Vulcanoid asteroid
Encyclopedia
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s that may orbit the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
in a dynamically stable zone inside the orbit of the planet Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
. They are named after the hypothetical planet Vulcan
Vulcan (hypothetical planet)
Vulcan was a small planet proposed to exist in an orbit between Mercury and the Sun. In an attempt to explain peculiarities of Mercury's orbit, in the 19th-century French mathematician Urbain Jean Joseph Le Verrier hypothesized that they were the result of another planet, which he named Vulcan...
, whose existence was disproven in 1915. No vulcanoids have yet been discovered, and it is not clear if any exist.
If they do exist, the vulcanoids could easily evade detection because they would be very small and drowned out by the bright glare of the nearby Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. Due to their proximity to the Sun, searches from the ground can only be carried out during twilight or solar eclipses. Any vulcanoids must be between about 100 metres (328.1 ft) and 60 kilometres (37.3 mi) in diameter and are probably located in nearly circular orbits near the outer edge of the gravitationally stable zone.
The vulcanoids, should they be found, may provide scientists with material from the first period of planet formation, as well as insights into the conditions prevalent in the early Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. Although every other gravitationally stable region in the Solar System has been found to contain objects, non-gravitational forces, such as the Yarkovsky effect
Yarkovsky effect
The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum...
, or the influence of a migrating
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...
planet in the early stages of the Solar System's development may have depleted this area of any asteroids that may have been there.
History and observation
Celestial bodies interior to the orbit of Mercury have been hypothesized, and searched for, for centuries. The German astronomer Christoph ScheinerChristoph Scheiner
Christoph Scheiner SJ was a Jesuit priest, physicist and astronomer in Ingolstadt....
believed he had seen small bodies passing in front of the Sun in 1611, but these were later shown to be sunspots. In the 1850s, Urbain Le Verrier made detailed calculations of Mercury's orbit and found a small discrepancy in the planet's perihelion precession from predicted values. He postulated that the gravitational influence of a small planet or ring of asteroids within the orbit of Mercury would explain the deviation. Shortly afterward, an amateur astronomer named Edmond Lescarbault
Edmond Modeste Lescarbault
Edmond Modeste Lescarbault , was a French doctor and an amateur astronomer, best remembered for his 1859 observation of Vulcan....
claimed to have seen Le Verrier's proposed planet transit
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
the Sun. The new planet was quickly named Vulcan
Vulcan (hypothetical planet)
Vulcan was a small planet proposed to exist in an orbit between Mercury and the Sun. In an attempt to explain peculiarities of Mercury's orbit, in the 19th-century French mathematician Urbain Jean Joseph Le Verrier hypothesized that they were the result of another planet, which he named Vulcan...
but was never seen again, and the anomalous behaviour of Mercury's orbit was explained by Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
's General theory of relativity in 1915. The vulcanoids take their name from this hypothetical planet. What Lescarbault saw was probably another sunspot.
Solar eclipse
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...
s. Several searches during eclipses were conducted in the early 1900s, which did not reveal any vulcanoids, and observations during eclipses remain a common search method. Conventional telescopes cannot be used to search for them because the nearby Sun could damage their optics.
In 1998, astronomers analysed data from the SOHO
Solar and Heliospheric Observatory
The Solar and Heliospheric Observatory is a spacecraft built by a European industrial consortium led by Matra Marconi Space that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December 2, 1995 to study the Sun, and has discovered over 2100 comets. It began normal operations in May...
spacecraft's LASCO instrument, which is a set of three coronagraph
Coronagraph
A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved...
s. The data taken between January and May of that year did not show any vulcanoids brighter than magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
7. This corresponds to a diameter of about 60 kilometres (37.3 mi), assuming the asteroids have an albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
similar to that of Mercury. In particular a large planetoid at a distance of 0.18AU, predicted by the theory of Scale relativity
Scale relativity
Scale relativity is a theory of space-time initially developed by Laurent Nottale, working at the French observatory of Meudon, near Paris. It is an extension of the concept of relativity found the special relativity and general relativity to physical scales . If scales in nature are always...
