NASA's Voyager 2 spacecraft has gone interstellar!

Monday, December 10, 2018, 4:21 PM - The Voyager 2 space probe just went interstellar! Just like its twin, Voyager 1, this spacecraft has slipped beyond our local 'bubble' of space, and is now on a journey through the interstellar cosmos!

Anticipated for the past six years, and hinted at for weeks now, NASA scientists have finally confirmed: After a journey of more than 41 years and over 18 billion kilometres, the Voyager 2 spacecraft is now in interstellar space!

This artist's rendition of the local space environment of our solar system shows the heliosphere, the heliopause (where the heliosphere ends) and the confirmed location of both Voyager probes outside of these environments, in interstellar space. Credit: NASA/JPL-Caltech

Back in 2012, Voyager 1 was returning some very interesting clues about how the space environment it was flying through was changing. Most notably, its instruments were picking up what appeared to be a radical shift in the type of high energy particles (known as 'cosmic ray particles') that were whizzing by it.

Inside the Sun's heliosphere - the immense protective 'bubble' in the interstellar medium formed by the Sun's magnetic field and solar wind - the probe had detected a fairly constant stream of two types of cosmic ray particles. The first kind were known to originate from inside the Sun, while the second type was from sources out in the galaxy, beyond the Sun's protective shield. When this shift happened, however, the number of solar cosmic ray particles dropped drastically, while the number of galactic cosmic ray particles spiked.

These were two important pieces of evidence mission scientists were expecting to see when Voyager 1 finally left the heliosphere and entered interstellar space. Unfortunately, it took some time to verify this, with reports going back and forth about exactly where Voyager 1 was, due to one unfortunate fact. The one instrument that could have immediately given them the data they needed, and confirmed the spacecraft's position in interstellar space - the Plasma Science Experiment or PLS - hadn't functioned since 1980.

It was only by using a different, functioning, instrument on the probe, that the science team was able to infer what was happening with the cosmic ray particles, and confirm what kind of environment Voyager 1 was flying through by watching what happened when three different coronal mass ejections swept past the spacecraft.

Watch below as Voyager 1 tracks three 'tsunami waves' of solar particles sweeping past it in interstellar space

Based on these events, the NASA scientists were finally able to confirm, once and for all, that Voyager 1 had entered interstellar space.

VOYAGER 2 FOLLOWS CONFIDENTLY

With Voyager 2, however, the announcement comes far more quickly this time, and far more confidently!

Not only did the science team know far more about what to look for, due to their experiences with Voyager 1, but Voyager 2's Plasma Science Experiment is still working! So, rather than having to rely on second-hand data and wait for fortuitous encounters with solar eruptions, this time, the team was able to detect all the crucial evidence they needed, directly!

Watch Below: See what Voyager 2's Plasma Science Experiment (PLS) saw as the probe entered interstellar space

Above, the animation shows the rate of detection of cosmic ray particles by Voyager 2 - galactic in the upper line and solar (heliospheric) in the lower line - and how these rates changed dramatically in late October and early November. With the number of galactic cosmic rays spiking, and the number of solar cosmic rays dropping dramatically, the science team determined that the probe had transitioned into interstellar space on November 5, 2018.

Additionally, at the same time, Voyager 2 picked up that crucial third piece of evidence, as well, as the same instrument detected the changes in the magnetic field as the spacecraft moved outside the influence of the Sun's magnetic field.

The Plasma Science Experiment instrument (pictured right) measures the amount of electric current generated in it, as it moves through magnetic fields in space, recorded in three dimensions, as shown in the graphs on the left. The precipitous drop in the current levels in late October, and the near constant background current that was reached in early November, show how the probe has moved beyond the influence of the Sun's magnetic field. Credit: NASA/JPL-Caltech/MIT

One of the fundamental laws about how our universe works is that when you have a conducting wire inside of a moving or changing magnetic field, and you hook that wire up to a closed circuit, electric current is produced along that wire. This is known as electromagnetic induction. So, while Voyager 2 was still inside the heliosphere, and thus subjected to the Sun's moving and changing magnetic field, electric currents were generated inside the PLS instrument. When the spacecraft moved beyond the heliosphere, and was then surrounded by the much weaker galactic magnetic field, the amount of current detected dropped to a very low background level.

So, that's all the evidence we need! Voyager 2 has left the heliosphere, and humanity officially has two interstellar probes!

VOYAGER SCIENCE CONTINUES

Both Voyager probes have sent back important science from the outer reaches of our solar system, and their mission is far from over.

"There is still a lot to learn about the region of interstellar space immediately beyond the heliopause," Ed Stone, Voyager project scientist at Caltech, said in a NASA statement. 

"Voyager has a very special place for us in our heliophysics fleet," Nicola Fox, the director of NASA's Heliophysics Division, added. "Our studies start at the Sun and extend out to everything the solar wind touches. To have the Voyagers sending back information about the edge of the Sun’s influence gives us an unprecedented glimpse of truly uncharted territory."

It is quite remarkable that both Voyager spacecraft are still operational, and still sending back whatever data they can, after traveling through space for over 41 years!

One of the exciting parts about this, for heliophysics scientists, is that each Voyager has exited the heliosphere in a different place, and at a different point in the Sun's 11-year solar cycle.

While Voyager 1 headed "up" and out, leaving through the northern hemisphere of the heliosphere, during the 2012 solar maximum, Voyager 2 has now exited through the heliosphere's southern hemisphere, during the 2018 solar minimum. These differences are enough to give researchers plenty to examine and compare, to see how the edge of the Sun's influence changes with time.

Watch below to follow Voyager 1 and Voyager 2's journey out to Saturn, then continue to chase Voyager 2 on its path to interstellar space

Given that their power sources - radioisotope thermoelectric generators or RTGs - are expected to provide them with enough energy to communicate with home until around 2025, the Voyagers should continue to send back data for years.

"I think we’re all happy and relieved that the Voyager probes have both operated long enough to make it past this milestone," said Suzanne Dodd, Voyager project manager at NASA’s Jet Propulsion Laboratory. "This is what we've all been waiting for. Now we’re looking forward to what we’ll be able to learn from having both probes outside the heliopause."

STILL IN THE SOLAR SYSTEM

An important thing to note here is that Voyager 2 has NOT left the solar system. This goes for Voyager 1, as well.

While the heliopause marks the edge of the direct influence of the solar wind and Sun's magnetic field, there's one thing that reaches even farther - the Sun's gravity.

While Voyager 1 has travelled over 21.6 billion kilometres from the Sun, and Voyager 2 is now over 18 billion km away, the Sun's gravitational pull reaches to somewhere between 6 trillion and 32 trillion kilometres away, out to the edge of the Oort Cloud - the cloud of ancient icy objects, left over after the formation of the solar system. For a little perspective, Alpha Centauri, the next-closest star system to ours, is around 41 trillion kilometres away (around 4.3 light years).

This graphic shows the scale of the solar system, in astronomical units (1 AU = 1 Earth-distance from the Sun), along with the current locations of Voyager 1 and Voyager 2. The 'outer edge' of the solar system is considered to extend all the way out past the Oort Cloud. Graphic adapted from NASA's 'Solar System, in Perspective', published Sept 12, 2013. Credit: NASA/JPL-Caltech/Scott Sutherland

So, the Voyagers have a LONG WAY TO GO before they fully exit the solar system!

Sources: NASA | NASA Voyager | NASA Goddard

WATCH BELOW: HAVE ASTRONOMERS FOUND THE VERY FIRST EXOMOON?