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This episode of Real Engineering is brought to you by CuriosityStream, watch over 2,400
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On March 27th 2019, Indian Prime Minister Narendra Modi announced to the world that
India had conducted its first successful anti-satellite test.
Launching a three-stage missile from Abdul Kalam island on the north-eastern coast of
India, with a trajectory taking it over the Bay of Bengal.
A trajectory that would eventually lead to it intercepting India’s military satellite,
Microsat-R, 283 kilometres overhead.
The seven hundred and forty-kilogram satellite met its end when the kinetic kill vehicle
ploughed through it.
Shattering it into hundreds of pieces which proceeded to spread in earth’s orbit.
The test was universally condemned.
A completely unnecessary political posturing move that added significantly to Earth’s
growing space debris problem.
This isn’t some far off distant problem that we need to worry about, and the probability
of collisions occurring is not only continually rising, but there have already been several
collision events that have damaged the international space station and other high-value satellites.
We can’t blame the current state of affairs entirely on anti-satellite tests like this.
Every single time we launch into space we generate some sort of unwanted waste.
Solid-fuel rockets deposit aluminium-oxide particles.
Explosive bolts fragment into pieces.
Even chips of paint can cause issues.
In 1983 the Challenger space shuttle was struck by a 0.2 mm chip of paint that managed to
gouge this pit out of one of its windows.
In fact, in the first 67 Space Shuttle launches 177 impacts were found in the windows.
45 of which were large enough to warrant a replacement window.
Post-mission analysis determined all of these impacts we caused by space debris, with 44%
being caused by aluminium alloys, 37% by paint chips, 12% by steel, 5% by copper and 2% by
titanium.
[11]
At fifty thousand dollars a pop, this did not come cheap.
Based on these numbers, the Space Shuttle had 67% chance of impact causing significant
damage to the windows during their 10 day missions, and these probabilities have only
risen over time.
In 2007 the likelihood of a collision between any satellite in low earth orbit and a piece
of debris over 1 centimeter in size was 17-20% in a single year.
[2] That statistic increased to 25-33% later that year when China tested their own anti-satellite
missile.
By 2010 the chance of a 1-centimetre piece of debris striking a satellite had increased
to 50% a year, after two full-sized satellites Iridium 33, a US communications satellite,
and Kosmos 2251, a retired Russian communications satellite, collided at 42,000 kilometres per
hour.
Obliterating both satellites and producing over 1000 fragments over 10 centimetres in
size, and many more too small to be tracked.
So the chances of collision are not small by any measure, they are common and it’s
only a matter of time before another serious incident like this 2009 collision occurs again,
so it is prudent that we design satellites to be capable of not only withstanding small
impacts but be capable of dodging larger ones.
The ISS, for example, is designed to withstand objects up to 1 centimetre in size [3], and
can dodge larger trackable objects 10 centimetres wide.
The biggest danger to the occupants of the ISS are objects in between these sizes.
That are untrackable but large enough to cause serious damage to the international space
station.
[3]
When the ISS was being planned, NASA laid out a basic risk management policy.
That the probability of any critical component of the ISS being penetrated by space debris
would be less than 19% over 10 years.
[4][5] A critical component is characterized by anything that could potentially lead to
a loss of life if it is damaged, and designers are careful to correctly categorise each component
on the ISS to ensure this is the case, which often involves performing hypervelocity impact
test here on earth.
This resulted in things like batteries and ammonia accumulators being categorized as
non-critical when they didn’t explode during tests, and so received less shielding that
other critical components.
Space debris is just a fact of everyday life on the ISS that astronauts need to be aware
of.
To get a better sense of what it’s like living with this.
I spoke with former ISS commander Chris Hadfield on the phone.
When you are onboard a spaceship you have a constant undercurrent awareness of the ever
prevalent risk of something hitting your spaceship and causing a leak.
Sort of like when you are driving a car, you always know at some point you could have some
kind of accident.
Yo u know, It’s not heavy on your mind.
You know it happens.
You know the odds are that eventually it will happen for sure, and you just have to find
a way to live with it.
And so the way we live with it is to understand the risk as accurately as we can.
We know the statistics.
