&gt;  Scientists believe they must occur thousands of times a day, but until this most recent study, only 16 had ever been detected. They seem randomly distributed throughout the sky, and no one is sure what causes them.<p>They are said to happen more frequently than a few times a year.  I&#x27;m guessing it&#x27;s just that we are not detecting the bulk of them because they&#x27;re randomly distributed.