When was Voyager launched? 2. What mathematical path do all spacecraft follow? 3. How long does it take for a spacecraft to get from Earth to Mars? 4. How long do most interplanetary launch "opportunities" last? 5. How often can we launch a spacecraft to Mars? 6. How often do the four outer planets line up for a spacecraft to visit them all? 7. Explain how "gravity assist" works: 8. What does the "Shepherding Theory" explain