This is the corona, our sun's outer
atmosphere. This fiery hell can reach
temperatures of 10 million°,
but just 1,500 km beneath this inferno.
The surface is significantly cooler.
This arguably makes no sense. How can
the temperature increase as it moves
away from the source? Surely that's the
wrong way round. Known as the coronal
heating problem, this is one of the
greatest unanswered questions in
astrophysics, and it's been baffling
scientists for decades. Over the years,
there's been theories and clues about
its cause, but it's hard to get answers
about something you can't easily see
other than the occasional fleeting
glimpse during solar eclipses. So, the
inner corona has remained largely
unobserved.
But in December 2024, all that changed.
The European Space Agency launched two
tiny satellites that together are
revolutionizing our understanding of the
sun, working in perfect synchrony to
make the once invisible visible.
I'm Alex Mccoan and you're watching
Astramm. Today we're heading into orbit
with the Prober 3 mission as its first
findings are released. We'll explore how
these two minissats have opened up a
window into the sun's most inaccessible
region and get to grips with the
extraordinary engineering that makes
flying in millimeter precision formation
possible.
On the 5th of December 2024, ISA's
Proverb 3 lifted off on a PSLV XL rocket
from the Satish Dawan Space Center,
Shrih Harikotta, India. This mission
didn't necessarily garner headlines, but
it was quietly set to change the face of
solar exploration and break a few
engineering records in the process.
Because Proverb 3 isn't just one
satellite, but two. This pair of fully
autonomous mini sets have to fly in
millimeter perfect formation through a
highly elliptic orbit of Earth all
whilst positioned 1 and a half football
pitches apart. And that's easier said
than done. Why would we want to do this,
you might ask? Well, they were being
sent to study the sun and one area in
particular, the corona. Why is the sun's
atmosphere hotter than its surface? What
drives the charged particles in the
solar wind? And how do billions of tons
of magnetized plasma erupt from our star
in one go? These questions have haunted
solar physicists for decades. And the
answers to all of them are believed to
be hiding in the same uncharted region
of the sun. Yes, the corona. But
observing this outermost part of the
sun's atmosphere has always been a
challenge. It's a million times dimmer
than our stars visible surface, the
photosphere, meaning this region gets
somewhat lost in the sun's light.
Observations aren't impossible, per se.
We do get a fleeting glimpse during
eclipses. These spectacular events offer
an excellent snapshot of the corona, and
over the centuries have not only led to
the discovery of the corona itself, but
also that of ionized iron within. The
first evidence of the coroner's extreme
temperatures. The problem with total
eclipses, though, is that they only last
for a few brief minutes, and they're
rare, only occurring every 18 months or
so. Add to that the fact that the paths
of total eclipses are often in remote
areas or over the ocean, which is pretty
impractical when it comes to using high
power telescopes, as they aren't exactly
what you'd call portable. Thankfully,
though, we do have another alternative.
In the 1930s, a French astronomer called
Bernard Leot started using an occultting
disc to act like an artificial moon,
blocking out the sun's surface to create
a mini eclipse. Instantly, observing
time went from minutes to hours. And
using these new instruments called
coronagraphs, we've been able to keep
almost constant tabs on the corona. We
can track its temperature and
composition and look for changes in the
flow of the solar wind. But yes, there's
a butt. There are once again
limitations. Groundbased chronographs
have to compete with daytime brightness
and atmospheric light scattering, making
the dimmest parts of the corona, or the
bit closest to the sun, almost
impossible to see. It therefore remains
mostly unseen and unstudied.
To solve the mystery of the sun's
corona, we needed a better view. And for
that we needed to get rid of at least
some of the background light. We can't
make the sun dimmer. So the easiest
thing to do is get rid of the
atmospheric scattering. In other words,
we needed to get coronagraphs into
space.
Space-based coronagraphs have
revolutionized our coronal
understanding. On the 14th of December
1971, an early space-based mission of
this kind called the Orbiting Solar
Observatory 7 saw a coronal mass
ejection for the first time. Since then,
we've seen tens of thousands of them.
