Revolving on an orbit about the star Altair, Tenebra is an example of one of Hal Clement's "hellworlds". 27 times the mass of Earth, Tenebra is three times Earth's diameter (38,226 kilometers; 23,746 miles) and has three times Earth's gravity at its surface (29.43 meters per second per second; 96.6 feet per second per second), which gives Tenebra an escape velocity of 33.558 kilometers per second (nearly 21 miles per second). Clement also describes the planet as rotating on its axis in a little less than four Earth days.
