Voyager 2 is almost outside the protective bubble of the sun

The space is vast, but it is not entirely empty. Take a closer look and you will find that even the abysses between the stars are filled with fast particles, survivors of ancient cataclysmic events that propelled them into the universe almost at the speed of light.

NASA announced that the Voyager 2 spacecraft, which had left the Earth more than 40 years ago to monitor the outer planets, had detected a 5% increase in the prevalence of these cosmic refugees. The new measurements have long indicated that the probe is moving closer to the protective influence of the sun, which for many researchers defines the line of demarcation between interplanetary space and interstellar space. Crossing this boundary will provide a second valuable data point in the search for its shape, but the precise moment when the probe leaves the solar system remains unknown.

"We are witnessing a change in the environment around Voyager 2, no doubt," said Ed Stone of Caltech, Voyager project scientist, in a press release. "We will learn a lot in the months to come, but we still do not know when we will hit the heliopause – we are not there yet – that is one thing I can say with confidence. "

If you feel that you've already heard this story, it's because you've already done Voyager 1, the faster brother of the gear, Voyager 1, returned data showing a similar increase in interstellar cosmic rays in May 2012. In addition to light, the sun also emits a plasma of charged particles in all directions. Too hot to be mastered, this solar wind is spreading beyond planets, repelling interstellar particles and creating a bubble that researchers call the heliosphere. Three months later, on August 25, Voyager's aging instruments detected a drop in solar particles and a new increase in interstellar particles, a clear evidence that they had finally left the heliosphere.

Many limits

From the point of view of the solar wind, leaving the heliosphere means leaving the solar system. In the months or years to come, Voyager 2 should join Voyager 1, which will allow it to taste the particles and magnetic fields present in the relative immobility between the stars. But the solar wind is not the only way the sun can exert its influence. If you consider the solar system as a collection of rocks and gas balls stuck by gravitation around the sun, the two Voyageurs still have a long way to go.

The easiest way to visualize such great distances is to use astronomical units, or AU. An AU is the distance between the sun and the earth. They are not heading in the same direction, but Voyager 1 is currently about 143 AU from the sun, while Voyager 2 is trailing at 118 AU. These distances place them well beyond Neptune at 30 AU, but there are many things even further. The newly discovered "goblin" orbit, for example, enters 2000 UA, and a collection of icy rocks known as the Oort Cloud extends tens of thousands, or even hundreds of thousands of AU beyond . NASA expects Voyager probes to emerge from this region in around 20,000 years.

Setting aside the high bar to escape the sun's alluring companions, a functional Voyager 2 joining Voyager 1 in interstellar space will be a landmark achievement. The exchange of signals with such remote machines requires huge resources, and NASA devotes seven to ten hours a day to precious time listening to its Deep Space network to capture more and more data. darker. The Pioneer 10 probe is also somewhere near the limit, but she stopped calling home in 2003. A total of five spacecraft travel fast enough to completely escape the sun, but two are missing and the third , New Horizons, should turn off before leaving. the heliosphere in a few decades.

A leaking balloon

Voyager 2's nuclear power source is expected to last until around 2025 – probably long enough to sample interstellar space – but everyone will be clear about the time when this data will arrive. While Voyager 1 crossed the border three months after the recording of an increase in cosmic rays, Voyager 2 is heading for a different part of the heliosphere, which can itself quickly change shape.

"All these boundaries could evolve with solar activity and we do not really know where they are located," says Merav Opher, an astronomer who studies the shape of the heliosphere at Boston University. At more active moments in the eleven-year solar cycle, like the one we are currently in, the sun blows the heliosphere like a balloon, pushing the boundaries of a handful of DU within a few years . Later in the cycle, a less active sun lets it deflate.

The shape of the heliosphere may not be as uniform as its name indicates. "In general, nature does not have very smooth borders. The question is to what extent and on what scale, "says Opher. His earlier simulations suggested that he might even have a shape more similar to the wake of a ship than a sphere.

The close observation of Voyager 1 at a point on this boundary has brought a number of revelations. On the one hand, the heliosphere is more permeable than thought physicists, equipped with a magnetic connection that allows a particle exchange in certain areas. Voyager 1 also revealed surprising features just beyond the edge. The probe "surfed" on a series of outward-directed pulses, triggered by sudden splinters of solar matter, and detected an unexpected stack of interstellar material, suggesting that the solar system traverses the galaxy in a less savage way than did not imagine it. Opher is even more excited about Voyager 2 because, unlike Voyager 1, it still has a working plasma instrument and should be able to record unpublished data on the boundary and interstellar medium.

Even with their unprecedented details, the two Voyager data points will probably not be enough to define the shape of the sun's protective bubble. Researchers will also need data from the current IBEX and future IMAP missions to remotely study the general geometry of the heliosphere. Opher compares the project to the resolution of a mystery murder with the help of several observers with different perspectives. "Someone took all the little tools in one room and the other took pictures away from home," she says. "You have to rebuild with both what the heliosphere does."

The resolution of this local mystery could have universal ramifications. Astronomers may find that the "astrospheres" surrounding the other stars also produce waves in the space around them, but their plasma remains too sparse to allow a direct probe. Understanding how the sun shapes our neighborhood and protects us from certain cosmic rays will be an essential step in understanding where and if the same process occurs elsewhere.

"If you want to study the development of life in other star systems, you have to understand the behavior of other astrospheres," says Opher. "Let's start in our own backyard."