Voyager 1 crossed the heliopause, or the edge of the heliosphere – the protective bubble of particles and magnetic fields created by our sun – in August 2012. Heading in a different direction, Voyager 2 crossed another part of the heliopause on today’s date a year ago, November 5, 2018. Thus Voyager 2 became only the second earthly spacecraft to cross into interstellar space, at a distance of some 11 billion miles (18 billion km) from Earth, well beyond the orbit of Pluto. Today, five new research papers in the peer-reviewed journal Nature Astronomy describe what scientists observed during and since Voyager 2’s historic crossing (see links to the papers below). A statement from NASA said:
Each paper details the findings from one of Voyager 2’s five operating science instruments: a magnetic field sensor, two instruments to detect energetic particles in different energy ranges and two instruments for studying plasma (a gas composed of charged particles). Taken together, the findings help paint a picture of this cosmic shoreline, where the environment created by our sun ends and the vast ocean of interstellar space begins.
Before their historic crossing into interstellar space, the Voyagers had already served humanity well. Taking advantage of a rare alignment of planets in the outer solar system, both Voyagers visited mighty Jupiter and ringed Saturn, and Voyager 2 performed the first – and only – flybys of the ice giants Uranus and Neptune.
Consider that – before Voyager 1 reached the edge of the heliosphere in 2012 – this edge was entirely theoretical in nature. We had never been to the boundary of interstellar space before. Scientists weren’t entirely sure how far this boundary was located from our sun, although their predictions turned out to be amazingly precise. You might know that the sun undergoes an 11-year cycle of activity. Scientists expected the heliopause – or boundary region of the heliosphere – to move with the changes in activity on the sun:
… sort of like a lung expanding and contracting with breath. This was consistent with the fact that the two probes encountered the heliopause at different distances from the sun.
And so – due to the malleable nature of the heliopause – the two Voyagers crossed into interstellar space at different times, one six years before the other, and at different distances from the sun. The new papers now confirm that Voyager 2 is not yet in undisturbed interstellar space. Like its twin, Voyager 1, Voyager 2 appears to be in a perturbed transitional region just beyond the heliosphere. Ed Stone, project scientist for Voyager and a professor of physics at Caltech, commented:
The Voyager probes are showing us how our sun interacts with the stuff that fills most of the space between stars in the Milky Way galaxy. Without this new data from Voyager 2, we wouldn’t know if what we were seeing with Voyager 1 was characteristic of the entire heliosphere or specific just to the location and time when it crossed.
The five new papers in Nature Astronomy describe various measurements made by Voyager 2. According to NASA:
The two Voyager spacecraft have now confirmed that the plasma in local interstellar space is significantly denser than the plasma inside the heliosphere, as scientists expected. Voyager 2 has now also measured the temperature of the plasma in nearby interstellar space and confirmed it is colder than the plasma inside the heliosphere.
In 2012, Voyager 1 observed a slightly higher-than-expected plasma density just outside the heliosphere, indicating that the plasma is being somewhat compressed. Voyager 2 observed that the plasma outside the heliosphere is slightly warmer than expected, which could also indicate it is being compressed. (The plasma outside is still colder than the plasma inside.) Voyager 2 also observed a slight increase in plasma density just before it exited the heliosphere, indicating that the plasma is compressed around the inside edge of the bubble. But scientists don’t yet fully understand what is causing the compression on either side.
Speaking of Voyager 2’s findings, NASA also said:
If the heliosphere is like a ship sailing through interstellar space, it appears the hull is somewhat leaky. One of Voyager’s particle instruments showed that a trickle of particles from inside the heliosphere is slipping through the boundary and into interstellar space. Voyager 1 exited close to the very ‘front’ of the heliosphere, relative to the bubble’s movement through space. Voyager 2, on the other hand, is located closer to the flank, and this region appears to be more porous than the region where Voyager 1 is located.
And NASA explained:
An observation by Voyager 2’s magnetic field instrument confirms a surprising result from Voyager 1: The magnetic field in the region just beyond the heliopause is parallel to the magnetic field inside the heliosphere. With Voyager 1, scientists had only one sample of these magnetic fields and couldn’t say for sure whether the apparent alignment was characteristic of the entire exterior region or just a coincidence. Voyager 2’s magnetometer observations confirm the Voyager 1 finding and indicate that the two fields align, according to Stone.
Voyager 1, the faster of the two probes, is currently over 13.6 billion miles (22 billion kilometers) from the sun.
Voyager 2 is 11.3 billion miles (18.2 billion kilometers) from the sun. Traveling at the speed of light, a signal from Voyager 2 requires about 16.5 hours to travel to Earth. By comparison, light traveling from the sun takes about eight minutes to reach Earth. The 22.4-watt transmitter on Voyager 2 has a power equivalent to the light that pops on when you open your refrigerator door. This dim signal from Voyager – which is more than a billion billion times dimmer by the time it reaches Earth – is picked up by the 70-meter antennas at three facilities spaced equidistant from each other – approximately 120 degrees apart in longitude – around the world. These are the sites of NASA’s Deep Space Network at Goldstone, near Barstow, California; near Madrid, Spain; and near Canberra, Australia.
The two Voyagers are powered by steadily decaying plutonium batteries. NASA scientists have been slowly powering down the crafts’ scientific instruments for some years now, attempting to stretch out the amount of time we can continue to communicate with them. Both craft are projected to drop below critical energy levels in the mid-2020s, after which they will fall silent.
Bottom line: Voyager 2 crossed into interstellar space on November 5, 2018 – one year ago today – becoming the 2nd craft ever to do so. This week, the journal Nature Astronomy published 5 new papers describing what Voyager 2 has been seeing on its journey into the unknown.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.