One Light-Day Away: Time incomprehensible

Sometime around mid-November, Voyager 1 will cross the one light-day mark. That means the signal from a spacecraft we launched in September of 1977 now takes a full 24 hours to reach us at the speed of light. It launched before the original Star Wars was in theaters, before the IBM PC existed, before most people alive today were born, and it has been running continuously for 49 years. And in roughly 60 days, it will officially be one light-day from Earth. The number sounds manageable until you think about what "one light-day" actually means. Light crosses roughly 26 billion kilometers in a year, which works out to about 26 million kilometers per day. Voyager is moving at close to 60,000 kilometers per hour, which is genuinely extraordinary by any human engineering standard. At that speed it still took nearly half a century to cover a single light-day. The nearest star is about four light-years away. Do the math: at Voyager's pace, reaching Alpha Centauri would take something on the…