- "Reaching a stable orbit around Kerbin is one of the first things budding space programs strive for. It is said that he who can get his ship into orbit is halfway to anywhere."
- ―Kerbal Astronomical Society
Kerbin is the third planet in orbit around the star Kerbol. It is the third largest celestial body that orbits Kerbol, following Jool and Eve. Jool's moon Tylo has the same radius of Kerbin, though it may be classified as larger, as the highest point on Tylo is about 5 km higher than the highest point on Kerbin. However, Tylo has only 80% of Kerbin's mass.
Kerbin has a roughly equal distribution of surface liquid water and solid land, with polar icecaps and scattered deserts. Some of its mountains exceed 6 km in height, with the tallest peak being 6767 m in altitude at the coordinates. The lowest point is almost 1.4 km deep.
Unlike other bodies in its system, Kerbin has few visible craters because its environment would erode craters from the few meteors that avoid the gravity or surface of the Mun and survive entry. Nevertheless, some geological formations indicate that bodies have violently collided with Kerbin: two planetary features appear to be impact craters that are coincidentally separated by nearly 180 degrees. The least eroded, and therefore presumably youngest, of the two (both are in excess of 100 km diameter) lies along the coastline. The uplift is easily visible as a series of islands, and the feature has a central peak that pokes up through the water (also known as a rebound peak.) The other, and older of the two, is near the prime meridian in the northern hemisphere and is more easily missed, but its uplift rims are visible, and it has a central rebound peak.
Kerbin has a thick, warm atmosphere with a mass of approximately 4.7×1016 kilograms, a sea level pressure of 101.325 kilopascals (1 atmosphere), and a depth of 70,000 meters. The atmosphere contains oxygen and can support combustion. Kerbin likely has an earth-like nitrogen-oxygen atmosphere. The air-fuel ratio of jet engines operating in Kerbin's atmosphere suggests that the percentage of oxygen is similar to that of Earth's atmosphere (about 21%).
Kerbin's atmosphere can be divided into three major layers, comparable to Earth's troposphere, stratosphere, and mesosphere. In the lower and upper layers, temperature decreases as altitude increases, while the middle layer spans of a region of increasing temperature. The boundary between the lower and middle layers occurs at an altitude of about 16 km at low latitudes, and about 9 km at high latitudes. The boundary between the middle and upper layer occurs at an altitude of about 38 km.