How do satellites not crash into each other?

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How do satellites not crash into each other?

Postby liveboy21 » Wed Feb 24, 2016 1:50 am UTC

There are thousands of satellites in orbit. How do they not crash into each other? Is there a governing body to ensure that satellites are launched in a clear path?

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Re: How do satellites not crash into each other?

Postby Sizik » Wed Feb 24, 2016 1:51 am UTC

Space is big.
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Re: How do satellites not crash into each other?

Postby Xanthir » Wed Feb 24, 2016 1:52 am UTC

Space is *really big*, yo. And satellites are *really tiny* in comparison.

(There might be some body that helps make sure collisions don't happen, I dunno, but even without one, it'd be unlikely to happen.)
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Re: How do satellites not crash into each other?

Postby Copper Bezel » Wed Feb 24, 2016 3:32 am UTC

I'm 99% certain I remember something about commercial satellite launches having to "buy" an orbit in the way that websites buy domain names, but I can't find anything about it now.
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Re: How do satellites not crash into each other?

Postby ijuin » Wed Feb 24, 2016 3:51 am UTC

https://en.wikipedia.org/wiki/Space_debris

There are on the order of a thousand currently active satellites in orbit, and about twenty thousand known objects larger than 5cm in diameter which are detectable by ground-based radar. About two thousand tonnes of debris exist in low orbit (below the inner Van Allen belt). To put this to scale, that is approximately the mass of a WWII era destroyer scattered over a volume one hundred times as great as the entire volume of Earth's oceans. Pretty empty, eh?

Anyway, there has been only one publicly-known accidental collision between two active (non-dead) satellites. https://en.wikipedia.org/wiki/2009_satellite_collision

Launching organizations generally get permission from their own nation's government for a given orbit, but for Geosynchronous orbits, which are all at the same altitude and therefore have fewer slots available, slots are granted through the International Telecommunications Union. Penalties for violations are mostly of the "you break it, you bought it" type--i.e. the party in violation is liable for any damage caused to other partys' satellites.

Satellites that still have active propulsion are able to maneuver in order to avoid potential collisions, but when they have run completely out of propellant (usually after several years of service), they have to trust to luck. This is one of the reasons why most satellites are shut down when they are no longer able to maneuver, especially in Geosynchronous orbit, where station-keeping is a must in order to stay within one's assigned "slot". Geosynchronous satellites generally use the last of their propellant at the time of decommissioning to maneuver into a higher "graveyard orbit" in order to free up their orbital slot and to avoid collision with still-active satellites.

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Re: How do satellites not crash into each other?

Postby p1t1o » Wed Feb 24, 2016 9:05 am UTC

Satellites also have a finite lifespan. Low-Earth Orbit satellites still experience a small amount of atmospheric drag (the atmosphere doesn't have a distinct boundary, it just sort of "fades out") and their orbits decay over time.
Satellites in higher orbits are often "parked" in a "graveyard" or "disposal" orbit and "passivated" (turned off), away from the more popular orbits, here they will sit for a long time.

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Re: How do satellites not crash into each other?

Postby jewish_scientist » Wed Feb 24, 2016 1:36 pm UTC

I would imagine that the organization launching the satellite would independently cross reference its planned orbit with the orbits of all known satellites. I would not want to tell to my boss that a decade long, multimillion dollar project is a complete failure, because someone forgot to look both ways before crossing the street.
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Re: How do satellites not crash into each other?

Postby Neil_Boekend » Wed Feb 24, 2016 1:40 pm UTC

That is rather difficult. There are millions of pieces that can destroy a satellite, and below 100mm earth based radar cannot track it (while a 100 mm piece of steel at orbital velocities can punch right through a satellite). The chances are small, although I imagine it is possible to insure against it.
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Re: How do satellites not crash into each other?

Postby p1t1o » Wed Feb 24, 2016 2:41 pm UTC

Apparently insurance costs are one of the most significant factors in the price of putting things in space.

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Re: How do satellites not crash into each other?

Postby malmensa » Fri Jul 08, 2016 6:16 am UTC

Occasionally they do!

It is actually going to be quite a big problem soon. A lot of satellites are in geostationary orbit, this means a set distance from Earth. The probability of a satellite hitting another is very low, HOWEVER, a particle the size of a grain of sand can easily smash a satellite into fragments. More fragments, means a higher probability of another collision. A satellite in a million pieces has a million times the chance of hitting another.

When you make some assumptions and "do the math", it seem that a mess is inevitable; once a chain reaction of fragmenting satellites gets going, it will become impossible to use geostationary orbits.

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Re: How do satellites not crash into each other?

Postby Eebster the Great » Fri Jul 08, 2016 1:42 pm UTC

malmensa wrote:When you make some assumptions and "do the math", it seem that a mess is inevitable; once a chain reaction of fragmenting satellites gets going, it will become impossible to use geostationary orbits.

It is possible to harden satellites against some types of debris, but nobody actually does this currently.

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Re: How do satellites not crash into each other?

Postby Soupspoon » Fri Jul 08, 2016 2:12 pm UTC

The tougher they are, the worse tbe debris wben they do succumb. They should do the opposite. Make them so fragile that when they inevitably explode into bits, the bits are submicroscopic and harmless. Except maybe to especially fragile satellites... erm... But ignore that minor detail.

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Re: How do satellites not crash into each other?

