Named for a nymph of Greek mythology, Castalia is an hourglass-shaped space rock that's 1.8 kilometers long. Korycansky describes it as an ideal candidate for the team's simulations. By sending the irregularly shaped Castalia into Venus at different orientations, the modelers are able to assess how an asteroid's shape influences how well it penetrates the atmosphere. According to Zahnle's formula, furthermore, Castalia is more or less the smallest object Venus' atmosphere should allow through.

The simulations start with Castalia 100 kilometers above the cratered plains of Venus. Because the coordinate system of the team's computations moves with the asteroid’s center of mass, animations of the data depict Castalia as if fixed in place even as the asteroid hurtles full tilt toward the surface. The resulting perspective is what you might see from a second asteroid diving in alongside.

As Castalia enters the atmosphere, a gauzy cloak of shocked Venusian air forms around it, and the rock starts lurching like a whiffle ball. On Castalia's leading surface, ripples appear, shift, swell, and break like waves. Then—FWOOSH!—the asteroid's face erupts, scattering rocky shards and billowing plumes of dust.

The growing undulations that undo Castalia are Rayleigh-Taylor instabilities, according to Korycansky. "The same thing that causes coffee to come out of a cup when you turn it upside-down," he explains. The instabilities appear whenever a light fluid (such as air) pushes a heavy one (such as coffee or a simulated asteroid). In the coffee situation, the lumps give gravity handles on which to yank. For the asteroid, they are sheets of supersonic wind, which tear apart the rock wherever they catch.