Solution

For a circular orbit, the gravitational force provides the centripetal force:
$\frac{GmM}{r^2}=mw^2 r$
where $G$ is the gravitational constant, $M$ is the mass of the Sun, $m$ the asteroid's mass, $r$ is the orbital radius and $w$ is the angular velocity
we get
$\frac{GM}{r^2}=w^2 r$
and from that
$r=(\frac{GM}{w^2})^1/3$