2 Pallas is one of the largest
asteroids and is located in the main
asteroid belt. It was the second asteroid to be discovered, by
astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802. Pallas was at first considered a planet, as were the other early asteroids
1 Ceres,
3 Juno, and
4 Vesta, until the discovery of many additional asteroids led to their re-classification.
With a mass estimated to be 7% of the total mass of the asteroid belt,
[ ] Pallas is one of the largest asteroids. Its diameter is some 530–565 km, comparable to or slightly larger than that of
4 Vesta, but it is 20% less massive,
placing it third among the asteroids. The Palladian surface appears to be a silicate material; the surface spectrum and estimated density resemble
carbonaceous chondrite meteorites. The Palladian orbit, at 34.8°, is unusually highly inclined to the plane of the main
asteroid belt, and the
orbital eccentricity is nearly as large as that of
Pluto, making Pallas relatively inaccessible to spacecraft.
Name
2 Pallas is named after
Pallas Athena, an alternate name for the goddess Athena.
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.
As with other asteroids, the
astronomical symbol for Pallas is its discovery number circled, . However, it also has dedicated symbols,
or sometimes
Variant symbol of Pallas
.
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.
The size of Pallas as estimated by
Schröter in 1811.
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.
When Pallas was discovered some estimates of its size were as large as 3,380 km in diameter. Even as recently as 1979, Pallas was estimated to be 673 km in diameter (26% greater than the currently accepted value).
Rotating frame animation of Pallas relative to Jupiter. Each frame lasts 100 years
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.
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. Since 1994 more than 10 members of this family have been identified, and these have semi-major axes between 2.50–2.82 AU and inclinations of 33–38°. The existence of this family was finally confirmed in 2002 by a comparison of their spectra.
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 resulted in the first accurate measurements of its diameter.
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 could not be 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.
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.
The
Dawn Mission team was granted viewing time on the
Hubble Space Telescope in September 2007 for a once-in-twenty-year opportunity to view the asteroid at closest approach, to obtain comparative data for Ceres and Vesta.
Characteristics
Size comparison: the first 10 asteroids profiled against Earth's
Moon. Pallas is at second left.
False colored image of Pallas
Both Vesta and Pallas have assumed the title of second largest asteroid from time to time.
However, while Pallas is similar to
4 Vesta in volume, it is significantly less massive. The mass of Pallas is only 22% of Ceres,
and about 0.3% that of the
Moon.
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.
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. During rare perihelic oppositions, Pallas can reach a magnitude of +6.4, right on the edge of naked-eye visibility.
During late February 2014, Pallas will shine at magnitude 6.96.
Pallas has unusual dynamic parameters for such a large body. Its orbit is highly inclined and somewhat eccentric, despite being at the same distance from the sun as the central part of the main belt. Furthermore, its axial tilt is very high, either 78±13° or 65±12° (based on ambiguous lightcurve data, the pole points towards either ecliptic coordinates (β, λ) = (−12°, 35°) or (43°, 193°) with a 10° uncertainty;
data from the Hubble Space Telescope obtained in 2007 as well as the observations by the Keck telescope in 2003–2005 favour the first solution.
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.
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.
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.
The near 18:7 resonance pattern with Jupiter only marches clockwise. It never halts and reverses course (i.e.
librates).
It is possible that the largest asteroids, including Pallas, are protoplanets. During the planetary formation stage of the solar system, objects grew in size through an accretion process to approximately this size. Many of these objects were incorporated into larger bodies, which became the planets, while others were destroyed in collisions with other protoplanets. Pallas is a likely survivor from the early stages of planetary formation.
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, if it is found to have a surface shaped by hydrostatic equilibrium. However, recent Hubble images make that prospect unlikely, as they reveal a slightly uneven surface.Near resonances
Pallas is in a near 1:1 mean-motion orbital resonance with Ceres.Transits of planets from Pallas
From Pallas, Mercury, Venus, Mars, and the Earth can occasionally appear to transit, or pass in front of, the Sun.
The Earth last did so in 1968 and 1998, and will next transit in 2224. Mercury does in October 2009. The last and next by Venus are in 1677 and 2123, and for Mars they are in 1597 and 2759[ (numbers generated by Solex)]Exploration
Pallas has not been visited by spacecraft, but if the Dawn probe is successful in studying 4 Vesta and 1 Ceres, it is possible its mission may be extended to include a flyby of Pallas as Pallas crosses the ecliptic in 2018. However, due to the high orbital inclination of Pallas, it will not be possible for Dawn to enter orbit.See also
