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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.
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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
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You can sign up to this amazing deal by clicking on this button on screen right now, or if
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