2 Pallas
Pallas is the second asteroid to have been discovered, after 1 Ceres. It is the third-largest asteroid in the Solar System by both volume and mass, and is a likely remnant protoplanet. With an estimated 7% of the mass of the asteroid belt, it is three quarters the mass of 4 Vesta and slightly under a quarter the mass of Ceres. It is about in diameter, only slightly smaller than Vesta.
When Pallas was discovered by the German astronomer Heinrich Wilhelm Matthäus Olbers on 28 March 1802, it was counted as a planet, as were other asteroids in the early 19th century. The discovery of many more asteroids after 1845 eventually led to their reclassification.
Pallas's surface is most likely composed of a silicate material; its spectrum and estimated density resemble carbonaceous chondrite meteorites. With an orbital inclination of 34.8°, Pallas's orbit is unusually highly inclined to the plane of the asteroid belt, making Pallas relatively inaccessible to spacecraft, and its orbital eccentricity is nearly as large as that of Pluto.
It was considered as a potential dwarf planet in 2006, but it has since been determined that its shape departs significantly from an ellipsoid, thus disqualifying it.
History
Discovery
On the night of 5 April 1779, Charles Messier recorded Pallas on a star chart he used to track the path of a comet that he observed in the spring of 1779, but apparently assumed it was nothing more than a star.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 that year by the Baron von Zach and Heinrich W. M. Olbers after a preliminary orbit was computed by Carl 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 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. Pallas has a relatively high orbital inclination to the plane of the ecliptic.
Later observations
In 1917, the Japanese astronomer Kiyotsugu Hirayama 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, after the largest member of the group. Since 1994 more than 10 members of this family have been identified, with semi-major axes between 2.50–2.82 AU and inclinations of 33–38°. The validity of the family was confirmed in 2002 by a comparison of their spectra.Pallas has been observed occulting stars several times, including the best-observed of all asteroid occultation events, by 140 observers on 29 May 1983. These measurements resulted in the first accurate calculation of its diameter.
After an occultation on 29 May 1979, the discovery of a possible tiny satellite with a diameter of about 1 km was reported, which was never confirmed.
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 team was granted viewing time on the Hubble Space Telescope in September 2007 for a once-in-twenty-year opportunity to view Pallas at closest approach, to obtain comparative data for Ceres and Vesta.
Naming
2 Pallas is named after Pallas Athena, an alternate name for the goddess Athena.In some versions of the myth, Athena killed Pallas, then adopted her friend's name out of mourning.
The adjectival form of the name is Palladian. The d is part of the oblique stem of the Greek name, which appears before a vowel but disappears before the nominative ending -s. The oblique form is seen in the Italian and Russian names for the asteroid, Pallade and Паллада Pallada.
The stony-iron Pallasite meteorites are not Palladian, being named instead 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 a disk with its discovery number, ②. It also has an older, more iconic symbol, ⚴ or sometimes ?.
Orbit and rotation
Pallas has unusual dynamic parameters for such a large body. Its orbit is highly inclined and moderately eccentric, despite being at the same distance from the Sun as the central part of the asteroid belt. Furthermore, Pallas has a very high axial tilt of 84°, with its north pole pointing towards ecliptic coordinates = with a 5° uncertainty in the Ecliptic J2000.0 reference frame. This means that every Palladian summer and winter, large parts of the surface are in constant sunlight or constant darkness for a time on the order of an Earth year, with areas near the poles experiencing continuous sunlight for as long as two years.Near resonances
Pallas is in a, likely coincidental, near-1:1 orbital resonance with Ceres. Pallas also has a near-18:7 resonance and an approximate 5:2 resonance with Jupiter.Transits of planets from Pallas
From Pallas, the planets Mercury, Venus, Mars, and Earth can occasionally appear to transit, or pass in front of, the Sun. 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.Physical characteristics
Both Vesta and Pallas have assumed the title of second-largest asteroid from time to time. At in diameter, Pallas is slightly smaller than Vesta. The mass of Pallas is that of Vesta, that of Ceres, and a quarter of one percent that of the Moon.Pallas is farther from Earth and has a much lower albedo than Vesta, and hence is dimmer as seen from Earth. Indeed, the much smaller asteroid 7 Iris marginally exceeds Pallas in mean opposition magnitude. Pallas's 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 shone with magnitude 6.96.
Pallas is a B-type asteroid. Based on spectroscopic observations, the primary component of the material on Pallas's surface is a silicate containing little 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 meteorites, which are 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. Pallas's visible and near-infrared spectrum is almost flat, being slightly brighter in towards the blue. There is only one clear absorption band in the 3-micron part, which suggests an anhydrous component mixed with hydrated CM-like silicates.
Very little is known of Pallas's surface features. Hubble images from 2007, with a resolution around per pixel, show pixel-to-pixel variation, but Pallas's albedo of 0.12 placed such features at the lower end of detectability. There is little variability between lightcurves obtained through visible-light and infrared filters, but there are significant deviations in the ultraviolet, suggesting large surface or compositional features near 285°. Pallas's rotation appears to be prograde.
Pallas is thought to have undergone at least some degree of thermal alteration and partial differentiation, which suggests that it is a remnant protoplanet. 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, whereas others were destroyed in collisions with other protoplanets. Pallas and Vesta are likely survivors from this early stage of planetary formation.
status under the IAU's 2006 draft proposal on the definition of planet. Pallas is second from the right, bottom row.
Pallas was on a "watchlist" of objects possibly meeting a provisional definition of "planet" in an early draft of the IAU's 2006 definition of planet.