2 Pallas

From Wikipedia, the free encyclopedia

2 Pallas  

A black-and-white image of Pallas roughly showing a flattened octahedron shape. Taken by the Hubble Telescope.
Discovery
Discovered by	Heinrich Wilhelm Olbers
Discovery date	March 28, 1802
Designations
Alternative names	none
Minor planet category	Pallas family
Orbital characteristics
Epoch November 26, 2005 (JD 2453700.5)
Aphelion	510.425 Gm (3.412 AU)
Perihelion	319.143 Gm (2.133 AU)
Semi-major axis	414.784 Gm (2.773 AU)
Eccentricity	0.231
Orbital period	1686.333 d (4.62 yr)
Average orbital speed	17.65 km/s
Mean anomaly	92.795°
Inclination	34.841°
Longitude of ascending node	173.152°
Argument of perihelion	310.448°
Physical characteristics
Dimensions	570×525×500 km[1][2]
Mass	2.2×1020 kg[3][4]
Mean density	2.8 g/cm³
Equatorial surface gravity	0.18 m/s²
Escape velocity	0.32 km/s
Rotation period	0.32555 d[5]
Albedo	0.159 (geometric)[6]
Temperature	~164 K max: ~265 K (-8 °C)
Spectral type	B-type asteroid[7]
Apparent magnitude	6.4[8] to 10.6
Absolute magnitude	4.13[6]
Angular diameter	0.59"[9] to 0.17"

2 Pallas (pronounced /ˈpæləs/, Greek: Παλλάς) is an asteroid located in the asteroid belt region of the solar system and was the second to be discovered. It was found and named by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802. The adjectival form of the name is Palladian.

Pallas is the third most massive object in the asteroid belt and is estimated to contain 7% of the region's entire mass.^[3] Pallas, as well as Ceres, 3 Juno and 4 Vesta, were once considered planets until the discovery of many other asteroids led to re-classifications. The orbit of Pallas is highly inclined to the plane of the main asteroid belt at 34.8°, and the orbital eccentricity is nearly as large as Pluto's. The surface is a silicate material with a spectrum that is similar to carbonaceous chondrite meteorites.

Contents 1 Name 2 History of observation 3 Characteristics 4 Exploration 5 See also 6 References 7 External links

[edit] Name

The asteroid is named after Pallas Athena, an alternate name for the goddess Athena.^[10][11] In some mythologies Athena killed Pallas, then adopted her friend's name out of mourning. There are several male characters of the same name in Greek mythology, but the first asteroids were invariably given female names.

The stony-iron Pallasite meteorites are not connected to the Pallas asteroid, being instead named 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.^[12]

Pallas' astronomical and astrological symbol is or sometimes   .

[edit] History of observation

In 1801, the astronomer Giuseppe Piazzi discovered an object which he initially believed to be a comet. Shortly thereafter he announced his observations of this object, noting that the slow, uniform motion was uncharacteristic of a comet, suggesting it was a different type of object. This was lost from sight for several months, but was recovered later in the year by the Baron von Zach and Heinrich W. M. Olbers after a preliminary orbit was computed by Friedrich Gauss. This object came to be named Ceres, and was the first asteroid to be discovered.

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.^[13]

The orbit of Pallas was determined by Gauss, who found the period of 4.6 years was similar to the period for Ceres. However, Pallas had a relatively high orbital inclination to the plane of the ecliptic.^[13]

In 1917, the Japanese astronomer Kiyotsugu Hirayama began to study asteroid motions. By plotting a set of asteroids based on their mean orbital motion, inclination and eccentricty, 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.^[14] Since 1994 more than 10 members of this family have been identified. (Members have semi-major axis = 2.50–2.82 AU; inclination = 33–38°.)^[15] The existence of this family was finally confirmed in 2002 by a comparison of their spectra.^[16]

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.^[1][2] During the occultation of May 29, 1979 the discovery of a possible tiny satellite with a diameter of about 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.^[17]

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.^[3]

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. Due to the high orbital inclination of Pallas, it is more difficult for a spacecraft to reach than the other large asteroids.^[18]

[edit] Characteristics

Pallas is the third largest object in the asteroid belt, similar to 4 Vesta in volume (to within uncertainty), but somewhat less massive. By comparison, the mass of Pallas is only about 0.3% of the Moon's mass. Both Vesta and Pallas have assumed the title of second largest asteroid from time to time.^[18]

