The steep drop in air travel is passing a milestone. Fewer than 100,000 people went through airport checkpoints on Tuesday, the lowest number since the Transportation Security Administration started keeping track. That’s down 95% from a year ago, and could be the smallest number since the 1950s. There was no commercial air travel in the U.S. for several days after the terror attacks in September, 2001, but people gradually got back on planes over the following months. It could be a slower recovery this time, according to outfits that have surveyed people about when they’ll feel safe flying again.