We know the relatively risk of man made debris versus naturally occurring debris, and we
know how the space station is designed to resist it with the multilayer shielding, and
then we also have procedures.
So let’s talk about that shielding first.
Shielding the ISS with heavy plates, as tanks do here on earth, is not an option.
This is the damage a 13-millimetre spherical bullet will do to a 18-centimetre aluminium
plate when travelling at 7 kilometres per second.
It prevented penetration and only just managed to prevent a large chunk of spall to break
off from the interior surface.
[6]
At 13 centimetres a 1 metre squared aluminium plate like this would weigh about 338 kilograms.
When the ISS started construction in 1998 the per kilo cost to launch to the International
Space Stations orbit using the Space Shuttle was about 93,400 dollars.
[7] Placing a 1 metre squared shield like this at a cost of 31.5 million dollars.
This is an extremely inefficient use of material, and the ISS uses something much more elegant
called a Whipple shield.
The Whipple shield uses the debris’ own velocity to stop it.
At 4 kilometres per second and higher, the energies involved are so immense that the
projectile itself breaks apart and vaporises on impact.
[8] Whipple shields take advantage of this by creating a shield that is composed of several
thin sheets of armour separated by space.
So when a meteoroid or debris does strike it, it first breaks up into thousands of smaller
superheated fragments.
Thereby spreading the energy of the impact over a larger area for the following shield
layer.
The European Space Agency conducted tests of their kevlar Whipple shields which protect
their ATV vehicle.
They did this by shooting a 7.5 mm diameter aluminium bullet at 7 kilometres per second,
which tore straight through the kevlar shield, but left only a scorch mark on the 3-millimetre
aluminium wall behind it.
[9]
These kind of impacts occur fairly frequently and as Chris Hadfield told me during the call:
If you just sit quietly by the wall of the space station and wait a while, you can hear
things hit your ship.
And that’s kind of an interesting thing.
It doesn’t happen too often and sometimes all you are hearing is the vehicle cooling
and heating in the sun, so you are hearing the natural popping of metal expanding or
contracting, but occasionally you hear just like the sound of a small bullet or high speed
stone banging into the thin aluminum hull of your ship
So astronauts are occasionally reminded of the space debris problem, and have to be careful
not to cut their suits while on space walks on sharp impact edges.
While this is a highly effective form of shielding that minimises the weight of shielding needed,
it is only effective for smaller debris.
Larger particles would tear right through this shield, and for those circumstances,
the ISS and other satellites literally have to dodge the incoming shrapnel.
Ground-based radar like the Haystack Radar are the main source of spatial data we have
on space debris.
It is an X-band radar system that simple stares at selected points in space and waits for
debris to pass through its radar beam.
[12] This gives us size, speed and direction information, which is fed into a database
that allows NASA and other space agencies around the world to predict potential collisions.
When a collision is predicted maneuvres can be planned to allow the international space
station to dodge it, but these maneuvres come with a cost, and mission control needs to
assess if the risk is worth that cost.
They start by drawing an imaginary box around the international space station.
50 kilometres squared and point seven five kilometres deep.
This acts as an exclusion zone and any tracked debris that passes through it will send an
alert to mission control.
From there careful risk analysis begins.
If there is a one in ten thousand to one in one hundred thousand chance of collision the
ISS receives a yellow warning.
[14]Which means flight controllers must perform avoidance maneuvres if they do not interfere
with mission objectives.
This can be as simple as interfering with microgravity experiments to forcing the Soyuz
to miss a launch window.
If there is a greater than one in ten thousand risk [14], then a red warning is received
and the international space station must take action.
Control momentum gyros can be used to alter the stations orientation, while thrusters
on the Zvezda module or on docked vehicles can be used to provide the necessary acceleration.
Boosting to a higher orbit requires expensive propellant, but the ISS already needs to perform
reboosts every few months to maintain its orbit, so these dodging maneuvers will just
alter the scheduling of these already needed boost burns.
These exclusion zones exist for all satellites in NASA’s database and on March 29th 2012,
Julie McEnery (hup the Irish) the project scientist for the Fermi Gamma-Ray Space Telescope
received an automated email alert.
Informing her of a predicted incursion between Fermi and Cosmos 1805, a retired Russian spy
satellite, where the two would pass within 700 feet of each other.
