PICA-3—was developed by SpaceX during
30096-424: Was initially in hydrostatic equilibrium but has since departed from this condition. It has a geochemically distinct crust , mantle , and core . The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with
30272-544: Was just sufficiently understood to ensure Apollo's success. However, radiative heat flux in carbon dioxide (Mars entry) is still barely understood and will require major research. The frozen gas model describes a special case of a gas that is not in equilibrium. The name "frozen gas" can be misleading. A frozen gas is not "frozen" like ice is frozen water. Rather a frozen gas is "frozen" in time (all chemical reactions are assumed to have stopped). Chemical reactions are normally driven by collisions between molecules. If gas pressure
30448-498: Was possible. However, the Mk-6 was a huge RV with an entry mass of 3,360 kg, a length of 3.1 m and a half-angle of 12.5°. Subsequent advances in nuclear weapon and ablative TPS design allowed RVs to become significantly smaller with a further reduced bluntness ratio compared to the Mk-6. Since the 1960s, the sphere-cone has become the preferred geometry for modern ICBM RVs with typical half-angles being between 10° and 11°. Reconnaissance satellite RVs (recovery vehicles) also used
30624-604: Was the Mk-2 RV (reentry vehicle), which was developed in 1955 by the General Electric Corp. The Mk-2's design was derived from blunt-body theory and used a radiatively cooled thermal protection system (TPS) based upon a metallic heat shield (the different TPS types are later described in this article). The Mk-2 had significant defects as a weapon delivery system, i.e., it loitered too long in the upper atmosphere due to its lower ballistic coefficient and also trailed
30800-401: Was twice the thickness of that of present-day Mars . The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space. A permanent Moon dust cloud exists around the Moon, generated by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. The dust stays above
30976-483: Was written by Bonnie J. McBride and Sanford Gordon at NASA Lewis (now renamed "NASA Glenn Research Center"). Other names for CEA are the "Gordon and McBride Code" and the "Lewis Code". CEA is quite accurate up to 10,000 K for planetary atmospheric gases, but unusable beyond 20,000 K ( double ionization is not modelled). CEA can be downloaded from the Internet along with full documentation and will compile on Linux under
#718281