Voyager 1 has no engine running. The 38,000-mile-per-hour speed it has held since its Saturn flyby in 1980 is borrowed momentum from planets it grazed more than four decades ago, and that free ride ca

Voyager 1 is often imagined as a spacecraft still pushing itself outward, an engine burning faintly somewhere beyond the planets. That is not how its journey works. The probe is moving because it was launched onto a carefully chosen path and then res... [6795 chars]