Final answer:
The shape of the small rocky astronomical body in question is irregular and undefined, as such bodies like asteroid Ida do not have sufficient gravity to form a spherical shape.
Step-by-step explanation:
The rocky astronomical body in question, given that it is small and shares space with other objects, is likely an asteroid. Based on the provided information, especially considering asteroid Ida as referenced in the question, we can deduce that the shape of such an object is typically irregular or elongated. This is because they do not possess enough gravity to pull themselves into a spherical shape. In astronomy, the larger a body is, the more likely it is to be spherical due to gravitational forces. It is noted that for silicate bodies, the limit where gravity will generally overcome the material strength of the rocks and lead to a spherical shape is around 400 kilometers in diameter. Objects smaller than this, such as asteroid Ida, can maintain irregular shapes. Therefore, the correct shape for this astronomical body, according to the descriptions and examples given, would be irregular and undefined.