Interstellar, here we come!

The amount of interstellar particles being detected by Voyager 2 may indicate that the spacecraft is about to pass through the heliopause into interstellar space.

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The universe is huge, the distances between things are enormous and it’s really only when we see our impact on the universe that we can appreciate just how huge it is, or conversely, just how tiny we are by comparison. Voyager 2 is about 118 Astronomical Units* from the Earth or nearly 16 and a half light hours away.

(*an Astronomical Unit is the distance from the Sun to the Earth, about 150,000,000 km)

The spacecraft was launched in 1977 within a short time of Voyager 1 primarily to conduct a mission around the Solar System that took advantage of a favourable alignment of planets in the late 1970s and early 1980s. Voyager 2 was launched first on 20 August 1977 and followed a month later by Voyager 1 on 5 September. While Voyager 1 got as far as Saturn and then headed off out of the plane of the Solar System, after Saturn, Voyager 2 was sent have a look at Uranus and Neptune before heading off.

Both of the spacecraft are now transversing the space between the Sun’s influence and interstellar space and on a mission called the Voyager Interstellar Mission.

Launch of Voyager 2 on a Titan-Centaur rocket in 1977 (Credit: NASA)

It is fantastic that these spacecraft are still providing valuable science after over 40 years of use. They are so far from Earth that the signal that is sent back to Earth is picked up on huge 34 and 70 metre Deep Sky Network receiver dishes. Both spacecraft haven’t got long left in their planed lives as by 2020 their batteries are expected to run out and no longer have the power to transmit data back to Earth. But in the mean time we are learning a lot about the interstellar medium and the boundary between that and where the Sun reigns supreme. Voyager 1 crossed into interstellar space back in 2012 after a lengthy trip through the heliosheath. Both spacecraft are motoring along at about 3.5 AU per year on their way out of the Solar System.

The relative positions of both Voyager spacecraft (Credit: NASA via Wikipedia)

The Sun’s influence is enormous and even out at the distances that the Voyager spacecraft are, there is a tremendous amount of high energy particles and and the influence of the magnetic field. This tends to shape the interface between the Solar System and interstellar space creating a termination shock. This is where the high energy particles whizzing out of the Sun at 100s of kilometres per second eventually slow down to less than the speed of sound as the pressure for interstellar particles interacts with the Sun’s solar wind. Next is the heliosheath and inside this region there are patches of almost no solar wind. Voyager 1 crossed the next part of the Solar System, the heliopause in 2012 and this is the boundary between the Sun’s influence and the presence of the interstellar wind. The interstellar particles are colder and scientists predicted that these would increase in proportion to the hot particles from the Sun. Their prediction was correct and in May 2012 Voyager 1 detected a rapid increase in the amount of interstellar particles. NASA determined that Voyager 1 had crossed the heliopause into interstellar space on 25 August 2012 and it looks like that Voyager 2 is not far off doing the same.

The regions on beyond the termination shock (Credit: NASA)

Since August NASA has picked up about a 5% increase in the number of cosmic particles hitting the spacecraft. This is very similar to what they saw in May 2012 with Voyager 1 so they believe that Voyager 2 might also be about to cross the heliopause and head out into interstellar space. Though it might not be so simple as the Sun’s influence is cyclical based on the 11 year solar cycle so Voyager 2 may have to spend a bit longer crossing the heliopause depending on what it happening out there.

The Sun is a bit like a comet as it races through the interstellar space orbiting the galactic centre it creates a bit of a bow-shock in front of it and long stream behind it as the boundary with interstellar space is stretched out – not all that different to how a short stubby comet might look – if you could see the high energy particles.

The Voyager spacecraft haven’t finished with the Solar System, even when they are both beyond the heliopause as they will have still have to pass through the Oort Cloud in the years to come. By that point we won’t be hearing from them anymore unfortunately.