The decision on what to do with this information was left to her, and the lessons learned from
the previous satellite collision were not lost on her.
In order to ensure they did not collide Fermi would need to rotate away from its view of
the sky to point its thrusters in the direction of travel.
It then performed a 1-second burn that would separate the two satellites crossing temporally.[15]
These thrusters had not been tested before, as they were designed to take the satellite
out of orbit at the end of its life, and so there was significant anxiety within the team
that they could malfunction and end Fermi’s mission prematurely.
Thankfully that did not happen, and Fermi continues to give us valuable information
about the Universe to this day.
These issues are only going to grow as human activities in space grows, and it’s time
we began thinking more seriously about how we manage our cosmic neighbourhood.
Mainstream media tends to incredibly alarmist about this issue.
“So all of that is debris that you are looking at there, so your concern for debris is well
placed.
We maybe putting so much debris is space that we will close ourselves off from space travel
because of the dangers it would take to get through our own garbage heap.
Space debris, obviously does not look like this, the vast majority of it is too small
to see.
Occasionally we have massive debris, like the upper stage of Apollo 12s Saturn V rocket,
which is still in orbit and expected to return to earth in the next couple of decades, but
this material is easy to dodge.
After all, space is a big place.
We are not going to be trapped on the planet, we are not going to lose all technology related
to satellites.
Even now, just a few weeks after India’s anti-satellite test, a decent amount of the
debris will have drifted back to earth.
We can take Operation Burnt Frost, the US’s own anti-satellite test in 2008, as proof
of this.
There were multiple similarities between this and India’s own test, with similar orbits
and altitudes.
Data from the Combined Space Operations Centre shows that the majority of the debris from
this test had fallen back to earth within 2 months, while other pieces that managed
to be ejected into higher orbits eventually return to earth about 2 years later [1]
This was just the larger trackable debris.
Smaller debris decays faster as it has a larger area to mass ratio, making them more sensitive
to atmospheric drag.
Even in orbit, molecules do exist that collide with satellites and debris, causing them to
lose slow down and lose altitude.
This isn’t a reason to ignore the problem.
If the problem continues to grow as our space activities grow, the potential loss in money
from damaging collisions and the potential chain reaction this could cause in a busy
earth orbit is going to motivate efforts to fix the problem.
It’s not just the cost of the satellite that will motivate efforts.
Entire economies have been built upon the services they provide.
International treaties dictating space operations need to be updated to mitigate the issue,
ensuring any satellite placed in Earth orbit will be required to be capable of bringing
itself out of orbit and be capable of dodging debris when needed.
As we saw earlier with Fermi this is a feature of some satellites, but not all.
Some satellites simply become giant bullets when they retire.
This cannot be allowed to continue.
This alone will not be enough to ensure space debris is kept to a reasonable level, and
active clearance may be needed if trends continue.
Launching more objects into orbit to solve the problem seems to me to be a very expensive
and ineffective way to deal with the problem.
Someone will have to fund it, which will be difficult as most companies want to add things
to space not remove them.
Not only that, but it will also add to the space debris problem with the natural bi-products
of launches, while only being capable of taking down large objects.
A more promising technology will involve using high power lasers that will be powerful enough
to ablate material from the object, which will provide thrust to slow its orbit and
thus increase its rate of decay.
There are issues with a technology like this, as it could be used as an anti-satellite weapon
which would not sit well with other space faring nations.
To overcome that it would need to be a joint venture between all space faring nations.
[17] Just as the ISS became an international effort to unite mankind, cleaning up our cosmic
neighbourhood can also become a uniting problem.
International cooperation and rivalries have long been the driving force for advancement
in the field of aerospace.
World War 1 and 2 advanced aviation at a unprecedented rate, and it’s conclusion led to an international
race to the moon.
If you want to learn more about this troubled birth of aerospace.
I highly recommend watching this documentary titled “Pioneers in Aviation” on curiositystream.
It will take you from the early years of the Wright Brothers to the foundation of the Boeing
and Douglas Aircraft Companies through the difficult years of the great depression and
the rapid advancement during the world wars, culminating in the space race.
You can watch all 3 hours of this 3 part series for free, by signing up to curiositystream
using the code realengineering, or using the link the description.
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