I'm sure you've seen some of the
spectacular images sent back by the
Solar and Heliospheric Observatory or
SOHO that launched in 1995 and the Solar
Terrestrial Relations Observatory or
STEREO that launched in 2006. I
personally love how you can see the
solar weather events erupting from the
surface when the star itself is blocked
out and how each craft specializes in
different regions above the sun, giving
us one large image when put together.
each of them their own part of the
puzzle. But once again, even
coronagraphs in space have issues. Just
like on Earth, the glare of the sun can
be too strong. Soho, in particular,
cannot see closer than 1.7 solar radi.
And most of the interesting phenomena
start much closer than that. Parker
Solar Probe and Solar Orbiter, both much
closer to the sun, also have chronograph
instruments, but they suffer with the
same issue, as well as internal light
scattering and defraction that degrade
images when the occultter, the bit
blocking the sun, is too close to the
camera lens. And that's something that
you can't really get around on a 3 m
tall probe. Scientists needed yet
another way to look at the sun if they
ever hope to unlock the secrets of the
corona. You may have heard that a total
code astron 5 when purchasing from the dwarf lab store. Now that you're sorted,
how are scientists tackling the
challenges of seeing the corona when 3 m
space telescopes aren't getting the job
done? The solution to put the occulta
further away, specifically on another
spacecraft. And to do that, engineers
had to create the world's first pair of
fully autonomous satellites that can
simulate artificial solar eclipses on
demand. And most crucially, without
having to worry about other solar
radiation, so-called parasitic light,
leaking into the image. Proverb 3 is the
fourth in issa's project for onboard
autonomy, a line of experimental
missions designed to work without
constant human monitoring on the ground.
And this is the most ambitious mission
yet. Two small satellites, one
containing the coronagraph and the other
the occulta fly 144 m apart, holding
their relative positions with
millimetric precision to simulate a
single giant spacecraft. This gets
around the issue other space
chronographs have with the two parts
being too close together. And perhaps
most impressively of all, the idea is
that they're able to do this
autonomously.
That might not necessarily sound overly
impressive, but trust me, it is. In
fact, it's never been done before. Just
to prove my point, let's get into the
nitty-gritty of how the mission works.
First up, the satellites take a highly
elliptical path around Earth. If they
had been launched into a standard low
Earth orbit, the constant need for
corrective thruster use would have
burned up the propellant in as little as
half an hour, making for a very short
mission. Instead, an elongated
elliptical orbit was chosen, starting at
an altitude of 600 km above Earth at its
nearest and reaching the farthest point
of 60,500 km once every 19 hours and 36
minute long orbit.
Imagine it like a roller coaster loop.
At the bottom of the loop near the
Earth, the satellites are moving the
fastest. As they begin to go around the
loop, they slow down from 10 km/s to 1
km/s.
Because of the slower speed, they spend
more time at the top of the loop, or in
this case, they spend more time at the
furthest point of their orbit away from
Earth, called the apogee. This gives
them maximum solar observation time. And
when they're closer to Earth, the pair
are set for a safe flyby. Then as they
approach Apogee, they are given a signal
to move into active formation, taking
about two hours to prepare to observe
the corona. Then like self-driving cars,
only way more precise, the satellites
align themselves using a suite of
absolute and relative positioning
technologies from GPS receivers and
radio links to optical cameras and LEDs,
a laser link, and even shadow position
sensors. Each of these hands off to the
next with increasing precision. It
starts with cameras that recognize the
constellations around them called star
trackers. These allow each spacecraft to
know exactly which direction is pointing
at any given moment during their low
orbit or below 20,200 km. The spacecraft
uses GPS signals and continuously
exchanges ranging information and data
through interatellite radio links. Then
comes the vision based system to help
two spacecraft determine their position
relative to each other. A wide angle
camera on each spacecraft tracks a
pattern of flashing LEDs on the other,
giving a rough first read on distance
and orientation. A narrower camera then
locks onto a smaller LED target,
refining that to around 1 cm of
accuracy. But even a centimeter isn't
precise enough. The occulta spacecraft
fires a laser at the reflector on the
chronograph which bounces it straight
back. That system called the fine
lateral and longitudinal sensor narrows
positioning down to a single millimeter.