, was ruled out.
Later attempts to detect the vulcanoids involved taking astronomical equipment above the interference of Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
, to heights where the twilight sky is darker and clearer than on the ground. In 2000, planetary scientist Alan Stern
Alan Stern
S. Alan Stern is an American planetary scientist. He is the principal investigator of the New Horizons mission to Pluto....
performed surveys of the vulcanoid zone using a Lockheed U-2
Lockheed U-2
The Lockheed U-2, nicknamed "Dragon Lady", is a single-engine, very high-altitude reconnaissance aircraft operated by the United States Air Force and previously flown by the Central Intelligence Agency . It provides day and night, very high-altitude , all-weather intelligence gathering...
spy plane. The flights were conducted at a height of 21300 metres (69,881.9 ft) during twilight. In 2002, he and Dan Durda performed similar observations on an F-18 fighter jet. They made three flights over the Mojave desert
Mojave Desert
The Mojave Desert occupies a significant portion of southeastern California and smaller parts of central California, southern Nevada, southwestern Utah and northwestern Arizona, in the United States...
at an altitude of 15000 metres (49,212.6 ft) and made observations with the Southwest Universal Imaging System—Airborne (SWUIS-A).
Even at these heights the atmosphere is still present and able to interfere with vulcanoid searches. In 2004, a sub-orbital spaceflight
Sub-orbital spaceflight
A sub-orbital space flight is a spaceflight in which the spacecraft reaches space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it does not complete one orbital revolution....
was attempted in order to get a camera above Earth's atmosphere. A Black Brant
Black Brant (rocket)
The Black Brant is a Canadian-designed sounding rocket built by Bristol Aerospace in Winnipeg, Manitoba. Over 800 Black Brants of various versions have been launched since they were first produced in 1961, and the type remains one of the most popular sounding rockets ever built...
rocket was launched from White Sands, New Mexico
White Sands, New Mexico
White Sands is a census-designated place in Doña Ana County, New Mexico, United States. The population was 1,323 at the 2000 census. It is part of the Las Cruces Metropolitan Statistical Area...
, on January 16, carrying a powerful camera named VulCam, on a ten-minute flight. This flight reached an altitude of 274000 metres (898,950.1 ft) and took over 50,000 images. Due to technical problems, none of the images were able to reveal any vulcanoids.
The MESSENGER
MESSENGER
The MErcury Surface, Space ENvironment, GEochemistry and Ranging space probe is a robotic NASA spacecraft in orbit around the planet Mercury. The spacecraft was launched aboard a Delta II rocket in August 2004 to study the chemical composition, geology, and magnetic field of Mercury...
space probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...
may provide evidence regarding vulcanoids. Its opportunities will be limited because its instruments need to be pointed away from the Sun at all times to avoid damage. The spacecraft has already taken a few of a planned series of images of the outer regions of the vulcanoid zone.
Orbit
A vulcanoid is an asteroid in a stable orbit with a semi-major axisSemi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
less than that of Mercury (i.e. 0.387 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
). This does not include objects like sungrazing comet
Sungrazing comet
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion – sometimes within a few thousand kilometres of the Sun's surface. While small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages...
s which, although they have a perihelion inside the orbit of Mercury, have a far greater semi-major axis.
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. All other similarly stable regions in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
have been found to contain objects, although non-gravitational forces such as radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
, Poynting–Robertson drag and the Yarkovsky effect
Yarkovsky effect
The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum...
may have depleted the vulcanoid area of its original contents. There may be no more than 300–900 vulcanoids larger than 1 kilometre (0.621372736649807 mi) in radius remaining, if any. The gravitational stability of the vulcanoid zone is due in part to the fact that there is only one neighbouring planet. In that respect it can be compared to the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
.
