A large but overlooked asteroid: (704) Interamnia

 I was looking at a list of the largest asteroids and was surprised to see one that had previously escaped my attention: (704) Interamnia.  The largest asteroid is (1) Ceres, which is also now classified as a dwarf planet.  The next three are (4) Vesta, (2) Pallas, and (10) Hygiea.  Interamnia is number 5, the fifth largest asteroid, with a mean diameter of 332 km, about one third the size of Ceres.  Interamnia was discovered in 1910, over 60 years after the discovery of the fourth largest asteroid, Hygiea (1849).

As it turns out, Interamnia is currently very close in the sky to Vesta, which I have recently photographed.  So I went back to the photos of Vesta and found it within the same field-of-view (FOV) near the edge of the frame.   The image below was taken on 1 March with a Rokinon 135mm f/2 lens.  The horizontal FOV is about 5.5°. 

The white circles mark M1 (Crab Nebula), Vesta, and Interamnia.

Vesta is the brightest asteroid and is periodically bright enough (mag 5.2) to seen with the unaided eye from a dark-sky location.  Interamnia, on the other hand, never gets brighter than magnitude 9.9.   At the time this image was taken, Vesta was at magnitude 8.0 and Interamnia was magnidute 11.8.

Last night (10 Mar) I went out and deliberately targeted Interamnia.

(704) Interamnia.  E-M5 + Rokinon 135mm f/2.  ISO 800, 30 s. 2° FOV.

Here is an expanded (3x) view.  Interamnia, marked by the white lines, is passing just below a magnitude 13.2 background star.

This region of the sky has many star clusters.  Here is a finder chart for 10 March:

credit: SkySafariAstronomy.com

Between the two asteroids and the large Hyades cluster are two small open clusters: NGC 1746 and NGC 1647.  

NGC 1746.  E-M5 + Rokinon 135mm f/2.  ISO 800, 30s.

NGC 1647.  E-M5 + Rokinon 135mm f/2.  ISO 800, 30 s.

For all of these images I was using the ZWO AM3 mount, unguided.  As usual, click to get access to the full-size versions.