NASA's Voyager 2 probe has detected an increase in cosmic rays that originate outside our solar system, the space craft has revealed.
Currently on a journey toward interstellar space, Voyager 2 is a little less than 11 billion miles from Earth, or more than 118 times the distance from Earth to the Sun.
It was launched in 1977, but since 2007 the probe has been traveling through the outermost layer of the heliosphere, including the bubble around the Sun and the planets dominated by solar material and magnetic fields.
We're seeing a change in the environment around Voyager 2, there's no doubt about that
Voyager scientists have been watching for the spacecraft to reach the outer boundary of the heliosphere, known as the heliopause. Since late August, they have discovered that the probe's Cosmic Ray Subsystem instrument has measured about a five per cent increase in the rate of cosmic rays hitting the spacecraft compared to early August.
The probe's Low-Energy Charged Particle instrument has also detected a similar increase in higher-energy cosmic rays.
Cosmic rays are fast-moving particles that originate outside the solar system. Some of these cosmic rays are blocked by the heliosphere, so mission planners expect that Voyager 2 will measure an increase in the rate of cosmic rays as it approaches and crosses the boundary of the heliosphere.
We're going to learn a lot in the coming months, but we still don't know when we'll reach the heliopause
"We're seeing a change in the environment around Voyager 2, there's no doubt about that," said Voyager Project Scientist Ed Stone, based at Caltech in Pasadena.
"We're going to learn a lot in the coming months, but we still don't know when we'll reach the heliopause. We're not there yet - that's one thing I can say with confidence."
The Voyager spacecraft were built by NASA's Jet Propulsion Laboratory in Pasadena, California.
The Voyager missions are a part of the NASA Heliophysics System Observatory, managed by the Heliophysics Division of the Science Mission Directorate in Washington.
Voyager 2 isn't the first space probe to reach interstellar space - that 'first' belongs to Voyager 1. However, it is expected to continue transmitting weak signals until around 2025 as its electrics slowly fail.
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