This problem concerns the electrically charged interstellar dust grain that was the subject of Problem 2.22. Its mass, which was not involved in that problem, may be taken as 10~13 gm. Suppose it is moving quite freely, with speed v<£c, in a plane perpendicular to the interstellar magnetic field which in that region has a strength of 3 X 10~6 gauss. How many years will it take to complete a circular orbit?