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141-507: Caloris was discovered on images taken by the Mariner 10 probe in 1974. Its name was suggested by Brian O'Leary , astronaut and member of the Mariner 10 imagery team. It was situated on the terminator —the line dividing the daytime and nighttime hemispheres—at the time the probe passed by, and so half of the crater could not be imaged. Later, on January 15, 2008, one of the first photos of

282-428: A mascon , is centered on Caloris Planitia. Most large impact basins on the moon, such as Mare Imbrium and Mare Crisium , are also the site of mascons. The giant impact believed to have formed Caloris may have had global consequences for the planet. At the exact antipode of the basin is a large area of hilly, grooved terrain, with few small impact craters that are known as chaotic terrain (also "weird terrain"). It

423-453: A 39-volt nickel–cadmium battery . The flyby past Mercury posed major technical challenges for scientists to overcome. Due to Mercury's proximity to the Sun, Mariner 10 would have to endure 4.5 times more solar radiation than when it departed Earth; compared to previous Mariner missions, spacecraft parts needed extra shielding against the heat. Thermal blankets and a sunshade were installed on

564-412: A camera whereas the prior mission lacked one. As Mariner 10 veered around Venus, from the planet's night side to daylight, the cameras snapped the probe's first image of Venus, showing an illuminated arc of clouds over the north pole emerging from darkness. Engineers initially feared that the star-tracker could mistake the much brighter Venus for Canopus, repeating the mishaps with flaking paint. However,

705-446: A central hub, which was roughly the shape of an octagonal prism. The hub stored the spacecraft's internal electronics. The Mariner 10 spacecraft was manufactured by Boeing. NASA set a strict limit of US$ 98 million for Mariner 10's total cost, which marked the first time the agency subjected a mission to an inflexible budget constraint. No overruns would be tolerated, so mission planners carefully considered cost efficiency when designing

846-417: A close encounter with a large body. During its flyby of Venus, Mariner 10 discovered evidence of rotating clouds and a very weak magnetic field. Using a near- ultraviolet filter, it photographed Venus's chevron clouds and performed other atmospheric studies. The spacecraft flew past Mercury three times. Owing to the geometry of its orbit – its orbital period was almost exactly twice that of Mercury's –

987-424: A conduit. Scientists could not quantify the age of the volcanic complex system but reported that it could be on the order of a billion years. The surface temperature of Mercury ranges from 100 to 700 K (−173 to 427 °C; −280 to 800 °F). It never rises above 180 K at the poles, due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. At perihelion ,

1128-474: A general paucity of smaller craters below about 30 km (19 mi) in diameter. Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to lunar maria. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localization and rounded, lobate shape of these plains strongly support volcanic origins. All

1269-566: A layer of regolith that inhibits sublimation . By comparison, the Antarctic ice sheet on Earth has a mass of about 4 × 10  kg, and Mars's south polar cap contains about 10  kg of water. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet's interior and deposition by impacts of comets. Mercury is too small and hot for its gravity to retain any significant atmosphere over long periods of time; it does have

1410-528: A layered, chemically heterogeneous crust with large-scale variations in chemical composition observed on the surface. The crust is low in iron but high in sulfur, resulting from the stronger early chemically reducing conditions than is found on other terrestrial planets. The surface is dominated by iron-poor pyroxene and olivine , as represented by enstatite and forsterite , respectively, along with sodium-rich plagioclase and minerals of mixed magnesium, calcium, and iron-sulfide. The less reflective regions of

1551-414: A long boom, one closer to the octagonal hub, the other one further away. Data from the two magnetometers would be cross-referenced to filter out the spacecraft's own magnetic field. Drastically weakening the probe's magnetic field would have increased costs. This experiment investigated the mass and gravitational characteristics of Mercury. It was of particular interest because of the planet's closeness to

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1692-441: A malfunction thought to be caused by a short-circuited diode occurred in the power subsystem. As a result, the main booster regulator and inverter failed, leaving the spacecraft dependent on the redundant regulator. Mission planners feared that the same problem could recur in the redundant system and cripple the spacecraft. In January 1974, Mariner 10 made ultraviolet observations of Comet Kohoutek . Another mid-course correction

1833-528: A more distant range of 48,069 kilometers (29,869 mi) below the southern hemisphere. After losing roll control in October 1974, a third and final encounter, the closest to Mercury, took place on 16 March 1975, at a range of 327 kilometers (203 mi), passing almost over the north pole. With its maneuvering gas just about exhausted, Mariner 10 started another orbit of the Sun. Engineering tests were continued until 24 March 1975, when final depletion of

1974-591: A pair of detectors made from antimony-bismuth thermopiles . The instrument was designed to measure temperatures as cold as −193 °C (−315.4 °F) and as hot as 427 °C (801 °F). Stillman C. Chase, Jr. of the Santa Barbara Research Center headed the infrared radiometer experiment. Two ultraviolet spectrometers were involved in this experiment, one to measure UV absorption, the other to sense UV emissions. The occultation spectrometer scanned Mercury's edge as it passed in front of

2115-487: A possibly separate subsequent episode called the Late Heavy Bombardment that ended 3.8 billion years ago. Mercury received impacts over its entire surface during this period of intense crater formation, facilitated by the lack of any atmosphere to slow impactors down. During this time Mercury was volcanically active; basins were filled by magma , producing smooth plains similar to the maria found on

2256-551: A prolonged interval. A "rimless depression" inside the southwest rim of the Caloris Basin consists of at least nine overlapping volcanic vents, each individually up to 8 km (5.0 mi) in diameter. It is thus a " compound volcano ". The vent floors are at least 1 km (0.62 mi) below their brinks and they bear a closer resemblance to volcanic craters sculpted by explosive eruptions or modified by collapse into void spaces created by magma withdrawal back down into

