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2 Pallas

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2 Pallas

2 Pallas
An ultraviolet image of Pallas showing its flattened shape, taken by the Hubble Telescope
Discovered by Heinrich Wilhelm Olbers
Discovery date March 28, 1802
Named after
Pallas Athena
Minor planet category main belt
(Pallas family)
Adjectives Palladian [1]
Orbital characteristics[2][3]
Epoch 2010-Jul-23 (JD 2455400.5)
Aphelion 3.412 AU (510.4 Gm)
Perihelion 2.132 AU (318.9 Gm)
2.772 AU (414.7 Gm)
Eccentricity 0.231
4.62 a (1685.87 d)
Average orbital speed
17.65 km/s
Inclination 34.841° to Ecliptic
34.21° to Invariable plane[4]
Satellites None
Proper orbital elements[5]
2.7709176 AU
Proper eccentricity
Proper inclination
Proper mean motion
78.041654 deg / yr
4.61292 yr
(1684.869 d)
Precession of perihelion
-1.335344 arcsec / yr
Precession of the ascending node
−46.393342 arcsec / yr
Physical characteristics
Dimensions 582 × 556 × 500±18 km[6]
544 km (mean)[3]
Mass (2.11±0.26)×1020 kg[7]
Mean density
≈ 2.8 g/cm³[6]
≈ 0.18 m/s² / .018g
≈ 0.32 km/s
0.32555 d
(7.8132 h)[8]
likely 78°±13°[9]
Albedo 0.159 (geometric)[10]
Temperature ≈ 164 K
max: ≈ 265 K (−8 °C)
Spectral type
B-type asteroid[11]
6.49[12] to 10.65
0.629″ to 0.171″[13]

Pallas, minor-planet designation 2 Pallas, is the second asteroid to have been discovered (after Ceres), and it is one of the largest asteroids in the Solar System. It is estimated to comprise 7% of the mass of the asteroid belt,[14] and its diameter of 544 kilometres (338 mi) is slightly larger than that of 4 Vesta. It is 10–30% less massive than Vesta,[7] placing it third among the asteroids. It is likely a remnant protoplanet.

When Pallas was discovered by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 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.

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 asteroid belt, and the orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft.[* 1][16]


2 Pallas is named after Pallas Athena, an alternate name for the goddess Athena.[17][18] In some mythologies Athena killed Pallas, then adopted her friend's name out of mourning.[19] (There are several male characters of the same name in Greek mythology, but the first asteroids were invariably given female names.)

Pallas is a Greek name and some other languages use variant versions of it for the asteroid: in Italian, Pallade, Russian Pallada, Spanish Palas, and Arabic Bālās, for example. In Chinese, however, the asteroid has a different name, 智神星 zhìshénxīng (which means "the wisdom-god(dess) star"), even though the Chinese call the goddess Pallas herself by the Greek name (帕拉斯 pàlāsī).

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.[20]

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  Variant of Pallas symbol).[21]

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.[22][23]

Diagram illustrating Johann Hieronymus Schröter's inaccurate 1811 estimate of the size of Pallas. Schröter wrongly believed Pallas to be over 3,000km in diameter, which would have made it larger than Pluto (which had not been discovered at the time of Schröter's estimate).

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.[24] When Pallas was discovered, some estimates of its size were as high as 3,380 km in diameter.[25] Even as recently as 1979, Pallas was estimated to be 673 km in diameter (26% greater than the currently accepted value).[26]

Size comparison of several objects with potential for dwarf planet status under the IAU's 2006 draft proposal on the definition of planet.[27] Pallas is second from the right, bottom row.

