I don't know how random your asteroids have to be, so this is maybe not good enough. It uses the dots pattern, so the 'asteroids' are evenly distributed. The border of the belt is randomized with the random steps decoration.
\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{patterns,decorations.pathmorphing}
\begin{document}
\begin{tikzpicture}[decoration={random steps,segment length=5pt}]
\draw [black!40] circle[radius=2.65cm];
\filldraw [pattern color=black,pattern=dots,even odd rule,decorate,draw=none] circle[radius=2.5cm] circle[radius=2.8cm];
\end{tikzpicture}
\end{document}
