2 Pallas is one of the largest
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
s and is located in the main
asteroid beltThe 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...
. It was the second asteroid to be discovered, by
astronomerAstronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
Heinrich Wilhelm Matthäus OlbersHeinrich 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. Pallas was at first considered a planet, as were the other early asteroids 1 Ceres,
3 JunoJuno , formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main belt asteroids, being one of the two largest stony asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid...
, and
4 Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
, until the discovery of many additional asteroids led to their re-classification.
With a mass estimated to be 7% of the total mass of the asteroid belt, Pallas is one of the largest asteroids. Its diameter is some 550 km, comparable to that of
4 Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
, but it is 20% less massive, placing it third among the asteroids. The Palladian surface appears to be a silicate material; the surface spectrum and estimated density resemble
carbonaceous chondriteCarbonaceous 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 main
asteroid beltThe 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 the
orbital eccentricityIn astrodynamics, under standard assumptions, any orbit must be of conic section shape. The eccentricity of this conic section, the orbit's eccentricity, is an important parameter of the orbit that defines its absolute shape...
is nearly as large as that of
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
, making Pallas relatively inaccessible to spacecraft.
Name
2 Pallas is named after
Pallas AthenaIn Greek mythology, Athena is the goddess of wisdom, peace, warfare, strategy, handicrafts and reason, shrewd companion of heroes and the goddess of heroic endeavour...
, an alternate name for the goddess Athena.
In some mythologies Athena killed
PallasIn Greek mythology, Pallas was the daughter of Triton. Acting as a foster parent to Zeus’ daughter Athena, Triton raised her alongside his own daughter Pallas. During a fight between the two goddesses, Athena was protected from harm by Zeus, however she mortally wounded Pallas. Out of sadness...
, 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
PallasiteA pallasite is a type of stony-iron meteorite. It consists of cm-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching. Minor constituents are schreibersite, troilite, chromite, pyroxenes, and phosphates...
meteoriteA meteorite is a natural object originating in outer space that survives an impact with the Earth's surface. 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 PallasPeter Simon Pallas was a German zoologist and botanist who worked in Russia.- Life and work :...
. The chemical element
palladiumPalladium is a chemical element with the chemical symbol Pd and an atomic number of 46. Palladium is a rare and lustrous silvery-white metal that was discovered in 1803 by William Hyde Wollaston, who named it after the asteroid Pallas, which in turn, was named after the epithet of the Greek...
, on the other hand, was named after the asteroid, which had been discovered just before the element.
As with other asteroids, the
astronomical symbolAstronomical symbols are symbols used to represent various celestial objects, theoretical constructs and observational events in astronomy. The symbols listed here are commonly used by professional and amateur astronomers. Many of the symbols are shared with western astrology, which uses multiple...
for Pallas is its discovery number circled, . However, it also has dedicated symbols,
or sometimes
.
History of observation
In 1801, the astronomer
Giuseppe PiazziGiuseppe Piazzi was an Italian Theatine monk, 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
cometA comet is a Small Solar System Body that has coma and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma , and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus...
. 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 ZachBaron Franz Xaver von Zach was a German astronomer born at Pest in Hungary....
and
Heinrich W. M. OlbersHeinrich 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 GaussJohann 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...
. 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
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
and
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
. Now, unexpectedly, a second such body had been found. When Pallas was discovered some estimates of its size were as large 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
inclinationInclination 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
eclipticThe ecliptic is the apparent path that the Sun traces out in the sky during the year, appearing to move eastwards on an imaginary spherical surface, the celestial sphere, relative to the fixed stars. More accurately, it is the intersection of the celestial sphere with the ecliptic plane, which is...
.
In 1917, the Japanese astronomer
Kiyotsugu Hirayamawas 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 a set of asteroids based on their mean orbital motion, inclination and eccentricty, he discovered several distinct groupings. In a later paper he reported a group of three asteroids associated with Pallas, which became named the
Pallas familyThe Pallas family of asteroids is a grouping of B-type asteroids at very high inclinations in the intermediate main 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 existence of this family was finally confirmed in 2002 by a comparison of their spectra.
Pallas has been observed
occultingAn 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 and can also be used in a general sense to describe when an object in the foreground occults objects in the background...
a
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
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
satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
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
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
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
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
.
