In just a couple of days’ time from the making of this video, NASA will be crashing
its only Saturn orbiter into the planet.
As Cassini enters Saturn’s atmosphere, it will aim its antenna towards Earth, transmitting
data about the composition of the atmosphere in real-time, until the thrusters can no longer
keep the probe steady due to the atmospheric drag.
Cassini will then burn up in Saturn’s atmosphere, ending a 20 year long mission from launch
to final descent.
This spacecraft has been an absolute trooper, surviving and performing in the harsh environment
of space, and personally I will be sad to see it go.
The legacy of Cassini will be felt for years to come though, as scientists are able to
trawl over the huge amount of data Cassini has collected during its mission.
During this final part of Cassini’s mission called the Grand Finale, it has been able
to approach the planet closer than ever before, darting in between the rings.
But what has it seen?
Has this unique perspective shown anything we’ve never seen before?
Well, starting with the moons of Saturn, it has seen some of the shepherd moons in unprecedented
detail.
This is a close approach of Atlas, a 40km wide moon near the outskirts of the A ring.
What looks remarkable about this moon is the lack of impact craters on its apparently smooth
surface, making it look absolutely bizarre.
Dust from the rings is collecting over the surface, particularly around the equator of
the moon, smoothing it over and giving it this disk shape.
A similar thing happens with the second-innermost moon of Saturn, Pan.
At 30km wide and found in the Encke Gap, any particles from the rings that stray into the
350km wide path get swept up by Pan.
This keeps the Encke gap steady and constant.
Daphnis is another shepherd moon, sadly not seen in quite so much detail.
But due to the gap it is located in, its effect can be seen for hundreds of kilometres.
It is only 8km in diameter, and is found in a very narrow gap in the A ring called the
Keeler gap.
Its gravity is very weak, but is just enough to whisk the nearby dust particles as it brushes
by.
This creates these waves, or ripple effect in the nearby rings, sometimes even ripping
material directly out of the ring, visible in this little trail here.
Not only do these ripples move side to side, but up and down too as can be seen by the
shadows they create.
I can only imagine what it would be like to sit on Daphnis and watch as waves follow its
orbital path, with glorious Saturn and its many other moons in the background.
It would be quite the sight to behold.
Talking of the rings though, Cassini has been able to capture some spectacular images.
One of my favourites from the Grand Finale is this one, showcasing the Janus 2:1 spiral
density wave.
Amazingly, what you’re looking at here is the result of the same process that creates
spiral galaxies, just much more tightly wound.
What appears to be many separate rings, is actually only two spiral arms looping around
the planet many times, so every second line you see in the image belongs to the same spiral
arm.
This image is part of the B ring, at a position where the ring orbits twice for every one
orbit of Saturn’s moon Janus, causing an orbital resonance.
The photo gives the illusion that the image is tilted away at the top left, but this isn’t
the case.
The illusion is caused by the way density waves propagate from the planet, the wavelength
decreasing with the distance from the resonance.
And this is where this resonance gets even more mind-blowing.
Janus, the moon that contributes to the resonance, switches orbital positions every four years
with its close neighbour moon, Epimetheus.
Every time this switch takes place, the rings respond, creating a new crest in the waves.
NASA says “The distance between any pair of crests corresponds to four years' worth
of the wave propagating downstream from the resonance, which means the wave seen here
encodes many decades' worth of the orbital history of Janus and Epimetheus.
According to this interpretation, the part of the wave at the very upper-left of this
image corresponds to the positions of Janus and Epimetheus around the time of the Voyager
flybys in 1980 and 1981".
This encoding reminds me a bit of a tree trunk encoding how many years it’s been alive
by the amount of rings it has.
Simply amazing.
Apart from some other beautiful and detailed images of the rings, other interesting sightings
have been these little propeller features dotted around the rings in a number of locations.
This image shows both sides of the rings, the top image shows the illuminated side and
the bottom the unlit side.
Even though the scale of the image is only about 500m per pixel, the moonlet might not
even be able to be resolved.
You might just be able to see some trace of it in this top image, but what can be seen
is that the moonlet is physically connected to the rings by this band of materials.
As I mentioned, this wasn’t the only moonlet trying to create a gap in the rings.
Here is another found right next to the Encke gap, and here is another and probably the
biggest out of all three.
None of these moonlets are thought to be bigger than 2km and probably have the density of
a snowball.
The last interesting thing Cassini has been able to image within the rings is extremely
small but solid objects which have formed around the F ring, potentially caused by the
perturbations of some of the shepherd moons around there.
They seem to be solid as they have survived crashing into the F ring a number of times,
kicking out dust and particles which sometimes then even follow their orbit, as can be seen
by the haze around them.
The objects themselves are not actually visible due to the dust obscuring the view.
Lastly, let’s look at the planet itself.
As Saturn’s northern hemisphere is in full summer at the moment, its remarkable hexagon
around the pole is in full view.
In the centre of the hexagon is found a permanent polar vortex with the eye wall of a massive
hurricane.
Interestingly, the pole seems to be changing colour with the season, as you can see quite
clearly in comparison to 2012 where the pole appeared quite dark in colour.
With the assistance of other wavelengths of light, other storms are visible and can be
seen dotted all over the planet, as well as bands reminiscent of Jupiter, just not quite
so vivid in natural light.
Also, because of the proximity of Cassini to the planet, it is able to get a good look
at the planet’s horizon.
On the left of the image can be seen a haze in the stratosphere of the atmosphere that
disappears towards the right of the image.
When Cassini does enter the atmosphere on its final approach, it is thought it will
not survive to even reach this haze.
Because of the composition of Saturn, it is not expected Cassini will be able to see much
more detail of the atmosphere than what you would find in this image, but what is of particular
interest to scientists is what the atmosphere consists of.
And sadly, that will be the last thing Cassini will ever send back to Earth before burning
up in the atmosphere.
And there we have it.
A final look at Cassini’s grand finale.
I would just like to extend my thanks to the Cassini team that made all of this space exploration
and discovery possible.
We have seen and learned things that would have never been possible if it wasn’t for
these intelligent and dedicated individuals, and I can only hope the discoveries will continue
as Cassini’s data is further analysed and also future missions get underway.
If you would like to learn more about Cassini, Saturn and its moons, then check out the playlist
I have for them here.
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