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.
8 Near-Earth Objects: Friend or Foe? Richard Binzel gives an evenhanded appraisal of the situation.
12 The Crash on Jupiter: Looking for Answers in the Impacts: Paul Weissman reports on the various hypotheses put forward as researchers attempt to explain what we saw.
3 Members' Dialogue Planetary protection; Golden's view for NASA.
7 World Watch NASA budget cuts; SETI program threats.
15 Basics of Spaceflight Getting there.
18 News & Reviews How safe is the assumption that an object from space killed the dinosaurs?
19 Society News Asteroid programs, the New Millennium Committee, and Steps to Mars.
20 Q&A Why is Earth the only planet with plate tectonics?