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2 Pallas
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2 Pallas (or as ) is one of the largest asteroids and is located in the main asteroid belt. It was the second asteroid to be discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802. Pallas was at first considered a planet, as were the other early asteroids 1 Ceres, 3 Juno, and 4 Vesta, 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.
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Encyclopedia
2 Pallas (or as ) is one of the largest asteroids and is located in the main asteroid belt. It was the second asteroid to be discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802. Pallas was at first considered a planet, as were the other early asteroids 1 Ceres, 3 Juno, and 4 Vesta, 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 Vesta, 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 chondrite meteorites. The Palladian orbit, at 34.8°, is unusually highly inclined to the plane of the main asteroid belt, and the orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft.
Name
2 Pallas is named after Pallas Athena, an alternate name for the goddess Athena.
In some mythologies Athena killed Pallas, 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 adjectival form of the name is Palladian.
The stony-iron Pallasite meteorites are not connected to the Pallas asteroid, being instead named after the German naturalist Peter Simon Pallas. The chemical element palladium, on the other hand, was named after the asteroid, which had been discovered just before the element.
As with other asteroids, the astronomical symbol for Pallas is its discovery number circled, . However, it also has dedicated symbols, or sometimes , which are not in common use.
History of observation
In 1801, the astronomer Giuseppe Piazzi discovered an object which he initially believed to be a 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 after a preliminary orbit was computed by Friedrich Gauss. 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 and Jupiter. Now, unexpectedly, a second such body had been found. When Pallas was discovered some estimates of its size were as large as 3380 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 to the plane of the ecliptic.
In 1917, the Japanese astronomer Kiyotsugu Hirayama 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 family 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 occulting a star 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 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 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.
The Dawn Mission team was granted viewing time on the Hubble Space Telescope 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 in volume, it is significantly less massive. The mass of Pallas is only 22% of Ceres, and about 0.3% that of the Moon.
Pallas is farther from the Earth with a much lower albedo than Vesta, and consequently appears dimmer. Indeed, the much smaller 7 Iris marginally exceeds Pallas in mean opposition magnitude.
Pallas' mean opposition magnitude is +8.0, which is well within the range of 10×50 binoculars, 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 is highly inclined and somewhat eccentric, despite being at the same distance from the sun as the central part of the main belt. Furthermore, its axial tilt is very high, either 78±13° or 65±12° (based on ambiguous lightcurve data, the pole points towards either ecliptic coordinates (ß, ?) = (−12°, 35°) or (43°, 193°) with a 10° uncertainty;
data from the Hubble Space Telescope 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 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 olivine and
pyroxene, which are found in CM chondrules.
The surface composition of Pallas is very similar to the Renazzo carbonaceous chondrite (CR) meteorites, which is even lower in hydrous minerals than the CM type. The Renazzo meteorite was discovered in Italy 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 protoplanets. During the planetary formation stage of the solar system, objects grew in size through an accretion process to approximately this size. Many of these objects were incorporated into larger bodies, which became the planets, 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 IAU'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, if it is found to have a surface shaped by hydrostatic equilibrium. 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 Jupiter.
Exploration
Pallas has not been visited by spacecraft, but if the Dawn probe is successful in studying 4 Vesta and 1 Ceres, it is possible its mission may be extended to include a flyby of Pallas as Pallas crosses the ecliptic. However, due to the high orbital inclination of Pallas, it will not be possible for Dawn to enter orbit.
See also
External links
—Horizons can be used to obtain a current ephemeris.
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