News of the Chinese Long March 5 rocket burning up on re-entry has made waves through the
world- in some cases, quite literally, as the debris from the rocket burned up over Saudi Arabia
and splashed down in the Indian Ocean near the Maldives.
And while it made a lot of people very nervous to know a 22 ton rocket was going to fall
out the sky, this sort of thing happens all the time.
Boosters, dead satellites, and sometimes even old space stations get dropped out of the
sky fairly often.
While the litter seems a little inconsiderate, this is probably far safer than the alternative:
leaving junk in orbit.
Just last month a hole was found in a robotic arm on the international space station - punctured
clean through by a fast-moving piece of shrapnel.
The accumulation of space junk poses a huge risk to all human operations in space especially
if we cross the threshold into the chain reaction of exponentially growing collisions known
as Kessler Syndrome.
The launch of Sputnik in 1957 kicked off a space race, and the number of satellites in
space has grown ever since, and in recent years it’s really taken off like a rocket.
There are more than 3300 operational satellites buzzing around up there.
But they aren’t alone.
They orbit in the midst of a giant cloud of debris - rocket boosters, protective shields,
old broken satellites and more, around 10,000 objects that are large enough to be tracked
by radar.
They are uncontrolled and they are moving very, very fast.
The quickest low-altitude orbits circle the Earth 16 times a day.
A collision with one of these means instant obliteration into countless shards.
And speaking of shards, there may be over 100 million pieces of debris so small they can’t be tracked
And while a loose screw or paint chip may not be all that scary on earth, at orbital speeds
of 7 km/s they can carry as much energy as a cannonball.
Right now there are probably over 3 million kilograms of space junk, vastly outweighing
our operational satellites and nearby natural meteoroids combined.
- forming a giant space-tornado of death above our heads.
The main thing protecting satellites from space junk is space itself.
Space is, as the name implies, spacious, and so collisions are relatively rare, or at least
for the moment they're rare.
But as the number of satellites increases, the risk of collisions also increases.
And collisions produce more space junk that produces more collisions.
This sort of collision cascade is known as the Kessler Syndrome, after Donald Kessler
who pointed out the danger in 1978.
The movie Gravity depicted this domino-chain of destruction as unfolding over minutes to
hours.
That’s not how it would actually play out.
In fact the Kessler Syndrome is already playing out - albeit a lot more slowly.
But it may soon speed up.
To understand the risk we face, let’s think a bit about exponential processes.
The Kessler Syndrome is due to what we call a positive feedback cycle - that’s any process
whose outcome tends to increase the rate of the process, leading to exponential increase.
The more space junk we have, the more space junk we make.
But a positive feedback process can be prevented from entering that exponential mode if it’s
suppressed in some way.
In the case of satellite collisions, exponential increase is inevitable unless debris is removed
from orbit or the risk of collision is mitigated.
And that needs to happen faster and better than the rate that space junk is made.
Let’s talk about mitigation first.
Large pieces of space junk can be tracked using ground based radar and more precise
optical and lidar methods.
And their future trajectories can be calculated to predict possible future collisions.
A functional satellite in harm's way can then steer itself into a safe orbit.
The international space station has to dodge tracked space junk roughly once a year.
But this method is far from perfect.
Below around 1000 km, Earth’s atmosphere is incredibly diffuse but still present.
It produces a drag that causes orbits to slowly decay.
That’s ultimately a good thing, as I’ll show.
But it also means we can lose track of space junk because the amount of drag can change
- in particular, during times of high solar activity when more energy is pumped into the
atmosphere, causing it to puff up and cause increased drag to higher altitudes.
The smaller the space junk the more susceptible it is to atmospheric drag.
That gives us a sweet spot for ‘lethal’ debris - pieces of junk in the size range
of 1 to 10 cm are the hardest to track, and almost impossible to keep track of.
And yet are still large enough to cause ‘lethal’ collisions- the kind that can obliterate a
satellite if they hit right in the main body.
And there are something like half a million of these hyper-velocity space bullets flitting
around up there.
Improved tracking facilities can help here - better, faster observations will help us
survive this game of cosmic dodgeball a little longer.
But tracking only goes so far.
Junk smaller than around a centimeter can’t be tracked at all - your paint flecks and
rocket exhaust and shards from prior collisions.
The only way this stuff gets cleaned up is if its orbit decays.
I already mentioned that orbits decay due to interaction with the upper reaches of Earth’s
atmosphere.
