The effect of weightlessness on the human body (mostly in the form of bone and muscle loss) is considered one of the biggest obstacles to long term human habitation of space. One solution, first proposed about 100 years ago, is to design a space station in the form of a wheel and to spin that wheel at a frequency such that the tangential acceleration at the rim of the wheel is equal to the acceleration due to gravity at the Earth's surface. Spinning the space station on its axis effectively creates what is sometimes called "artificial gravity." Obviously, the smaller the diameter of the wheel, the faster the wheel will have to rotate in order to achieve 1 g acceleration at its edge. Centrifuge studies on human subjects indicate that rotational frequencies greater than about 3-4 RPM (revolutions/minute) will likely cause astronauts to experience balance problems, dizziness, and motion sickness (and no one wants to puke in space). What radius must a space station have in order to achieve a tangential acceleration of 1 g at its rim without rotating faster than 4 RPM?