On August 25, 2012, a 722-kilogram spacecraft built with 1970s technology crossed a boundary no human artifact had ever reached. NASA announced the fact on September 12, 2013. Voyager 1 had entered interstellar space. The evidence came not from a camera but from the spacecraft's plasma wave instrument, which detected a 40-fold increase in the density of charged particles surrounding the probe. This density matched predictions for the cool, dense plasma of interstellar space, beyond the hot, tenuous bubble of solar wind that defines our Sun's domain.
The probe was 18.8 billion kilometers from Earth. A signal from mission control, traveling at light speed, took 17 hours and 22 minutes to reach it. Voyager 1's primary mission to Jupiter and Saturn ended in 1980. For 33 years it flew a silent, pre-programmed trajectory into darkness, its systems powered by three radioisotope thermoelectric generators. Engineers had to shut down instruments one by one to conserve dwindling power; the last science instrument will likely go silent around 2025.
This moment is often misunderstood as leaving the solar system. Voyager 1 has left the heliosphere, the Sun's magnetic and particle influence. It has not, however, exited the Oort Cloud, the distant shell of icy bodies still gravitationally bound to the Sun. That final frontier lies roughly 300 years of travel ahead. The craft carries a golden record containing sounds and images of Earth, intended for any intelligence that might find it.
Voyager's crossing provided the first direct measurements of the interstellar medium. It transformed a theoretical boundary into a place with measurable properties. The spacecraft continues to transmit data about magnetic fields and cosmic rays, a faint signal from a realm where the light from our nearest star is just another bright point in the sky.