2397-413: A revolution would have caused a libration of 23.65° in longitude. For the same reason, there are two points on Mercury's equator, 180 degrees apart in longitude , at either of which, around perihelion in alternate Mercurian years (once a Mercurian day), the Sun passes overhead, then reverses its apparent motion and passes overhead again, then reverses a second time and passes overhead a third time, taking

2538-489: A second option for calculating the attitude. Nitrogen gas thrusters were used to adjust Mariner 10 ' s orientation along three axes. The spacecraft's electronics were intricate and complex: it contained over 32,000 pieces of circuitry, of which resistors, capacitors, diodes, microcircuits, and transistors were the most common devices. Commands for the instruments could be stored on Mariner 10 ' s computer, but were limited to 512 words. The rest had to be broadcast by

2679-429: A series of smaller "corpuscules") might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation. Other explanations considered included a slight oblateness of the Sun. The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place faith in this possible explanation, and the hypothetical planet was named Vulcan , but no such planet

2820-410: A set of detectors and were counted. Two fluxgate magnetometers were entrusted with discerning whether Mercury produced a magnetic field , and studying the interplanetary magnetic field between flybys. In designing this experiment, scientists had to account for interference from the magnetic field generated by Mariner 10's many electronic parts. For this reason, the magnetometers had to be situated on

2961-458: A significant source of sodium and potassium, indicating that the fractures created by the impact facilitate the release of gases from within the planet. The unusual terrain is also a source of these gases. Mariner 10 Mariner 10 was an American robotic space probe launched by NASA on 3 November 1973, to fly by the planets Mercury and Venus . It was the first spacecraft to perform flybys of multiple planets. Mariner 10

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3102-461: A smaller wide angle optic, but using the same tube. It had an 8-position filter wheel, with one position occupied by a mirror for the wide-angle bypass. TV camera exposures ranged from 3 ms to 12 s with each camera being able to take a picture every 42 s. The picture resolution was 832 x 700 pixels, 8-bit coded. The entire imaging system was imperiled when electric heaters attached to the cameras failed to turn on immediately after launch. To avoid

3243-465: A solid, metallic outer core layer, a deeper liquid core layer, and a solid inner core. The composition of the iron-rich core remains uncertain, but it likely contains nickel, silicon and perhaps sulfur and carbon, plus trace amounts of other elements. The planet's density is the second highest in the Solar System at 5.427 g/cm , only slightly less than Earth's density of 5.515 g/cm . If

3384-408: A spacecraft's instruments and antennas aimed in the correct direction. During course correction maneuvers, the spacecraft may need to rotate so that its rocket engine faces the proper direction before being fired. Mariner 10 determined its attitude using two optical sensors, one pointed at the Sun, and the other at a bright star, usually Canopus ; additionally, the probe's three gyroscopes provided

3525-446: A tenuous atmosphere consisting primarily of helium , as well as a magnetic field and a large iron-rich core . Its radiometer readings suggested that Mercury has a nighttime temperature of −183 °C (−297.4 °F) and maximum daytime temperatures of 187 °C (369 °F). Planning for MESSENGER , a spacecraft that surveyed Mercury until 2015, relied extensively on data and information collected by Mariner 10 . In 1975,

3666-409: A tenuous surface-bounded exosphere at a surface pressure of less than approximately 0.5 nPa (0.005 picobars). It includes hydrogen , helium , oxygen , sodium , calcium , potassium , magnesium , silicon , and hydroxide , among others. This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen atoms and helium atoms probably come from

3807-409: A total of about 16 Earth-days for this entire process. In the other alternate Mercurian years, the same thing happens at the other of these two points. The amplitude of the retrograde motion is small, so the overall effect is that, for two or three weeks, the Sun is almost stationary overhead, and is at its most brilliant because Mercury is at perihelion, its closest to the Sun. This prolonged exposure to

3948-471: Is 17%. Research published in 2007 suggests that Mercury has a molten core. The mantle-crust layer is in total 420 km (260 mi) thick. Projections differ as to the size of the crust specifically; data from the Mariner 10 and MESSENGER probes suggests a thickness of 35 km (22 mi), whereas an Airy isostacy model suggests a thickness of 26 ± 11 km (16.2 ± 6.8 mi). One distinctive feature of Mercury's surface

4089-424: Is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres (1,516.0 mi). Mercury is also smaller —albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan . Mercury consists of approximately 70% metallic and 30% silicate material. Mercury appears to have a solid silicate crust and mantle overlying

4230-525: Is a series of geologic formations thought to have been produced by the basin's ejecta, collectively called the Caloris Group . Mercury has a very tenuous and transient atmosphere, containing small amounts of hydrogen and helium captured from the solar wind , as well as heavier elements such as sodium and potassium . These are thought to originate within the planet, being "out-gassed" from beneath its crust. The Caloris basin has been found to be

4371-461: Is in May or November. This occurs about every seven years on average. Mercury's axial tilt is almost zero, with the best measured value as low as 0.027 degrees. This is significantly smaller than that of Jupiter , which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury's poles, the center of the Sun never rises more than 2.1 arcminutes above

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4512-436: Is stabilized by the variance of the tidal force along Mercury's eccentric orbit, acting on a permanent dipole component of Mercury's mass distribution. In a circular orbit there is no such variance, so the only resonance stabilized in such an orbit is at 1:1 (e.g., Earth–Moon), when the tidal force, stretching a body along the "center-body" line, exerts a torque that aligns the body's axis of least inertia (the "longest" axis, and

4653-427: Is that Mercury originally had a metal–silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass. Early in the Solar System's history, Mercury may have been struck by a planetesimal of approximately 1 ⁄ 6 Mercury's mass and several thousand kilometers across. The impact would have stripped away much of

4794-419: Is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified. Mercury's core has a higher iron content than that of any other planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory

4935-406: Is thought by some to have been created as seismic waves from the impact converged on the opposite side of the planet. Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode. This hypothetical impact is also believed to have triggered volcanic activity on Mercury, resulting in the formation of smooth plains. Surrounding Caloris

