Over the years, we’ve gotten to know the solar system quite well. The inner, terrestrial planets;
the outer, gas and ice giants; the asteroid belt that cuts through the middle of it all. But one
of the solar system’s greatest mysteries still remains the star at its heart. We know that the
sun is an incredible source of energy, and the provider for life on Earth… but there are still
plenty more questions about this fiery, explosive mass left to answer. And now we’re getting as
close as possible in our pursuit of knowledge. This is Unveiled, and today we’re answering the
extraordinary question: did a NASA probe really just touch the sun?
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The Parker Solar Probe stands as one of the most unique and interesting NASA missions
in recent times. While so many space probes tend to head outwards from Earth, to explore
the rest of the solar system - and, indeed, universe - Parker flies in the other direction.
Toward the gravitational centre of the solar system, much further in than even the
innermost planet, Mercury, to our sun. Parker launched on August 12th, 2018,
and is fast approaching the halfway point of its planned seven-year mission. And, while
the sun is its target for research, it’s not as though it just beelined straight for it. Rather,
Parker has made (and will continue to make) use of multiple gravity assists around Venus.
This means that it cuts a tighter and tighter orbital path through the solar system, toward
and away from the sun, but taking it progressively closer to the star with each repetition. As such,
on October 29th, 2018, just eleven weeks after it left Earth, Parker passed closer to the sun than
any other artificial object in history - beating a decades old record set by the Helios 2 probes,
back in 1976. Since then, though, it has moved nearer and nearer, breaking new ground,
until in December 2021 news broke that Parker had completed one of its most
audacious mission goals by touching the sun. To understand what this means, we need to first
look at how the sun is structured. Our star might look like just a flaming ball of fire,
but it’s actually made up of multiple layers. In the very centre there’s the core… on top of that
there’s the radiative zone where energy releases outwards from the core over thousands of years…
and then there’s the convection zone, cycling that energy toward the surface. These take up
most of the sun’s bulk, but then we reach the outer layers - the photosphere, chromosphere,
transition region, and the corona. The photosphere is sometimes known as the solar surface,
although the sun doesn’t have a solid surface like terrestrial planets do… the chromosphere is
a little above the photosphere, where temperatures actually rise the further out into it you go…
and then the transition region is a thin zone marking yet another temperature increase.
Finally, the corona encases it all, a wispy but blistering outer layer of atmosphere,
with temperatures now hitting around 900,000 degrees Fahrenheit. And it’s here that the
Parker Solar Probe has now reached, which is why it can claim to have touched the sun.
Parker first breached the Corona on April 28th, 2021, during its eighth solar approach,
although the news wasn’t announced until December 14th, 2021. According to NASA, the probe flew
“through the sun’s upper atmosphere - the corona - and sampled particles and magnetic fields”.
The Agency has also described the feat as a “monumental moment” and “one giant leap for
solar science”. It represents such an important breakthrough because until now we’ve only
ever been able to study the sun from afar, but thanks to this mission we’re starting to gather
first-hand data from the star itself. And, indeed, the corona isn’t even visible to us under usual
conditions, although we know it’s there… so Parker is truly lifting a veil of mystery on that front.
As far as findings go, it’s still early days. Parker only spent a few hours in the corona
on this, its first visit, and there hasn’t yet been anything like a huge revelation of
brand-new science to make us look at the sun in a totally different way… but scientists do expect to
gradually gather more information over the coming months. Upon announcing Parker’s latest milestone,
one Thomas Zurbuchen - the associate administrator for the Science Mission Directorate at NASA HQ
in Washington - said that not only would Parker “provide us with deeper insights into our Sun’s
evolution” but that it would also “teach us more about the stars in the rest of the universe”.
One thing that has already been readdressed is the Alfvén critical surface… which is the
boundary between the corona and the rest of the solar system. While solar wind containing solar
particles does, of course, spread throughout the system, with solar flares carrying a particular
threat to Earth and the other surrounding planets, there is a kind of outer edge to the sun itself. A
final port of call before the most energetic solar particles pull away from the gravity of the star,
and get released as solar wind. But Parker has reportedly found that outer edge - the
Alfvén critical surface - not to be round and uniform, but jagged and staggered,
instead. This had already been predicted… but that prediction is now supported by first-hand data.
Another property of the sun that’s already been granted deeper analysis thanks to Parker
is switchbacks. These refer to a repeating pattern in the solar wind released by the star,
wherein solar waves appear to briefly double back on themselves before rushing out into space. This
results in a steady, zigzagging, ripple effect, and scientists still aren’t wholly sure why the
sun releases in this way. It was Parker that first provided any direct evidence of them, during an
early flyby of the sun in 2019… from which point our star, in our minds, has been imbued with a
kind of pulsing rhythm. It clearly isn’t a random pattern, so there has to be a reason for it, but
the debate continues as to what that reason is. Parker’s more recent contribution is to seemingly
discover that switchbacks originate much closer to the solar surface than originally suspected.
At a much lower level of the solar atmosphere. In this case, as with so much about how the sun
works, we’re really only just beginning along the road to solar enlightenment. On a basic level,
we’ve understood for centuries that this shining ball of light in our sky is also our life-giver.
A spectacular celestial entity that floods out energy, a tiny fraction of which manages
to travel the 93 million miles between us and it to reach whichever side of Earth happens to
be facing it at the time. But, while we have staged various missions in the past to get
to know the sun better, the Parker Solar Probe is breaking new ground over and over again.
In April 2021, when Parker first breached the Corona, it was still some 8.1 million
miles away from the solar surface itself. It has since passed even closer than that,
to within six million miles. But, over the coming years, as the probe continues to orbit the sun,
that distance is predicted to be cut again to just 3.8 million miles by the time of its closest
approach and mission end in 2025. It’s one reason why this mission has been variously described as
one of the most exciting space projects in modern times, because although records have already
fallen and a wealth of new data has already been captured, it still has years left to run.
By the end of Parker’s unique journey through space we will have learnt even more about the
solar system and the star at its centre. While most other space missions look outwards with
wonder at the rest of the universe, with Parker NASA has bucked the trend to get a grip on what’s
ultimately one of the most important locations in all the cosmos from our point of view.
We know that the sun won’t be there forever… that in around one billion years’ time
it’ll most likely become too hot for life on Earth to withstand,
on its way toward expanding into a red giant in about five billion years’ time. But, between now
and then, there’s ample opportunity to remove the mystery of our solar centrepiece, and to improve
our knowledge of stellar science in general. The Parker Solar Probe is leading the way, on
the frontline of this new frontier. And that’s how a NASA probe really has managed to touch the sun.
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