As the United States considers new goals for NASA after the loss of the Columbia, some space enthusiasts have renewed calls for a mission to Mars. But a team of physicists and biologists here at a laboratory on Long Island is demonstrating that even if the nation wanted to commit to such a goal, it would be far more complex than the Moon mission that gripped the country in the 60's. One reason is radiation, in the form of heavy ions from distant stars, zipping through everything in their path. Others include price, estimated at $30 billion to $60 billion,...