After a 30 year journey, NASA's twin Voyager probes are about to leave the solar system and enter interstellar space.
It's about to leave the heliosphere, the giant bubble of solar plasma and magnetic fields around the sun, about three times wider than the orbit of Pluto.
Its edge, the heliosheath, is filled with a magnetic 'froth' that no spacecraft has ever before encountered, and with low-frequency radio bursts heard nowhere else.
"In many ways, the heliosheath is not like our models predicted," says says Voyager project scientist Ed Stone of Caltech.
Last year, Voyager 1 beamed back the information that the outward velocity of the solar wind where it is right now is a rather startling zero. The assumption is that it's turned a corner - but nobody knows in which direction.
Voyager 1 is attempting to find out through a series of "weather vane" maneuvers, in which V1 turns itself in a different direction to track the local breeze.
The Voyagers are now moving into the heliosheath, but nobody knows just when they will emerge into interstellar space. "The heliosheath is three to four billion miles in thickness," estimates Stone. "That means we'll be out within five years or so."
In the late 1970s when the probes took advantage of a rare alignment of outer planets to allow Voyager 1 to visit Jupiter and Saturn, while Voyager 2 flew past Jupiter, Saturn, Uranus and Neptune.
The've been able to establish the existence of of volcanoes on Jupiter's moon Io; evidence for an ocean beneath the icy surface of Europa; hints of methane rain on Saturn's moon Titan; the crazily-tipped magnetic poles of Uranus and Neptune; icy geysers on Neptune's moon Triton and planetary winds that blow faster and faster with increasing distance from the sun.
Both Voyagers are powered by the radioactive decay of a Plutonium 238 heat source. This should keep critical subsystems running through at least 2020.
After that, says Stone, "Voyager will become our silent ambassador to the stars."