This episode of Real Engineering is brought
to you by the CuriosityStream & Nebula bundle deal. Watch until the end of the video to
see a trailer for our upcoming Nebula Original series, The Battle of Britain. For
the first time in human history, a spacecraft has flown through the atmosphere of the Sun.
Sweeping through the super heated particles of the Corona. A momentous moment that has
provided scientists vital information on the nature of our closest star, that may help
us unlock its mysteries. The Sun’s atmosphere, much like our own
atmosphere, is composed of layers. Where we have the troposphere, stratosphere and mesosphere,
the sun has the photosphere, the chromosphere and the corona. [1] We have a saying: hotter than the surface
of the Sun, but the surface of the sun, the photosphere, is not actually that hot.It ranges
from about 6200 degrees celsius (6500 Kelvin) at the bottom and 3700 degrees celsius (4000
Kelvin) at the top. That’s about the same temperature as a welding
arc [2], and the air around a lightning bolt can reach temperatures 5 times hotter than
the photosphere. [3] What’s weird, is that the outermost layer
of the Sun’s atmosphere, the corona, is much much hotter than the photosphere. The
corona, starting about 2100 kilometers above the surface of the sun, reaches half a million
degrees celsius, 80 times higher than the surface. That’s like walking away from a fire and
the temperature getting warmer as you get further away from it. This is a strange anomaly that makes the Parker
Solar Probe’s achievement even more impressive, as at first glance it’s easy to dismiss
that the probe “only” flew through the upper atmosphere [4], when in reality the
upper atmosphere is vastly hotter than the surface. The reason this occurs is one of
the many unsolved mysteries of the universe, and as such, is one of the primary missions
of Parker Solar Probe. To gather information on the magnetic fields and charged particles
in this region, and attempt to answer this riddle. To understand the achievement of the Parker
Solar Probe, let’s dive into the engineering and physics of its solar mission. The first
big problem facing the Parker Solar is actually reaching the Sun. Despite the Sun’s gravity
acting as the anchor for our entire solar system, getting close to it is not easy. In order to bring a satellite out of orbit
around the Earth, we have to shed its angular momentum so that it falls back to Earth. The
same has to be done when trying to bring something out of orbit around the Sun, except in this
case, we are located 1 AU, or 150 million kilometers from the Sun, and traveling at
30 kilometers per second [5]. And anything launching from Earth will be imbued with the
same orbital velocity around the Sun. This means in order to achieve a tighter orbit
to the sun we need to lower the spacecraft's orbital velocity around the Sun, and this
is an incredibly energy intensive task, especially when you add the energy needed to escape earth’s
gravity. So, let’s say we want to first get our satellite
from the Earth's surface into orbit around Earth, this will require us to accelerate
our satellite by around 9.2 km/s kilometers per second relative to Earth’s surface.
The satellite is now in orbit around earth, and traveling with it at 30 kilometers per
second around the Sun. From here we need to perform something called
a Hohmann transfer. This is an orbital maneuver where we either change the spacecraft’s
orbital energy to adjust it’s perihelion, its closest approach to the Sun, or it’s
aphelion, its furthest approach from the Sun. To visit an outer planet, like Mars, we want
to increase our aphelion by adding to the spacecraft’s orbital energy. While reaching
an inner planet requires us to reduce our perihelion by decreasing our orbital energy.
[6] To reach Mars from earth’s orbit requires
a delta v of around 2.9 kilometers per second. To reach Venus requires around 2.5 kilometers
per second. This is found using this equation: Where mu, this greek letter that looks like
a u, is the planetary parameter of the sun, which is a product of the Sun’s mass. R1
is the orbital radius we are starting from, in this case Earth’s distance from the sun
at 150 million kilometers, and r2 is the desired perihelion or aphelion. If we calculate the
delta v required to reach the solar parker probe's closest approach of 6.2 million kilometers.
