2 Pallas
From Freepedia
| Discovery A | |
|---|---|
| Discoverer | Heinrich Wilhelm Olbers |
| Discovery date | March 28, 1802 |
| Alternate designations | none B |
| Category | Main belt |
| Orbital elements C | |
| Eccentricity (e) | 0.230 |
| Semi-major axis (a) | 414.717 Gm (2.772 AU) |
| Perihelion (q) | 319.358 Gm (2.135 AU) |
| Aphelion (Q) | 510.077 Gm (3.410 AU) |
| Orbital period (P) | 1685.927 d (4.62 a) |
| Mean orbital speed | 17.89 km/s |
| Inclination (i) | 34.852° |
| Longitude of the ascending node (Ω) | 173.166° |
| Argument of perihelion (ω) | 310.529° |
| Mean anomaly (M) | 346.022° |
| Physical characteristics | |
| Dimensions | 570×525×500 km (4)(5) |
| Mass | 2.2×1020 kg [1](6) |
| Density | 2.8 g/cm³ |
| Surface gravity | 0.18 m/s² |
| Escape velocity | 0.32 km/s |
| Rotation period | 0.32555 d |
| Spectral class | B-type asteroid |
| Absolute magnitude | 4.13 |
| Albedo | 0.159 [2] |
| Mean surface temperature | ~164 K |
2 Pallas (pal'-us) was the first asteroid discovered after 1 Ceres. It was found and named by H. Wilhelm Olbers on March 28, 1802.
Contents |
Name
The asteroid is named after Pallas, the daughter of Triton and friend of Athena in Greek mythology. (There are several male characters of the same name in Greek mythology, but the first asteroids were invariably given female names.)
According to the OED, the adjectival form of Pallas is Palladian.
Characteristics
Pallas is the second largest main belt asteroid, a little larger than 4 Vesta in volume, but only three-quarters Vesta's mass.
Pallas is currently the largest Solar System body (barring trans-Neptunian objects) whose surface has not been directly imaged by spacecraft or telescopes. It is also the largest irregularly-shaped body, meaning that it has not been compressed by gravity into a spheroid shape (again apart from some trans-Neptunian objects).
Pallas also has unusual dynamical parameters for such a large body. Its orbit is highly inclined and somewhat eccentric despite being located in the central part of the Main belt.
Furthermore, its axial tilt is very high, being around 70° (in fact estimates vary from 56° to 81°) (4)(7). 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. Consensus has not been reached as to whether Pallas' rotation is prograde or retrograde, but one of the poles points in a direction around right ascension 5 h, declination +6°, with a 25° uncertainty (4).
There are indications that the surface composition of Pallas is very similar to the Renazzo carbonaceous chondrite meteorite (8).
Observations
Some notable observation milestones for Pallas include:
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 have helped determine an accurate diameter (5). During the occultation of May 29, 1979 the discovery of a possible tiny satellite with a diameter of ~1 km was reported. However, it has not been 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. [3]
Radar 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 (1).
There have not been any telescopic observations of Pallas that have resolved any features on its disk. Pallas has not yet been visited by a spacecraft, but if the Dawn probe is successful in studying 1 Ceres and 4 Vesta, its mission may be extended to Pallas.
Trivia
The chemical element palladium (atomic number 46) was named after Pallas.
Aspects
References
- E. V. Pitjeva, Estimations of Masses of the Largest Asteroids and the Main Asteroid Belt From Ranging to Planets, Mars Orbiters And Landers Solar System Resarch, Vol. 39 pp. 176 (2005).
- Supplemental IRAS Minor Planet Survey
- Other Reports of Asteroid Companions, compiled by Wm. Robert Johnston
- J. D. Drummond and W. J. Cocke Triaxial ellipsoid dimensions and rotational pole of 2 Pallas from two stellar occultations, Icarus, Vol. 78, pp. 323 (1989).
- D. W. Dunham et al The size and shape of (2) Pallas from the 1983 occultation of 1 Vulpeculae, The Astronomical Journal, Vol. 99, pp. 1636 (1990).
- E. Goffin New determination of the mass of (2) Pallas, Astronomy and Astrophysics, Vol. 365, pp. 627 (2001).
- D. L. Mitchell et al Radar observations of asteroids 1 Ceres, 2 Pallas, and 4 Vesta, Icarus, Vol. 124, pp. 113 (1996).
- Sato et al Absorption bands near three micrometers in diffuse reflectance spectra of carbonaceous chondrites: Comparison with asteroids, Meteoritics and Planetary Science, Vol. 32, pp. 503 (1997).
- James L. Hilton, U.S. Naval Observatory Ephemerides of the Largest Asteroids The Astronomical Journal, Vol. 117 pp. 1077 (1999)
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