Mean Radius of Venus

Venus is the one planet whose surface no optical telescope has ever seen: the sulphuric-acid cloud deck is total and permanent. Its radius therefore had to be measured by radar — first crudely from Earth in the 1960s, then definitively by Magellan, which mapped 98 per cent of the surface from orbit between 1990 and 1994. The mean radius is the honest single figure here more than anywhere: Venus rotates so slowly that it has no rotational bulge at all, and its total relief, from the Maxwell Montes peaks to the deepest rift, spans only 13 km against Earth's 20.

A radius 5 per cent short of Earth's, with 81.5 per cent of the mass, gives nearly the same density and surface gravity — the twin-planet arithmetic that makes the two worlds' divergence the sharpest lesson in comparative planetology. Same size, same materials; one temperate, one at 737 K under 92 bar of carbon dioxide.