expressed as a multiple of the Earth's (mean) radius (6,371 km). For instance, Mars has a mass of 6.4185·1023 kg = 0.107 Earth masses and a mean radius of 3,390 km = 0.532 Earth radii. The surface gravity of Mars is therefore approximately times that of Earth. Without using the Earth as a reference body, the surface gravity may also be calculated directly from Newton's law of universal gravitation, which gives the formula where M is the mass of the object, r is its radius, and G is the gravitational constant. If we let ρ = M/V denote the mean density of the object, we can also write this as so that, for fixed mean density, the surface gravity g is proportional to the radius r. Since gravity is inversely proportional to the square of the distance, a space station 400 km above the Earth feels almost the same gravitational force as we do on the Earth's surface. A space station does not plummet to the ground because it is in an orbit around the Earth. Gas giants For gas giant planets such as Jupiter, Saturn, Uranus, and Neptune, the surface gravity is given at the 1 bar pressure level in the atmosphere. Non-spherically symmetric objects Most real astronomical objects are not perfectly spherically symmetric. One reason for this is that they are often rotating, which means that they are affected by the combined effects of gravitational force and centrifugal force. This causes stars and planets to be oblate, which means that their surface gravity is smaller at the equator than at the poles. This effect was exploited by Hal Clement in his SF novel Mission of Gravity, dealing with a massive, fast-spinning planet where gravity was much higher at the poles than at the equator. To the extent that an object's internal distribution of mass differs from a symmetric model, we may use the measured surface gravity to deduce things about the object's internal structure. This fact has been put to practical use since 1915–1916, when Roland Eötvös's torsion balance was used to prospect for oil near the city of Egbell (now Gbely, Slovakia.), p. 1663;, p. 223. In 1924, the torsion balance was used to locate the Nash Dome oil fields in Texas., p. 223. It is sometimes useful to calculate the surface gravity of simple hypothetical objects which are not found in nature. The surface gravity of