Pallas is farther from the Earth with a much lower albedo than Vesta, and consequently appears dimmer. Indeed, the much smaller 7 Iris marginally exceeds Pallas in mean opposition magnitude.^[19] Pallas' mean opposition magnitude is +8.0, which is well within the range of 10×50 binoculars, but unlike Ceres and Vesta, it will require more powerful optical aid to view at small elongations, when its magnitude can drop as low as +10.6. But during rare perihelic oppositions, Pallas can reach a magnitude of +6.4.^[8] During late February 2014, Pallas will shine at magnitude 6.96.^[9]

Pallas was among the "candidate planets" in an early draft of the IAU's 2006 definition of planet, but does not qualify in the final definition because it has not "cleared the neighborhood" around its orbit. In the future, it is possible that Pallas may be classified as a dwarf planet,^[20] but only if it is found that its shape is consistent with hydrostatic equilibrium.^[21]

There is some consideration that the largest asteroids, such as Pallas, are actually protoplanets. During the planetary formation stage of the solar system, objects grew in size through an accretion process. Many of the objects the size of Ceres and Pallas were accreted by the largest bodies, which became the planets. Other protoplanetary bodies were destroyed through collisions with similar-size bodies. Pallas is likely a survivor of this early planetary-formation phase.^[22]

Pallas has unusual dynamical parameters for such a large body. Its orbit is highly inclined and somewhat eccentric despite being located at the same distance from the sun as the central part of the main belt. Furthermore, its axial tilt is very high, being around 60° (in fact estimates vary from 56° to 81°).^[1][23][24] 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. The most recent analysis of lightcurves indicates that the pole points towards ecliptic coordinates (β, λ) = (-12°, 35°) or (43°, 193°) with a 10° uncertainty.^[24] This gives axial tilts of 57° or 65°, respectively.^{[citation needed]}

Based on spectroscopic observations, the primary component of the Pallas surface material is a silicate that is low in iron and water. Minerals of this type include olivine and pyroxene, which are found in CM chondrules.^[25] There are indications that the surface composition of Pallas is very similar to the Renazzo carbonaceous chondrite (CR) meteorites, which is even lower in hydrous minerals than the CM type.^[26] The Renazzo meteorite was discovered in Italy in 1824, and is one of the most primitive meteorites known.^[27]

[edit] Exploration

An orbit of Pallas cannot be achieved by the Dawn Mission due to the fuel requirements of reaching the asteroid's highly inclined orbit. However, a flyby mission may be theoretically possible whenever Pallas is crossing an orbital node, as this briefly places the asteroid in the plane of the ecliptic.^[28] The Dawn team has been granted viewing time on the Hubble Space Telescope, for a once-in-twenty-years opportunity to view the asteroid at closest approach, to obtain comparative data.^[29]

[edit] See also

• Pallas in fiction
• List of Solar System bodies formerly considered planets

[edit] References

[edit] External links

• 2 Pallas. JPL Small-Body Database Browser. Retrieved on 2007-03-29.
• Tedesco, Edward F.; Noah, Paul V.; Noah, Meg; Price, Stephan D. (2002). "The Supplemental IRAS Minor Planet Survey". The Astronomical Journal 123 (2): 1056-1085. doi:10.1086/338320. 
• Hilton, James L. (April 1, 1999). U.S. Naval Observatory Ephemerides of the Largest Asteroids. U.S. Naval Observatory. Retrieved on 2007-03-14.
• Dunn, Tony (2006). Ceres, Pallas Vesta and Hygeia. GravitySimulator.com. Retrieved on 2007-03-15.
• Yeomans, Donald K.. Horizons system. NASA JPL. Retrieved on 2007-03-20.—Horizons can be used to obtain a current ephemeris.

v • d • e Minor planets navigator 1 Ceres       ·       2 Pallas       ·       3 Juno

v • d • e Small Solar System bodies Asteroids (Vulcanoids · Near-Earth · Main belt · Jupiter Trojans · Centaurs · Damocloids · Neptune Trojans · Groups · Families · Moons) Comets · Trans-Neptunians (Kuiper belt · Scattered disc objects · Oort cloud) · Minor planets · Meteoroids Lists: Minor planets · Asteroid groups and families · Binary asteroids · Asteroid moons See also: Near-Earth object · Dwarf planet · Planet · Solar System · Pronunciation of asteroid names · Meanings of asteroid names.