This would require a delta V of 21.4, over 8.5 times greater than the delta V required
to reach Venus. That’s an incredibly high delta V. One beyond
the capabilities of any rocket ever made. But, on December 8th 2018, the Parker space
probe launched from Cape Canaveral aboard the Delta IV heavy, the world’s second highest
capacity rocket in the world, second only to the Falcon Heavy. [7] To give the probe an extra push, the Delta
IV was fitted with a special solid rocket third stage, providing an additional 3 km/s
of delta v for the typically two staged rocket design. [8] Yet, even with this added power, the probe
would never have gotten close to the Sun. To achieve it’s record breaking flight,
one seventh that of the previous record hold of Helios 2, the Parker Solar Probe completed
an astounding 5 gravity assists by Venus with an additional 2 flybys due in 2023 and 2024. This number of flybys was needed because Venus
is a relatively low mass planet. The magnitude of velocity change a planet
can provide is largely determined by its gravity, determined by its mass. In fact, the original
plan for the probe was to do a single planetary assist by Jupiter, which would have brought
the probe 3 times closer to the Sun, but this trajectory came with some issues. [9] Because Jupiter's orbit is so much further
from the sun, the sunlight reaching the panels at its aphelion would have been 25 times dimmer,
requiring much larger solar panels to power the spacecraft. This poses an issue when the
spacecraft makes its way around Jupiter and begins to accelerate towards the Sun. The
heat of the Sun would destroy the solar panels, and at this size they could not retract and
hide behind the sunshield. Other options were available. A radioisotope thermal generator
could have been used, but would have driven the cost, weight and complexity of the spacecraft
up significantly. The real selling point for this radical new
flight trajectory was the added time and data it would provide scientists to fulfill the
probe's mission, to study the Sun. With the original Jupiter plan the probe would have
had just 100 hours inside the desired zone around the Sun, completing just 2 solar passes
before the probe reached the end of it’s 8 year mission duration. While the new lower orbit plan would mean
the Parker solar probe would take less than 150 days to complete its orbit around the
Sun, allowing scientists to gather over 900 hours worth of data over the probe's 24 orbits
around the Sun. The change in plan came be with a change of
design, moving away from the original conical shaped heat shield [10] to the familiar and
compact flat shield. This shield is primarily constructed from 11.4 centimeter carbon foam.
A truly fascinating material developed by one of the most prolific material innovation
labs, Ultramet. [11] Under a scanning electron microscope the carbon
foam looks like this. An incredibly porous material, dominated by open space, making
its internal volume 97% empty space, providing the heat shield fantastic insulation properties,
while also benefiting from carbon's thermal stability. [12] Next a carbon carbon composite, which is made
by combining graphite with an organic binder, such as pitch or an epoxy resin. This mixture
was applied to each side of the foam, before being super heated to transform the binder
into a pure form of carbon. Creating a carbon-carbon composite. Finally a white ceramic paint was applied
to the Sun facing side to reflect as much of that heat away from the heat shield before
it had a chance to even enter the labyrinth of carbon beneath. From here the rest of the spacecraft, besides
a few specialized sensors and solar panels, had to be designed to fit within the umbra,
or shadow, of the shield. There are several instruments that bravely
peak out beyond the safety of the heat shield. Like the solar probe cup, one of the many
sensors on board. This thing is easily the most impressive bit of technology aboard.
Completely unprotected by the sunshield, its designers had to get very creative with materials. The solar probe cup is a faraday cup, which
is a device that can count and measure the properties of electrons and ions coming from
the Sun, essentially giving the spacecraft the capability of studying solar winds and
mass ejections of particles coming from the Sun. This is the cross section of the solar probe
cup. It essentially works by applying an electric field over the grid at the cup's opening.
By varying the voltage, we can select or filter out particles that can enter the cup, giving
us more data on what is causing current as these charged particles strike the collecting
plate at the bottom of the cup. It’s a very simple device in practice, but with the temperatures
it’s facing, 1400 degrees celsius, just below the melting point of pure iron, the
solar probe cup needed some innovative engineering. The first challenge was selecting a material
for the electric grid that generates the selecting electric field at the entrance of the cup.
This grid needed to be conductive and heat resistant, while also being machinable to
create the tiny 100 micron spaced grid. Tungsten was chosen, the same material used
in incandescent light bulbs here on earth, as they are capable of surviving the extremely
high temperatures needed to generate light. Tungsten light filaments operate at temperatures
as 3000 degrees celsius [13], so are more than capable of surviving these temperatures,
however, machining tungsten into a grid this fine is difficult. Micron scale machining like this is not done
with traditional tools. You would immediately break the grid with the force required to
shave the metal away. Instead lasers are typically used to etch away material, but because tungsten
is so heat resistant, lasers would not be capable of melting the tungsten to form the
grid. Instead acid etching was used. [14] Next we needed electrical cables capable of
supplying the grid with power, and carrying electrical signals away from the collecting
plate. The two most commonly used conductors here on earth, copper and aluminum, would
turn to a pool of molten metal in these conditions, so these were most definitely not an option. Any conducting cables in this part of the
spacecraft had to be made out of niobium C-103, a special alloy of 89% niobium 10% hafnium
and 1% titanium. All the external casing components were also constructed from this exotic aerospace
material. Normally wires would be insulated from outer casings with plastic, but this
obviously wasn’t an option for the Parker Space Probe, and the engineers were forced
to use sapphire to ensure the niobium wires were insulated.[15]
These are some extremely exotic materials to perform what is a relatively mundane job
here on earth. Other portions of sensors peaking beyond the
sunshield are built in a similar manner. The magnetic field measuring instruments hidden
behind the shield need antennas that reach beyond the sunshield in order to make it’s
measurements of the Sun’s magnetic fields. These 4 antennas are also made from niobium
C-103. [16] The solar panels were the next challenge.
