2 Pallas

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This article is about the asteroid. For other uses, see Pallas (disambiguation).

2 Pallas
arcsec / yr
Physical characteristics
Dimensions(568 km × 532 km × 448 km) ± 12 km[9]
550 km × 516 km × 476 km[10]
Mean diameter
511±4 km[11]
513±6 km
Synodic rotation period
7.8132 h[14]
Equatorial rotation velocity
65 m/s[c]
84°±5°[10]
0.155[11]
0.159[15]
B[6][16]
6.49[17] to 10.65
4.13[15]
0.629″ to 0.171″[f]

Pallas (

Ceres, and is a likely remnant protoplanet. Like Ceres, it is believed to have a mineral composition similar to carbonaceous chondrite meteorites, though significantly less hydrated than Ceres. It is 79% the mass of 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 507 to 515 kilometers (315 to 320 mi) in diameter, 90–95% 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.[18]

When Pallas was discovered by the German astronomer

Heinrich Wilhelm Matthäus Olbers on 28 March 1802, it was considered to be a planet,[19] 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.[20]

The high inclination of the orbit of Pallas results in the possibility of close conjunctions to stars that other solar objects always pass at great angular distance. This resulted in Pallas passing Sirius on 9 October 2022, only 8.5 arcminutes southwards,[21] while no planet can get closer than 30 degrees to Sirius.

History

Size comparison: the first 10 asteroids profiled against the Moon. Pallas is number two.

Discovery

On the night of 5 April 1779,

C/1779 A1 (Bode), that he observed in the spring of 1779, but apparently assumed it was nothing more than a star.[22]

In 1801, the astronomer

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.[23][24]

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.[25] When Pallas was discovered, some estimates of its size were as high as 3,380 km in diameter.[26] Even as recently as 1979, Pallas was estimated to be 673 km in diameter, 26% greater than the currently accepted value.[27]

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.[25]

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

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.[31][32] 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.[33]

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.[34][35]

High-resolution images of the north (at left) and south (at right) hemispheres of Pallas, made possible by the Adaptive-Optics (AO)-fed SPHERE imager on the Very Large Telescope (VLT) in 2020.[36] Two large impact basins could have been created by asteroid family–forming impacts. The bright spot in the southern hemisphere is reminiscent of the salt deposits on Ceres.

Name and symbol

The symbols for Ceres and Pallas, as published in 1802

Pallas is an epithet of the Greek goddess

Ancient Greek: Παλλάς Ἀθηνᾶ).[37][38] In some versions of the myth, Athena killed Pallas, daughter of Triton, then adopted her friend's name out of mourning.[39]

The adjectival form of the name is Palladian.

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).[g]
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.[40]

The old

leaf shape, a cordate leaf shape (♤: Cordate variant of Pallas symbol), and a triangle (△); the last made it effectively the alchemical symbol for sulfur, ⟨🜍⟩. The generic asteroid symbol of a disk with its discovery number, ⟨②⟩, was introduced in 1852 and quickly became the norm.[41][42] The iconic lozenge symbol was resurrected for astrological use in 1973.[43]

Orbit and rotation

Pallas has a high eccentricity and a highly inclined orbit

Pallas has unusual dynamic parameters for such a large body. Its

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 (β, λ) = (30°, −16°) with a 5° uncertainty in the Ecliptic J2000.0 reference frame.[10] 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.[10]

Near resonances

Pallas is in a near-1:1 orbital resonance with Ceres, which is probably coincidental.[44] Pallas also has a near-18:7 resonance (91,000-year period) and an approximate 5:2 resonance (83-year period) with Jupiter.[45]

  • Animation of the Palladian orbit in the inner Solar System   Pallas   Ceres   Jupiter   Mars   Earth   Sun
    Animation of the Palladian orbit in the inner Solar System
    •   Pallas
    •   Ceres
    •   Jupiter
    •   Mars
    •   Earth
    •   Sun
  • An animation of Pallas's near-18:7 resonance with Jupiter. The orbit of Pallas is green when above the ecliptic and red when below. It only marches clockwise: it never halts or reverses course (i.e. no libration). The motion of Pallas is shown in a reference frame that rotates about the Sun (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.
    An animation of Pallas's near-18:7 resonance with Jupiter. The orbit of Pallas is green when above the ecliptic and red when below. It only marches clockwise: it never halts or reverses course (i.e. no libration). The motion of Pallas is shown in a reference frame that rotates about the Sun (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.

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.[46]

Physical characteristics

Relative sizes of the four largest asteroids. Pallas is second from right.