The
Dawn MissionDawn is a robotic spacecraft sent by NASA on a space exploration mission to the two most massive members of the asteroid belt: the asteroid Vesta and the dwarf planet Ceres. Launched September 27, 2007, Dawn is scheduled to explore Vesta between 2011 and 2012, and Ceres in 2015...
team was granted viewing time on the
Hubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...
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 Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
in volume, it is significantly less massive. The mass of Pallas is only 22% of Ceres, and about 0.3% that of the
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
.
Pallas is farther from the Earth with a much lower albedo than Vesta, and consequently appears dimmer. Indeed, the much smaller
7 Iris7 Iris is a large main belt asteroid. Among 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
magnitudeThe apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere...
is +8.0, which is well within the range of 10×50
binocularsBinoculars, 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 with binocular vision when viewing distant objects. Most are sized to be held...
, 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
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
is highly
inclinedInclination 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 main belt. Furthermore, its
axial tiltIn astronomy, axial tilt is the angle between an object's rotational axis and a line perpendicular to its orbital plane. The angle is measured between the line perpendicular to object's orbital plane and object's rotational axis passing through north pole at which the planet appears to rotate...
is very high, either 78±13° or 65±12° (based on ambiguous lightcurve data, the pole points towards either
ecliptic coordinatesThe 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 sky over the course of a year. It is also the projection of the Earth's orbital plane onto the celestial sphere...
(β, λ) = (−12°, 35°) or (43°, 193°) with a 10° uncertainty;
data from the
Hubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...
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
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
year.
Based on spectroscopic observations, the primary component of the Pallas surface material is a silicate that is low in iron and water. Minerals of this type include
olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
and
pyroxeneThe pyroxenes are a group of important rock-forming silicate 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 chondrulesMost meteorites that fall on Earth are chondrites, which are characterized by the presence of round grains called chondrules . Chondrules formed 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 chondriteCarbonaceous 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 is even lower in hydrous minerals than the CM type. The Renazzo meteorite was discovered in
ItalyItaly , officially the Italian Republic , is a country located on the Italian Peninsula in Southern Europe and on the two largest islands in the Mediterranean Sea, Sicily and Sardinia. Italy shares its northern, Alpine boundary with France, Switzerland, Austria and Slovenia...
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 and 75° west longitude. Rotation appears to be prograde.
It is possible that the largest asteroids, including Pallas, are
protoplanetProtoplanets are moon-sized planets, or larger embryos within protoplanetary discs. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...
s. During the planetary formation stage of the
solar systemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
, objects grew in size through an
accretionIn 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
planetA 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 is a likely survivor from the early stages of planetary formation.
Pallas was among the "candidate planets" in an early draft of the
IAUThe International Astronomical Union 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 planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
, if it is found to have a surface shaped by
hydrostatic equilibriumHydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient wihch creates a pressure gradient force in the opposite direction...
. However, recent Hubble images make that prospect unlikely, as they reveal a slightly uneven surface.
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
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
.
Transits of planets from Pallas
From Pallas, Mercury, Venus, Mars, and the Earth can occasionally appear to
transitThe 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, as seen by an observer at some particular vantage point.* A transit occurs when a celestial...
, or pass in front of, the Sun.
The Earth last did so in 1968 and 1998, and will next transit in 2224. Mercury does 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
DawnDawn is a robotic spacecraft sent by NASA on a space exploration mission to the two most massive members of the asteroid belt: the asteroid Vesta and the dwarf planet Ceres. Launched September 27, 2007, Dawn is scheduled to explore Vesta between 2011 and 2012, and Ceres in 2015...
probe is successful in studying
4 Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
and 1 Ceres, it is possible its mission may be extended to include a flyby of Pallas as Pallas crosses the
eclipticThe ecliptic is the apparent path that the Sun traces out in the sky during the year, appearing to move eastwards on an imaginary spherical surface, the celestial sphere, relative to the fixed stars. More accurately, it is the intersection of the celestial sphere with the ecliptic plane, which is...
. However, due to the high orbital inclination of Pallas, it will not be possible for
Dawn to enter orbit.
See also
- Pallas in fiction
- List of Solar System bodies formerly considered planets
External links
—Horizons can be used to obtain a current ephemeris.