On the Cover: Craters are not the only traces that asteroids and comets leave on Earth. A thin layer of iridium-rich clay found outside Gubbio, Italy, was the first recognized evidence that some large near-Earth object had once obliterated itself by colliding with the planet. (The layer is seen here as the greenish bray-to-red segment between the white limestone below and the pinkish tan limestone above.) While rare on the surface of Earth, iridium is plentiful in asteroids and comets, and one of these struck the planet 65 million years ago, leaving its signature in this clay layer. It also left a mark by wiping out the dinosaurs. Evidence such as this warns us that danger from the skies is real and should not be ignored. Frank Asaro
4 The Planetary Exploration Survey: What Society Members Think About Planetary Protection: Donald MacGregor and Paul Slovic present their analysis of the results of this survey.
8 Near-Earth Objects: Friend or Foe? Richard Binzelgives an evenhanded appraisal of the situation.
12 The Crash on Jupiter: Looking for Answers in the Impacts: Paul Weissmanreports on the various hypotheses put forward as researchers attempt to explain what we saw.