Voyager 1 and 2 probes amaze us again: another discovery from the confines of the Solar System
We thought that once we passed the Hercules Columns of the heliosphere the probes would
stop talking to us.
Not because of the energy exhaustion, but because from then on we imagined a desert
- the interstellar space! - almost completely empty of matter and therefore also of physical
phenomena to detect and to tell.
Far from the protective embrace of the Sun, the boundary of our solar system seems in
fact to be a cold, empty and dark place.... a vast expanse of frightening nothingness.
But we were wrong.
The Voyager probes have left the Solar System behind, but the great void they had in front
of them is instead proving to be an inexhaustible source of new phenomena.
After the one on the unexpected density of the interstellar medium, given to us by Voyager
2 a few months ago, here is now the joint discovery of Voyager 1 and Voyager 2, which
let us know that in those parts have suddenly detected a real bombardment of electrons.
And this time it was friendly fire, since the particles came directly from the Sun...
Launched back in 1977, the twin probes Voyager 1 and Voyager 2 have done an extraordinary
job in all these years, increasing our knowledge about the Solar System at least tenfold, and
even now they continue to provide significant scientific data.
After visiting all the outer planets, from Jupiter to Neptune, the two aging spacecrafts
are in fact studying the deepest recesses of the Solar System.
For quite some time they have been going beyond the heliopause, the "no man's land" between
the heliosphere - the bubble of hot solar plasma that encloses our entire planetary
system - and the region that surrounds it, dominated by the interstellar medium.
A region that Voyager 1 has already reached in 2012, and Voyager 2, which followed a different
and longer path, in 2018.
Despite their incredible distance of 23 and 19 billion kilometers respectively, communication
with the two spacecraft is maintained by the antennas of the Deep Space Network, and sporadically
reaches us an imperceptible signal whose power does not even reach a billionth of a billionth
of a watt and contains extraordinary data that allow us for the first time to understand
the true nature of what exists outside the heliosphere.
Over the past century, scientists have managed to understand the composition of the interstellar
medium, largely through observations using radio telescopes and X-ray telescopes.
They revealed that it is composed of extremely diffuse ionized hydrogen atoms, dust and cosmic
rays, interspersed with dense molecular clouds of gas that are believed to be the birthplace
of new stars.
But knowing the components of a soup is not the same as knowing how it tastes.
You have to go there and taste it, as they are doing in this moment the two probes, which
are experiencing completely new phenomena in an environment not reproducible in laboratories.
But we were talking about new discoveries...
You will certainly remember that last October, Voyager 2 let us know that the density of
cosmic rays coming from "outside", in practice the interstellar wind that hits the extreme
border of the heliosphere, was much higher than what could be assumed.
This time the discovery concerns instead the wind that blows behind the probes, and that
from the Sun pushes a continuous flow of electrons towards the interstellar spaces.
In practice, it happens that our Sun, although we can say a star quite "quiet" if compared
to some turbulent and dangerous monsters of the sky, is produced sometimes in giant and
sudden releases of energy, resulting in the expulsion of ionized plasma particles that
expand rapidly in interplanetary space.
It's like a cosmic tsunami... a shockwave moving from the surface of the Sun outward,
moving up the solar wind stream.
Something similar happened in 2014, when a giant solar flare kicked off a huge particle
stream.
The explosion crossed the orbits of Mercury and Venus at nearly 800 km per second, and
after two days it reached Earth, fortunately protected by its solid magnetic field.
The next day it hit Mars, and the day after it swept through the asteroid belt, continuing
towards the gas giants: Jupiter, Saturn, Uranus... and after more than two months, Neptune, 4.5
billion kilometers away from the Sun.
Six months passed, and finally the plasma stream arrived more than 13 billion kilometers
from the Sun, in a boundary region known as the "termination shock", where its speed abruptly
reduced by half due to the external pressure of the interstellar medium.
At the end of 2015, the shock wave reached Voyager 2, when the spacecraft was still inside
the heliosphere, and only in 2017 arrived at Voyager 1, which had been sailing in interstellar
space for five years.
The solar wind hit the probes at different regions, distances, and times, which gave
us additional clues about the nature, shape, and properties of the heliopause.
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These encounters at the edge of nowhere have only recently become known to us, since we
were able to process the data that the spacecraft had provided to transmit to us.
But the most important thing, the real news is another: from that same data we learned
that 13 days before Voyager 1 recorded the arrival of the shock wave and 30 days before
Voyager 2 did the same, the two probes had detected the unexpected arrival of a huge
flow of electrons!
