2 Pallas

Discovery A
Discoverer	Heinrich Wilhelm Olbers
Discovery date	March 28, 1802
Alternate designations	none B
Category	Main belt
Orbital elements C D
Epoch November 25, 2005 (JD 2453700)
Eccentricity (e)	0.2306
Semi-major axis (a)	414.784 Gm (2.773 AU)
Perihelion (q)	319.14 Gm (2.133 AU)
Aphelion (Q)	510.43 Gm (3.412 AU)
Orbital period (P)	1686.33 d (4.62 a)
Mean orbital speed	17.89 km/s
Inclination (i)	34.841°
Longitude of the ascending node (Ω))	173.152°
Argument of perihelion (ω)	310.448°
Mean anomaly (M)	92.795°
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, Greek Παλλάς) was the first asteroid discovered after 1 Ceres. It was found and named by H. Wilhelm Olbers on March 28, 1802.

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

Size comparison: the first 10 asteroids profiled against Earth's Moon. Pallas is at second left. The first 10 asteroids profiled against the Earth's Moon. From left to right, 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, 5 Astraea, 6 Hebe, 7 Iris, 8 Flora, 9 Metis, and 10 Hygiea. [Source]

Pallas is the third largest main belt asteroid, similar to 4 Vesta in volume (to within uncertainty), but significantly less massive.

Pallas is currently the largest Solar System body (barring trans-Neptunian objects) whose surface has not been directly imaged by spacecraft or telescopes. It may also be the largest irregularly-shaped body, meaning that it has not been compressed by gravity into a spheroid shape (other candidates may be trans-Neptunian objects such as 2003 EL61).

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

Stationary, retrograde	Opposition	Distance to Earth (AU)	Maximum brightness (mag)	Stationary, prograde	Conjunction to Sun
February 14, 2005	March 23, 2005	1.37020	7.1	May 7, 2005	November 18, 2005
May 2, 2006	July 1, 2006	2.52251	9.5	August 24, 2006	February 1, 2007
July 6, 2007	September 3, 2007	2.25303	8.8	October 22, 2007	March 29, 2008
October 28, 2008	December 4, 2008	1.56731	8.0	January 21, 2009	September 12, 2009
March 24, 2010	May 3, 2010	1.94004	8.6	July 2, 2010	December 22, 2010
May 25, 2011	July 29, 2011	2.54744	9.5	September 16, 2011	February 22, 2012
August 9, 2012	September 24, 2012	1.96443	8.3	November 17, 2012	May 9, 2013
January 5, 2014	February 20, 2014	1.23636	7.0	March 23, 2014	October 25, 2014
April 19, 2015	June 11, 2015	2.38719	9.4	August 7, 2015	January 19, 2016
June 18, 2016	August 20, 2016	2.40255	9.2	October 7, 2016	March 14, 2017
September 24, 2017	October 27, 2017	1.70519	8.2	December 23, 2017	August 5, 2018
March 4, 2019	April 8, 2019	1.57083	7.8	June 1, 2019	December 2, 2019
May 10, 2020	July 12, 2020	2.55643	9.6	September 1, 2020	February 9, 2021

Links

Computer generated image

References

1. E. V. Pitjeva, Estimations of Masses of the Largest Asteroids and the Main Asteroid Belt From Ranging to Planets, Mars Orbiters And Landers, Vol. 39 pp. 176 (2005).
   
2. Supplemental IRAS Minor Planet Survey
   
3. Other Reports of Asteroid Companions, compiled by Wm. Robert Johnston
   
4. 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).
   
5. 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).
   
6. E. Goffin New determination of the mass of (2) Pallas, Astronomy and Astrophysics, Vol. 365, pp. 627 (2001).
   
7. D. L. Mitchell et al Radar observations of asteroids 1 Ceres, 2 Pallas, and 4 Vesta, Icarus, Vol. 124, pp. 113 (1996).
   
8. 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, Vol. 117 pp. 1077 (1999)
     
     
     
     
     

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