Thetis.space

Main-belt asteroid (S-type) · Inner main belt · 2.15–2.80 AU

Thetis

(17) Thetis · discovered 1852 at Düsseldorf-Bilk

A 90-kilometre stony asteroid whose surface may be basalt — a clue that it once melted inside.

Key factsMeteorite picker
Thetis at a glance
Object typeMain-belt asteroid; S-type (Tholen), Sl (SMASS)[1]
DiscoveredApril 17, 1852 by Robert Luther, Bilk Observatory, Düsseldorf[2]
Named byFriedrich W. A. Argelander, at Luther’s request[2]
Named forThetis, sea nymph and mother of Achilles[2]
Diameter≈ 85 km (JPL); ≈ 90 km often quoted[1]
Mass≈ 1.23 × 10¹⁸ kg (Baer & Chesley, 2007)[2]
Density≈ 3.21 g/cm³[2]
Geometric albedo≈ 0.19[1]
Rotation period≈ 12.27 hours[1]
Distance from Sun2.15–2.80 AU[1]
Orbital period≈ 3.9 years (1,419 days)[2]
SymbolA dolphin and a star — Unicode 17.0, U+1CECA[2]

Robert Luther’s first asteroid

Thetis was discovered on 17 April 1852 by Robert Luther at the Bilk Observatory in Düsseldorf. It was his first asteroid discovery, and he deferred the naming to Friedrich Wilhelm August Argelander, who chose Thetis, the sea nymph who was the mother of Achilles.[2]

Luther went on to find 24 asteroids between 1852 and 1890 and won the Lalande Prize seven times.[3]

A surface of basalt?

Thetis’s spectrum shows both low- and high-calcium pyroxene, with less than 20% olivine. High-calcium pyroxene makes up 40% or more of the pyroxene, which points to a history of igneous rock.[2]

That suggests Thetis underwent differentiation by melting, leaving a surface of basalt — the same broad kind of rock that forms from lava on Earth.[2]

Weighed by its neighbours

Thetis’s mass was calculated from its gravitational tugs on 4 Vesta and 11 Parthenope. In 2007, Baer and Chesley found about 1.23 × 1018 kg and a density of 3.21 g/cm³ — dense enough to be mostly solid rock.[2]

Orbit

Thetis orbits in the inner asteroid belt between about 2.15 and 2.80 AU from the Sun, once every 3 years and 11 months, on an orbit tilted about 6°.[1][2]

Sun Earth (1 AU)Mars (1.52 AU) aphelion ≈ 2.80 AU perihelion ≈ 2.15 AU
Top-down orbit of Thetis to scale, computed from JPL orbital elements (a ≈ 2.47 AU, e ≈ 0.131).[1] Earth and Mars orbits shown for comparison.[5]

Meteorite picker

Thetis may be covered in basalt from ancient melting — and some meteorites are exactly that kind of rock. Answer five quick questions and we’ll suggest meteorite types that match your budget and interests — with links to browse current listings.

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A dolphin and a star

Early asteroids had symbols, like the planets. Thetis’s was a dolphin and a star. In Unicode 17.0 it was encoded as character U+1CECA, so it can now appear in digital text.[2]

One stellar occultation by Thetis was observed from Oregon in 1999, though the event was not timed.[2]

Basalt from space

Some meteorites are basaltic achondrites from asteroids that melted, such as the HED meteorites linked to Vesta.[4] Our meteorite buying guide explains the main types and how to buy authentic specimens.

Read more

Thetis: an asteroid that may have melted

What pyroxene and olivine in Thetis’s spectrum say about its hot early history.

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Robert Luther and the Bilk Observatory

How a student of Encke found 24 asteroids from Düsseldorf — starting with Thetis.

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From a dolphin sketch to Unicode: Thetis’s symbol

Why early asteroids had symbols, why they were dropped — and how Thetis’s returned in Unicode 17.0.

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Sources

  1. Small-Body Database Lookup: 17 Thetis — NASA JPL Solar System Dynamics
  2. 17 Thetis — Wikipedia
  3. Robert Luther — Wikipedia
  4. 4 Vesta — Wikipedia
  5. Planetary Fact Sheet (orbit comparison) — NASA NSSDCA

Figures are rounded; where sources differ we note it. Spotted an error? Tell us.