17 Thetis

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17 Thetis 17 Thetis symbol.png
Star field showing asteroid Thetis in the center
Discovered by R. Luther
Discovery date April 17, 1852
Pronunciation /ˈθtɨs/ THEE-tis
Named after Thetis
Alternative names A913 CA; A916 YF;
1954 SO1
Minor planet category Main belt
Adjective Thetidian
Orbital characteristics[1]
Epoch October 22, 2004 (JD 2453300.5)
Aphelion 419.069 Gm (2.801 AU)
Perihelion 319.991 Gm (2.139 AU)
Semi-major axis 369.530 Gm (2.470 AU)
Eccentricity 0.134
Orbital period 1418.027 d (3.88 a)
Average orbital speed 18.87 km/s
Mean anomaly 38.435°
Inclination 5.587°
Longitude of ascending node 125.622°
Argument of perihelion 135.906°
Physical characteristics
Dimensions 90 ± 3.7 km (IRAS)[1]
Mass 1.2×1018 kg[2][3]
Mean density 3.21 ± 0.92 g/cm³[2]
Equatorial surface gravity 0.0252 m/s²
Escape velocity 0.0476 km/s
Rotation period 0.5113 d (12.27 h)[1][4]
Albedo 0.1715[1][5]
Temperature ~173 K
Spectral type S[1]
Apparent magnitude 9.89 to 13.51[6]
Absolute magnitude (H) 7.76[1]

17 Thetis is a large main-belt asteroid that was discovered by R. Luther on April 17, 1852. It was his first asteroid discovery. The name comes from Thetis, the mother of Achilles in Greek mythology.[7]

One Thetidian stellar occultation was observed from Oregon in 1999. However, the event was not timed.

The spectrum of this object indicates that it is an S-type asteroid with both low and high calcium forms of pyroxene on the surface, along with less than 20% olivine. The high-calcium form of pyroxene forms 40% or more of the total pyroxene present, indicating a history of igneous rock depopsits. This suggests that the asteroid underwent differentiation by melting, creating a surface of basalt rock.[8]


The mass of Thetis has been calculated from perturbations by 4 Vesta and 11 Parthenope. In 2007, Baer and Chesley calculated Thetis to have a mass of 1.2×1018 kg[2][3] with a density of 3.21 g/cm³.[2]


  1. ^ a b c d e f "JPL Small-Body Database Browser: 17 Thetis". 2008-08-04 last obs. Retrieved 2008-11-12. 
  2. ^ a b c d Baer, James; Steven R. Chesley (2007). "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. Bibcode:2008CeMDA.100...27B. doi:10.1007/s10569-007-9103-8. Retrieved 2008-11-12. 
  3. ^ a b (Mass of Thetis 0.00617 / Mass of Ceres 4.75) * Mass of Ceres 9.43E+20 = 1.224E+18
  4. ^ "LIGHTCURVES AND MAP DATA ON NUMBERED ASTEROIDS N° 1 TO 52225". AstroSurf. Archived from the original on 2005-11-27. Retrieved 2008-11-03. 
  5. ^ "Asteroid Data Archive". Planetary Science Institute. Archived from the original on 2006-06-23. Retrieved 2008-11-03. 
  6. ^ apmag 9.89 (1977-Jul-25) to 13.51 (1963-Aug-02) JPL Horizons daily output for 1950 to 2099
  7. ^ Schmadel, Lutz D. (2003). Dictionary of minor planet names, Volume 1 (5th ed.). Berlin Heidelberg New York: Springer-Verlag. p. 17. ISBN 3-540-00238-3. 
  8. ^ Sunshine, Richard P. et al. (August 2004), "High-calcium pyroxene as an indicator of igneous differentiation in asteroids and meteorites", Meteoritics & Planetary Science 39 (8): 1343–1357, Bibcode:2004M&PS...39.1343S, doi:10.1111/j.1945-5100.2004.tb00950.x. 

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