On the Cover: After it encounters Neptune in August 1989 (see page 21),<i> Voyager 2</i> will join its sister spacecraft—<i>Pioneers 10</i> and <i>11</i> and <i>Voyager 1</i>—now racing through the outer reaches of our solar system. By carefully analyzing the radio signals from these spacecraft, scientists can determine what, if any, objects might be affecting the space craft gravitationally. If there is a Planet X orbiting the Sun outside of Pluto's orbit, these plucky robotic explorers may help us find it. The search begins on page 6.
4 The Saying of Science: Jonathan Eberhart discusses how science often leads along converging successions of questions rather than from answer to specific answer.
6 Planet X: Fact or Fiction? John Anderson looks at the evidence for a tenth planet in our solar system.
10 Humans to Mars: The Mission That NASA Did Not Fly: Edward Ezell examines why we haven't yet sent humans to Mars.
3 Members' Dialogue SETI, expanding beyond Earth, and remembering a son.
16 World Watch NASA appropriation, CRAF and Cassini, and Russia's Phobos mission.
17 News & Reviews Heavenly chaos; struggling U.S. space science.
18 Society Notes Teaching the teachers, the Caltech Balloon Symposium, and dolphin communication.
28 Q&A How does an icy body in the Oort cloud "fall into" our solar system and become a comet?