The surface area of any three-dimensional shape is the total measure of the area of its two-dimensional surface. Surface areas of common shapes, such as spheres and pyraminds have simple formulas, while most more complicated shapes can have their surface area measured by using calculus. Fractals are pathological mathematical objects with finite volume but infinite surface area.
Astronomers are often interested in the surface area of celestial bodies. For an object in a hydrostatic-equilibrium or near-spherical shape, the surface area is a function of the equatorial and polar radii of the object. But some objects, like the Martian moons Phobos and Deimos, have highly irregular shapes, and their surface areas do not lend themselves to an accurate calculation.
Surface areas of solar system bodies are often given not only in traditional units of area but also as a fraction or multiple of the surface area of the earth.