Though the animated data are vivid, the simulations do cut some corners, Korycansky says. The virtual Castalia's descent is not fiery, and the computations currently leave out this physics. Real asteroids move too fast to absorb heat from the atmosphere and wouldn't exhaust much energy illuminating it, Korycansky says.

So far, the researchers have only modeled Castalia as a liquid, based on reasoning that is partly theoretical but partly practical. In principle, the aerodynamic forces generated by Castalia's cosmic plunge might make fast work of any material. In practice, assuming Castalia is solid makes the impact problem tougher and more computationally intensive. It also would mean speculation, Korycansky says, because no one knows whether asteroids are generally hard, powdery, or in between. And no one knows where cracks and fissures might run through Castalia in particular.

But the simulations are growing steadily richer in detail. The team ran its previous round of simulations on only four processors, but Korycansky is retooling the computations to exploit an updated and more parallel version of NCSA's ZEUS code. Mac Low predicts the team will soon be running simulations on hundreds of processors at once. Upcoming runs on NCSA's Origin2000 should give the team their first indications of whether the Venusian atmosphere deals differently with Castalia as a solid. Within a year, they expect to incorporate structural heterogeneity and flaws into the asteroid.

According to Mac Low, the more details that go in, the simpler the problem looks. In 2D runs with a spherical asteroid-like impactor, even minuscule adjustments of the impactor's velocity on entry sizably affected how much mass and energy reached the planet's surface. "In 3D, we took these lumpy, bumpy things, and we turned them around, and it made virtually no difference," he says.

Such results bode well for the ability of Zahnle's formula to extract the basics of an impact for a wide variety of asteroid shapes, speeds, and geometries of impact, Mac Low says. The formula closely captures how Castalia's cratering capacity would decline on a beeline descent through Venus's atmosphere, simulations show so far. Next the modelers will be simulating oblique impacts and trying larger asteroids.

With a formula calibrated to cope with the gamut of objects in the sky, the team will be able to enact impacts by the thousands and to discover how long Venus' craters have taken to accumulate. The result will tell them how long ago its surface hardened and hint at what kinds of churning could be going on inside the planet.