17 Thetis

17 Thetis is a stony asteroid from the inner regions of the asteroid belt, approximately 90 kilometers in diameter. It was discovered on 17 April 1852, by German astronomer Robert Luther at Bilk Observatory in Düsseldorf, Germany who deferred to Friedrich Wilhelm August Argelander the naming his first asteroid discovery after Thetis from Greek mythology.^[3][4] Its historical symbol was a dolphin and a star; it is in the pipeline for Unicode 17.0 as U+1CECA 𜻊 ().^[12][13]

17 Thetis

Star field showing asteroid Thetis in the center
Discovery[1]
Discovered by	R. Luther
Discovery site	Düsseldorf-Bilk Obs.
Discovery date	17 April 1852
Designations
MPC designation	(17) Thetis
Pronunciation	/ˈθiːtɪs/[2]
Named after	Thetis (Greek mythology)[3]
Alternative designations	1954 SO1 · A913 CA A916 YF
Minor planet category	main-belt[4] · (inner)
Adjectives	Thetidian /θɛˈtɪdiən/[5]
Symbol	(historical)
Orbital characteristics[1]
Epoch 16 February 2017 (JD 2457800.5)
Uncertainty parameter 0
Observation arc	164.55 yr (60,102 days)
Aphelion	2.7987 AU
Perihelion	2.1436 AU
Semi-major axis	2.4712 AU
Eccentricity	0.1325
Orbital period (sidereal)	3.88 yr (1,419 days)
Average orbital speed	18.87 km/s
Mean anomaly	100.44°
Mean motion	0° 15m 13.32s / day
Inclination	5.5902°
Longitude of ascending node	125.56°
Argument of perihelion	136.10°
Physical characteristics
Dimensions	84.899±2.027[6] 90±3.7km (IRAS)[7] 93.335±2.627[8]
Mass	1.23×1018 kg[9][a]
Mean density	3.21±0.92 g/cm3[9]
Synodic rotation period	12.27048±0.00001[10][11]
Geometric albedo	0.193±0.028[6]
Spectral type	B–V = 0.829[1] U–B = 0.438[1] S (Tholen)[1] Sl (SMASS)[1] · S[10]
Absolute magnitude (H)	7.76[1][7][8] · 7.85[11]

Description

The asteroid orbits the Sun at a distance of 2.1–2.8 AU once every 3 years and 11 months (1,419 days). Its orbit has an eccentricity of 0.13 and an inclination of 6° with respect to the ecliptic.^[1]

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 deposits. This suggests that the asteroid underwent differentiation by melting, creating a surface of basalt rock.^[14]

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 1.23×10¹⁸ kg^[a] with a density of 3.21 g/cm³.^[9]

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

This minor planet was named after Thetis, the mother of Achilles in Greek mythology.^[3]

Notes

1. Mass of Thetis = (6.17±0.64)×10⁻¹³ M_☉