5076-400: The Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable. It is likely that this magnetic field is generated by a dynamo effect, in a manner similar to the magnetic field of Earth. This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal heating effects caused by

5217-524: The Earth 's Moon , Mercury's surface displays an expansive rupes system generated from thrust faults and bright ray systems formed by impact event remnants . Mercury's sidereal year (88.0 Earth days) and sidereal day (58.65 Earth days) are in a 3:2 ratio. This relationship is called spin–orbit resonance , and sidereal here means "relative to the stars". Consequently, one solar day (sunrise to sunrise) on Mercury lasts for around 176 Earth days: twice

5358-517: The Mariner 10 camera system was tested by taking five photographic mosaics of the Earth and six of the Moon . It also obtained photographs of the north polar region of the Moon where prior coverage was poor. These photographs provided a basis for cartographers to update lunar maps and improve the lunar control net . Far from being an uneventful cruise, Mariner 10 ' s three-month journey to Venus

5499-486: The VLA in the early 1990s revealed that there are patches of high radar reflection near the poles. Although ice was not the only possible cause of these reflective regions, astronomers thought it to be the most likely explanation. The presence of water ice was confirmed using MESSENGER images of craters at the north pole. The icy crater regions are estimated to contain about 10 –10  kg of ice, and may be covered by

5640-459: The antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around Mercury, converging at the basin's antipode (180 degrees away). The resulting high stresses fractured the surface. Alternatively, it has been suggested that this terrain formed as a result of

5781-472: The core , Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron. The radius of Mercury's core is estimated to be 2,020 ± 30 km (1,255 ± 19 mi), based on interior models constrained to be consistent with a moment of inertia factor of 0.346 ± 0.014 . Hence, Mercury's core occupies about 57% of its volume; for Earth this proportion

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5922-534: The interplanetary medium and to obtain experience with a dual-planet gravity assist mission. Mariner 10 ' s science team was led by Bruce C. Murray at the Jet Propulsion Laboratory . Mariner 10 was the first mission to use a gravity assist from one planet (in this case, Venus) to reach another planet. It used Venus to bend its flight path and bring its perihelion down to the level of Mercury's orbit. This maneuver, inspired by

6063-462: The maria of the Moon . These plains are superposed by explosive vents associated with pyroclastic material. Outside the walls, material ejected in the impact which created the basin extends for 1,000 km (620 mi), and concentric rings surround the crater. In the center of the basin is a region containing numerous radial troughs that appear to be extensional faults , with an unrelated 40 km (25 mi) crater, Apollodorus , located near

6204-585: The orbital mechanics calculations of the Italian scientist Giuseppe Colombo , put the spacecraft into an orbit that repeatedly brought it back to Mercury. Mariner 10 used the solar radiation pressure on its solar panels and its high-gain antenna as a means of attitude control during flight, the first spacecraft to use active solar pressure control. The components on Mariner 10 can be categorized into four groups based on their common function. The solar panels, power subsystem, attitude control subsystem, and

6345-488: The solar wind . A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting , which meant that lighter particles were lost from the accreting material and not gathered by Mercury. Each hypothesis predicts a different surface composition, and two space missions have been tasked with making observations of this composition. The first MESSENGER , which ended in 2015, found higher-than-expected potassium and sulfur levels on

6486-403: The 1980s–1990s, and are thought to result primarily from the vaporization of surface rock struck by micrometeorite impacts including presently from Comet Encke . In 2008, magnesium was discovered by MESSENGER . Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and

6627-530: The Atlas-Centaur carrying Mariner 10 lifted off from pad SLC-36B . The Atlas stage burned for around four minutes, after which it was jettisoned, and the Centaur stage took over for an additional five minutes, propelling Mariner 10 to a parking orbit . The temporary orbit took the spacecraft one-third of the distance around Earth: this maneuver was needed to reach the correct spot for a second burn by

6768-428: The Caloris Basin was so powerful that it caused lava eruptions and left a concentric mountainous ring ~2 km (1.2 mi) tall surrounding the impact crater . The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they were volcanic lava flows induced by the impact or a large sheet of impact melt. At

6909-420: The Centaur engines, which set Mariner 10 on a path towards Venus. The probe then separated from the rocket; subsequently, the Centaur stage diverted away to avoid the possibility of a future collision. Never before had a planetary mission depended upon two separate rocket burns during the launch, and even with Mariner 10 , scientists initially viewed the maneuver as too risky. During its first week of flight,

7050-582: The Greek Hermes, because it moves across the sky faster than any other planet, though some associated the planet with Apollo instead, as detailed by Pliny the Elder . The astronomical symbol for Mercury is a stylized version of Hermes' caduceus ; a Christian cross was added in the 16th century: [REDACTED] . Mercury is one of four terrestrial planets in the Solar System , which means it

7191-512: The Mission Sequence Working Group from Earth. Supplying the spacecraft components with power required modifying the electrical output of the solar panels. The power subsystem used two redundant sets of circuitry, each containing a booster regulator and an inverter , to convert the panels' DC output to AC and alter the voltage to the necessary level. The subsystem could store up to 20 ampere hours of electricity on

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7332-442: The Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including solar wind and micrometeorite impacts. There are two geologically distinct plains regions on Mercury. Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces, predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show

7473-524: The Moon. One of the most unusual craters is Apollodorus , or "the Spider", which hosts a series of radiating troughs extending outwards from its impact site. Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that

7614-596: The Solar System, Mercury was formed approximately 4.5 billion years ago. Its mantle is highly homogeneous, which suggests that Mercury had a magma ocean early in its history, like the Moon. According to current models , Mercury may have a solid silicate crust and mantle overlying a solid outer core, a deeper liquid core layer, and a solid inner core. There are many competing hypotheses about Mercury's origins and development, some of which incorporate collision with planetesimals and rock vaporization. Historically, humans knew Mercury by different names depending on whether it

