2 Pallas
 VLT-SPHERE image of Pallas[1] | |
| Discovery [2] | |
|---|---|
| Discovered by | Heinrich Wilhelm Olbers |
| Discovery date | 28 March 1802 |
| Designations | |
MPC designation | (2) Pallas |
| Pronunciation | /ˈpæləs/[5] |
Named after | Pallas Athena (Greek goddess)[3] |
Minor planet category | asteroid belt · (central) Pallas family [4] |
| Adjectives | Palladian /pæˈleɪdiən/ [6] |
| Orbital characteristics [8] | |
| Epoch 1 July 2021 (JD 2459396.5) | |
| Uncertainty parameter 0 | |
| Observation arc | 217 yr |
| Aphelion | 3.41112 AU (510.296 million km) |
| Perihelion | 2.13651 AU (319.617 million km) |
| 2.77381 AU (414.956 million km) | |
| Eccentricity | 0.229758 |
| 4.62 yr (1687.39 d) | |
| 229.230 | |
| 0° 12m 46.8s / day | |
| Inclination | 34.8977° (34.43° to invariable plane[7]) |
| 172.920° | |
| 310.437° | |
| Proper orbital elements[9] | |
Proper semi-major axis | 2.7709176 AU |
Proper eccentricity | 0.2812580 |
Proper inclination | 33.1988686° |
Proper mean motion | 78.041654 deg / yr |
Proper orbital period | 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 | c/a = 0.79±0.03[10] 568 ±12 km × 532 ±12 km × 448 ±12 km[11] 550 km × 516 km × 476 km[12] |
Mean diameter | 511±4[10]
513±6 km[11] 512±6 km[12] |
Surface area | (8.3±0.2)×105 km2 (2020)[a][13] |
| Volume | (7.1±0.3)×107 km3 (2020)[a][14] |
| Mass | (2.04±0.03)×1020 kg average est.[11] (2.01±0.13)×1020 kg[b][15] |
Mean density | 2.92±0.08 g/cm3[10] 2.89±0.08 g/cm3[11] 2.57±0.19 g/cm3[15] |
Equatorial surface gravity | ≈ 0.21 m/s² (average)[c] 0.022 g |
Equatorial escape velocity | 324 m/s[11] |
| 7.8132 h[16] | |
Equatorial rotation velocity | 65 m/s[a] |
| 84°±5°[12] | |
| 0.155[10] 0.159[17] | |
| B[8][18] | |
| 6.49[19] to 10.65 | |
Absolute magnitude (H) | 4.13[17] |
| 0.629″ to 0.171″[20] | |
Pallas (minor-planet designation: 2 Pallas) is the second asteroid to have been discovered, after 1 Ceres. Like Ceres, it is believed to have a mineral composition similar to carbonaceous chondrite meteorites, though significantly less hydrated than Ceres. It is the third-largest asteroid in the Solar System by both volume and mass, and is a likely remnant protoplanet. It is 79% the mass of 4 Vesta and 22% the mass of Ceres, constituting an estimated 7% of the mass of the asteroid belt. Its estimated volume is equivalent to a sphere 505 to 520 kilometers (314 to 323 mi) in diameter, 90–96% the volume of Vesta.
During the planetary formation era of the Solar System, objects grew in size through an accretion process to approximately the size of Pallas. Most of these 'protoplanets' were incorporated into the growth of larger bodies, which became the planets, whereas others were ejected by the planets or destroyed in collisions with each other. Pallas, Vesta and Ceres appear to be the only intact bodies from this early stage of planetary formation to survive within the orbit of Neptune.[21]
When Pallas was discovered by the German astronomer Heinrich Wilhelm Matthäus Olbers on 28 March 1802, it was counted as a planet,[22] as were other asteroids in the early 19th century. The discovery of many more asteroids after 1845 eventually led to the separate listing of 'minor' planets from 'major' planets, and the realization in the 1950s that such small bodies did not form in the same way as (other) planets led to the gradual abandonment of the term 'minor planet' in favor of 'asteroid' (or, for larger bodies such as Pallas, 'planetoid').
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.[23]
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 (now known as C/1779 A1 (Bode)) that he observed in the spring of 1779, but apparently assumed it was nothing more than a star.[24]
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.[25][26]
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.[27] When Pallas was discovered, some estimates of its size were as high as 3,380 km in diameter.[28] Even as recently as 1979, Pallas was estimated to be 673 km in diameter, 26% greater than the currently accepted value.[29]
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.[27]
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.[31] Since 1994 more than 10 members of this family have been identified, with semi-major axes between 2.50 and 2.82 AU and inclinations of 33–38°.[32] The validity of the family was confirmed in 2002 by a comparison of their spectra.[33]
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.[34][35] 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.[36]
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.[37][38]
Name and symbol[]
2 Pallas is named after Pallas Athena (Ancient Greek: Παλλάς Ἀθηνᾶ), an alternate name for the goddess Athena.[39][40] In some versions of the myth, Athena killed Pallas, then adopted her friend's name out of mourning.[41]
The adjectival form of the name is Palladian.[6] 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.[42]
(There are several male characters with a similar same name in Greek mythology, Pállas rather than Pallás, but the first asteroids were invariably given female names. Because the oblique stem is different, the male name would have been Pallante in Italian and Паллант Pallant in Russian.)
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.[43]
The old astronomical symbol of Pallas, still used in astrology, is a spear or lance, ⚴ (
), one of the symbols of the goddess. The blade was most often a lozenge ◊ shape, but various graphic variants were published, including an acute/elliptic leaf shape, a cordate ♤ leaf shape, and a triangle △; the last made it effectively the alchemical symbol for sulfur,