2 Pallas

2 Pallas

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Pallas, formally designated 2 Pallas, is the second asteroid
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...

 to have been discovered (after Ceres), and one of the largest. It is estimated to constitute 7% of the mass 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...

, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

. It is however 20% less massive than Vesta, placing it third among the asteroids. It is possibly the largest irregularly shaped body 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...

 (that is, the largest body not rounded under its own gravity), and a remnant protoplanet
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...

.

When Pallas was discovered by astronomer
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 Heinrich Wilhelm Matthäus Olbers
Heinrich Wilhelm Matthäus Olbers
Heinrich Wilhelm Matthäus Olbers was a German physician and astronomer.-Life and career:Olbers was born in Arbergen, near Bremen, and studied to be a physician at Göttingen. After his graduation in 1780, he began practicing medicine in Bremen, Germany...

 on March 28, 1802, it was counted as a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

, as were other asteroids in the early 19th century. The discovery of many more asteroids after 1845 eventually led to their re-classification.

The Palladian surface appears to be a silicate material; the surface spectrum and estimated density resemble carbonaceous chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...

 meteorites. The Palladian orbit, at 34.8°, is unusually highly inclined to the plane of the asteroid belt, and the orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 is nearly as large as that of Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, making Pallas relatively inaccessible to spacecraft.

Name


2 Pallas is named after Pallas Athena
Athena
In Greek mythology, Athena, Athenê, or Athene , also referred to as Pallas Athena/Athene , is the goddess of wisdom, courage, inspiration, civilization, warfare, strength, strategy, the arts, crafts, justice, and skill. Minerva, Athena's Roman incarnation, embodies similar attributes. Athena is...

, an alternate name for the goddess Athena.
In some mythologies Athena killed Pallas
Pallas (daughter of Triton)
In Greek mythology, Pallas was the daughter of Triton. Acting as a foster parent to Zeus’s daughter Athena, Triton raised her alongside his own daughter. During a friendly fight between the two goddesses, Athena was protected from harm by Zeus but mortally wounded Pallas. Out of sadness and...

, then adopted her friend's name out of mourning.
(There are several male characters of the same name in Greek mythology, but the first asteroids were invariably given female names.)

The stony-iron Pallasite
Pallasite
A pallasite is a type of stony–iron meteorite.-Structure and composition:It consists of cm-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching...

 meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

s are not connected to the Pallas asteroid, being instead named after the German naturalist Peter Simon Pallas
Peter Simon Pallas
Peter Simon Pallas was a German zoologist and botanist who worked in Russia.- Life and work :Pallas was born in Berlin, the son of Professor of Surgery Simon Pallas. He studied with private tutors and took an interest in natural history, later attending the University of Halle and the University...

. The chemical element palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...

, on the other hand, was named after the asteroid, which had been discovered just before the element.

As with other asteroids, the astronomical symbol
Astronomical symbols
Astronomical symbols are symbols used to represent various celestial objects, theoretical constructs and observational events in astronomy. The earliest forms of these symbols appear in Greek papyri of late antiquity. The Byzantine codices in which the Greek papyri were preserved continued and...

 for Pallas is a disk with its discovery number, ②. However, it also has an older, more iconic symbol, ( or sometimes  
).

History of observation


In 1801, the astronomer Giuseppe Piazzi
Giuseppe Piazzi
Giuseppe Piazzi was an Italian Catholic priest of the Theatine order, mathematician, and astronomer. He was born in Ponte in Valtellina, and died in Naples. He established an observatory at Palermo, now the Osservatorio Astronomico di Palermo – Giuseppe S...

 discovered an object which he initially believed to be a comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

. Shortly thereafter he announced his observations of this object, noting that the slow, uniform motion was uncharacteristic of a comet, suggesting it was a different type of object. This was lost from sight for several months, but was recovered later in the year by the Baron von Zach and Heinrich W. M. Olbers
Heinrich Wilhelm Matthäus Olbers
Heinrich Wilhelm Matthäus Olbers was a German physician and astronomer.-Life and career:Olbers was born in Arbergen, near Bremen, and studied to be a physician at Göttingen. After his graduation in 1780, he began practicing medicine in Bremen, Germany...

 after a preliminary orbit was computed by Friedrich Gauss
Carl Friedrich Gauss
Johann Carl Friedrich Gauss was a German mathematician and scientist who contributed significantly to many fields, including number theory, statistics, analysis, differential geometry, geodesy, geophysics, electrostatics, astronomy and optics.Sometimes referred to as the Princeps mathematicorum...