And finally, to make sure the shadow is
falling exactly where it needs to, photo
detectors monitor its edges in real
time. If it drifts even slightly, a
correction fires instantly. To keep the
whole system as stable as possible,
neither spacecraft has any moving parts.
Apart from a single rotating filter
wheel on the chronograph, everything
else is locked in place.
With the two spacecraft aligned
perfectly, a precisely controlled shadow
is cast from one satellite to the other,
blocking our stars fiery disc and thus
creating artificial eclipses that allow
us to collect sustained observations of
the sun's corona.
Usually from Earth, the corona can only
be glimpsed naturally for a matter of
minutes during a total solar eclipse.
But the idea with prob 3 was for it to
reproduce the same effect as a solar
eclipse for hours at a time, as much as
5 and 1/2 hours of its nearly 20our
orbit. For the first time ever, these
tiny satellites have opened up the
chance for us to observe the corona on a
consistent basis. But it was all
uncharted territory. Even the slightest
slip in the satellite's formation and
the eclipse is gone. taking with it the
opportunity to study the corona. As the
team turned the spacecraft on for
calibration, no one really knew if it
would all work.
Just as stray light can ruin images on
Earth, it could hear, too. The team
really weren't sure whether or not light
would defract and leak around the edges
of the occulta. Thankfully, though, they
had nothing to worry about. As the first
data came through on the 25th of March
2025, it was clear that the prob 3 team
had achieved the millimeter precision
needed. Andre Zurkov, the chronograph's
principal investigator, said it was so
unbelievable that it just worked from
the first time. Before Proverb 3, we
could see as close as 0.7 solar radi
from the sun's surface. Now, we could
see down to 70,000 km above the sun's
surface, which is just 0.1 solar radi.
Put simply, no other space-based
chronograph can observe the light
scattering of particles in the sun's
corona this close to the sun. Proof of
concept complete. It was time to move on
to the science. The mission carries two
scientific instruments. The first of
which is the chronograph. is called the
ASPIX or the association of spacecraft
for polytric and imaging investigation
of the corona of the sun and it images
both the quiescent and eruptive corona
in other words both when it's calm and
during more specific solar weather
events. The second instrument is DRA the
digital absolute radiometer designed to
measure the total solar irradiance or
TSI. Whilst calibration was ongoing, the
team kept the results under wraps. But
on the 16th of June 2025, images from
the first artificial solar eclipse were
released. And not only were they
spectacular to look at, they gave a
glimpse into the valuable data yet to
come. And by December of 2025, a year
into the mission, the satellites have
created more than 50 artificial eclipses
and provided hundreds of hours of
invaluable observational data about our
sun's corona. So, what has it found?
The corona is where the solar wind picks
up speed before streaming outward across
the entire solar system, eventually
washing over the Proverb 3 spacecraft
and reaching Earth itself. It's a
continuous stream of charged particles
that are constantly flowing outward from
the sun in all directions, filling the
entire solar system. The corona is where
most coronal mass ejections are born.
massive eruptions of magnetized plasma
that when aimed at Earth can knock out
satellites, disrupt GPS, and overwhelm
power grids. Take a look at this
composite image made up of three sets of
images taken over an hour and a half by
instruments on different missions. All
observing the same coronal mass ejection
or CME event on the 16th of July, 2025.
The yellow at the center shows the sun's
disc and lower atmosphere captured in
ultraviolet by an extreme ultraviolet
telescope carried by probe 2, a previous
issa mission that also studies the sun.
In red, it shows the outer corona as
seen by the last code 2 chronograph on
Soho. And in green is the middle layer,
the inner corona imaged for the first
time with this kind of clarity and
continuity by probe the aspix
chronograph filling in the gap that has
existed between those two instruments
for decades. You can watch the CME
forming at the sun's edge, expanding
through the inner corona and pushing
outward into the outer atmosphere in one
continuous unbroken view. And in April
2026, the first science from Prober 3
followed being published in the
Astrophysical Journal Letters. It
revealed that solar wind structures in
the inner corona can travel up to four
times faster than previously thought.
This set of images shows the sun in
ultraviolet light as seen artificially
colored in yellow captured by prob 2.
