2 Pallas

2 Pallas  Pallas symbol.svg
PallasHST2007.jpg
An ultraviolet image of Pallas showing a flattened roughly octahedron shape.
Discovery
Discovered by Heinrich Wilhelm Olbers
Discovery date March 28, 1802
Designations
Pronunciation /ˈpæləs/ [note 1]
Named after Pallas Athena
Minor planet
category		 Pallas family
Adjective Palladian[1]
Epoch August 22, 2008 (JD 2454700.5)
Aphelion 510.468 Gm (3.412 AU)
Perihelion 319.005 Gm (2.132 AU)
Semi-major axis 414.737 Gm (2.772 AU)
Eccentricity 0.231
Orbital period 1686.044 d (4.62 a)
Average orbital speed 17.65 km/s
Mean anomaly 306.605°
Inclination 34.838° to Ecliptic
34.21° to Invariable plane[3]
Longitude of ascending node 173.134°
Argument of perihelion 310.274°
Physical characteristics
Dimensions 582×556×500±9 km[4]
544 km (mean)[2]
Mass (2.11±0.26) × 1020 kg[5]
Mean density ~2.8 g/cm³[4]
Equatorial surface gravity ~0.18 m/s²
Escape velocity ~0.32 km/s
Rotation period 0.325 55 d
(7.8132 h)[6]
Axial tilt likely 78 ± 13°[7]
Albedo 0.159 (geometric)[8]
Temperature ~164 K
max: ~265 K (-8 °C)
Spectral type B-type asteroid[9]
Apparent magnitude 6.4[10] to 10.6
Absolute magnitude (H) 4.13[8]
Angular diameter 0.59"[11] to 0.17"

Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802. It was at first considered a planet, as were other early asteroids, until the discovery of many more led to their re-classification. Pallas appears to be the largest irregularly shaped body in the Solar System (that is, the largest body not rounded under its own gravity), and a remnant protoplanet.

With a mass estimated to be 7% of the total mass of the asteroid belt,[12] 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,[5] 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.[13][14]

Contents

Name

2 Pallas is named after Pallas Athena, an alternate name for the goddess Athena.[15][16] In some mythologies Athena killed Pallas, then adopted her friend's name out of mourning.[17] (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.[18]

As with other asteroids, the astronomical symbol for Pallas is its discovery number circled, ②. However, it also has dedicated symbols, Pallas symbol.svg 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.[19][20]

Diagram illustrating Johann Hieronymus Schröter's inaccurate 1811 estimate of the size of Pallas. Schröter wrongly believed Pallas to be over 3,000km in diameter, which would have made it larger than Pluto (which had not been discovered at the time of Schröter's estimate).

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.[21] When Pallas was discovered some estimates of its size were as high as 3,380 km in diameter.[22] Even as recently as 1979, Pallas was estimated to be 673 km in diameter (26% greater than the currently accepted value).[23]

Size comparison of several objects with potential for dwarf planet status under the IAU's 2006 draft proposal on the definition of planet.[24] Pallas is second from the right, bottom row.

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.[21]

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.[25] 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°.[26] The existence of this family was finally confirmed in 2002 by a comparison of their spectra.[27]

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.[28][29] 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.[30]

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.[31]

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.[4][32]

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.[33] However, while Pallas is similar to 4 Vesta in volume,[34] it is significantly less massive. The mass of Pallas is only 22% of Ceres,[12] 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.[35] 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.[10] During late February 2014, Pallas will shine at magnitude 6.96.[11]

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;[7] 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.[4][36]) 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.[37] 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.[38] The Renazzo meteorite was discovered in Italy in 1824 and is one of the most primitive meteorites known.[39]

The animation illustrates Pallas' near 18:7 resonance pattern with Jupiter. The motion of Pallas is shown in a reference frame that rotates about the sun (i.e., the center dot) with a period equal to Jupiter's orbital period. Accordingly, Jupiter's orbit appears almost stationary as the pink ellipse at top left. Mars' motion is orange, and the Earth-moon system is blue and white. The orbit of Pallas is green when above the ecliptic, and red when below. The near 18:7 resonance pattern with Jupiter only marches clockwise: it never halts or reverses course (i.e., no libration).

