Ceres

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Largest Asteroid May Be 'Mini Planet' with Water Ice
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This is a NASA Hubble Space Telescope color image of Ceres, the largest object in the asteroid belt.

Astronomers enhanced the sharpness in these Advanced Camera for Surveys images to bring out features on Ceres' surface, including brighter and darker regions that could be asteroid impact features. The observations were made in visible and ultraviolet light between December 2003 and January 2004.

The colors represent the differences between relatively red and blue regions. These differences may simply be due to variation on the surface among different types of material.

Ceres' round shape suggests that its interior is layered like those of terrestrial planets such as Earth. Ceres may have a rocky inner core, an icy mantle, and a thin, dusty outer crust inferred from its density and rotation rate of 9 hours. Ceres is approximately 590 miles (950 kilometers) across and was first discovered in 1801.

Source

Here is a list of the largest asteroids with their widths:

Ceres	975 x 909 km
Pallas	582 x 500
Vesta	578 x 458
Hygiea	530 x 370
Davida	357 x 231
Interamnia	350 x 303
Sylvia	384 x 232
Europa	360 x 240
Eunomia	357 x 212
Juno	320 x 200
Psyche	240 x 145
Lutetia	132 × 101
Ida	116 x 46
Mathilde	57 x 50
Gaspra	34 x 20
Eros	35 x 13
Geographos	5.1 x 1.8
Toutatis	4 x 2.5
Castalia	1.8 x 0.75
Itokawa	0.54 x 0.27

Source: Wikipedia