Surrounding it is a grayscale area
captured in visible light by prob 3. You
can see how the solar wind is moving
away from the sun in all directions, but
in some regions, such as around the
bottom of the video, material also falls
back towards the sun. In the second half
of the video, a coronal mass ejection is
then visible, expanding out towards the
right. What we've learned is that the
inner corona is far busier and more
turbulent than previously expected. It's
filled with tiny, constantly moving
blobs and streams of plasma. Some
flowing outward and some actually
flowing back toward the sun. These
structures are moving at speeds ranging
from 14 to 520 km/s.
So exceptionally quick and 3 to four
times faster than previous instruments
had measured in this region. This means
our existing models of how solar wind
accelerates have been significantly off.
Understanding these small-cale dynamics
is key to answering how the slow solar
wind forms, how the corona gets so hot,
and how solar eruptions are triggered.
All of which directly affect our ability
to forecast space weather. But there's
still a long way to go. Proba 3 is only
at the start of its life and scientists
behind the scenes are scouring the data.
I've been assured that new publications
and findings are already on the way.
That's as long as the precision
formation flying can continue though. I
know it sounds as if the mission has all
gone smoothly, but as ever, it's not
been without hiccups.
Earlier this year, the Proviser team
actually lost the Chronograph
spacecraft. It vanished uncontactable
for a whole month. So what happened?
Well, in Andre's own words, there was a
software glitch and it lost the
orientation towards the sun. And well,
when the solar panel is not looking
towards the sun, then the battery is not
charging. And if the battery is not
charging, then the power is down and the
spacecraft is in trouble. Facing away
from the sun, Proverb 3's chronograph
was unable to enter safe mode to
preserve its battery. And as this
happened at a weekend, the operators
weren't working, so it wasn't corrected
in time. This might sound surprising,
but remember, Proverb 3 is meant to be
autonomous, so it should be able to
survive a weekend unsupervised.
Apparently not. Tracking the missing
half of Proverb 3 using the camera on
the occulting satellite, the team were
eventually able to find the chronograph
and managed to command it to turn
towards the sun. At worst, the two
spacecraft drifted 80 km apart, an
enormous separation for a pair designed
to fly less than 150 m from one another.
Thankfully, both satellites are now
online and back together. They recently
turned both spacecraft back on and it
appears there's been no lasting damage.
As of early June, they are back on their
observing schedule. Perhaps we're a
little way off fully autonomous after
all.
Even having a month-long sabbatical,
Robber 3 has collected more than 250
hours of highresolution video of the
corona over 57 artificial eclipses. To
put this into perspective, the average
total eclipse on Earth lasts for about
7.5 minutes. Proverb 3 has already
captured 2,000 times that. In that time,
it hasn't quite solved the three great
coronal questions. But for the first
time in the history of solar science,
there is an instrument dedicated to
doing so. Watching the solar corona with
the consistency, resolution, and
sensitivity needed to see what is
actually happening. And we really are
only at the start. Andre Sukov and his
team are already preparing the next
batch of images and discoveries to be
released. And in December 2026, Proverb
3 will surpass its nominal mission
period of 2 years. After that point,
researchers from around the world can
begin requesting specific observations
they want Proverb 3 to make, opening up
the mission to the greater global
physics community. Looking beyond solar
science, Proverb 3 is also quite simply
an engineering marvel. No one expected
its precision formation flying to work
as well as it does, and the mission has
gone a long way to proving the
experimental concept of autonomous
satellites flying in precise unity. I
personally am looking forward to seeing
what future missions may use this
technology and whether it can unlock any
of the other big questions that still
linger out in the cosmos.
I really hope you enjoyed learning about
how important eclipses are. We're
actually live streaming the upcoming
solar eclipse on the 12th of August.
We'll see you there at 6:30 p.m. BST and
we'll be online to answer all your
questions, too. See you there.
Thanks for watching. If you've been
enjoying Astramm's videos and want to
help keep this channel thriving, I want
to ask you to take less than a minute to
check out the Astramm Patreon. It's not
just ad free videos, but it's a way to
make Astramm's videos less reliant on
sponsors and algorithms. The link is
below. Your membership directly helps
ensure that future videos can stay
independent, highquality, and
consistent. created for curiosity, not
clicks. Thanks so much for considering
it. I'll see you next time.