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Well since it's in LEO and since the sats will have been destroyed or damaged they won't be able to maintain their orbits, so the debris orbit will be decaying from the moment of the incident and should re-enter in a matter of a few months. The scenario I'm sure will be simulated by numerous people/orgs by the time this project is in full swing.

This is not true of the orbit that the SpaceX satellites are going in to. LEO is a bit of a general term that covers altitudes anywhere from 100 km to 2000 km. While it is true that something at 400 km altitude will deorbit in a few months, they will be orbiting at an altitude of 1100-1300 km. At that altitude it will take hundreds of years from them to deorbit. The drag goes down exponentially as you get in to higher orbits. The FCC filing says that the SpaceX satellites will have enough fuel remaining in order to deorbit the satellite after the satellite has reached its usable mission life.

This was a big concern for NASA though. Their typical guidelines were that commercial satellites have to have >90% reliability to deorbit the satellite. But they want to increase this for the SpaceX satellites because if 10% (~400) of the satellites end up failing then they are effectively up there forever.

I believe you're thinking about their initial application. They've updated it [0] to an altitude of 550km, so atmospheric drag should still de-orbit broken satellites fairly promptly.

[0] - <https://fcc.report/IBFS/SAT-MOD-20181108-00083>

Thank you, I was thinking about their initial application. Yes, de-orbit times from 550 km are on the order of years to possibly tens of years. The requirement from the FCC is to deorbit your satellite within 25 years after the end of mission.

Edit: Their most recent application only moved the Phase 1 satellites in to the lower orbit. In later phases they are still planning on having more than 1000 satellites at the higher orbital altitude.

Yes, de-orbit times from 550 km are on the order of years to possibly tens of years.

So it wouldn't stop progress for a century, but might it put a damper on things on the same scale as the Great Depression?

SpaceX recently updated their plans to put most satellites between 300 and 400km. That ensures a quick deorbit of the satellites and debris and cuts down the ping times.

Interesting. That does make me feel a bit more secure about the whole thing, but it does raise the question of how much they will need to invest in orbital upkeep/replacement of decayed spacecraft. I can see this getting expensive quick.

Definitely expensive, but it is likely still much less expensive than putting all of the satellites in to the higher orbit. It takes a lot more fuel to get the satellites in to the higher orbit, as well as still having enough fuel to get it back down to de-orbit the satellite.

Plus, in the ~1100 km orbit you are getting in to the lower Van Allen belt where the radiation will degrade electronics much more quickly. This can be mitigated with radiation-resistant parts and more shielding, but it adds a lot more cost.

There are new technologies like air breathing ion engines and electrodynamic tethers, which have the potential to sustain low orbit satellites for years.

they filed that because it's believed that they have mechanical problems on the satellite. Not because they wanted to:

https://mobile.twitter.com/TMFAssociates/status/106076080100...

Isn't the lower orbit even better for them in terms of latency though? So long as the orbit doesn't decay faster than the planned 5 year life span of the sats, it's actually a win.

The lower orbit means the satellite coverage on Earth is smaller now, so they need more satellites to compensate for delivering the same capacity in a given area. It's better in terms of latency, but it's not a significant difference.

Very good point, thanks for responding.

Not to mention a struck satellite (probably) no longer has the capability to deorbit, at least not in a controlled manner.

Little pieces of smashed satellites are not obligated by the rules of physics to decay.

An example of how long shit can stay in space: A Saturn V booster rocket is still out there, slowly orbiting the planet: https://en.wikipedia.org/wiki/J002E3

I'd think that only a fraction of the debris will head down into the influence of atmospheric drag. The rest would fly away at an angle that either takes it parallel to the path of the orbit, or higher in the orbit (however short that may last). These satellites, while in a LEO orbital configuration, I would assume would still be high enough to where there is a decent amount of room before significant drag comes into play. Otherwise, it would not be economical to have these in orbit as they would decay and need to be replaced too quickly to justify the cost.

They’re already in place that has enough drag. The satellites will deorbit in months to years without periodic boosts. Debris will come down faster, since they have more surface area relative to their mass. A single impulse always results in an orbit that intersects the original orbit at the location of the impulse, so it’s impossible for a collision to raise debris entirely beyond that altitude.