7755-411: The Sun and could move along with the rotatable scan platform on the spacecraft. The experiment's most important goal was to ascertain whether Mercury had an atmosphere, but would also gather data at Earth and Venus and study the interstellar background radiation. The plasma experiment's goal was to study the ionized gases ( plasma ) of the solar wind, the temperature and density of its electrons, and how

7896-400: The Sun at its brightest makes these two points the hottest places on Mercury. Maximum temperature occurs when the Sun is at an angle of about 25 degrees past noon due to diurnal temperature lag , at 0.4 Mercury days and 0.8 Mercury years past sunrise. Conversely, there are two other points on the equator, 90 degrees of longitude apart from the first ones, where the Sun passes overhead only when

8037-442: The Sun's apparent motion ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes. A similar effect would have occurred if Mercury had been in synchronous rotation: the alternating gain and loss of rotation over

8178-446: The Sun's damaging heat, the cameras were deliberately placed on the spacecraft side facing away from the Sun. Consequently, the heaters were needed to prevent the cameras from losing heat and become so cold that they would become damaged. JPL engineers found that the vidicons could generate enough heat through normal operation to stay just above the critical temperature of −40 °C (−40 °F); therefore they advised against turning off

8319-448: The Sun) on Mercury last exactly two Mercury years, or about 176 Earth days. Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic ), the largest of all eight known solar planets. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between Earth and the Sun, which

8460-449: The Sun, so that the perihelion now coincided with the orbit of Mercury. The spacecraft flew past Mercury three times. The first Mercury encounter took place at 20:47 UTC on 29 March 1974, at a range of 703 kilometers (437 mi), passing on the shadow side. After looping once around the Sun while Mercury completed two orbits, Mariner 10 flew by Mercury again on 21 September 1974, at

8601-401: The Sun, and detected whether solar ultraviolet radiation was absorbed in certain wavelengths, which would indicate the presence of gas particles, and therefore an atmosphere. The airglow spectrometer detected extreme ultraviolet radiation emanating from atoms of gaseous hydrogen, helium, carbon, oxygen, neon, and argon. Unlike the occultation spectrometer, it did not require backlighting from

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8742-551: The Sun, collide with Venus, be ejected from the Solar System, or even disrupt the rest of the inner Solar System. In 1859, the French mathematician and astronomer Urbain Le Verrier reported that the slow precession of Mercury's orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead

8883-495: The Sun, large orbital eccentricity, and unusual spin-orbit resonance. As the spacecraft passed behind Mercury on the first encounter there was an opportunity to probe the atmosphere and to measure the radius of the planet. By observing phase changes in the S-band radio signal, measurements of the atmosphere could be made. The atmosphere was assessed as having a density of about 10  molecules per cm . Boeing finished building

9024-422: The Sun. This varying distance to the Sun leads to Mercury's surface being flexed by tidal bulges raised by the Sun that are about 17 times stronger than the Moon's on Earth. Combined with a 3:2 spin–orbit resonance of the planet's rotation around its axis, it also results in complex variations of the surface temperature. The resonance makes a single solar day (the length between two meridian transits of

9165-490: The US Post Office issued a commemorative stamp featuring the Mariner 10 space probe. The 10-cent Mariner 10 commemorative stamp was issued on 4 April 1975, at Pasadena, California. Mercury (planet) Mercury is the first planet from the Sun and the smallest in the Solar System . In English, it is named after the ancient Roman god Mercurius ( Mercury ), god of commerce and communication, and

9306-625: The United States. Out of forty-six submissions, JPL selected seven experiments on the basis of maximizing science return without exceeding cost guidelines: together, the seven scientific experiments cost US12.6 million dollars, about one-eighth of the total mission budget. The imaging system, the Television Photography Experiment, consisted of two 15 centimeters (5.9 in) Cassegrain telescopes feeding vidicon tubes. The main telescope could be bypassed to

9447-416: The amount of hydrazine fuel Mariner 10 would carry, and also equipped the spacecraft with more nitrogen gas for the thrusters than the previous Mariner mission had held. These upgrades proved crucial in enabling the second and third Mercury flybys. The mission still lacked the ultimate safeguard: a sister spacecraft. It was common for probes to be launched in pairs, with complete redundancy to guard against

9588-463: The area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity. According to International Astronomical Union rules, each new crater must be named after an artist who was famous for more than fifty years, and dead for more than three years, before the date the crater is named. The largest known crater is Caloris Planitia , or Caloris Basin, with a diameter of 1,550 km (960 mi). The impact that created

9729-482: The atmosphere is stratified into upper and lower layers that do not mix with each other; photographs of the upper and lower cloud layers corroborated this hypothesis. Mariner 10 ' s ultraviolet photographs were an invaluable information source for studying the churning clouds of Venus' atmosphere. The mission researchers believed the cloud features they photographed were located in the stratosphere and upper troposphere, created by condensation; they also concluded that

9870-425: The atmosphere of Venus. Data from the radio science experiment measured the extent to which radio waves passing through the atmosphere were refracted, which was used to calculate the density, pressure, and temperature of the atmosphere at any given altitude. Overall, atmospheric temperature is higher closer to the planet's surface, but Mariner 10 found four altitudes where the pattern was reversed, which could signify

10011-410: The axis of the aforementioned dipole) to always point at the center. However, with noticeable eccentricity, like that of Mercury's orbit, the tidal force has a maximum at perihelion and therefore stabilizes resonances, like 3:2, ensuring that the planet points its axis of least inertia roughly at the Sun when passing through perihelion. The original reason astronomers thought it was synchronously locked

10152-452: The cameras during the flight. Test photos of the Earth and Moon showed that image quality had not been significantly affected. The mission team was pleasantly surprised when the camera heaters started working on 17 January 1974, two months after launch. Later investigation concluded that a short circuit in a different location on the probe had prevented the heater from turning on. This allowed

