15 Eumonia – S-type Astroid

15 Eumonia is a stony asteroid (S-type). It was magnitude ~9 last night.

15 Eunomia is a large asteroid in the inner asteroid belt. 15 Eunomia was discovered by Annibale de Gasparis in July 1851, and named after Eunomia, a personification of order and law in Greek mythology.

For science fiction afficionados, you may recognize 15 Eumonia from…

One of the hidden levels in the video game Descent and, “a secret mine station.”

As one of the five largest asteroids in the science-fiction novel Rendezvous With Rama by Arthur C. Clarke.

In the science-fiction story, Space Apprentice by Arkady and Boris Strugatsky, 15 Eunomia is nearly completely annihilated by physicists and astrophysicists who experimentally test models of cosmology and general theory of relativity. One episode shows the study of gravitational waves at the expense of a chunk of Eunomia the size of Everest.

15 Eunomia is the inspiration for the look of the heptapod ships in Arrival.

The Stony Bright Beast

Eunomia is the largest S-type asteroid, with a mean diameter of 250 km, and is about 300 km across along its longest axis. As the largest S-type asteroid, Eunomia has attracted a moderate amount of scientific attention.

Eunomia’s orbit has a period of 4.3 years, and a semi-major axis of 2.64 AU. This places it in a 7:16 resonance with Mars, a chaotic orbit that will change randomly over time, because of gravitational perturbations by the planets. I am so lucky to have captured it last night.

The Eunomia family of asteroids is a large grouping of S-type asteroids named after 15 Eunomia. It is the most prominent family in the asteroid belt, containing about 5% of all asteroids in the main belt. 15 Eunomia is the largest member, and is estimated to contain 1% of the mass of the entire asteroid belt.

Like other members of the family, Eunomia’s surface is composed of silicates and some nickel-iron, and is quite bright. Pyroxene and olivine have also been detected on Eunomia’s surface. This composition indicates that Eunomia was at least partially differentiated under the influence of internal heating during the early period of the Solar System. The range of compositions of the other Eunomian asteroids is encompasses all the surface variations on Eunomia itself. 

This evidence suggest that Eunomia is the parent body of its family, which was formed by a collision millions of years ago. Most of Eunomia’s crustal material was stripped by the disrupting impact, but Eunomia was not disrupted itself. Eunomia is estimated to have contained about 75% of the mass of the original parent body.

Keynote from Apple is an excellent light table for astronomical work with both its opacity level and arrangement tilt.

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