SpaceX gave their proposals for this (very valid) concern here:

https://licensing.fcc.gov/myibfs/download.do?attachment_key=...

Think how big the Earth is. How hard it would be to find a Xerox machine in the Amazon or the middle of the Atlantic if someone stuck it there. Space is much, much, bigger.

> How hard it would be to find a Xerox machine in the Amazon or the middle of the Atlantic if someone stuck it there.

Part of the problem, though, is that said Xerox machine is traveling at 17,000 miles per hour.

We've already had a collision (https://en.wikipedia.org/wiki/2009_satellite_collision) between two of the ~3-4k satellites in orbit. SpaceX's constellation is planned to be double that number.

That single collision turned two satellites into two thousand high-velocity projectiles, too.

> That single collision turned two satellites into two thousand high-velocity projectiles, too.

Just to add to that point: https://en.wikipedia.org/wiki/Envisat#Space_safety

A single 10kg. chunk of debris could result in over 8 tonnes of additional debris, triggering a rather nasty chain reaction.

I'm no stranger to the vast physical space at play here (amateur astronomer, here). I'm aware there is a LOT of room for this stuff to move around, but there is still a threshold, however high, to where the amount of debris would become dangerously numerous.

And you think that we can only fit <10k objects in an area larger than the surface of the earth?

<10K isn't the problem. It's when that 10K becomes much, much larger because of an unforeseen collision with an unrelated spacecraft/piece of space junk. It only takes one big collision to utterly litter that particular orbit, and thousands of slightly different orbits caused by the new trajectories of the various bits of the original spacecraft.

So whats your solution? Never put a bunch of satellites in space?

He's not necessarily saying he know's a solution, just that such a problem should be considered. Satellites do collide - https://en.wikipedia.org/wiki/2009_satellite_collision

The idea of satellite collisions and space junk is an actual concern https://en.wikipedia.org/wiki/Kessler_syndrome .

Sure a few washing machines in space isn't a big concern. Now imagine a few collide, hundreds of bits and bolts flying around earth, at unknown trajetories, at thousands of miles per hour.

There are a variety of potential mitigations. Legal requirements that satellites deorbit or park in a safe orbit before going dark, technology like https://en.wikipedia.org/wiki/E.Deorbit to capture and deorbit large chunks, better tracking of orbital debris, etc.

At any one time there are usually <10k airplanes in the sky,and they still hit each other sometimes despite going 23x slower than satellites

The vast majority of those collisions occur in areas where planes are forced to congregate (around airports, etc).

Isn't there high traffic where polar intersect? See: https://en.wikipedia.org/wiki/2009_satellite_collision.

To be fair, a lot of that is because of the fact that planes follow paths that are designed to keep collisions from happening, but aren't always adhered to.

There has been a collision between an Iridium satellite and another one before. It's a valid concern.

If they're travelling 7 km a second, maybe??

Now imagine that Xerox machine is constantly moving at 17,000 mph. How interested would you be in figuring out where its going?

I think this issue is over played a bit sometimes.

1. Satellites can kill themselves by dropping back into the atmosphere by their own propulsion system

2. Even if they fail, most of these are low enough that the time until they fall is reasonable short.

3. The objects are known and tracked so even if they are out of control, the others will avoid them.

4. We are moving into a world with cheap access to space and lots of satellites we need service satellites anyway and multiple are in development. This could be a legal requirement but even without it will develop because SpaceX doesn't want to have debris flying around on 'their' plane.

This is a well informed answer to your question:

https://www.quora.com/Why-dont-satellites-crash-into-each-ot...

Thank you for the article, but it really doesn't answer my question. That thread's answers assume that we are talking about the current situation we have around the Earth. We have no precedent for this many satellites orbiting at the same altitude, but at different latitudes/longitudes (which negates the safeguard that they are moving at the same speed, it doesn't matter if they can intersect). The satellites would all need to be moving at both the same speed AND the same direction to not have a risk of collisions (which would of course be a circle, useless to this application).

