| Object type | Main-belt asteroid, C-type (Tholen) / Cb (SMASS)[1] |
|---|---|
| Discovered | Sept. 1, 1854 by James Ferguson — first asteroid found from North America[2] |
| Named for | Euphrosyne, one of the Charites (Graces)[4] |
| Diameter | ≈ 267 km[1] |
| Shape | Third-roundest known asteroid, after Ceres and Hygiea[2] |
| Mass | (1.7 ± 0.3) × 10¹⁹ kg[2] |
| Density | 1.7 ± 0.2 g/cm³[2] |
| Geometric albedo | ≈ 0.053 (very dark)[1] |
| Rotation period | ≈ 5.53 hours[1] |
| Distance from Sun | 2.48–3.84 AU[1] |
| Inclination | ≈ 26.3°[1] |
| Moon | One, found 2019; ≈ 4 km, ≈ 1.2-day orbit at ≈ 670 km[2] |
A first for the New World
On September 1, 1854, James Ferguson discovered Euphrosyne — the first asteroid found from North America.[2][3] Ferguson was a Scottish-born American astronomer and engineer.[3]
The name comes from Greek myth: Euphrosyne was one of the three Charites, or Graces — goddesses of beauty and grace — with Aglaia and Thalia.[4]
Nearly round, but not a dwarf planet
About 267 km across, Euphrosyne is roughly tied for seventh-largest asteroid within measurement uncertainty.[1][2] It is the third-roundest asteroid known, after Ceres and Hygiea.[2]
The roundness is thought to come from re-accretion: after a collision disrupted it, the pieces fell back together into a rounded pile. It is not close to hydrostatic equilibrium.[2]
A small moon, found in 2019
In 2019 astronomers discovered a small companion. Preliminary orbits show it circling about every 1.2 days at roughly 670 km. VLT images suggest it is about 4 km across, assuming it is as dark as Euphrosyne itself.[2]
The moon made it possible to weigh the asteroid accurately for the first time in 2020: (1.7 ± 0.3) × 1019 kg, giving a density of 1.7 ± 0.2 g/cm³.[2]
Head of a big family
Euphrosyne gives its name to a complex family of about two thousand asteroids with similar spectra and orbits, thought to have formed in a collision roughly 280 million years ago. All members have fairly steep orbital tilts.[2][5] The moon likely came from the same collision.[2]
Orbit
Euphrosyne orbits between about 2.48 and 3.84 AU from the Sun every 5.6 years, on a steep 26° tilt.[1] Its nodes lie near perihelion and aphelion, so perihelion falls at the northernmost point of its orbit.[2]
Observing Euphrosyne
Euphrosyne is never visible in binoculars. At the best possible opposition it reaches only about magnitude +10.2, as in November 2011 — fainter than any of the 30 asteroids found before it.[2] At perihelic oppositions it rides very high for northern observers and cannot be seen at all from places like New Zealand and Chile.[2] Our picker suggests gear for faint asteroids.
Read more
The first asteroid found from North America
In 1854 the asteroid count passed 30 — and for the first time, a discovery came from the other side of the Atlantic.
Read →How a 4 km moon weighed Euphrosyne
The 2019 discovery of a tiny companion gave the first accurate mass — and a density that points to lots of ice.
Read →The Euphrosyne family: two thousand steeply tilted siblings
A collision about 280 million years ago scattered fragments that still share Euphrosyne’s orbit — and maybe left it a moon.
Read →Sources
- Small-Body Database Lookup: 31 Euphrosyne — NASA JPL Solar System Dynamics
- 31 Euphrosyne — Wikipedia
- James Ferguson (American astronomer) — Wikipedia
- Charites — Wikipedia
- Asteroid family — Wikipedia
- Rubble pile — Wikipedia
- Planetary Fact Sheet (orbit comparison) — NASA NSSDCA
Figures are rounded; where sources differ we note it. Spotted an error? Tell us.