A solar sail allows a spacecraft to use radiation pressure for propulsion, similar to the way wind propels a sailboat. The sails of such spacecraft are made out of enormous reflecting panels. The area of the panels is maximized to catch the largest number of incident photons, thus maximizing the momentum transfer from the incident radiation.
For such spacecraft to work, the force from the radiation pressure exerted by the photons must be greater than the gravitational attraction to the star emitting the photons. The critical parameter is the area density (mass per unit area) of the sail.

A.) Consider a perfectly reflecting mirror oriented so that solar radiation of intensity I is incident upon, and perpendicular to, the reflective surface of the mirror. If the mirror has surface area A, what is Frad, the magnitude of the average force due to the radiation pressure of the sunlight on the mirror? 2IA/c and this is correct.
To solve the second part of this problem you will need to know the following:
* the mass of the sun, Msun = 2 x 10^30 kg
* the intensity of sunlight as a function of the distance R from the sun,
IsunR = 3.2 x 10^25 1/R^2 (W/m^2) and
* the gravitational constant G = 6.67 x 10^-11 m^3/(kg x s^2)
B.) Suppose that the mirror described in Part A is initially at rest a distance R away from the sun. What is the critical value of area density for the mirror at which the radiation pressure exactly cancels out the gravitational attraction from the sun?