2011 was an exciting year for astronomers. For over 200 years mankind had known about
the existence of gigantic asteroids found in the asteroid belt, but for most of that,
they’ve only been specks in the night sky. The first asteroid to be discovered, Ceres,
was found in 1801, and was added to the list of planets. A year later, Pallas was discovered, and in
the following years Juno and Vesta. Because of this, in 1845, our Solar System
had 11 planets, the original 7 from Mercury to Uranus, as Neptune hadn’t been discovered
yet, plus the 4 asteroids. As more and more asteroids were discovered,
it became clear they couldn’t all be listed as planets. A good thing too, as today there are millions
of known asteroids of various shapes and sizes. However, before 2011, we had never seen any
of the original 4 asteroids up close. Enter the Dawn spacecraft. Launched in 2007, it had a very special mission,
to explore and investigate not just one, but two of these large asteroids. First Vesta, and then Ceres. But what did it find and discover while it
was there? I’m Alex McColgan, and you’re watching
Astrum, and in this video, we will find out everything Dawn saw and discovered around
the asteroids Vesta and Ceres, and examine what made the Dawn spacecraft one of the most
technically impressive probes ever produced, and how it’s success paved the way for NASA
missions in production right now. Let’s start with Dawn itself. It’s trip to Vesta took four years, utilising
a slingshot from Mars’ gravity along the way. And you’ll be forgiven for not seeing anything
special about this trip from this perspective. However, zooming in on the Dawn spacecraft
itself reveals its very special feature, an ion engine. Ion engines had been tested already on NASA’s
Deep Space 1, however the way Dawn utilised it pushed this technology to a whole new level. Thanks to its ion engine, it was the first
ever spacecraft to go into orbit around two separate extra-terrestrial bodies. You see, while ion thrusters aren’t very
powerful, they are extremely efficient, and so can remain on for extended periods of time. Unlike chemical thrusters, which rely on reactions
causing heat and pressure to push gas away from the rocket, ion thrusters simply ionise
neutral Xenon gas with electricity to create acceleration. These ionised gas particles rush out of the
engine at 150,000 km/h, which pushes the spacecraft in the opposite direction. As the ionised gas is expended slowly – Dawn
can only create so much electric charge after all – the acceleration is really slow. It would take Dawn 4 days to accelerate from
0 to 100 km/h. But over extended periods, it really adds
up. Dawn was firing its thrusters for 85% of the
time during the transit to Mars, expending only 72kg of xenon propellant, and gaining
over 1.8km/s velocity. Upon reaching Vesta in July 2011, the images
it started returning wowed the science community. It really was not what they were expecting. After being captured by Vesta’s gravity,
Dawn lowered its orbit to get a closer look at this unique asteroid. So, what made Vesta an expectedly nice surprise? The first thing you’ll notice about Vesta
is that it has an unusual shape. It kind of looks like a squashed ball. There are two reasons for this. The first is that it is not very big. Yes, these asteroids, although big for asteroids,
are pretty tiny on astronomical scales. Vesta is not quite big enough for it to be
in hydrostatic equilibrium, or in other words, to be rounded by its own gravity, as it is
only about 500km in diameter. This gives it the surface area of Pakistan,
about 800,000 sq km. You’ll see how small it is if you compare
it to our Moon. Although it should be noted that even at this
size, it still contributes towards 9% of the total mass of the asteroid belt, which can
help you appreciate just how dispersed the asteroid belt really is. The second reason for this unusual shape is
the two giant impacts it experienced in its past. Estimated to have occurred over a billion
years ago, Vesta was impacted not once, but twice around its south pole with planetary
scale objects. These impacts produced craters so large, they
penetrated all the way to the mantle of the asteroid. The crust has since cooled off and solidified,
leaving a complex crater called Rheasilvia. As these craters have overlapped, Rheasilvia
is the most recent and thus the most prominent crater that remains. As is typical with complex craters, terraced
walls can be seen around the edge of the crater, seen in the form of huge troughs around the
equator that put the Grand Canyon to shame in terms of size. Also typical of complex craters, a prominent
peak can be found at the centre. This peak was once thought to be the tallest
mountain in the solar system, but more accurate Dawn observations shows that the title has
returned to Olympus Mons, although Rheasilvia is still 20-25km high and over 100km across. Due to its size compared to the size of Vesta,
it appears like a giant pimple around the South Pole, and it is easily visible from
orbit. The two collisions that carved out the South
Pole of Vesta flung a massive amount of ejecta into space. So much debris ended up in the asteroid belt
that this debris has been given its own asteroid spectral type classification, namely V-Type
asteroids. V-Type asteroids are thought to have originated
from Vesta, and a lot of them can be traced back to those impacts. But these asteroids didn’t just end up in
the asteroid belt. They are scattered all across the solar system,
and in fact, 5% of the meteorites that end up on Earth come from Vesta, known as howardite–eucrite–diogenite
meteorites. This is very handy, as we haven’t needed