Below around 1000 km, that drag slows satellites, causing them to fall into lower orbits where
the drag is even greater.
This is another positive feedback loop - orbital decay accelerates until the satellite finally
burns in a glorious blaze of re-entry.
Satellites in low earth orbit may need to be boosted a few times a year to prevent this
inspiral.
During a maximum of solar activity when our atmosphere extends further out, the lowest
satellites need to be boosted every 2-3 weeks.
That’s annoying, but orbital decay is the most important process for cleaning low-earth
orbit of defunct satellites and debris.
Ultimately, the Kessler Syndrome can be avoided if orbital decay cleans up low-earth orbit
faster than new launches and new collisions fill it.
OK, let’s look a bit harder at the sources of space junk to see if these mitigating processes
have any hope of saving us.
Over 40% of the catalogued space junk is debris from the upper stages of a handful of US rockets
that exploded after releasing their payloads.
These plus few other catastrophic events are responsible for most of the fragmentary space
junk.
Notably we have the 2009 collision between the Iridium 33 communication satellite and
Kosmos 2251, a deactivated Soviet military satellite.
With a collision velocity of 11.6km/s, it was less a smash than a splash, creating hundreds
of fragments large enough to destroy other satellites, and 10s of thousands of that could
cause damage or deactivate satellites.
Relatively few events have produced as much of our most dangerous space junk.
For example, we have the times that nations deliberately destroyed their own satellites.
The US did this in 1985 with an air-to-space missile, and China demonstrated their capability
in 2007 with a ground-launched “kinetic kill vehicle”.
The US followed a year later with operation burnt frost, ostensibly to eliminate the risk
of reentry of a tank of toxic hydrazine - and the reentry of that stuff would have been
very bad.
But the timing after the Chinese launch does make you wonder.
The issue with destroying satellites is that the fragments don’t stay in the same orbit
as the progenitor satellite.
They quickly smear out in a ring around the Earth, making tracking and avoidance that
much more difficult.
John Gabbard, who was working at NORAD tracking missile test debris in 1960, noticed that
debris also spreads out up and down.
This increases the altitude range of the risk, and can also mean that collisions in fast-decaying
orbits can still generate long-lasting debris.
So bringing this all together we can paint a picture of our trashed orbital space.
Each collision tends to populate a wide swath of low-earth-orbit with dangerous debris.
That debris then decays in orbit.
Very slowly for the highest orbits, quite quickly for the lowest.
But those lowest orbits are continuously repopulated as debris rains down from higher up.
For example, even though the International Space Station orbits several hundred kilometers
below where the Iridium-Kosmos collision happened, on multiple occasions it's had to perform
collision avoidance maneuvers to avoid debris that’s sinking through its orbit.
As long as the self-cleaning of the lowest orbits outpaces the introduction of new debris,
we avoid exponential buildup.
But these lowest orbits are the most useful and cheapest to launch to - and so are the
most densely populated.
Add to that the rain of debris from above, and these are the most at risk of the Kessler
Syndrome.
So how long until the collision cascade?
Actually, it’s already happening.
More debris is produced than is removed.
Exponential buildup doesn’t have to happen over the course of a 90 minute Hollywood movie
to be dangerous.
In 1978, Donald Kessler estimated that the first satellite collision would probably happen
some time in the next two to three decades, which was pretty spot on with the Iridium-Kosmos
destruction just over a decade ago.
But the time to the next collision should be shorter - and indeed there have been several
now, although thankfully none as cataclysmic as the first.
We’re currently in the slow part of the exponential rise, so we have years or decades
of useful space ahead of us.
But that can change if we change our launching habits for the worse.
This is where StarLink deserves a mention.
SpaceX plans to launch constellation of over 40,000 satellites as a global internet network.
This will increase the number of satellites in orbit by a factor of more than 10.
Now SpaceX insists that they have collision mitigation systems planned, even if they haven’t
told us exactly what those are.
But even the best collision dodger can’t do much against an untracked and untrackable
shard of debris.
One hit can turn your agile, debris-dodging ninja satellite into just another hunk of
debris.
I should add that StarLink will be in very low orbit, and so has a fast decay time - under
5 years.
That might be enough to mitigate the dangers in this case.
But StarLink isn’t going to be the last giant swarm of internet satellites - in fact,
it’s sure to inspire other companies and nations to launch their own.
Presumably, at some point, some of them without the same care that SpaceX claims to be taking.
OK, let’s take the worst case scenario.
