|Discovered by||Heinrich Wilhelm Olbers|
|Discovery date||March 28, 1802|
|Named after||Pallas Athena|
|Minor planet category||main belt
|Epoch 2010-Jul-23 (JD 2455400.5)|
|Aphelion||3.412 AU (510.4 Gm)|
|Perihelion||2.132 AU (318.9 Gm)|
|Semi-major axis||2.772 AU (414.7 Gm)|
|Orbital period||4.62 a (1685.87 d)|
|Average orbital speed||17.65 km/s|
|Inclination||34.841° to Ecliptic
34.21° to Invariable plane
|Longitude of ascending node||173.12°|
|Argument of perihelion||310.15°|
|Proper orbital elements|
|Proper semi-major axis||2.7709176 AU|
|Proper mean motion||78.041654 deg / yr|
|Proper orbital period||4.61292 yr
|Precession of perihelion||-1.335344 arcsec / yr|
|Precession of the ascending node||−46.393342 arcsec / yr|
544 km (mean)
|Mean density||≈2.8 g/cm³|
|Equatorial surface gravity||≈0.18 m/s²|
|Escape velocity||≈0.32 km/s|
|Rotation period||0.325 55 d
|Axial tilt||likely 78 ± 13°|
max: ≈265 K (-8 °C)
|Spectral type||B-type asteroid|
|Apparent magnitude||6.49 to 10.65|
|Absolute magnitude (H)||4.13|
|Angular diameter||0.629" to 0.171"|
Pallas, minor-planet designation 2 Pallas, is the second asteroid to have been discovered (after Ceres), and one of the largest in the Solar System. It is estimated to comprise 7% of the mass of the asteroid belt, and its diameter of 544 kilometres (338 mi) is slightly larger than that of 4 Vesta. It is however 10–30% less massive than Vesta, placing it third among the asteroids. It is possibly the largest irregularly shaped body in the Solar System (that is, the largest body not rounded under its own gravity)[not verified in body], and 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 re-classification.
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]
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.)[* 2]
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.
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 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. 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 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, and these have semi-major axes between 2.50–2.82 AU and inclinations of 33–38°. The validity of this grouping was 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.
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.
Both Vesta and Pallas have assumed the title of second-largest asteroid from time to time. However, while Pallas is slightly larger than 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 and has a much lower albedo than Vesta, and consequently appears dimmer. Indeed, the much smaller 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 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 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; 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 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. 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. 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's 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° (75° west longitude). Rotation appears to be prograde.
Pallas is believed to have undergone at least some degree of thermal alteration and partial differentiation, which suggests that it was a 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, while others were destroyed in collisions with other protoplanets. Pallas and Vesta are likely survivors from this early stage 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 in hydrostatic equilibrium.
Transits of planets from Pallas
From Pallas, Mercury, Venus, Mars, and the Earth can occasionally appear to transit, or pass in front of, the Sun. The 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.
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. However, problems that have since developed with Dawn's reaction wheels have precluded such a possibility. Moreover, 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.[* 3]
- Pallasite meteorites are not named for the asteroid, but for a scientist who characterized them, and they do not necessarily share origin with Pallas.
- Since Pallas is already a Greek name, the asteroid has the same name in Greek, unlike 1 Ceres, 3 Juno, and 4 Vesta. All other languages, with one exception, use Pallas or national variants of that name: Italian Pallade, Russian Pallada, Spanish Palas, Arabic Bālās. The one exception is Chinese, in which Pallas is called the 'wisdom-god(dess) star' (智神星 zhìshénxīng). This is in contrast to the goddess Pallas, where Chinese uses the Greek name (帕拉斯 pàlāsī).
- 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."
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|Wikimedia Commons has media related to (2) Pallas.|
- Computer-generated shape of Pallas (Gable, 2009)
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