After a journey of 36 years, NASA has announced that the Voyager 1 space probe officially left the Solar System on August 25, 2012. Based on new analysis of data sent by by the unmanned spacecraft, the space agency has declared that it is now in the first man-made object to travel into interstellar space, 12 billion miles (19 billion km) from the Sun.

Originally sent out to study the giant planets of the outer Solar System, scientists knew that Voyager 1 would eventually leave the system when it launched back in 1977 at a speed fast enough to escape the Sun’s gravity. The questions were, when would it do so and would it be still operating. Thirty six years later, the answers are 25 August 2012, and yes, it is.

Given that Voyager 1 is so far from Earth that a radio signal takes 17 hours to reach it, you’d think that it was long past the limits of the Solar System. Having left the planets far behind, that’s a fair assumption. The probe many billions of miles from the nearest planet and the vacuum is so hard that atoms are collector’s items.

This may seem as if the edge of the Solar System is a matter of mere opinion, but it is, in fact very important. Though the terrestrial real estate is far behind it, Voyager has been traveling through an area where the Sun’s influence can be still be observed in the form of solar winds and magnetic fields. So long as Voyager was inside this area of influence, called the heliosphere, it was technically inside the Solar System. Where this area ends and how it ends can tell scientists a great deal about how the Solar System is put together and how it fits into the cosmic neighborhood.

The big question is, how do you mark the edge of the heliosphere? When Voyager was launched, it seemed pretty straightforward. There should have been a noticeable change in the environment around the spacecraft as it left the Solar System. Specifically, there should have been a drop in the number of cosmic rays coming from inside the system and a dramatic rise in the ones coming from interstellar space. In addition, there should have been a shift of the magnetic field as Voyager left the Sun’s and entered the magnetic fields between the stars.

Unfortunately, when the cosmic ray shift occurred around August 25, 2012, it wasn't as neat as planned. The interstellar cosmic rays jumped to record levels, while the solar ones dropped by a factor of a thousand, but the magnetic field, though it was 60 percent stronger, barely shifted. NASA contended that the heliopause, as the boundary between solar and interstellar is called, is more complex than scientists thought and that the magnetic field shift was still the deal breaker. Others claimed that the heliopause was a complex, fragmented area and that the magnetic field was a local phenomenon in a more porous barrier where inside and outside forces mixed.

Artist's impression of the relative positions of Voyagers 1 and 2 (Image: NASA/JPL-Caltech)

The breakthrough came, strangely enough, in March of 2012, though no one realized it at the time. On Saint Patrick’s Day, the Sun shot out a massive jet of plasma that hit Voyager 13 months later in April 2013. At this distance, the discharge was so weak that it could only be detected as an oscillation in the local plasma, which NASA compared to plucking a violin string. What’s clever here is that the oscillations allowed scientists to calculate the density of the plasma, which turned out to be 40 times denser than it should have been. In fact, it was dense enough for interstellar space.

That’s when the lights started burning late as NASA started going over older data and found the same density in October and November 2012. With these points of reference, they calculated that Voyager 1 had left the Solar System on August 25, 2012 – the same time as the shift in cosmic rays.

"We literally jumped out of our seats when we saw these oscillations in our data – they showed us the spacecraft was in an entirely new region, comparable to what was expected in interstellar space, and totally different than in the solar bubble," says Don Gurnett of the University of Iowa. "Clearly we had passed through the heliopause, which is the long-hypothesized boundary between the solar plasma and the interstellar plasma."

So, has Voyager 1 finally left the Solar System? The quick answer is, yes and no. Yes, it is in interstellar space where the solar winds and magnetic field no longer holds sway, but there is still gravity to contend with. The very outer frontier where the Sun’s gravity is finally eclipsed by the collective gravitational pull of the rest of the Galaxy is the Oort cloud (and technically it's outer edge), where the comets that periodically visit the inner Solar System originate and where the pull of the Sun or passing stars sometimes disturb them and send them arcing towards us.

Astronomers have regarded the outer edge of the Solar System as being between 1,000 and 100,000 AU distant (1 AU = 149,597,870.7 km) since the 1960s, but it is a bit like claiming that the edge of Japan is Scotland because you can detect Japanese earthquakes from there. Nevertheless, there is one good reason to make August 25, 2012 the official day. Voyager is only 125 AU from Earth and reach the Oort cloud for 300 years and won’t pass through it for another 30,000 years, so the champagne would be bit off by then.

Regardless of when it has or will leave, Voyager 1’s next encounter with another celestial body won’t be for 40,000 years when it passes within 1.6 light years of the star Gliese 445 (AC +79 3888). Voyager 2 is following, though on a different trajectory, so scientists can’t predict when it will leave the system.

“Voyager has boldly gone where no probe has gone before, marking one of the most significant technological achievements in the annals of the history of science, and adding a new chapter in human scientific dreams and endeavors,” says John Grunsfeld, NASA’s associate administrator for science in Washington. “Perhaps some future deep space explorers will catch up with Voyager, our first interstellar envoy, and reflect on how this intrepid spacecraft helped enable their journey.”

The report on Voyager 1’s departure was published this week in Science.

The video below describes Voyager 1 leaving the Solar System.

Editor's note: Whether or not Voyager 1 has technically left the Solar System depends on where you set the boundary. It now seems certain that Voyager 1 has left the heliosphere, but there's the question of whether it has left the sphere inside which the Sun's gravity takes precedence over the galactic tide. As the BBC's Jonathan Amos points out, NASA's Ed Stone "has always been very careful not to use the phrase 'leave the Solar System', mindful that his spacecraft still has to pass through the Oort cloud where there are comets gravitationally bound to the Sun." This despite the fact that Voyager 1's current distance from the Sun puts it beyond most estimates of the outer edge of the Oort cloud. Nevertheless, the BBC and many others seem happy to report that Voyager 1 has indeed left the Solar System, though it's worth noting that this specific wording is not present in NASA's statement.

Source: NASA

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