10293-433: The center of the pattern. The exact cause of this pattern of troughs is not currently known. The feature is named Pantheon Fossae . The impacting body is estimated to have been at least 100 km (62 miles) in diameter. Bodies in the inner Solar System experienced a heavy bombardment of large rocky bodies in the first billion years or so of the Solar System. The impact that created Caloris must have occurred after most of

10434-544: The computer kept the spacecraft operating properly during the flight. The navigational system, including the hydrazine rocket, would keep Mariner 10 on track to Venus and Mercury. Several scientific instruments would collect data at the two planets. Finally, the antennas would transmit this data to the Deep Space Network back on Earth, as well as receive commands from Mission Control. Mariner 10 ' s various components and scientific instruments were attached to

10575-497: The contrast between darker and lighter features was due to differences in the cloud's absorptivity of UV light. The subsolar region was of particular interest: as the sun is straight overhead, it imparts more solar energy to this area than other part of the planet. Compared to the rest of the planet's atmosphere, the subsolar region was highly active and irregular. "Cells" of air lifted by convection, each up to 500 kilometers (310 mi) wide, were observed forming and dissipating within

10716-433: The convergence of ejecta at this basin's antipode. Overall, 46 impact basins have been identified. A notable basin is the 400 km (250 mi)-wide, multi-ring Tolstoj Basin that has an ejecta blanket extending up to 500 km (310 mi) from its rim and a floor that has been filled by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a 625 km (388 mi)-diameter rim. Like

10857-503: The crust are high in carbon, most likely in the form of graphite. Names for features on Mercury come from a variety of sources and are set according to the IAU planetary nomenclature system. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to

10998-473: The data is needed. Mercury's surface is similar in appearance to that of the Moon, showing extensive mare -like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is more heterogeneous than the surface of Mars or the Moon, both of which contain significant stretches of similar geology, such as maria and plateaus. Albedo features are areas of markedly different reflectivity, which include impact craters,

11139-420: The detail seen by Mariner was a surprise to most researchers. The probe continued photographing Venus until 13 February 1974 Among the encounter's 4165 acquired photographs, one resulting series of images captured a thick and distinctly patterned atmosphere making a full revolution every four days just as terrestrial observations had suggested. The mission revealed the composition and meteorological nature of

11280-438: The early 20th century, Albert Einstein 's general theory of relativity provided the explanation for the observed precession, by formalizing gravitation as being mediated by the curvature of spacetime. The effect is small: just 42.980 ± 0.001 arcseconds per century (or 0.43 arcsecond per year, or 0.1035 arcsecond per orbital period) for Mercury; it therefore requires a little over 12.5 million orbits, or 3 million years, for

11421-408: The effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm versus Earth's 4.4 g/cm . Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at

11562-404: The effects of the eccentricity, showing Mercury's orbit overlaid with a circular orbit having the same semi-major axis . Mercury's higher velocity when it is near perihelion is clear from the greater distance it covers in each 5-day interval. In the diagram, the varying distance of Mercury to the Sun is represented by the size of the planet, which is inversely proportional to Mercury's distance from

11703-402: The efficiency of the spacecraft's nitrogen jet thrusters, which could now be placed on the panel tips. The panels could be rotated a maximum of 76°. Additionally, Mariner 10 ' s hydrazine rocket nozzle had to face the Sun to function properly, but scientists rejected covering the nozzle with a thermal door as an undependable solution. Instead, a special paint was applied to exposed parts on

11844-427: The equatorial subsolar point is located at latitude 0°W or 180°W, and it climbs to a temperature of about 700 K . During aphelion , this occurs at 90° or 270°W and reaches only 550 K . On the dark side of the planet, temperatures average 110 K . The intensity of sunlight on Mercury's surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m ). Although daylight temperatures at

11985-429: The failure of one or the other. The budget constraint ruled this option out. Even though mission planners stayed sufficiently under budget to divert some funding for constructing a backup spacecraft, the budget did not permit both to be launched at the same time. In the event that Mariner 10 failed, NASA would only allow the backup to be launched if the fatal error was diagnosed and fixed; this would have to be completed in

12126-420: The features has suggested a total shrinkage of Mercury's radius in the range of ~1–7 km (0.62–4.35 mi). Most activity along the major thrust systems probably ended about 3.6–3.7 billion years ago. Small-scale thrust fault scarps have been found, tens of meters in height and with lengths in the range of a few kilometers, that appear to be less than 50 million years old, indicating that compression of

12267-632: The heavy bombardment had finished, because fewer impact craters are seen on its floor than exist on comparably-sized regions outside the crater. Similar impact basins on the Moon such as the Mare Imbrium and Mare Orientale are believed to have formed at about the same time, possibly indicating that there was a 'spike' of large impacts towards the end of the heavy bombardment phase of the early Solar System. Based on MESSENGER ' s photographs, Caloris' age has been determined to be between 3.8 and 3.9 billion years. A gravitational high, also known as

12408-516: The horizon. By comparison, the angular size of the Sun as seen from Mercury ranges from 1 + 1 ⁄ 4 to 2 degrees across. At certain points on Mercury's surface, an observer would be able to see the Sun peek up a little more than two-thirds of the way over the horizon, then reverse and set before rising again, all within the same Mercurian day . This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity equals its angular rotational velocity so that

12549-429: The interior and consequent surface geological activity continue to the present. There is evidence for pyroclastic flows on Mercury from low-profile shield volcanoes . Fifty-one pyroclastic deposits have been identified, where 90% of them are found within impact craters. A study of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over

12690-462: The largest risks of failure for the Mariner 10 mission; Mariner 1 , Mariner 3 , and Mariner 8 all failed minutes after lift-off due to either engineering failures or Atlas rocket malfunctions. The mission had a launch period of about a month in length, from 16 October 1973, to 21 November 1973. NASA chose 3 November as the launch date because it would optimize imaging conditions when the spacecraft arrived at Mercury. On 3 November at 17:45 UTC,