a sample return mission to be able to study samples of Vesta, because Vesta has delivered
some right to our front door. Their structure and composition reveal some
clues about how Vesta was formed, and the Dawn mission attempted to broaden that understanding. From a combination of all the data collected,
it has been revealed that Vesta is very unique in our solar system. It is the only remaining rocky protoplanet,
or in other words, it is a planetary embryo that never finished forming. The theory goes that as the solar system was
forming, dust from the early protoplanetary disk coalesced into thousands of different
planetesimals. These planetesimals collided with each other
over time, building up into the large planets we see today. Our moon is thought to have formed from such
an impact with a large planetesimal impacting Earth, called Theia, the debris from the collision
coalescing in Earth’s orbit, and over time rounding under its own gravity to form the
Moon we know today. Vesta obviously got started on its way to
becoming a planet. It experienced plenty of large impacts with
planetesimals. As a result of all these impacts, the heat
generated by them meant that at one-point Vesta had an active mantle under the surface. Even today it is believed to still have a
core of iron about 220km in diameter, a core similar to the other terrestrial worlds like
Mercury, Venus, Earth and Mars. However, Vesta’s interior has since cooled
off, meaning the interior has solidified. But because of this differentiated interior,
it likely would be called a dwarf planet today if not for those two collisions we talked
about earlier. One of the criteria for a dwarf planet is
that it is rounded under its own gravity. But as the collisions happened roughly one
billion years ago, a few billion years after Vesta formed, Vesta had already cooled off
too much for it to be elastic enough to return to a shape in hydrostatic equilibrium. And the reason Vesta never became a planet? Fingers are currently being pointed at Jupiter,
which stole mass that would have otherwise formed Vesta, or at least disturbed enough
of it to stop Vesta from ever getting going. Now, to the naked eye, Vesta does appear quite
bland. This is a true colour image of Vesta, appearing
as you would see it. However, if you have a camera that can see
in a wide variety of wavelengths of light, suddenly Vesta’s true variety becomes apparent. In this composite image, the black material
is likely ejecta brought by a large meteor impact, the red material is likely also from
an impact, but material that melted before solidifying again. Dawn also made some unexpected discoveries
on Vesta’s surface. Vesta is thought to be very dry, with little
to no volatiles found in its crust. Why then has evidence of past flowing been
observed? In this false colour image, you can see a
crater a couple of kilometres across with a flow channel coming out of it, the different
colours indicating it consists of a different material to the surrounding area. The exact origin of this material is unknown,
but perhaps it was brought by the impactor and melted on collision. Another fascinating discovery was found in
one of Vesta’s youngest craters, Marcia. Near the bottom of the crater, Dawn observed
something called pitted terrain. Why pitted terrain was found of Vesta is a
bit of a mystery, as we had only seen it on Mars before that. However, scientists believe hydrated mineral
rocks on the surface may have been rapidly heated, perhaps by another impact, releasing
the water in the rocks, which exploded as the water degassed into space, leaving these
craters you see here. And I would be remiss to mention that this
Marcia crater is part of a chain of craters which makes up the famous “snowman” found
on Vesta. What is interesting though is if you notice
the terrain is relatively smooth around these craters. That is believed to be because a blanket of
ejecta covered the region from the impacts, smoothing it over. Dawn was only around Vesta for a year before
it left Vesta’s orbit and moved on to the second leg of its journey towards the actual
dwarf planet, Ceres. Using its ion engine, it was able to build
up speed until it was finally fast enough to escape Vesta’s gravity altogether, and
then begin its journey towards Ceres. The anticipation within the scientific community
to reach Ceres was palpable. Even with the aid of the Hubble Space Telescope,
the best image we had of Ceres was this. It was still a mysterious body. What was lying in wait there? Would Dawn hold up to years in the unforgiving
environment of space? And what would Ceres reveal about our own
solar system? After 2 years in transit between the two bodies,
Dawn finally began the approach to Ceres. As days passed, the resolution of Ceres got
better and better. Details like craters could finally be resolved,
and most interestingly of all, bright white dots could be seen. I remember at the time that as these images
were coming in speculation was rife about what they could be. As higher resolution images were received,
it looked like the brightest spot was actually two separate spots, and then the increased
resolution revealed it was in fact several different spots. Dawn also observed the scarred nature of Ceres
up close, with craters littering the surface. Although, there aren’t as many craters here
as previously expected. Taking this into consideration and the bright
spots I already mentioned, it became clear that Ceres is not as inactive and inert as
we may have previously thought. Before we delve deeper into that, let’s
first give you some context. Ceres is a very unusual body, seemingly out
of place in the asteroid belt. Most asteroids are mainly composed of non-volatile
