Viktor Safronov

Safronov argued that planets grow through repeated collisions and gravitational accumulation of smaller solid bodies called planetesimals. This accretion framework remains fundamental to modern models of terrestrial-planet and planetary-core formation.

The Safronov number compares the gravitational scattering strength of a body with the relative speed of nearby objects. Planetary scientists use it to describe whether encounters are more likely to produce strong deflection, capture, collision, or ejection.

His 1969 Russian work on the evolution of the protoplanetary cloud presented a systematic theory of planetary accretion. An English translation published in the early 1970s helped bring that framework to a wider planetary-science community.

Modern telescopes can observe dusty disks around young stars, while Safronov supplied a physical account of how solid material within such disks could grow into planetesimals and planets. Current models add detailed gas dynamics, migration, pebble accretion, and computer simulations to that basic accretion picture.