While cruising through it’s distant orbit around the Sun, the spacecraft can fully deploy
it’s solar panels without issue, but as the probe begins its sweep towards the Sun
heat will become an ever growing issue. This can be counteracted somewhat by retracting
the solar panels, but the spacecraft needs to maintain some power to operate it’s scientific
equipment during this vital stage of flight. Here two smaller secondary panels remain sticking
out in view of the Sun and are cooled with water, which is pumped through the solar panels
and into these black radiators attached to the titanium truss just below the sunshield. This truss is exceptionally light for how
big it looks. The entire truss only weighs 22.7 kilograms (50 pounds) [17] , which even
for low density titanium is very low for the size of the truss. The engineers at NASA have
clearly triple checked their stress calculations to ensure this thing could use as little material
as possible, which of course saves launch weight, but also minimizes the material available
to conduct heat from the heat shield to the spacecraft bus. Testing these systems in the heat they are
expected to meet is difficult on earth. The Odeillo Solar Furnace is our best approximation
of the environment they would need to endure. This facility, built on a hillside in rural
France, uses 10,000 adjustable mirrors to focus light onto one concave mirror. The facility
is capable of reaching temperatures as high as 3500 degrees celsius, over double the temperature
that even the sunshield will experience. Parts like the faraday cup and sunshield were placed
in the focal point of this concave mirror and exposed to the temperatures they will
need to endure. However, components like the faraday cup also needed to be tested while
performing their sensory tasks, and for this the engineers need a particle accelerator
to simulate the electrons and ions it will encounter from solar wind. Combining a particle
accelerator with this solar furnace was not an option, so the researchers at the University
of Michigan came up with the bright idea of using 4 high powered IMAX projectors to simulate
the heat of the sun, and they found that the faraday cup actually performs better when
heated, as the heat decontaminates the system. Much of the data we have received from these
instruments is of little interest to the average space enthusiast. Raw data that will provide
scientists with valuable clues to the nature of the Sun, but there is one sensor on board
relaying images back to Earth that we can all enjoy. During a solar eclipse we can observe a beautiful
phenomenon, bright loops of light dancing around the Sun. These fantastic patterns are
created by glowing electrons, sailing around the Sun on magnetic field lines, distorted
by solar winds. [18] We have been able to observe the streams of high energy electrons
from Earth and our solar observatories parked in Lagrange point 1, but we have never, until
very recently, been able to observe them from up close. As the Parker Space Probe dipped into the
Corona for its ninth encounter with the Sun, it began recording from its wideview imager,
here on the spacecraft. The images it delivered look like a passenger's view of a passing
snowstorm on a dark night. Bright sub-particles streaming by the probe as it dips into the
eye of the storm. Beautiful images that no doubt are giving scientists unparalleled data
on the nature of the coronal streamers. The Parker Space Probe has many more rendezvous
with the Sun, with the next due in September of 2022, and with another 2 Venus flybys the
Parker Space Probe will be breaking its own records in 2023 and 2024, bringing us even
closer to the Sun. [19] And while we are getting closer to the Sun
in our skies, we are also getting closer to recreating the Sun here on earth for boundless
fusion power. The current record for fusion power is setting at 70%, meaning we have managed
to reclaim 70% of the power needed to get the fusion reactor started. We are still well
away from being able to actually create power using fusion, but ITER, a new Fusion reactor
is due to be complete in 2025, with worldwide cooperation helping to fund its astronomical
cost. Upwards of 45 billion dollars, making it one of the world’s most expensive science
experiments in human history. This documentary on CuriosityStream shows
the behind the scenes of Europe’s previous generation magnetically confined plasma fusion
reactor. It’s a fascinating watch that you can watch by signing up to yearly CuriosityStream
membership for just over 1 dollar a month. This deal gets you access to incredible documentaries
on CuriosityStream for less than a dollar a month, and access to our upcoming Battle
of Britain series. We are just putting the final touches on the first episode, which
will be launching on the 25th of February. Here’s a quick trailer for what you can
expect: An unpresented war began in the Summer of
1940. A war that would change the face of modern warfare forever. The world’s first
war to take place entirely in the skys. A war of attrition. A battle that was won
and lost by the actions of the few. This is the Battle of Britain. I’m incredibly excited to share the series
we have been working on for over a year, and you can get access to it for just over 1 dollar
a month. This is by far the best way to support this channel and help us continue to develop
our abilities as documentary filmmakers. You can sign up to this amazing deal by clicking
on this button on screen right now, or if you are looking for something else to watch
right now you could watch our last video on the future of carbon taxes, or you could watch
Real Science’s latest video on the incredible biology of carnivorous plants.
