Voyager in interstellar space

Voyager 1 has become the first manmade spacecraft to leave our solar system.

But the only reason scientists have been able to say that Voyager 1 has reached interstellar space is that it produced a particular sound in April of this year. A coronal mass ejection from our sun erupted in March 2012 and when it reached Voyager 13 months later, the plasma around the spacecraft started to vibrate.

The vibrations were detected by Voyager's plasma wave instrument. NASA scientists say they used the pitch of the oscillations to calculate the density of plasma around the spacecraft. It was 40 times denser than what was encountered in the outer layer of the heliosphere. Such a density is expected in interstellar space.

Voyager 1 was launched 16 days after "its twin," Voyager 2, in 1977. They have long exceeded NASA's expectations. Voyager 1, which is about the size of a subcompact car, visited Jupiter and Saturn in 1979 and 1980 and sent back detailed images of their moons.

In 1990, it sent the first complete photo of the solar system.

Voyager 2, which is trailing at 15 billion kilometers from Earth, also sent images of Uranus and Neptune.

Both spacecraft still have signals' contact with Earth.

Eventually, the Voyagers will run out of nuclear fuel and will power down their instruments, perhaps by 2015.