2 Pallas

Discovery
Discoverer	Heinrich Wilhelm Olbers
Discovery Date	March 28, 1802
Alternate Designations
Category	Main belt
Orbital Elements Epoch July 14, 2004 (JD 2453200.5)
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	570×525×482 km
Mass	2.06×1020 kg
Density	4.2 g/cm³
Surface Gravity	0.16 m/s²
Escape Velocity	0.35 km/s
Rotation Period	0.32555 d
Spectral Class	U
Absolute Magnitude	4.13
Albedo	0.159
Mean Surface Temperature	~226 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. It was named after Pallas of Greek mythology, the daughter of Triton and friend of Athena. There is another Pallas in Greek mythology who is male, the god of wisdom; however, the first asteroids were invariably given female names.

Pallas is the second largest Main belt asteroid, only a little larger than 4 Vesta. Its orbit is located in the central part of the belt but it is somewhat inclined and eccentric for a large asteroid. The composition of Pallas is unique but quite similar to the C-type asteroids.

Pallas has been observed occulting a star several times. Careful occultation timing measurements have helped give it an accurate diameter.

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. (1) http://www.johnstonsarchive.net/astro/asteroidmoonsq.html

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.

The chemical element palladium (atomic number 46) was named after Pallas.

References

• James L. Hilton, U.S. Naval Observatory Ephemerides of the Largest Asteroids (1999) http://aa.usno.navy.mil/hilton/ephemerides/asteroid_ephemerides.html
• E. V. Pitjeva, Estimations of Masses of the Largest Asteroids and the Main Asteroid Belt From Ranging to Planets, Mars Orbiters And Landers (2004) http://www.cosis.net/abstracts/COSPAR04/02014/COSPAR04-A-02014.pdf
• IRAS Minor Planet Survey http://dorothy.as.arizona.edu/DSN/IRAS/iras.html#i.5