The orbit of Pallas was determined by Gauss, who found the period of 4.6 years was similar to the period for Ceres. Pallas had a relatively high orbital inclination to the plane of the ecliptic.[24]

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.[28] 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°.[29] The validity of this grouping was confirmed in 2002 by a comparison of their spectra.[30]

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.[31][32] During the occultation of May 29, 1979 the discovery of a possible tiny satellite with a diameter of about 1 km was reported. 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.[33]

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.[34]

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.[6][35]


Size comparison: the first 10 asteroids profiled against Earth's Moon. Pallas is at second left.
False-colored image of Pallas.

Both Vesta and Pallas have assumed the title of second-largest asteroid from time to time.[36] However, Pallas is slightly larger than 4 Vesta in volume, but significantly less massive. The mass of Pallas is only 22% of Ceres[14] and about 0.3% that of the Moon.

Pallas is farther from Earth and has a much lower albedo than Vesta, and so it appears dimmer. Indeed, the much smaller 7 Iris marginally exceeds Pallas in mean opposition magnitude.[37] 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.[12] During late February 2014 Pallas shone with magnitude 6.96.[38]

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 asteroid 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 (β, λ) = (−12°, 35°) or (43°, 193°) with a 10° uncertainty;[9] 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.[6][39]). 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.

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 and pyroxene, which are found in CM chondrules.[40] The surface composition of Pallas is very similar to the Renazzo carbonaceous chondrite (CR) meteorites, which are even lower in hydrous minerals than the CM type.[41] The Renazzo meteorite was discovered in Italy in 1824 and is one of the most primitive meteorites known.[42]

The animation illustrates Pallas's near-18:7 resonance pattern with Jupiter. The motion of Pallas is shown in a reference frame that rotates about the Sun (i.e. the center dot) with a period equal to Jupiter's orbital period. Accordingly, Jupiter's orbit appears almost stationary as the pink ellipse at top left. Mars's motion is orange, and the Earth–Moon system is blue and white. The orbit of Pallas is green when above the ecliptic and red when below. The near-18:7 resonance pattern with Jupiter only marches clockwise: it never halts or reverses course (i.e. no libration).

Very little is known of Palladian surface features. Hubble images from 2007 show pixel-to-pixel variation (pixel resolution is around 70 kilometres (43 mi)), 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° (75° west longitude). Pallas's rotation appears to be prograde.[6]

Pallas is believed to have undergone at least some degree of thermal alteration and partial differentiation,[6] 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.[43]

Pallas was among the "candidate planets" in an early draft of the IAU's 2006 definition of planet, but it does not qualify in the final definition because it has not "cleared the neighborhood" around its orbit.[44][45] In the future, it is possible that Pallas may be classified as a dwarf planet, if it is found to have a surface that is in hydrostatic equilibrium.

Near resonances

Pallas is in a near-1:1 mean-motion orbital resonance with Ceres.[46] Pallas also has a near-18:7 resonance (6500-year period) and an approximate 5:2 resonance (83-year period) with Jupiter.[47]

Transits of planets from Pallas

From Pallas, 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.[48]


There is no planned exploration of Pallas by spacecraft. It was originally hoped that if the Dawn probe was successful in studying 4 Vesta and 1 Ceres, sufficient fuel might remain for its mission to be extended for a brief flyby of Pallas as Pallas crossed the ecliptic in December 2018. Problems that have since developed with Dawn's reaction wheels have precluded such a possibility. Due to the high orbital inclination of Pallas, it would not have been possible for Dawn to match orbits, which would have required a different spacecraft design.[49][* 2]

See also


  1. ^ Pallasite meteorites are not named for the asteroid, but for a scientist who characterized them. They do not necessarily share origin with Pallas.[15]
  2. ^ A member of the Dawn team explained, "It is impossible to reach with a mission in the same class as Dawn because it takes too much thrust to reach Pallas. Pallas is highly inclined to the ecliptic plane. A lot of energy is needed to climb out of the ecliptic plane especially as far out of the plane as Pallas is. I did try to design a mission to reach Pallas and it was impossible with the Dawn spacecraft even if we went nowhere else than Pallas."[50]


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  21. ^ Unicode value U+26B4
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