. This object came to be named Ceres, and was the first asteroid to be discovered.


A few months later, Olbers was again attempting to locate Ceres when he noticed another moving object in the vicinity. This was the asteroid Pallas, coincidentally passing near Ceres at the time. The discovery of this object created interest in the astronomy community. Before this point it had been speculated by astronomers that there should be a planet in the gap between Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 and Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

. Now, unexpectedly, a second such body had been found. When Pallas was discovered some estimates of its size were as high as 3,380 km in diameter. Even as recently as 1979, Pallas was estimated to be 673 km in diameter (26% greater than the currently accepted value).


The orbit of Pallas was determined by Gauss, who found the period of 4.6 years was similar to the period for Ceres. However, Pallas had a relatively high orbital 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...

 to the plane of 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...

.

In 1917, the Japanese astronomer Kiyotsugu Hirayama
Kiyotsugu Hirayama
was a Japanese astronomer, best known for his discovery that many asteroid orbits were more similar to one another than chance would allow, leading to the concept of asteroid families, now called "Hirayama families" in his honour....

 began to study asteroid motions. By plotting the mean orbital motion, inclination and eccentricity of a set of asteroids, he discovered several distinct groupings. In a later paper he reported a group of three asteroids associated with Pallas, which became named the Pallas family
Pallas family
The Pallas or Palladian family of asteroids is a grouping of B-type asteroids at very high inclinations in the intermediate asteroid belt . It was first noted by Kiyotsugu Hirayama in 1928....

 after the largest member of the group. Since 1994 more than 10 members of this family have been identified, and these have semi-major axes between 2.50–2.82 AU and inclinations of 33–38°. The validity of this grouping was confirmed in 2002 by a comparison of their spectra.

Pallas has been observed occulting
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

 a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 several times, including the best observed of all asteroid occultation events on May 29, 1983, when careful occultation timing measurements were taken by 140 observers. These resulted in the first accurate measurements of its diameter.
During the occultation of May 29, 1979 the discovery of a possible tiny satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 with a diameter of about 1 km was reported. However, it could not be confirmed. In 1980, speckle interferometry was reported as indicating a much larger satellite with a diameter of 175 km, but the existence of the satellite was later refuted.

Radio signals from spacecraft in orbit around Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 and/or on its surface have been used to estimate the mass of Pallas from the tiny perturbations induced by it onto the motion of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

.

The Dawn Mission
Dawn Mission
Dawn is a NASA spacecraft tasked with the exploration and study of the two largest members of the asteroid belt – Vesta and the dwarf planet Ceres. The spacecraft was constructed with some European cooperation, with partners in Germany, Italy, and the Netherlands providing Dawns framing...

 team was granted viewing time on the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 in September 2007 for a once-in-twenty-year opportunity to view the asteroid at closest approach, to obtain comparative data for Ceres and Vesta.

Characteristics




Both Vesta and Pallas have assumed the title of second-largest asteroid from time to time.
However, while Pallas is similar to 4 Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

 in volume, it is significantly less massive. The mass of Pallas is only 22% of Ceres, and about 0.3% that of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

.

Pallas is farther from the Earth with a much lower albedo than Vesta, and consequently appears dimmer. Indeed, the much smaller 7 Iris
7 Iris
7 Iris is a large main-belt asteroid. Among the S-type asteroids, it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina....

 marginally exceeds Pallas in mean opposition magnitude.
Pallas' mean opposition 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...

 is +8.0, which is well within the range of 10×50 binoculars
Binoculars
Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

, but unlike Ceres and Vesta, it will require more powerful optical aid to view at small elongations, when its magnitude can drop as low as +10.6. During rare perihelic oppositions, Pallas can reach a magnitude of +6.4, right on the edge of naked-eye visibility.
During late February 2014, Pallas will shine at magnitude 6.96.