The Kessler syndrome accelerates, churning up low earth orbit into a cloud of shrapnel.
What comes next?
Some sci-fi writers like to present Kessler syndrome as an impassable maelstrom, a giant
space blender, effectively imprisoning us on the Earth.
That’s a bit dramatic.
The danger is for objects spending any lengthy period of time in those zones.
For a rocket passing straight through on its way to higher obits, or to more distant parts, the
risk will probably will be tolerablely low.
Higher orbits, like medium earth orbit where the GPS satellites live, will probably also
be okay due to the relative low density up there.
Still, there are rocket boosters left over from launches to high altitudes- maybe they
could be destroyed in collisions in low earth orbit, creating a train of debris that wreaks
havoc higher up.
But the real risk is for anything trying to stay in low earth orbit.
At the late stages of the Kessler Syndrome, the space below a couple of thousand km may
be largely unusable - at least for a while.
At the very least, the cost of maintaining a satellite presence will be prohibitively
expensive with satellite lifetimes dropping drastically.
We’ll have to curb our appetite for global digital communication for at least several
years while we wait for the junk field to fall back to Earth.
After about a decade, very low earth orbit - up to 400km - will become usable again.
Or we could start taking more care right now.
It’s possible to plan to deorbit booster rockets and old satellites, or send them up
into safer graveyard orbits.
And all sorts of new technology are being developed to do this stuff, like huge deployable
sails which increase drag and cause trash to fall faster, or even electromagnetic tethers
which push on earth’s magnetic field to deorbit a satellite.
The key will be better organization and cooperation between nations and between companies to maintain
our cosmic front yard, and continue our safe use of Earth’s orbital space time.
This show would be much, much harder to do without the help of the Space Time community
on Patreon.
Don’t worry, we’ll keep doing this no matter what - but I gotta give a huge shoutout
to the Patreon crew - you guys really help the show to make ends meet.
And today’s extra-extra-special shoutout goes to Tiffany Poindexter, who’s supporting
us at the big bang level.
Tiffany, we’ve used your contributions to fund a research program to solve the space
junk problem.
Our plan is to de-orbit all of the smaller pieces of that shiny debris right over your
house, where they’ll burn up on reentry in a beautiful fireworks display in celebration
of your awesomeness.
Obviously we can’t do that for the bigger stuff because that’ll hit
the ground, so please let us know where you’d like us to deliver, i mean de-orbit all of that.
Last episode was on Planck Relics, those subatomic scale black holes that could be literally everywhere and even
explain dark matter.
Let’s see what you guys had to say.
Matt Kelly asks what happens to normal matter when it interacts with one of these tiny black
holes.
Well first of all, Planck relics would be so impossibly tiny that all matter would be
like empty space to them.
Give them any velocity at all and they stream right through matter as though it isn’t
there.
The gravitational interaction is weak except right next to the relic - on the scale of
Planck lenfths.
If you could somehow get an elementary particle like an electron within that range then perhaps
it would be absorbed and the relic would start to grow.
The problem then is that the black hole will immediately Hawking-radiate back down to a
Planck relic.
That will happen long before the next particle is likely to hit it.
So Planck relics will stay Planck relics unless you can force feed them at an enormous rate.
Johanna Mueller never realized that the Planck mass is such a huge thing.
I know right?
Two hundredths of a milligram is giant compared to the 1.6x10^-35 meters of the Planck length
or the 5x10^-44 seconds of the Planck time.
But the planck mass defines the mass of a black hole one Planck length in radius,
and remember, black holes are massive!
Bleedingleaderq asked whether a region dense in Planck relics would emit high-energy radiation
as the relics merged and then Hawking-radiated again
This is the sort of extremely cool thought that astrophysicists love because it’s something we can look
for
Unfortunately these suckers are so small that they would never, ever merge - even if they
got within an atomic nucleus width apart.
They’d just fly by each other.
This is what makes them the perfect candidate for WIMPS - the weakly interacting massive
objects that may explain dark matter.
Several of you hypothesized that there being tiny black holes everywhere perfectly explains
one of the most vexing paradoxes in the field of domestic mechanics - as in it explains
what happened to your lost socks or screws or the money in your wallet.
It’s also a great excuse for why you didn’t finish your physics homework on time - no
self-respecting physics professor can confidently say that a planck relic didn’t eat your
homework. It also explains why I don’t have a funnier joke to end this episode.
I swear, I wrote one down and it just vanished.