First, check out the video explaining the orbits: https://www.youtube.com/watch?feature=youtu.be&v=AdKNCBrkZQ4...

Doing some quick math, the shell in which they'll be orbiting has a surface area of 373,367,808km. With ~7000 satellites, an even distribution would leave them 533,382 km between satellites. All the satellites have station keeping thrusters to keep them in LEO with drag. Keeping them from running into each other is probably pretty simple, in comparison to the other challenges they'll need to solve.

Sure, but this assumes nothing ever collides with one and causes debris to be introduced to the web. The web itself will clearly be ok as long as it remains intact, but if something does cause one of these to RUD catastrophically, we now have debris following a similar, but just different enough orbit to be dangerous.

You won't find an answer to your question because you don't seem willing to drop your alarmist attitude.

There is just far too much space for 100k objects, let alone 10k to ever run the risk of collission. These satellites are all meant to be in line-of-sight. They will communicate with optics as opposed or in conjunction to radio.

You fear unforseen space particles destroying one and causing a chain reaction. Why don't you hold that same fear for existing satelittes which typically operate at an orbit with far lower decay rate? The orbit for StarLink is very low, small particles would not sustain their energy long enough to be a risk factor. The StarLink sats maintain orbit with fuel, everything else without fuel in that orbit will fall. The chance that a particle will end up in the same orbit, on the same plane, at the same time is astronomically low.

Your comments just echo the same type of delusions of people who think in their brillance they've found problems others have not foreseen. SpaceX is really just full of amateurs you ought to submit your findings.

> There is just far too much space for 100k objects, let alone 10k to ever run the risk of collission.

No, there isn't. Satellites can and do collide despite the vast amount of space in a given orbit. It is improbable, but likely enough to warrant at least some concern.

> Why don't you hold that same fear for existing satelittes which typically operate at an orbit with far lower decay rate?

I do, as does the FCC. This is evident in the fact that they now require any body wishing to launch a spacecraft guarantee that the craft can be put into a graveyard orbit.

> Your comments just echo the same type of delusions of people who think in their brillance they've found problems others have not foreseen. SpaceX is really just full of amateurs you ought to submit your findings.

I'm genuinely sorry this post made you feel this way. If I already had the brilliance to answer this question as I'm sure many bright minds at SpaceX do, I would not have asked the question and rebutted the responses to begin with.

> Satellites can and do collide despite the vast amount of space in a given orbit.

Excluding planned collisions, or collisions during failed docking, I found 4 instances of collisions between satellites and space debris. Ever. Now, a couple of things:

* Yes this larger constellation will increase those odds in the future.

* No, a catastrophic doomsday scenario did not occur when those happened.

* SpaceX will have complete control of these satellites through their operational lifespan (excluding collisions or malfunctions).

* Orbital decay for debris at the altitude they're operating at is ~10 years.

Also, have to ask... is all this concern a result of reading Seveneves? :)

I'm afraid I haven't heard of Seveneves before today, although it does look like an interesting read.

My concern is just from a general interest in spaceflight. More of a wish to protect a resource rather than a Chicken Little fear of the sky literally falling out.

Satellites don't just cross their paths by magic. If their orbits don't intersect, they can't collide. Satellites don't collide with each other, it has never happened.

Satellites do collide with debris, but that's a different issue. Furthermore, they don't need a guarantee that they can be put into a graveyard orbit - the proposed satellites (and even any debris, if they'd explode for whatever reason) will be in a graveyard orbit from day one until they stop being pushed up and deorbit.

> Satellites don't collide with each other, it has never happened.

Actually, they do, and it has indeed happened before (https://en.wikipedia.org/wiki/2009_satellite_collision). Far from a Kessler syndrome scenario, but certainly something that should be examined.

I'm guessing LEO satellites has a much shorter orbital decay so more likely to burn up in the atmosphere is something goes wrong.

These types of satellites typically sag out of orbit and burn up after a few years

This is known as Kessler syndrome.

It could make orbital planes unusable, but is generally thought not to pose a substantial problem for launching through them, so worst case we just have to start putting satellites a bit higher.