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'2 Pallas' (, Greek ''Παλλάς)'' is an asteroid located in the asteroid belt region of the solar system and was the second to be discovered. It was found and named by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802. The adjectival form of the name is ''Palladian.''
Pallas is the third most massive object in the asteroid belt and is estimated to contain 7% of the region's entire mass. Pallas, as well as Ceres, 3 Juno and 4 Vesta, were once considered planets until the discovery of many other asteroids led to re-classifications. The orbit of Pallas is highly inclined to the plane of the main asteroid belt at 34.8°, and the orbital eccentricity is nearly as large as Pluto's. The surface is a silicate material with a spectrum that is similar to carbonaceous chondrite meteorites.

Contents

Name

History of observation

Characteristics

See also

References

External links

Name

The asteroid is named after Pallas Athena, an alternate name for the goddess Athena.^[8][9] (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 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.^[10]
Pallas' astronomical and astrological symbol is

or sometimes  

Variant symbol of Pallas

 .

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 that were 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. Prior to 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.^[11]
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.^[10] Since 1994 more than 10 members of this family have been identified. (Members have semi-major axis = 2.50–2.82 AU; inclination = 33–38°.)^[13] The existence of this family was finally confirmed in 2002 by a
comparison of their spectra.^[14]
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 have helped determine an accurate diameter. During the occultation of May 29, 1979 the discovery of a possible tiny satellite with a diameter of ~1 km was reported. However, it has not been 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.^[15]
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.
There have not been any telescopic observations of Pallas that have resolved any features on its disk. Pallas has not yet been visited by a spacecraft, but if the ''Dawn'' probe is successful in studying 1 Ceres and 4 Vesta, its mission may be extended to Pallas. Due to the high orbital inclination of Pallas, it is more difficult for a spacecraft to reach than the other large asteroids.^[16]

Characteristics

Pallas is the third largest object in the asteroid belt, similar to 4 Vesta in volume (to within uncertainty), but somewhat less massive. By comparison, the mass of Pallas is only about 0.3% of the Moon's mass. Both Pallas and Vesta have assumed the title of second largest asteroid from time to time. However, Vesta is closer to Earth with a much higher albedo than Pallas, and consequently appears brighter. Indeed, the much smaller 7 Iris marginally exceeds Pallas in mean opposition magnitude.^[17] 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 +11.
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,^[18] but only if it is found that its shape is consistent with hydrostatic equilibrium.^[19]
There is some consideration that the largest asteroids, such as
Pallas, are actually protoplanets. During the planetary formation stage of the solar system, objects grew in size through an accretion process. Many of the objects the size of Ceres and Pallas were accreted by the largest bodies, which became the planets. Other protoplanetary bodies were destroyed through collisions with similar-size bodies. Pallas is likely a survivor of this early planetary-formation phase.^[20]
Pallas has unusual dynamical parameters for such a large body. Its orbit is highly inclined and somewhat eccentric despite being located at the same distance from the sun as the central part of the main belt. Furthermore, its axial tilt is very high, being around 60° (in fact estimates vary from 56° to 81°).^[21][22] 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.
Consensus has not been reached as to whether Pallas' rotation is prograde or retrograde. The most recent
analysis of lightcurves indicates that the pole points towards ecliptic coordinates (β, λ) = (-12°, 35°) or (43°, 193°) with a 10° uncertainty. This gives axial tilts of 57° or 65°, respectively.
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.^[23] There are indications that 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.^[24] The Renazzo meteorite was discovered in Italy in 1824, and is one of the most primitive meteorites known.^[25]

See also

★ Pallas in fiction

★ List of Solar System bodies formerly considered planets

References

1. Triaxial ellipsoid dimensions and rotational pole of 2 Pallas from two stellar occultations, Drummond, J. D.; Cocke, W. J., , , Icarus, 1989
2. The size and shape of (2) Pallas from the 1983 occultation of 1 Vulpeculae, D. W. Dunham ''et al'', , , Astronomical Journal, 1990
3.
4. New determination of the mass of Pallas, , E., Goffin, Astronomy and Astrophysics, 2001
5. Asteroid Lightcurve Derived Data. EAR-A-5-DDR-DERIVED-LIGHTCURVE-V8.0.
6. Asteroid Taxonomy.EAR-A-5-DDR-TAXONOMY-V5.0.
7. IRAS Minor Planet Survey. IRAS-A-FPA-3-RDR-IMPS-V6.0.
8. Pallas: 2006-2015
9. Athena
10.
11. Astronomical Serendipity
12.
13. Description of the System of Asteroids
14. New clusters for highly inclined main-belt asteroids, Foglia, S.; Masi, G., , , The Minor Planet Bulletin, 1999
15. Other Reports of Asteroid/TNO Companions
16. Notable Asteroids
17. The Brightest Asteroids
18.
IAU 2006 General Assembly: Result of the IAU Resolution votes
19. Planets plan boosts tally to 12 Paul Rincon
20. Ceres, Vesta, and Pallas: Protoplanets, Not Asteroids, McCord, T. B.; McFadden, L. A.; Russell, C. T.; Sotin, C.; Thomas, P. C., , , Transactions of the American Geophysical Union, 2006
21. Radar Observations of Asteroids 1 Ceres, 2 Pallas, and 4 Vesta, D. L. Mitchell ''et al'', , , Icarus, 1996
22. Shapes and rotational properties of thirty asteroids from photometric data, J. Torppa ''et al'', , , Icarus, 1996
23. The 3 Micron Spectrum of Asteroid 2 Pallas., Feierberg, M. A.; Larson, H. P.; Lebofsky, L. A., , , Bulletin of the American Astronomical Society, 1982
24. Absorption bands near 3 m in diffuse reflectance spectra of carbonaceous chondrites: Comparison with asteroids, Sato, Kimiyasu; Miyamoto, Masamichi; Zolensky, Michael E., , , Meteoritics, 1997
25. Earliest Meteoritse Provide New Piece in Planetary Formation Puzzle.

External links

★ 2 Pallas

★ The Supplemental IRAS Minor Planet Survey, Tedesco, Edward F.; Noah, Paul V.; Noah, Meg; Price, Stephan D., , , The Astronomical Journal, 2002

★ U.S. Naval Observatory Ephemerides of the Largest Asteroids

★ Ceres, Pallas Vesta and Hygeia

★ Horizons system — Horizons can be used to obtain a current ephemeris.