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The Solar and Heliospheric Observatory, also know as SOHO, recently celebrated its 25th
anniversary in space.
During these 25 years, it has observed the solar wind, watched out for dangerous coronal
mass ejections, and observed the atmosphere of the Sun.
An unintended consequence of its observations around this region were the discoveries of
over 4000 Sun grazing comets, most of which we had no idea existed until they came into
SOHO’s view.
And SOHO isn’t the only solar observatory, the STEREO spacecraft, and the Solar Dynamics
observatory, have all seen new comets.
And what’s so cool about that is that they weren’t designed with that in mind at all.
I’m Alex McColgan, and you’re watching Astrum, and in this video I wanted to look
at some of the most impressive of these comets, try to understand how they interact with the
Sun, and how the Sun interacts with them.
Sun grazers are comets that do just that, they graze the Sun as they pass, with the
closest parts of their orbits taking them within a hair’s breadth of the Sun’s surface.
Often, they will pass through the Sun’s huge atmosphere, called its corona.
These comets are mainly long period comets, comets whose orbits take hundreds to thousands
of years to complete.
Because of their orbit’s extreme elliptical nature, they build up tremendous speed as
they approach the Sun, sometimes accelerating to 0.2% of the speed of light, an absolutely
incredible speed for a particle, let alone a house sized object.
The majority of Sun Grazer comets actually all originate from one, large comet that was
ripped apart several hundred years ago.
This group of Sun Grazers, shown in red in this video, is known as the Kreutz Sun Grazers.
What tends to happen over time is that the fragments from the larger comet spread out,
meaning that there is a reasonably steady flow of them.
As the largest of these fragments also pass by the Sun, they too may break apart into
even smaller comets.
And the reason we believe most Sun Grazers originated from the same comet?
Well, they all tend to follow to same orbital path.
The brightest comet in the last millennium, known as Comet Ikeya-Seki, was probably a
fragment.
It was so bright as it approached the Sun that it could even be seen during the day.
You may have heard of another famous fragment of this comet that dimly illuminated the sky
in the southern hemisphere in 2011, called Comet Lovejoy.
While Comet Lovejoy wasn’t as bright to the naked eye, it did make for some very impressive
long exposure images, and was seen by all the Sun observing satellites.
Comet Lovejoy was not expected to survive this close encounter, as it would have been
in the Sun’s one-million-degree Celsius corona for more than one hour.
However, astonishingly, it fizzed away from the other side of the disk, mainly intact,
although probably severely impacted from the experience.
The same thing happened to another comet, Comet ISON.
You may remember that comet ISON was expected to be a bright comet, potentially visible
to the naked eye when it passed by the Sun in 2013, alas that wasn’t to be.
However, it still made for good viewing for the Sun observing satellites.
ISON’s approach was bright and impressive, and upon reaching the other side of the Sun,
it faded out.
Scientists can’t be sure if the nucleus survived or not, but if it did, there are
certainly no volatile substances on it anymore, all that’s left is probably dust.
However, these are some of the biggest Kreutz Sun Grazers we’ve ever observed, their nucleus’
maybe being a few hundred metres in diameter.
Most of the smaller comets were not known about until they actually came into the view
of a sun observing satellite.
Due to their small size, being only 10s of metres across, many of the smaller comets
are completely snuffed out by the Sun immediately after passing by too closely.
Which means unfortunately, they were vapourised pretty much immediately after they got discovered.
Sometimes they will pass around the back of the disk of the Sun, never to re-emerge from
the other side.
Although at other times, the angle of the comets orbit means we can witness this vaporisation
as it happens.
At this distance from the Sun, the heat is incredible, and the gravity is overwhelming.
The icy comets not only evaporate quickly but the rocky elements of them are also ripped
apart from tidal forces.
Our own close encounters with comets show that they tend to be structurally weak and
very porous, sometimes nothing more than a pile of rubble held together by its own gravity.
So, combine that with the influence on the Sun, and even the largest sun grazer comets
will come away heavily scarred.
Now there’s an interesting phenomenon that sometimes happens when a Sun Grazer passes
by the Sun, and that is that a CME will go off at the exact moment the comet passes by.
There are numerous examples of this.
However, scientists are still of the opinion that there is no mechanism for a Sun Grazer
to cause a CME, these comets simply aren’t big enough to have any consequential impact
on the Sun, so it is currently believed that these examples you see here are purely coincidental.
What scientists enjoy about Sun Grazers though is that while we can’t send probes deep
into the Sun’s corona, it’s simply too hot for that, we do have these thousands of
comets that are willing to take the plunge for us.
And comets are perfect for us to observe what we are looking for, which is to better understand
the magnetic fields within the corona, so that we can better predict CMEs and space
weather generally.
Look how as a comet passes by its tail wiggles.
The particles in the tail get heated so much, they turn into a plasma which can be seen
easily by the UV cameras of the satellites.
Plasma reacts strongly to magnetic fields, so the wiggle you see in the tail is believed
to be due to the way in which the tail interacts with the magnetic field lines in the corona.
Currently, space weather is something we don’t have a complete understanding of, so as more
comets pass through, the more we will begin to understand that environment.
So, there we have it, some of the most impressive looking Sun Grazer comets caught on camera
by satellites that weren’t even designed for them.
Have you ever been fortunate enough to see a comet?
What was your experience like?
I’d be interested to hear your stories in the comments below.
As I mentioned, understanding the Sun’s Corona has very real implications for us here
on Earth.
That’s because the Sun’s Corona is the source of space weather and solar wind.
You see, on Earth our electronics are actually quite vulnerable to the particles ejected
from the Sun.
If a sudden burst of particles heads our way in what is known as a coronal mass ejection,
they can damage our satellites and in extreme cases, knock out our power grid and internet.
There’s a documentary on MagellanTV that discusses this in detail, called Solar Superstorms,
which looks at some of the largest solar storms on record, and how we are preparing for the
next big one.
You can watch this documentary and gain access to MagellanTV’s library of thousands more
by using my link in the description, which will give you a month free trial.
This was a really high-quality documentary, plus it is in 4K, so I highly recommend checking
it out.
Thanks for watching!
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