Voyager 1 left the solar wind behind

Even so, it has not yet crossed the boundary of the solar system into interstellar space. This thing is expected to happen in about four years

The venerable Voyager 1 spacecraft applied the phrase "going where no one has gone before". Voyager 1 has reached a remote point at the edge of the solar system where it no longer receives signals from the solar wind.

At a distance of 17.3 billion kilometers from the Sun, Voyager-1 crosses into an area where the speed of the plasma particles (the ionized gas), which are emitted directly out of the Sun, slows to zero. The scientists suspect that the solar wind is turned sideways due to the pressure of the interstellar wind in the region between the stars.
"The solar wind is moving sideways," says Voyager Project Scientist Ed Stone at the California Institute of Technology in Pasadena. Voyager 1 is approaching interstellar space.

The event is a milestone in Voyager-1's journey through the heliosheath, the outer shell of the Sun's influence, and which indicates that the spacecraft is leaving the Solar System. Since its launch on September 5, 1977, the Low Energy Particle Capture Instrument has been used to measure the speed of the solar wind. When the speed of the ionized particles heating the outer part of the spacecraft matched the speed of the spacecraft, the researchers knew that the net speed of the particles in the outward direction was zero. This happened in June when Voyager 1 was about 17 billion kilometers from the Sun.

But the speeds can vary, so the scientists watched another four months before they were convinced that the speed of the solar wind had indeed slowed to zero.

Analysis of the data showed that the speed of the solar wind has gradually slowed at a rate of 72 km/h each year since August 2007, when the solar wind accelerated outwards at a speed of about 210 km/h. The velocity in the direction out of the solar system has remained at zero since June.

"When I realized we were getting solid zero, I was amazed," said Rob Decker, operator of the low-energy particle detector and researcher at the Applied Physics Laboratory at Johns Hopkins University in Maryland. Here Voyager, a spacecraft that has been a workhorse for 33 years, is showing us something new again."

The scientists believe that Voyager 1 has not yet crossed the Heliosheath into interstellar space. This crossing means a sudden decrease in the density of the hot particles and an increase in the density of the cold particles. The scientists put the data coming from the spacecraft into models of the structure of the heliosphere and can better estimate when Voyager-1 will reach interstellar space. The researchers now estimate that it will cross this border in about 4 years.

Our sun is constantly launching ionized particles that bubble through the heliosphere around the solar system. The solar wind composed of those particles moves at supersonic speed until it crosses a shock wave known as a termination shock. At this point in time, the solar wind slows dramatically and warms the heliosheath.

A sister spacecraft, Voyager-2, was launched on August 20, 1977 and reached a distance of about 14 billion km from the Sun. The two spacecraft move in different directions and at different speeds. Voyager-1 moves faster at a speed of about 61 km/h, compared to Voyager-2's speed of 56 km/h. In the next two years, the scientists expect that Voyager-2 will experience the same phenomena that Voyager-1 experienced.

The findings were presented at the American Geophysical Union conference held last week in San Francisco.

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