Voyager 1 enters interstellar space

Nothing happened on 25 August 2012, at least nothing anyone could see. More than 18 billion kilometres from Earth, a spacecraft built in the disco era sailed on in silence. But aboard Voyager 1, the particle counters told a sudden story: charged particles from the Sun all but vanished, while galactic cosmic rays from dying stars elsewhere in the Milky Way surged. After a 35-year journey, the probe had slipped across the heliopause, the boundary where the Sun's wind yields to the matter between the stars. No one on Earth would be sure of it for more than a year.

The problem was cruel: the one instrument designed to measure the surrounding plasma directly had been dead since 1980. Proof required knowing the density of the gas Voyager was flying through, and there was no way to read it. The Sun itself provided the answer. A massive solar eruption in March 2012 finally washed over Voyager 1 in April 2013 and set the local plasma ringing like a struck bell. The plasma wave instrument heard the oscillation, and the pitch revealed a density forty times higher than the outer heliosphere. Only interstellar space could be that dense.

Working backwards through the data, Don Gurnett's team dated the crossing to around 25 August 2012, at 121.7 astronomical units from the Sun. NASA made it official on 12 September 2013, with the results published in the journal Science. Thirty-six years after leaving Florida in September 1977 on a mission to Jupiter and Saturn, Voyager 1 had become the first human-made object to enter the space between the stars.

It is still out there, still transmitting through NASA's Deep Space Network, its signal taking the better part of a day to arrive. Bolted to its side rides the Golden Record, a gold-plated copper disc carrying greetings in 55 languages, the sounds of Earth, and Chuck Berry. Long after its power fades, the spacecraft will drift on, our first envoy to the galaxy.