substances, mainly rocks and metals. Ceres, on the other hand, has a similar composition
to that of a comet, it is, in other words, an icy world. However, being this close to the Sun, any
ice directly on the surface sublimates. This means that the surface crust is rocky,
yet porous, with water locked into the gaps, with a ratio of about 90% rocks and 10% water. Beneath the surface, there is believed to
be a muddy mantle and a large core of hydrated rocks, such as clays, where rock and brine
are mixed together at a 50:50 ratio, although this has been hard to confirm. Other models suggest the core could be a lot
drier and smaller, with a greater ratio of water to be found in the mantle. Either way, water is definitely present in
Ceres in large quantities, making up perhaps 50% of its total volume due to Ceres’ low
density. And it’s this water that perhaps renews
the surface of Ceres, albeit over extremely long timescales. You see, these bright spots are what is known
as cryovolcanoes. Unlike regular volcanoes, which spew lava
out from the mantle, cryovolcanoes erupt water. This is a 3D model based on Dawn data of the
biggest bright spot on Ceres. Water on Ceres is packed full of salt, meaning
that when a cryovolcano on Ceres erupts, the water sublimates, and the salt is left behind. This was directly observed over the brightest
patch on Ceres, known as Spot 5, as a haze was periodically seen over this area, indicating
that water there had sublimated. These bright spots darken over time from exposure
to the Sun through space weathering, so it’s likely that many old cryovolcanoes exist on
the surface of Ceres, although we can now only see the most active and most recent ones. However, the ones we can see aren’t just
limited to just the bright spots we’ve looked at so far, there are many of them dotted around
the dwarf planet. Water was discovered in other regions around
Ceres too. Due to Ceres’ very minimal axial tilt of
only 4°, some craters at its poles are in perpetual darkness. The bottom of these craters never see direct
sunlight, meaning water ice can exist here without sublimating. This is very similar to our Moon, where water
ice is also thought to have been trapped at the bottom of perpetually dark craters for
billions of years. What else did Dawn spot on the surface of
Ceres? As I mentioned, it saw plenty of craters,
each with unique characteristics. Some were very round and defined, others had
scarps along the crater floor. There were plenty of examples of complex craters,
with tall peaks in the centre. There was even evidence of crater rims collapsing,
for example, this rock having fallen away from the crater walls. Smaller rocks were also spotted having fallen
down crater walls, evidenced by the trails they left behind. I also love these photos taken at an angle,
pointed along the limb of the dwarf planet. To me, these give a much better idea of the
scale of the features we are looking at, although without an atmosphere to provide a sense of
depth, it’s hard to judge sizes in images like these. Fractures were also spotted all over Ceres,
indicating there have been stresses in the crust. Some of them are relatively young, perhaps
only a few hundred million years old. Some of them come in the form of grooves and
troughs, where the crust has been stretched, and other fractures can be seen in the form
of rows of mountains, where the crust has been compressed. One particularly unusual feature was spotted
on Ceres, called Ahuna Mons. It’s a mountain about 20km wide and 5km
high, but what’s unusual about it is how it just sticks up from the surrounding area,
with no apparent cause. It would be less unusual if there were other
features like this on Ceres, because then it could be said that it’s a global phenomenon,
but it’s the only thing like it on the entire world. The best bet we currently have is that it
is an old cryovolcano, formed because of a large impact directly on the other side of
the dwarf planet. Seismic waves from large impact can propagate
through the crust of a planet, and where the waves meet again on the opposite side is known
as the antipode. Known antipodal regions around the solar system
tend to have some kind of weird terrain, like that found on Mercury. Ahuna Mons could fit this description, with
seismic waves from an impact on the opposite side of Ceres triggering volcanic activity
here. If it is a cryovolcano, it actually has some
analogues around the solar system. It would be classified as a dome volcano,
similar to Mt. St. Helens, or to some domes seen on Mars. In 2018, Dawn concluded its mission, having
been a tremendous success. It finally ran out of propellent, which meant
it could no longer stay pointed at Earth to send back data or receive commands. It’s been left in a stable, derelict orbit
around Ceres, a monument in space that will remain there for at least another 20 years. Dawn’s discoveries and data will be at the
heart of asteroid research for many years yet, but it also leaves behind another legacy. Its ion engines were the key to its success,
and there are now many other missions that currently use them. SpaceX’s Starlink satellites have ion thrusters
onboard, as well as China’s Tiangong space station. ESA’s BepiColomobo mission are using them
to get to Mercury, and NASA’s DART mission is using them to get to the binary asteroid
system Didymos and Dimorphos. If you want to know more about that – in
my opinion - intriguing mission, check out my video I made about it here. Thanks for watching! Want to help pick the next supercut video? Go to my channel page on the community tab
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