12831-469: The main body. After evaluating different choices for the sunshade cloth material, mission planners chose beta cloth , a combination of aluminized Kapton and glass-fiber sheets treated with Teflon . However, solar shielding was unfeasible for some of Mariner 10 ' s other components. Mariner 10 ' s two solar panels needed to be kept under 115 °C (239 °F). Covering the panels would defeat their purpose of producing electricity. The solution

12972-445: The messenger of the gods. Mercury is classified as a terrestrial planet , with roughly the same surface gravity as Mars . The surface of Mercury is heavily cratered , as a result of countless impact events that have accumulated over billions of years. Its largest crater, Caloris Planitia , has a diameter of 1,550 km (960 mi), which is about one-third the diameter of the planet (4,880 km or 3,030 mi). Similarly to

13113-523: The nitrogen supply was signaled by the onset of an un-programmed pitch turn. Commands were sent immediately to the spacecraft to turn off its transmitter, and radio signals to Earth ceased. Mariner 10 is presumably still orbiting the Sun , although its electronics have probably been damaged by the Sun's radiation. Mariner 10 has not been spotted or tracked from Earth since it stopped transmitting. The only ways it would not be orbiting would be if it had been hit by an asteroid or gravitationally perturbed by

13254-510: The orbital eccentricity of Mercury varies chaotically from nearly zero (circular) to more than 0.45 over millions of years due to perturbations from the other planets. This was thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), because this state is more likely to arise during a period of high eccentricity. However, accurate modeling based on a realistic model of tidal response has demonstrated that Mercury

13395-678: The original crust and mantle, leaving the core behind as a relatively major component. A similar process, known as the giant impact hypothesis , has been proposed to explain the formation of Earth's Moon. Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K. Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by

13536-407: The permanently shadowed polar craters. The detection of high amounts of water-related ions like O , OH , and H 3 O was a surprise. Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind. Sodium, potassium, and calcium were discovered in the atmosphere during

13677-528: The planet is at aphelion in alternate years, when the apparent motion of the Sun in Mercury's sky is relatively rapid. These points, which are the ones on the equator where the apparent retrograde motion of the Sun happens when it is crossing the horizon as described in the preceding paragraph, receive much less solar heat than the first ones described above. Mercury attains an inferior conjunction (nearest approach to Earth) every 116 Earth days on average, but this interval can range from 105 days to 129 days due to

13818-479: The planet taken by the MESSENGER probe revealed the crater in its entirety. The basin was initially estimated to be about 810 mi (1,300 km) in diameter, though this was increased to 960 mi (1,540 km) based on subsequent images taken by MESSENGER . It is ringed by mountains up to 2 km (1.2 mi) high. Inside the crater walls, the floor of the crater is filled by lava plains, similar to

13959-571: The planet's eccentric orbit. Mercury can come as near as 82,200,000 km (0.549 astronomical units; 51.1 million miles) to Earth, and that is slowly declining: The next approach to within 82,100,000 km (51 million mi) is in 2679, and to within 82,000,000 km (51 million mi) in 4487, but it will not be closer to Earth than 80,000,000 km (50 million mi) until 28,622. Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of an inferior conjunction. This large range arises from

14100-406: The planet's high orbital eccentricity would serve to keep part of the core in the liquid state necessary for this dynamo effect. Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within Earth, is strong enough to trap solar wind plasma . This contributes to the space weathering of

14241-441: The planet's high orbital eccentricity. Essentially, because Mercury is closest to the Sun, when taking an average over time, Mercury is most often the closest planet to the Earth, and—in that measure—it is the closest planet to each of the other planets in the Solar System. The longitude convention for Mercury puts the zero of longitude at one of the two hottest points on the surface, as described above. However, when this area

14382-512: The planet's sidereal year. This means that one side of Mercury will remain in sunlight for one Mercurian year of 88 Earth days; while during the next orbit, that side will be in darkness all the time until the next sunrise after another 88 Earth days. Combined with its high orbital eccentricity , the planet's surface has widely varying sunlight intensity and temperature, with the equatorial regions ranging from −170 °C (−270 °F) at night to 420 °C (790 °F) during sunlight. Due to

14523-442: The planet's surface. According to NASA, Mercury is not a suitable planet for Earth-like life. It has a surface boundary exosphere instead of a layered atmosphere, extreme temperatures, and high solar radiation. It is unlikely that any living beings can withstand those conditions. Some parts of the subsurface of Mercury may have been habitable , and perhaps life forms , albeit likely primitive microorganisms , may have existed on

14664-504: The planet's surface. Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles in the planet's magnetotail indicate a dynamic quality to the planet's magnetosphere. During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury's magnetic field can be extremely "leaky". The spacecraft encountered magnetic "tornadoes"—twisted bundles of magnetic fields connecting

14805-482: The planet. Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field . According to measurements taken by Mariner 10 , it is about 1.1% the strength of Earth's . The magnetic-field strength at Mercury's equator is about 300 nT . Like that of Earth, Mercury's magnetic field is dipolar and nearly aligned with the planet's spin axis (10° dipolar tilt, compared to 11° for Earth). Measurements from both

14946-439: The planetary magnetic field to interplanetary space—that were up to 800 km wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known as flux transfer events , form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface via magnetic reconnection . This also occurs in Earth's magnetic field. The MESSENGER observations showed

15087-517: The planets affected the velocity of the plasma stream. The experiment contained two components, facing in opposite directions. The Scanning Electrostatic Analyzer was aimed toward the Sun and could detect positive ions and electrons, which were separated by a set of three concentric hemispherical plates. The Scanning Electron Spectrometer has aimed away from the Sun, and detected only electrons, using just one hemispherical plate. The instruments could be rotated about 60° to either side. By gathering data on

15228-399: The presence of a layer of clouds. The inversions occurred at the 56, 61, 63, and 81 km (35, 38, 39, and 50 mi) levels, confirming previous observations made by the Mariner 5 encounter. The ultraviolet spectrometers identified the chemical substances that comprise Venus' atmosphere. The elevated concentration of atomic oxygen in the upper atmosphere showed that