The outer edge of the vulcanoid zone is approximately 0.21 AU from the Sun. More distant objects are unstable due to the gravitational influence of Mercury and would be perturbed into Mercury-crossing orbits on timescales of the order of 100 million years. The inner edge is not sharply defined: objects closer than 0.06 AU are highly susceptible to Poynting–Robertson drag and the Yarkovsky effect, and even out to 0.09 AU vulcanoids would have temperatures of 1,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
or more, which is hot enough for evaporation of rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
s to be the limiting factor in their lifetime.
The volume of the vulcanoid zone is very small compared to that of the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
. Collisions between objects in the vulcanoid zone would be frequent and highly energetic, tending to lead to the destruction of the objects. The most favourable location for vulcanoids is probably in circular orbits near the outer edge of the vulcanoid zone. Vulcanoids are unlikely to have inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
s of more than about 10° to the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
. Mercury trojans
Trojan (astronomy)
In astronomy, a Trojan is a minor planet or natural satellite that shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability , and , which lie approximately 60° ahead of and behind the larger body,...
, asteroids trapped in Mercury's Lagrange points, are also possible.
Physical characteristics
Any vulcanoids that exist must be relatively small. Previous searches, particularly from the SOHOSolar and Heliospheric Observatory
The Solar and Heliospheric Observatory is a spacecraft built by a European industrial consortium led by Matra Marconi Space that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December 2, 1995 to study the Sun, and has discovered over 2100 comets. It began normal operations in May...
spacecraft, rule out asteroids larger than 60 kilometres (37.3 mi) in diameter. The minimum size is about 100 metres (328.1 ft); particles smaller than 0.2 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
are strongly repulsed by radiation pressure, and objects smaller than 70 m would be drawn into the Sun by Poynting–Robertson drag. Between these upper and lower limits, a population of asteroids between 1 kilometre (0.621372736649807 mi) and 25 kilometres (15.5 mi) in diameter is thought to be possible. They would be almost hot enough to glow red hot.
It is believed that the vulcanoids would be very rich in elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
with a high melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
, such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
. They are unlikely to possess a regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
because such fragmented material heats and cools more rapidly, and is affected more strongly by the Yarkovsky effect
Yarkovsky effect
The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum...
, than solid rock. Vulcanoids are probably similar to Mercury in colour and albedo, and may contain material left over from the earliest stages of the Solar System's formation.
There is evidence that Mercury was struck by a large object relatively late in its development, a collision which stripped away much of Mercury's crust and mantle, and explaining the thinness of Mercury's mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
compared to the mantles of the other terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
s. If such an impact occurred, much of the resulting debris might still be orbiting the Sun in the vulcanoid zone.
Significance
Vulcanoids, being an entirely new class of celestial bodies, would be interesting in their own right, but discovering whether or not they exist would yield insights into the formation and evolution of the Solar SystemFormation and evolution of the Solar System
The formation and evolution of the Solar System is estimated to have begun 4.568 billion years ago with the gravitational collapse of a small part of a giant molecular cloud...
. If they exist they might contain material left over from the earliest period of planet formation, and help determine the conditions under which the terrestrial planets, particularly Mercury, formed. In particular, if vulcanoids exist or did exist in the past, they would represent an additional population of impactors that have affected no other planet but Mercury making that planet's surface appear older than it actually is. If vulcanoids are found not to exist, this would place different constraints on planet formation and suggest that other processes have been at work in the inner Solar System, such as planetary migration
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...
clearing out the area.
See also
- Vulcan (hypothetical planet)Vulcan (hypothetical planet)Vulcan was a small planet proposed to exist in an orbit between Mercury and the Sun. In an attempt to explain peculiarities of Mercury's orbit, in the 19th-century French mathematician Urbain Jean Joseph Le Verrier hypothesized that they were the result of another planet, which he named Vulcan...
- List of hypothetical Solar System objects
- Groups of minor planets
- List of Mercury-crossing minor planets
- Kreutz SungrazersKreutz SungrazersThe Kreutz Sungrazers are a family of sungrazing comets, characterized by orbits taking them extremely close to the Sun at perihelion. They are believed to be fragments of one large comet that broke up several centuries ago and are named for German astronomer Heinrich Kreutz, who first...