Ed Stone cannot say when the Voyager-1 spaceship will leave the Solar System, but he believes the instant is close. It could be any day, except it could also be several more years. The newest data from this extraordinary probe suggests it is surfing right on the very edge of our Sun's domain. Launched way back in 1977, the probe has now travelled so far from home that its steady chatter of data takes 17 hours to arrive at the US space agency's receiving system.
The particles streaming away from our star have condensed to a drip at its present position, 18.5 billion km from Earth. Particles soaring towards it from interstellar space, by contrast, have jumped clearly in the past year. It all points to a forthcoming departure, which would make Voyager the first man-made object to cross into the space between the stars.