Pallas has unusual dynamic parameters for such a large body. Its orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 is highly inclined
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...

 and somewhat eccentric, despite being at the same distance from the Sun as the central part 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...

. Furthermore, its axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

 is very high, either 78±13° or 65±12° (based on ambiguous lightcurve data, the pole points towards either ecliptic coordinates
Ecliptic coordinate system
The ecliptic coordinate system is a celestial coordinate system that uses the ecliptic for its fundamental plane. The ecliptic is the path that the sun appears to follow across the celestial sphere over the course of a year. It is also the intersection of the Earth's orbital plane and the celestial...

 (β, λ) = (−12°, 35°) or (43°, 193°) with a 10° uncertainty;
data from the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 obtained in 2007 as well as the observations by the Keck telescope in 2003–2005 favour the first solution.)
This means that, every Palladian summer and winter, large parts of the surface are in constant sunlight or constant darkness for a time of the order of an Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 year.

Based on spectroscopic observations, the primary component of the Palladian surface material is a silicate that is low in iron and water. Minerals of this type include olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

 and
pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...

, which are found in CM chondrules
Chondrule
Chondrules are round grains found in chondrites. Chondrules form as molten or partially molten droplets in space before being accreted to their parent asteroids...

.
The surface composition of Pallas is very similar to the Renazzo carbonaceous chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...

 (CR) meteorites, which are even lower in hydrous minerals than the CM type. The Renazzo meteorite was discovered in Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

 in 1824 and is one of the most primitive meteorites known.


Very little is known of Palladian surface features. Hubble images from 2007 show pixel-to-pixel variation (pixel resolution is ~70 km), but Pallas' 12% albedo placed such features at the lower end of detectability. There is little variability between lightcurves obtained through visible-light and infrared filters, but significant deviations in the ultraviolet, suggesting large surface or compositional features near 285° (75° west longitude). Rotation appears to be prograde.

Pallas is believed to have undergone at least some degree of thermal alteration and partial differentiation
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...

, which suggests that it was a protoplanet
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...

. During the planetary formation stage of 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...

, objects grew in size through an accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...

 process to approximately this size. Many of these objects were incorporated into larger bodies, which became the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s, while others were destroyed in collisions with other protoplanets. Pallas and Vesta are likely survivors from this early stage of planetary formation.

Pallas was among the "candidate planets" in an early draft of the IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

's 2006 definition of planet, but does not qualify in the final definition because it has not "cleared the neighborhood" around its orbit.
In the future, it is possible that Pallas may be classified as a dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

, if it is found to have a surface shaped by hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...

.

Near resonances


Pallas is in a near-1:1 mean-motion orbital resonance with Ceres. Pallas also has a near-18:7 resonance (6500-year period) and an approximate 5:2 resonance (83-year period) with Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

.

Transits of planets from Pallas


From Pallas, Mercury, Venus, Mars, and the Earth can occasionally appear to 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...

, or pass in front of, the Sun.
The Earth last did so in 1968 and 1998, and will next transit in 2224. Mercury did in October 2009. The last and next by Venus are in 1677 and 2123, and for Mars they are in 1597 and 2759.

Exploration


Pallas has not been visited by spacecraft, but if the Dawn
Dawn Mission
Dawn is a NASA spacecraft tasked with the exploration and study of the two largest members of the asteroid belt – Vesta and the dwarf planet Ceres. The spacecraft was constructed with some European cooperation, with partners in Germany, Italy, and the Netherlands providing Dawns framing...

probe is successful in studying 4 Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

 and 1 Ceres, and if sufficient fuel remains, it is possible its mission may be extended to include a flyby of Pallas as Pallas crosses 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...

in 2018. However, due to the high orbital inclination of Pallas, it will not be possible for Dawn to enter orbit.

External links