Imagine a spacecraft so far away that radio signals from Earth—traveling at the speed of light—take over 20 hours to reach it. That is Voyager 1's reality today. Launched by NASA on September 5, 1977, Voyager 1 has traveled farther than any human-made object ever built. In August 2012, it crossed the heliopause, the invisible boundary where the Sun's solar wind stops pushing outward and the interstellar medium takes over. It is now genuinely in interstellar space, drifting among the stars.

The spacecraft achieved this incredible journey through a clever trick called a 'gravity assist.' As Voyager 1 flew past Jupiter in 1979 and Saturn in 1980, the enormous gravity of these gas giants acted like a cosmic slingshot, flinging the probe faster and farther into space. This maneuver saved fuel and accelerated the spacecraft to speeds that allowed it to escape the Sun's pull forever. Today, Voyager 1 travels at about 17 kilometers per second—roughly 61,000 kilometers per hour—yet it is still so distant that Earth's most powerful telescopes cannot see it.

What makes Voyager 1 even more remarkable is that it still works. Powered by radioisotope thermoelectric generators (RTGs)—which convert heat from decaying plutonium into electricity—the spacecraft continues sending data back to Earth. Though most of its scientific instruments have been powered down to conserve energy, Voyager 1's basic systems remain functional. Its journey will not end soon: at its current speed, Voyager 1 will not approach another star for roughly 40,000 years.

Aboard Voyager 1 is the 'Golden Record,' a phonograph record containing sounds and images from Earth—music, greetings in 55 languages, and scientific diagrams. It is humanity's message in a bottle, cast into the cosmic ocean. Whether another civilization ever finds it remains one of humanity's most profound long-shot hopes.