NASA’s Voyager 1 spacecraft has reached a distant point at the edge of the solar system where there is no outward motion of solar wind.
Now hurtling toward interstellar space some 17.4bn kilometres (10.8bn miles) from the Sun, Voyager 1 has crossed into an area where the velocity of the hot ionised gas, or plasma, emanating directly outward from the Sun has slowed to zero. Scientists suspect the solar wind has been turned sideways by the pressure from the interstellar wind in the region between stars.
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, and the spacecraft’s upcoming departure from our solar system.
Our Sun gives off a stream of charged particles that form a bubble known as the heliosphere around our solar system. 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 5 September 1977, Voyager 1 crossed the termination shock in December 2004 into the heliosheath. Scientists have used data from Voyager 1’s low-energy charged particle instrument to deduce the solar wind’s velocity. When the speed of the charged particles hitting the outward face of Voyager 1 matched the spacecraft’s speed, researchers knew that the net outward speed of the solar wind was zero. This occurred in June, when Voyager 1 was about 17 billion kilometres (10.6 billion miles) from the Sun.
Because the velocities can fluctuate, scientists examined four more monthly readings before they were convinced that the solar wind’s outward speed actually had slowed to zero. Analysis of the data shows the velocity of the solar wind has steadily slowed at a rate of about 20km/sec each year (45,000mph each year) since August 2007, when the solar wind was speeding outward at about 60km/sec (130,000mph). The outward speed has remained at zero since June.
A sister spacecraft, Voyager 2, was launched on 20 August 1977 and has reached a position 14.2 billion kilometres (8.8 billion miles) from the Sun. Both spacecraft have been travelling along different trajectories and at different speeds.
Voyager 1 is travelling faster, at a speed of about 17km/sec (38,000mph), compared with Voyager 2’s velocity of 15km/sec (35,000mph). In the next few years, scientists expect Voyager 2 to encounter the same kind of phenomenon as Voyager 1.
The Voyagers were built by NASA’s Jet Propulsion Laboratory in Pasadena, California, which continues to operate both spacecraft.