15369-412: The probe could make course corrections, and attached squibs , whose detonation would signal Mariner 10 to exit the launch rocket and deploy its instruments. The planned gravity assist at Venus made it feasible to use an Atlas-Centaur rocket instead of a more powerful but more expensive Titan IIIC . The probe and the Atlas-Centaur were attached together ten days prior to liftoff. Launch posed one of

15510-458: The reconnection rate was ten times higher at Mercury, but its proximity to the Sun only accounts for about a third of the reconnection rate observed by MESSENGER . Mercury has the most eccentric orbit of all the planets in the Solar System; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 km (29,000,000 to 43,000,000 mi). It takes 87.969 Earth days to complete an orbit. The diagram illustrates

15651-450: The resulting ejecta, and ray systems . Larger albedo features correspond to higher reflectivity plains. Mercury has " wrinkle-ridges " (dorsa), Moon-like highlands , mountains (montes), plains (planitiae), escarpments (rupes), and valleys ( valles ). The planet's mantle is chemically heterogeneous, suggesting the planet went through a magma ocean phase early in its history. Crystallization of minerals and convective overturn resulted in

15792-436: The rocket so as to reduce heat flow from the nozzle to the delicate instruments on the spacecraft. Accurately performing the gravity assist at Venus posed another hurdle. If Mariner 10 was to maintain a course to Mercury, its trajectory could deviate no more than 200 kilometers (120 mi) from a critical point in the vicinity of Venus. To ensure that the necessary course corrections could be made, mission planners tripled

15933-445: The same face directed towards the Sun, in the same way that the same side of the Moon always faces Earth. Radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly stationary in Mercury's sky. The 3:2 resonant tidal locking

16074-707: The same side of Mercury was sunlit each time, so it was only able to map 40–45% of Mercury's surface, taking over 2,800 photos. It revealed a more or less Moon-like surface. It contributed enormously to the understanding of Mercury, whose surface had not been successfully resolved through telescopic observation. The regions mapped included most or all of the Shakespeare, Beethoven, Kuiper, Michelangelo, Tolstoj, and Discovery quadrangles , half of Bach and Victoria quadrangles, and small portions of Solitudo Persephones (later Neruda), Liguria (later Raditladi), and Borealis quadrangles. Mariner 10 also discovered that Mercury has

16215-756: The smooth plains of Mercury formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket. An unusual feature of Mercury's surface is the numerous compression folds, or rupes , that crisscross the plains. These exist on the Moon, but are much more prominent on Mercury. As Mercury's interior cooled, it contracted and its surface began to deform, creating wrinkle ridges and lobate scarps associated with thrust faults . The scarps can reach lengths of 1,000 km (620 mi) and heights of 3 km (1.9 mi). These compressional features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent. Mapping of

16356-516: The solar wind's movement around Mercury, the plasma experiment could be used to verify the magnetometer's observations of Mercury's magnetic field. Using the plasma detectors, Mariner 10 gathered the first in situ solar wind data from inside Venus' orbit. Shortly after launch, scientists found that the Scanning Electrostatic Analyzer had failed because a door shielding the analyzer did not open. An unsuccessful attempt

16497-452: The solar wind, diffusing into Mercury's magnetosphere before later escaping back into space. The radioactive decay of elements within Mercury's crust is another source of helium, as well as sodium and potassium. Water vapor is present, released by a combination of processes such as comets striking its surface, sputtering creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in

16638-412: The spacecraft and lost tracking on the guide star Canopus . An automated safety protocol recovered Canopus, but the problem of flaking paint recurred throughout the mission. The on-board computer also experienced unscheduled resets occasionally, which necessitated reconfiguring the clock sequence and subsystems. Periodic problems with the high-gain antenna also occurred during the cruise. On 8 January 1974,

16779-603: The spacecraft at the end of June 1973, and Mariner 10 was delivered from Seattle to JPL's headquarters in California, where JPL comprehensively tested the integrity of the spacecraft and its instruments. After the tests were finished, the probe was transported to the Eastern Test Range in Florida, the launch site. Technicians filled a tank on the spacecraft with 29 kilograms (64 lb) of hydrazine fuel so that

16920-402: The spacecraft's instruments. Cost control was primarily accomplished by executing contract work closer to the launch date than was recommended by normal mission schedules, as reducing the length of available work time increased cost efficiency. Despite the rushed schedule, very few deadlines were missed. The mission ended up about US$ 1 million under budget. Attitude control is needed to keep

17061-411: The span of a few hours; some had polygonal outlines. The gravity assist was also a success, coming well within the acceptable margin for error. In the four hours between 16:00 and 20:00 UTC on 5 February 1974, Mariner 10 ' s heliocentric velocity dropped from 37.008 km/s (82,785 mph) to 32.283 km/s (72,215 mph). This changed the shape of the spacecraft's elliptical orbit around

17202-510: The star-tracker did not malfunction. Earth occultation occurred between 17:07 and 17:11 UTC, during which the spacecraft transmitted X-band radio waves through Venus' atmosphere, gathering data on cloud structure and temperature. Although Venus's cloud cover is nearly featureless in visible light, it was discovered that extensive cloud detail could be seen through Mariner's ultraviolet camera filters. Earth-based ultra-violet observation had shown some indistinct blotching even before Mariner 10 , but

17343-514: The study of Mercury. Depressions or fossae are named for works of architecture. Montes are named for the word "hot" in a variety of languages. Plains or planitiae are named for Mercury in various languages. Escarpments or rupēs are named for ships of scientific expeditions. Valleys or valles are named for abandoned cities, towns, or settlements of antiquity. Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during

17484-411: The surface of Mercury and the atmosphere of Venus, from which the temperature could be calculated. How quickly the surface lost heat as it rotated into the planet's dark side revealed aspects about the composition of the surface, such as whether it was made out of rocks, or out of finer particles. The infrared radiometer contained a pair of Cassegrain telescopes fixed at an angle of 120° to each other and

