It happens so fast. For eons you're coasting at a cool 70,000 kilometers per hour on a seemingly endless circuit through space. You doze off for a moment. Suddenly you're riding a roaring shock wave through Venus' pea-soup atmosphere and lighting up like a second sun. You and your steadfast 10²⁰ joules of kinetic energy—worth 10,000 Titan-II thermonuclear missiles—are cashing in. And this isn't just a personal catastrophe. What about the planet? What about the atmosphere? What's happening?

Answers to such burning questions may not help the asteroid, but researchers are after them nonetheless. Astrophysicists Don Korycansky, Kevin Zahnle, and Mordecai-Mark Mac Low believe that craters on Venus will tell them how long ago the planet’s surface hardened—but only after they’ve learned what asteroids go through.

From radar images the Magellan spacecraft sent back in the 1990s, the researchers know the number and sizes of craters on Venus. From the observations of astronomers, they know the same statistics for asteroids, including how often different sizes hit Venus on average. But they can't connect the hits to the craters, because they can't predict how much oomph different asteroids have left—if any—after they encounter Venus' atmosphere. Once they understand what happens as asteroids come in, they will be able to calculate how many years of hits are really represented by the crater record on Venus.

Zahnle, who works at the NASA Ames Research Center in Moffett Field, CA, devised a formula several years ago for how atmospheres sap the strength of asteroids. But he didn't have data with which to calibrate or test his formula for impactors of different size, speed, and incoming angle—to name some of the parameters he believes will be key.

Zahnle might have set off with a notebook, binoculars and a suit that could withstand a million megatons for a billion-year expedition to Venus. Instead, he recruited Mac Low, a scientist with the American Museum of Natural History in New York, who crafted a software strategy for capturing on computer the detailed physics of atmospheric impacts. Zahnle also enlisted Korycansky, who built into the software a radar-imaged likeness of a real asteroid known as 4769 Castalia, which periodically crosses Venus' orbit. From his office at the University of California, Santa Cruz, Korycansky runs the team's impact simulations, employing an SGI Origin2000 supercomputer and NCSA's ZEUS community software for simulating astrophysical phenomena.

Access Online | Posted 4-24-2001