In practice, it was as if the shockwave of plasma, once arrived in contact with the interstellar
wind that came from the opposite direction, had suddenly accelerated its electrons to
push them towards the two probes at speeds close to those of light.
A very strange thing indeed.
The idea that shockwaves in certain conditions can accelerate the particles of which they
are formed is not new, only that until now this process had never been observed, but
only theorized.
And the extraordinary thing is that the phenomenon has been detected directly in the interstellar
medium, which is a very different environment from that of our Solar System, both for particle
density and for the lack of physical collision between the solar wind and the interstellar
wind.
In fact, as the authors of the paper reporting the news, published in Astronomical Journal,
point out, this interaction between shockwave and interstellar medium seems to accelerate
the electrons, pushing them forward as if they were messengers of what was coming.
As a result, as has been calculated, the energized electrons begin to move about 670 times faster
than the shock wave that originally pushed them toward the heliopause, meaning that they
are accelerated to near-relativistic speeds.
Basically, it's like seeing light reflected from the glow of a distant explosion and then
hearing the sonic boom come in later.
And in this case, the time interval between light and sound provides important information
about the properties of the interstellar medium and the penetrative capacity of the shock
wave in the interstellar medium.
The discovery could help physicists better understand the dynamics behind shockwaves
and cosmic radiation coming from stars that can suddenly change in brightness due to powerful
plasma emissions on their surface.
To this dangerous typology of stars belongs also Proxima Centauri, the red dwarf at the
center in these months of a particular attention in the SETI program...
It would be important to consider the physics of such phenomena when sending astronauts
on extended lunar or Martian excursions, for example, during which they would be exposed
to concentrations of cosmic rays far greater than those we experience on Earth.
However, more data will be needed to better understand these results, especially from
Voyager 2, which experienced this particular phenomenon when it had not yet left the heliosphere.
And to tell us something definitive will surely be in the future NASA's IMAP mission (Interstellar
Mapping and Acceleration Probe), whose launch is scheduled for 2024.
For the moment we can only count on the precious collaboration of the old Voyager, and the
increasingly feeble messages that still come to us.
But at this point it becomes impossible not to ask the question: how long can Voyager
last?
And in fact the situation does not appear rosy.
The main problem is obviously the energy.
The probes travel thanks to three Radioisotope Thermoelectric Generators that produce energy
using the heat emitted by the natural decay of radioisotopes of plutonium-238; the same
type of generators also used, among others, in the Viking, Pioneer, Cassini and New Horizons
missions, but also during the Apollo 12 mission.
The decay of plutonium atoms provides a truly enormous range, but plutonium also wears out
over time.
It is estimated that the probes have produced about 4 Watts of power less each year and
today, therefore, the generators provide about 40% less power than they produced at launch.
The energy is obviously used to run the on-board systems and scientific instruments, but not
only.
It is in fact also used to "keep warm" the instrumentation.
Recently the engineers of the mission, after careful discussions and in agreement with
the scientific team, have decided to intervene.
In order to maintain the activity of the probes as long as possible, it is necessary to rationalize
the use of energy ... and it becomes strategic to choose carefully what to continue to work
and what instead to cut.
Voyager 2 took precedence in this plan, having one more instrument and therefore also consuming
more energy over the years.
Among the first actions already put into practice is to turn off the ultraviolet spectrometer
and, recently, a heater on the cosmic ray detection instrument, but not the energy to
run it.
In fact, the instrument plays a crucial role in detecting the amount of cosmic rays in
interstellar space.
Information of vital importance for future human missions of space exploration.
The problem of radiation is in fact the main problem for long-duration missions such as
those that humanity is going to undertake.
What just can not risk to be left at the mercy of the cold is for obvious reasons the pointing
system, which allows the probes to orient themselves in the right direction to send
data to Earth.
If the power fails or if the cold damages it, we can no longer receive data or even
communicate with the probes to send new instructions.
The challenge is to understand what to remove energy, which of the two probes, which information
will be more important and which can be sacrificed, to continue to do science as long as possible
with both.
And it has been calculated that if everything goes in the best way, the two probes should
continue to send signals at least until 2025.
And then what?
And then Voyager will be silent, while continuing by inertia to travel farther and farther.
And from then on, they will turn from observers and explorers into ambassadors of our species;
indeed, of all the species on our planet.
Designed at a time when space exploration was a dream, the two Voyagers left carrying
scientific instruments and archaic computers, but also a message thrown like a bottle into
the sea to tell anyone who might encounter them: "Hello, here we are, this is our nature,
and our culture".
A bottle full of images and recordings of wind, thunder, animal cries, crying babies,
literary texts and music of all kinds.
We will never know about them again, but they will be like two seeds let go in the interstellar
wind.