(2) Pallas

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(2) PallasPallas symbol.svg
Recordings from different perspectives using the Very Large Telescope and SPHERE
Recordings from different perspectives using the Very Large Telescope and SPHERE
Properties of the orbit ( animation )
Epoch:  4th September 2017 ( JD 2,458,000.5)
Orbit type Main belt asteroid
Asteroid family Pallas family
Major semi-axis 2,773  AU
eccentricity 0.231
Perihelion - aphelion 2.134 AU - 3.413 AU
Inclination of the orbit plane 34.8 °
Length of the ascending node 173.1 °
Argument of the periapsis 310 °
Time of passage of the perihelion July 21, 2018
Sidereal period 4 a 226 d
Mean orbital velocity 17.645 km / s
Physical Properties
Medium diameter 582 km × 556 km × 500 km
Dimensions 2.34  x  10 20Template: Infobox asteroid / maintenance / mass kg
Albedo 0.16
Medium density 2.76 g / cm³
Rotation period 7 h 48 min 48 s
Absolute brightness 4.13 mag
Spectral class
(according to Tholen)
Spectral class
(according to SMASSII)
Explorer H. Olbers
Date of discovery March 28, 1802
Source: Unless otherwise stated, the data comes from JPL Small-Body Database Browser . The affiliation to an asteroid family is automatically determined from the AstDyS-2 database . Please also note the note on asteroid items.

(2) Pallas is the largest asteroid with a mean diameter of 546 km and the second largest body in the asteroid belt after Ceres , but its mass is still exceeded by Vesta . It is almost spherical in shape, but is somewhat deformed by collisions.


Pallas was discovered by Heinrich Wilhelm Olbers on March 28, 1802 and named after the Greek goddess Pallas Athene . The discovery happened by chance when Olbers discovered Ceres a year earlier and now classified as a dwarf planet . After the discovery of Ceres Olbers had, like other astronomers of his time, not expecting yet another planet between Mars and Jupiter to find because it was believed that the Titius-Bode law predicted planet in this region of the solar system found to have. Olbers therefore took the hypothesis that Ceres and Pallas were fragments of a larger, broken planet. This "debris hypothesis" of the formation of asteroids was, in competition with several other, repeatedly prevailing opinion until around 1960.

The discovery spurred the activity of the so-called sky police of the European observatories; this was founded in 1800 to search systematically for suspected minor planets. With Olbers and Harding , two members of this organization also succeeded in discovering the asteroids (3) Juno and (4) Vesta in 1804 and 1807 .

Just like Ceres, Pallas was regarded as a fully fledged planet immediately after its discovery, so nine celestial bodies were considered planets in 1802. After the discovery of Juno and Vesta, the number of "planets" increased to eleven by 1807. It stayed that way until 1845. Then with (5) Astraea the fifth asteroid was discovered, and this was initially listed as a planet. After the discovery of the “real” planet Neptune in 1846, even 13 celestial bodies were considered planets. However, since new discoveries of asteroids increased from 1847 onwards, these were from then on differentiated from the “big planets”, the number of which fell back to eight.

Wilhelm Herschel , who discovered Uranus in 1781 , tried to determine the diameter of Ceres and Pallas, which caused him some difficulties: the two objects appeared practically star-shaped in his telescope (i.e. without measurable dimensions like the classical planets). Herschel therefore suggested as early as 1802 that Ceres and Pallas should be called "asteroids" ( Greek for "star-like") and thus to be differentiated from the larger planets. At the time, however, only a few astronomers shared his view. It was not until around 1850 that the number of celestial bodies found between Mars and Jupiter rose rapidly that they were grouped under the names "asteroids", "planetoids", "small planets" or "minor planets".

The chemical element palladium , discovered in 1803, was named after this asteroid.


Pallas between the orbits of Mars and Jupiter

Pallas moves at a mean distance of 2.77 AU in 4.62 years around the Sun and is in an 18: 7 resonance with Jupiter. The orbital eccentricity is relatively large at 0.23: The distance from the sun therefore varies between 2.14 AU in perihelion and 3.41 AU in aphelion . The orbit is strongly inclined towards the ecliptic at 34.9 ° , which means that Pallas is often far away from the ecliptic in the sky.

Presumably it is the original body of the rest of the Pallas family, which was described by Kiyotsugu Hirayama (see Hirayama family ) in 1928 . All members have similarly high inclinations, eccentricities and orbital half-axes as well as the rarer spectral type B. This allows the conclusion that the smaller asteroids were blasted off by Pallas in collisions.

During opposition , it reaches an apparent magnitude of up to 6.5 mag. This makes it the second brightest asteroid in the night sky after (4) Vesta, but can no longer be seen with the naked eye.

Physical Properties

The rotation period of Pallas is 7.8 hours. Their surface area has an average albedo of 0.16. The size of Pallas could be determined with the Hubble space telescope : The shape of Pallas corresponds to a triaxial ellipsoid with axis lengths of 582 ± 10 km, 556 ± 3 km and 500 ± 9 km. The mass was determined to be 1.18 × 10 −10 solar masses (corresponds to 2.34 × 10 20  kg). A mean diameter of about 523 km had previously been derived from observations of the Infrared Astronomical Satellite and from star occultations .

It is assumed that Pallas, like Vesta , is a remaining protoplanet with a differentiated internal structure. However, the hydrostatic equilibrium is no longer present after cooling, so that Pallas has now assumed an irregular shape due to collisions.

See also


  • G. Gerstbach: The asteroids - drama and rubble in the planetary system . Star Messenger December 2002, Vienna.

Web links

Commons : (2) Pallas  - album with pictures, videos and audio files

Individual evidence

  1. v ≈ π * a / period (1 + sqrt (1-e²))
  2. McCord, TB; McFadden, LA; Russell, CT; Sotin, C .; Thomas, PC: Ceres, Vesta, and Pallas: Protoplanets, Not Asteroids . In: Transactions of the American Geophysical Union . 87, No. 10, 2006, p. 105. bibcode : 2006EOSTr..87..105M . doi : 10.1029 / 2006EO100002 .
  3. ^ BE Schmidt, PC Thomas, JM Bauer, J.-Y. Li, LA McFadden, JM Parker, AS Rivkin, CT Russell, SA Stern: Hubble takes a look at Pallas: Shape, size, and surface . (PDF) In: 39th Lunar and Planetary Science Conference (Lunar and Planetary Science XXXIX). Held March 10-14, 2008, in League City, Texas. . 1391, 2008, p. 2502. bibcode : 2008LPI .... 39.2502S . Retrieved August 30, 2016.