Very little is known of Palladian surface features. Hubble images from 2007 show pixel-to-pixel variation (pixel resolution is ~70 km), but Pallas' 12% albedo placed such features at the lower end of detectability. There is little variability between lightcurves obtained through visible-light and infrared filters, but significant deviations in the ultraviolet, suggesting large surface or compositional features near 285 and 75° west longitude. Rotation appears to be prograde.[4]

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.[40]

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.[41][42] 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.[4] Pallas is believed to have undergone at least some degree of thermal alteration and partial differentiation.[4]

Near resonances

Pallas is in a near 1:1 mean-motion orbital resonance with Ceres.[43] Pallas also has a near 18:7 resonance (6500 year period) and an approximate 5:2 resonance (83 year period) with Jupiter.[44]

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 did in October 2009. The last and next by Venus are in 1677 and 2123, and for Mars they are in 1597 and 2759[45]

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.[33][46]

See also

Notes

  1. ↑ In US dictionary transcription, us dict: păl′·əs. Or as Greek: Παλλάς.

References

  1. ↑ OED
  2. ↑ 2.0 2.1 "JPL Small-Body Database Browser: 2 Pallas". http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2. Retrieved 2009-09-12. 
  3. ↑ "The MeanPlane (Invariable plane) of the Solar System passing through the barycenter". 2009-04-03. Archived from the original on 2009-05-14. http://www.webcitation.org/5glwILykY. Retrieved 2009-04-10.  (produced with Solex 10 written by Aldo Vitagliano; see also Invariable plane). Retrieved 2009-04-25.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Schmidt, B. E., et al. (2008). "Hubble takes a look at Pallas: Shape, size, and surface" (PDF). 39th Lunar and Planetary Science Conference (Lunar and Planetary Science XXXIX). Held March 10–14, 2008, in League City, Texas. 1391: 2502. http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2502.pdf. Retrieved 2008-08-24. 
  5. ↑ 5.0 5.1 Baer, James; Steven R. Chesley (2008). "Astrometric masses of 21 asteroids, and an integrated asteroid ephemeris" (PDF). Celestial Mechanics and Dynamical Astronomy (Springer Science+Business Media B.V. 2007) 100 (2008): 27–42. doi:10.1007/s10569-007-9103-8. http://www.springerlink.com/content/h747307j43863228/fulltext.pdf. Retrieved 2008-11-11. 
  6. ↑ Harris, A. W.; Warner, B. D.; Pravec, P.; Eds. (2006). "Asteroid Lightcurve Derived Data. EAR-A-5-DDR-Derived-Lightcurve-V8.0.". NASA Planetary Data System. http://www.psi.edu/pds/resource/lc.html. Retrieved 2007-03-15. 
  7. ↑ 7.0 7.1 Torppa, J.; et al. (2003). "Shapes and rotational properties of thirty asteroids from photometric data". Icarus 164 (2): 346–383. doi:10.1016/S0019-1035(03)00146-5. http://adsabs.harvard.edu/abs/2003Icar..164..346T. Retrieved 2007-03-15. 
  8. ↑ 8.0 8.1 Tedesco, E. F.; Noah, P. V.; Noah, M.; Price, S. D. (2004). "IRAS Minor Planet Survey. IRAS-A-FPA-3-RDR-IMPS-V6.0.". NASA Planetary Data System. http://www.psi.edu/pds/resource/imps.html. Retrieved 2007-03-15. 
  9. ↑ Neese, C.; Ed. (2005). "Asteroid Taxonomy. EAR-A-5-DDR-Taxonomy-V5.0.". NASA Planetary Data System. http://www.psi.edu/pds/resource/taxonomy.html. Retrieved 2007-03-15. 
  10. ↑ 10.0 10.1 Menzel, Donald H.; Pasachoff, Jay M. (1983). A Field Guide to the Stars and Planets (2nd ed.). Boston, MA: Houghton Mifflin. p. 391. ISBN 0395348358. 
  11. ↑ 11.0 11.1 Calculated with JPL Horizons for 2014-Feb-24
