Launched in 1977, NASA’s Voyager spacecraft each carry a Golden Record—a gold-plated copper phonograph disc containing sounds and images of Earth, intended as a message to any intelligent life that might one day find it. On the record’s aluminum cover, scientists embedded a small sample of uranium-238, serving as a long-term radioactive clock. With a half-life of about 4.51 billion years, the decay of this uranium allows any technologically advanced finder to calculate precisely how long the record has been in space by measuring the ratio of remaining uranium to its decay products.

This method requires no shared language or knowledge of human history—only an understanding of nuclear physics. The uranium clock was not meant to work alone. It is paired with a pulsar map etched onto the same cover, which shows the position of the Sun relative to 14 known pulsars, with their rotational periods encoded in binary. Since pulsars slow down predictably over time, a finder could compare the recorded periods with current measurements to determine the elapsed time since the record’s creation.

Together, the uranium and pulsar map form two independent timekeeping systems that can cross-check each other. If they disagreed, that discrepancy could itself yield scientific insight. While the physical record is expected to survive roughly a billion years in interstellar space, the uranium clock remains readable for much longer—potentially several billion years. The real challenge isn’t the clock’s accuracy, but the near-improbability of either Voyager spacecraft ever being found. Voyager 1 won’t pass within a few light-years of another star for about 40,000 years. Still, the clock was built to endure, a silent testament to a civilization that prepared an answer to a question that may never be asked.