| Object type | Main-belt asteroid; Tholen F-type, SMASS B-type[1] |
|---|---|
| Discovered | Oct. 2, 1910 by Vincenzo Cerulli, Collurania Observatory, Teramo[2] |
| Named for | Interamnia, the Latin name of Teramo[3] |
| Diameter | ≈ 306 km (JPL); ≈ 332 km from VLT imaging[1] |
| Size rank | Fifth-largest asteroid, after Ceres, Vesta, Pallas and Hygiea[2] |
| Geometric albedo | ≈ 0.078 (dark)[1] |
| Rotation period | ≈ 8.73 hours[1] |
| Density | 1.98 ± 0.68 g/cm³ (VLT study)[2] |
| Distance from Sun | 2.58–3.53 AU[1] |
| Orbital period | ≈ 5.3 years[1] |
| Inclination | ≈ 17.3°[1] |
| Brightness | ≈ mag +11 at most oppositions; +9.9 at best[2] |
Big, dark and overlooked
Interamnia is the fifth-largest asteroid, after Ceres, Vesta, Pallas and Hygiea, yet it is described as a very little-studied body.[2] It is easily the largest F-type asteroid, a dark carbon-rich class.[2][4]
Size estimates differ by method: JPL lists about 306 km, while imaging with the Very Large Telescope gave about 332 km.[1][2]
An observatory in a garden in Teramo
Interamnia was found on October 2, 1910 by Vincenzo Cerulli, who had built his own observatory, “Collurania”, in his home town of Teramo in central Italy.[2][3] It was the only asteroid he discovered, and he named it after Teramo’s Latin name.[3]
Cerulli is also remembered for arguing that the famous “canals” of Mars were an optical illusion — a view later confirmed.[3]
A shape close to equilibrium
Observations with the VLT’s SPHERE imager in 2017–2019, combined with stellar occultations, found an ellipsoidal shape, similar to 4 Vesta.[2] That shape may be consistent with hydrostatic equilibrium for a body of its density spinning once every 7.6 hours. Interamnia spins in about 8.7 hours today, so it may have formed as an equilibrium body before impacts changed its spin.[2][5]
No deep basins show up in the VLT images. Any large craters must have flat floors, as expected for an icy body.[2]
Is Interamnia icy?
The VLT study gave a density of 1.98 ± 0.68 g/cm³ — not precise enough to pin down the composition. But hydrated materials on the surface and an overall spectrum similar to Ceres suggest that it is likely an icy body.[2]
Its mass is just as hard to pin down. Estimates since 2001 range from about 2.7 to 7 × 1019 kg; a 2019 consolidation of 21 earlier estimates gave (3.8 ± 1.3) × 1019 kg.[2]
Orbit
Interamnia orbits in the outer main belt, between about 2.58 and 3.53 AU from the Sun, once every 5.3 years, on an orbit tilted about 17° to the ecliptic.[1] It has no associated asteroid family, suggesting it has not suffered a giant impact in a long time.[2]
Can you see it?
Not in binoculars. Its dark surface and distance keep it around magnitude +11 at most oppositions, and only about +9.9 at the most favourable ones — fainter than Ceres, Vesta or Pallas ever get.[2] A small telescope, a good finder chart and two nights to confirm motion will do it. Our asteroid-hunter picker suggests gear for exactly this.
Read more
Vincenzo Cerulli and the Collurania Observatory
The Italian astronomer who built his own observatory in Teramo, doubted the Martian canals and found one big asteroid.
Read →Interamnia’s first real portrait
How VLT images and occultations finally gave the fifth-largest asteroid a shape — and hinted it may be icy.
Read →Weighing an asteroid with no moon
Why Interamnia’s mass estimates range from 2.7 to 7 × 10¹⁹ kg, and how astronomers combine them.
Read →Sources
- Small-Body Database Lookup: 704 Interamnia — NASA JPL Solar System Dynamics
- 704 Interamnia — Wikipedia
- Vincenzo Cerulli — Wikipedia
- F-type asteroid — Wikipedia
- Hydrostatic equilibrium — Wikipedia
- Very Large Telescope — Wikipedia
- Light curve — Wikipedia
- Planetary Fact Sheet (orbit comparison) — NASA NSSDCA
Figures are rounded; where sources differ we note it. Spotted an error? Tell us.