  12. ↑ 12.0 12.1 Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants" (PDF). Solar System Research 39 (3): 176. doi:10.1007/s11208-005-0033-2. http://iau-comm4.jpl.nasa.gov/EPM2004.pdf. 
  13. ↑ Pallasite meteorites are named for an involved scientist who characterised them, not for this asteroid and they do not necessarily share origin with Pallas. Anonymous (2007-11-05). "Pre-Dawn: The French-Soviet VESTA mission". Space Files. http://spacefiles.blogspot.com/2007/11/pre-dawn-french-soviet-vesta-mission.html. Retrieved 2008-06-28. 
  14. ↑ Anonymous. "Space Topics: Asteroids and Comets, Notable Comets". The Planetary Society. http://www.planetary.org/explore/topics/our_solar_system/asteroids_and_comets/asteroids.html. Retrieved 2008-06-28. 
  15. ↑ James, Andrew (September 1, 2006). "Pallas: 2006-2015". Southern Astronomical Delights. http://homepage.mac.com/andjames/PagePallas000.htm. Retrieved 2007-03-29. 
  16. ↑ "Athena". 1911 Edition of the Encyclopaedia Britannica. Encyclopaedia Britannica (Tim Starling). http://en.wikisource.org/w/index.php?title=User:Tim_Starling/ScanSet_TIFF_demo&vol=02&page=EB2A875. Retrieved 2008-08-16. 
  17. ↑ Dietrich, Thomas (2005). The Origin of Culture and Civilization: The Cosmological Philosophy of the Ancient Worldview Regarding Myth, Astrology, Science, and Religion. Turnkey Press. p. 178. ISBN 0976498162. 
  18. ↑ "Palladium". Los Alamos National Laboratory. http://periodic.lanl.gov/elements/46.html. Retrieved 2007-03-28. 
  19. ↑ Hoskin, Michael (1992-06-26). "Bode's Law and the Discovery of Ceres". Observatorio Astronomico di Palermo "Giuseppe S. Vaiana". http://www.astropa.unipa.it/HISTORY/hoskin.html. Retrieved 2007-07-05. 
  20. ↑ Forbes, Eric G. (1971). "Gauss and the Discovery of Ceres". Journal for the History of Astronomy 2: 195–199. http://adsabs.harvard.edu/abs/1971JHA.....2..195F. 
  21. ↑ 21.0 21.1 "Astronomical Serendipity". NASA JPL. http://dawn.jpl.nasa.gov/DawnCommunity/flashbacks/fb_06.asp. Retrieved 2007-03-15. 
  22. ↑ Hilton, James L. (2007-11-16). "When did asteroids become minor planets?". U.S. Naval Observatory. http://aa.usno.navy.mil/faq/docs/minorplanets.php. Retrieved 2008-06-22. 
  23. ↑ Hilton, James L.. "Asteroid Masses and Densities" (PDF). U.S. Naval Observatory. http://www.lpi.usra.edu/books/AsteroidsIII/pdf/3008.pdf. Retrieved 2008-09-07. 
  24. ↑ O. Gingerich (2006). "The Path to Defining Planets" (PDF). Harvard-Smithsonian Center for Astrophysics and IAU EC Planet Definition Committee chair. http://astro.cas.cz/nuncius/nsiii_03.pdf. Retrieved 2007-03-13. 
  25. ↑ Kozai, Yoshihide (November 29-December 3, 1993). "Kiyotsugu Hirayama and His Families of Asteroids (invited)". Proceedings of the International Conference. Sagamihara, Japan: Astronomical Society of the Pacific. http://adsabs.harvard.edu/abs/1976IAUS...73...75P. Retrieved 2007-01-08. 
  26. ↑ Faure, Gérard (May 20, 2004). "Description of the System of Asteroids". Astrosurf.com. http://www.astrosurf.com/aude/map/us/AstFamilies2004-05-20.htm. Retrieved 2007-03-15. 
  27. ↑ Foglia, S.; Masi, G. (1999). "New clusters for highly inclined main-belt asteroids". The Minor Planet Bulletin 31: 100–102. http://asteroidi.uai.it/family/. Retrieved 2007-03-15. 
  28. ↑ Drummond, J. D.; Cocke, W. J. (1989). "Triaxial ellipsoid dimensions and rotational pole of 2 Pallas from two stellar occultations". Icarus 78: 323–329. doi:10.1016/0019-1035(89)90180-2. http://adsabs.harvard.edu/abs/1989Icar...78..323D. Retrieved 2007-03-15. 
  29. ↑ Dunham, D. W.; et al. (1990). "The size and shape of (2) Pallas from the 1983 occultation of 1 Vulpeculae". Astronomical Journal 99: 1636–1662. doi:10.1086/115446. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1990AJ.....99.1636D. Retrieved 2007-03-14. 
