NASA's Voyager 1 spacecraft has reached a point at the edge of the solar system where there is no outward motion of solar wind.
The event signifies that after 33 years on the go, the spacecraft is about to reach interstellar space.
Mission controllers at the Jet Propulsion Laboratory, Pasadena California say the spacecraft is now some 17.4 billion kilometres from the Sun and has crossed into an area where the velocity of ionized gas, or plasma, emanating from the Sun has slowed to zero.
Scientists suspect that interstellar winds have deflected the solar wind to one side.
The event is a major milestone in Voyager 1's passage through the heliosheath, the turbulent outer shell of the Sun's sphere of influence.
The Sun gives off a stream of charged particles called the solar wind which forms a bubble around the solar system known as the heliosphere.
The solar wind travels at supersonic speed until it crosses a shockwave called the termination shock.
At this point, the solar wind dramatically slows down and heats up in the heliosheath.
Launched on September 5th, 1977, Voyager 1 crossed the termination shock and entered the heliosheath in December 2004.
Instruments aboard Voyager 1 have allowed scientists to monitor the solar wind's velocity.
When the speed of the charged particles hitting Voyager 1 matched the spacecraft's speed, researchers knew that the net outward speed of the solar wind was zero.
JPL say that occurred in June, when Voyager 1 was about 17 billion kilometres from the Sun.
Solar wind fluctuations
However because these velocities can fluctuate, scientists took readings for several more months before they were convinced the solar wind's outward speed has actually slowed to zero.
Scientists believe Voyager 1 has not yet crossed the heliosheath into interstellar space.
They believe that event will be marked by a sudden drop in the density of hot particles and a corresponding increase in the density of cold particles.
Scientists are putting the data into their models of the heliosphere's structure and should be able to better estimate when Voyager 1 will reach interstellar space.
Researchers currently believe Voyager 1 will cross that frontier in about four years.
Meanwhile, Voyager 1's sister ship Voyager 2, which was launched on August the 20th, 1977 is now some 14 billion kilometres from the Sun.
Both spacecraft have been travelling on different trajectories and at different speeds.
Voyager 1 is travelling faster, at a speed of about 62,000 kilometres per hour while Voyager 2's velocity is 57,000 kilometres per hour.
An amazing time
Dr Fred Watson from the Australian Astronomical Observatory says it's "an amazing time", as the furthest-travelled man made object prepares to leave the solar system.
"The solar wind has dwindled to a breeze," he says.
"Voyager 1 is still surrounded by the Sun's solar wind, it's just that these particles are now moving at right angles to the spacecraft because of the impact of the interstellar medium."
Watson says eventually the solar wind will just fade into the interstellar medium.
"That could be quite sudden but that will be the point at which Voyager 1 will have left the solar system."