17625-445: The surface of Mercury are generally extremely high, observations strongly suggest that ice (frozen water) exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K, far lower than the global average. This creates a cold trap where ice can accumulate. Water ice strongly reflects radar , and observations by the 70-meter Goldstone Solar System Radar and

17766-406: The surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because said potassium and sulfur would have been driven off by the extreme heat of these events. BepiColombo , which will arrive at Mercury in 2025, will make observations to test these hypotheses. The findings so far would seem to favor the third hypothesis; however, further analysis of

17907-514: The two and a half weeks between the scheduled launch on 3 November 1973 and the last possible launch date of 21 November 1973. The unused backup was sent to the Smithsonian museum for display. Mariner 10 conducted seven experiments at Venus and Mercury. Six of these experiments had a dedicated scientific instrument to collect data. The experiments and instruments were designed by research laboratories and educational institutions from across

18048-488: The very small axial tilt , the planet's poles are permanently shadowed . This strongly suggests that water ice could be present in the craters. Above the planet's surface is an extremely tenuous exosphere and a faint magnetic field that is strong enough to deflect solar winds . Mercury has no natural satellite . As of the early 2020s, many broad details of Mercury's geological history are still under investigation or pending data from space probes. Like other planets in

18189-435: The vidicons to be turned off as needed. Of the six main scientific instruments, the 43.6 kilograms (96 lb) cameras were by far the most massive device. Requiring 67 watts of electricity, the cameras consumed more power than the other five instruments combined. The system returned about 7,000 photographs of Mercury and Venus during Mariner 10's flybys. The infrared radiometer detected infrared radiation given off by

18330-480: The westerly direction on Mercury. The two hottest places on the equator are therefore at longitudes 0° W and 180° W, and the coolest points on the equator are at longitudes 90° W and 270° W. However, the MESSENGER project uses an east-positive convention. For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and always keeping

18471-487: Was an evening star or a morning star. By about 350 BC, the ancient Greeks had realized the two stars were one. They knew the planet as Στίλβων Stilbōn , meaning "twinkling", and Ἑρμής Hermēs , for its fleeting motion, a name that is retained in modern Greek ( Ερμής Ermis ). The Romans named the planet after the swift-footed Roman messenger god, Mercury (Latin Mercurius ), whom they equated with

18612-464: Was captured into the 3:2 spin-orbit state at a very early stage of its history, within 20 (more likely, 10) million years after its formation. Numerical simulations show that a future secular orbital resonant interaction with the perihelion of Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the orbit will be destabilized in the next five billion years. If this happens, Mercury may fall into

18753-520: Was ever found. The observed perihelion precession of Mercury is 5,600 arcseconds (1.5556°) per century relative to Earth, or 574.10 ± 0.65 arcseconds per century relative to the inertial ICRF . Newtonian mechanics, taking into account all the effects from the other planets and including 0.0254 arcseconds per century due to the oblateness of the Sun, predicts a precession of 5,557 arcseconds (1.5436°) per century relative to Earth, or 531.63 ± 0.69 arcseconds per century relative to ICRF. In

18894-472: Was first visited, by Mariner 10 , this zero meridian was in darkness, so it was impossible to select a feature on the surface to define the exact position of the meridian. Therefore, a small crater further west was chosen, called Hun Kal , which provides the exact reference point for measuring longitude. The center of Hun Kal defines the 20° west meridian. A 1970 International Astronomical Union resolution suggests that longitudes be measured positively in

19035-413: Was fraught with technical malfunctions, which kept mission control on edge. Donna Shirley recounted her team's frustration: "It seemed as if we were always just patching Mariner 10 together long enough to get it on to the next phase and next crisis". A trajectory correction maneuver was made on 13 November 1973. Immediately afterward, the star-tracker locked onto a bright flake of paint which had come off

19176-489: Was launched approximately two years after Mariner 9 and was the last spacecraft in the Mariner program . (Mariner 11 and Mariner 12 were allocated to the Voyager program and redesignated Voyager 1 and Voyager 2 .) The mission objectives were to measure Mercury's environment, atmosphere, surface, and body characteristics and to make similar investigations of Venus. Secondary objectives were to perform experiments in

19317-439: Was made on 21 January 1974. The spacecraft passed Venus on 5 February 1974, the closest approach being 5,768 kilometers (3,584 mi) at 17:01 UTC. It was the twelfth spacecraft to reach Venus and the eighth to return data from the planet, as well as the first mission to succeed in broadcasting images of Venus back to Earth. Mariner 10 built upon observations made by Mariner 5 six years earlier; importantly, Mariner 10 had

19458-459: Was made to forcibly unfasten the door with the first course correction maneuver. The experiment operators had planned to observe the directions taken by positive ions prior to the ions' collision with the Analyzer, but this data was lost. The experiment was still able to collect some data using the properly functioning Scanning Electron Spectrometer. The goal of the charged particles experiment

19599-460: Was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin-orbit resonance, a solar day lasts about 176 Earth days. A sidereal day (the period of rotation) lasts about 58.7 Earth days. Simulations indicate that

19740-423: Was to add an adjustable tilt to the panels, so the angle at which they faced the sun could be changed. Engineers considered folding the panels toward each other, making a V-shape with the main body, but tests found this approach had the potential to overheat the rest of the spacecraft. The alternative chosen was to mount the solar panels in a line and tilt them along that axis, which had the added benefit of increasing

19881-462: Was to observe how the heliosphere interacted with cosmic radiation . In connection with the plasma detectors and magnetometers, this experiment had the potential to provide additional evidence of a magnetic field around Mercury, by showing whether such a field had captured charged particles. Two telescopes were used to collect highly energetic electrons and atomic nuclei, specifically oxygen nuclei or less massive. These particles then passed through

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