  30. ↑ Johnston, William Robert (March 5, 2007). "Other Reports of Asteroid/TNO Companions". Johnson's Archive. http://www.johnstonsarchive.net/astro/asteroidmoonsq.html. Retrieved 2007-03-14. 
  31. ↑ Pitjeva, E. V. (2004). "Estimations of masses of the largest asteroids and the main asteroid belt from ranging to planets, Mars orbiters and landers". 35th COSPAR Scientific Assembly. Held 18–25 July 2004, in Paris, France. pp. 2014. http://adsabs.harvard.edu/abs/2004cosp.meet.2014P. 
  32. ↑ Staff (October 24, 2007). "Hubble Images of Asteroids Help Astronomers Prepare for Spacecraft Visit". JPL/NASA. http://dawn.jpl.nasa.gov/science/index.asp. Retrieved 2007-10-27. 
  33. ↑ 33.0 33.1 "Notable Asteroids". The Planetary Society. 2007. http://www.planetary.org/explore/topics/near_earth_objects/asteroids_and_comets/asteroids.html. Retrieved 2007-03-17. 
  34. ↑ The volume of Pallas is indistinguishable from that of Vesta given the uncertainties of current measurements.
  35. ↑ Odeh, Moh'd. "The Brightest Asteroids". Jordanian Astronomical Society. http://jas.org.jo/ast.html. Retrieved 2007-07-16. 
  36. ↑ Carry, B.; et al. (2007). "Asteroid 2 Pallas Physical Properties from Near-Infrared High-Angular Resolution Imagery" (PDF). http://www.eso.org/sci/activities/santiago/projects/PlanetaryGroup/journal_club/slides/ESO.JournalClub-2007.08.14-BenoitCARRY.pdf. Retrieved 2009-06-22. 
  37. ↑ Feierberg, M. A.; Larson, H. P.; Lebofsky, L. A. (1982). "The 3 Micron Spectrum of Asteroid 2 Pallas.". Bulletin of the American Astronomical Society 14: 719. http://adsabs.harvard.edu/abs/1982BAAS...14..719F. Retrieved 2007-03-19. 
  38. ↑ Sato, Kimiyasu; Miyamoto, Masamichi; Zolensky, Michael E. (1997). "Absorption bands near 3 m in diffuse reflectance spectra of carbonaceous chondrites: Comparison with asteroids". Meteoritics 32: 503–507. doi:10.1111/j.1945-5100.1997.tb01295.x. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997M%26PS...32..503S. Retrieved 2007-03-14. 
  39. ↑ "Earliest Meteorites Provide New Piece in Planetary Formation Puzzle.". Particle Physics and Astronomy Research Council. September 20, 2005. http://www.pparc.ac.uk/Nw/meteorite.asp. Retrieved 2006-05-24. 
  40. ↑ McCord, T. B.; McFadden, L. A.; Russell, C. T.; Sotin, C.; Thomas, P. C. (2006). "Ceres, Vesta, and Pallas: Protoplanets, Not Asteroids". Transactions of the American Geophysical Union 87 (10): 105. doi:10.1088/0034-4885/61/2/001. http://adsabs.harvard.edu/abs/1998RPPh...61...77K. Retrieved 2007-03-19. 
  41. ↑ "IAU 2006 General Assembly: Result of the IAU Resolution votes". IAU. http://www.iau.org/public_press/news/release/iau0603/. Retrieved 2008-08-16. 
  42. ↑ Rincon, Paul (August 16, 2006). "Planets plan boosts tally to 12". BBC News. http://news.bbc.co.uk/1/hi/sci/tech/4795755.stm. Retrieved 2007-03-17. 
  43. ↑ Goffin, E. (2001). "New determination of the mass of Pallas". Astronomy and Astrophysics 365: 627–630. doi:10.1051/0004-6361:20000023. http://adsabs.harvard.edu/abs/2001A&A...365..627G. Retrieved 2007-03-15. 
  44. ↑ Taylor, D. B. (1982). "The secular motion of Pallas". Royal Astronomical Society 199: 255–265. http://adsabs.harvard.edu/full/1982MNRAS.199..255T. Retrieved 2009-01-31. 
  45. ↑ "Solex by Aldo Vitagliano". Archived from the original on 2009-04-29. http://www.webcitation.org/query?id=1241027727416772. Retrieved 2009-03-19.  (numbers generated by Solex)
  46. ↑ Perozzi, Ettore; Rossia, Alessandro; Valsecchid, Giovanni B. (2001). "Basic targeting strategies for rendezvous and flyby missions to the near-Earth asteroids". Planetary and Space Science 49 (1): 3–22. doi:10.1016/S0032-0633(00)00124-0. 

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