Postby Echo244 » Fri Jul 08, 2016 4:02 pm UTC

What about encasing them in some kind of viscous goo, so that anything that hits them (up to a certain limit), sticks? Yes, you then have the problem that that will change their orbit, but better one big sticky slightly-out-of-place agglomeration of satellites than endlessly multiplying debris, no?
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Re: How do satellites not crash into each other?

Postby speising » Fri Jul 08, 2016 4:04 pm UTC

Are highspeed collisions actually possible in GEO? I'd think all satellites have to move in the same direction at the same speed, don't they?

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Re: How do satellites not crash into each other?

Postby Xenomortis » Fri Jul 08, 2016 4:08 pm UTC

Not all geosynchronous orbits are geostationary ones.
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Re: How do satellites not crash into each other?

Postby Soupspoon » Fri Jul 08, 2016 4:19 pm UTC

And there's nothing stopping high delta-Ved fragments from another orbiter's destruction (say one originally in a polar orbit) from now passing up/down through the geostationary 'ring' (or geosychronous zone in general) at various oblique angles with horribly intense closing velocities to intersect with the unsuspecting well-behaved satellite.

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Re: How do satellites not crash into each other?

Postby Zohar » Fri Jul 08, 2016 5:45 pm UTC

You also need to consider things get REALLY HOT when the satellites are in the sun and REALLY COLD when they're not, in addition to being bombarded by radiation and being in a vacuum. That really puts a limit on what sort of materials you can use up there.
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Re: How do satellites not crash into each other?

Postby sardia » Sat Jul 09, 2016 8:41 pm UTC

Echo244 wrote:What about encasing them in some kind of viscous goo, so that anything that hits them (up to a certain limit), sticks? Yes, you then have the problem that that will change their orbit, but better one big sticky slightly-out-of-place agglomeration of satellites than endlessly multiplying debris, no?

The key thing is to avoid collisions, which causes debris, which eventually causes kessler syndrome. The cure is orbital decay from drag. All debris will eventually fall back to earth. Wait long enough, and the problem goes away.

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Re: How do satellites not crash into each other?

Postby Sableagle » Sun Jul 10, 2016 1:44 pm UTC

So we need someone to build the Transporter from Elite, fit it with fuel scoops and send it up there to salvage all those pieces for sale to space museums around the world, or a gravity gun to yank them all down so they'll hit denser air and burn up?

There used to be a thing called Jane's Space Directory, which had all the orbits in it, but it got discontinued years ago.
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Re: How do satellites not crash into each other?

Postby Wolfkeeper » Mon Jul 11, 2016 12:15 am UTC

Echo244 wrote:What about encasing them in some kind of viscous goo, so that anything that hits them (up to a certain limit), sticks? Yes, you then have the problem that that will change their orbit, but better one big sticky slightly-out-of-place agglomeration of satellites than endlessly multiplying debris, no?


That doesn't really work, the impact speed is typically so high that everything just... explodes.

One deployed protection is the Whipple shield, it's just a thin layer of material in front of a thicker layer.

https://en.wikipedia.org/wiki/Whipple_shield

However, being hit by any large fragment is probably not survivable.

At GEO altitudes the problem is a bit easier; orbital speeds are lower and the space is bigger (volume goes as a cube law on distance), so impacts are rarer.

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Re: How do satellites not crash into each other?

Postby malmensa » Mon Jul 11, 2016 1:42 am UTC

Bear in mind, that in this context, a bullet is a large, slow moving object. The more common type of debris would be a fleck of paint, moving at ten times the speed of a bullet.
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Re: How do satellites not crash into each other?

Postby Neil_Boekend » Mon Jul 11, 2016 2:05 pm UTC

Wolfkeeper wrote:At GEO altitudes the problem is a bit easier; orbital speeds are lower and the space is bigger (volume goes as a cube law on distance), so impacts are rarer.

Only if both objects are geostationary. If one is a bit of paint from a lower satellite that has an eccentric weird orbit that happens to cross geostationary you're screwed. According to wiki the orbital speed at geostationary is just over 11000 km/h so even in the unlikely case the fleck of paint is hanging still in space the impact is rather significant.

Currently it about one satellite a year is destroyed by a collision in space, and all satellites that are being sent up for something that falls under the FCC must be able to go to a graveyard orbit 300 km higher.

The Chinese government isn't very concerned about such issues. Note: the USA did a similar test in the 80's.
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Re: How do satellites not crash into each other?

Postby gmalivuk » Mon Jul 11, 2016 7:29 pm UTC

Neil_Boekend wrote:even in the unlikely case the fleck of paint is hanging still in space the impact is rather significant.
You say "even" as though that is a less significant impact than one would normally get in GEO, but in fact it's a far greater one than average. With 3km/s as the orbital velocity for that distance, most things are going to be traveling close to 3km/s in close to the same direction.

If a fleck of paint falls off of one geostationary satellite and ends up in a slightly different orbit than GEO, it's going to re-intersect that orbit at the same speed it left. Which for typical flecks of paint, isn't that great a speed.

Something else in a geosynchronous orbit with a different plane isn't going to intersect GEO at 3km/s relative to the other things there unless that plane is 60º inclined from the equator.
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Re: How do satellites not crash into each other?

Postby Neil_Boekend » Mon Jul 11, 2016 7:55 pm UTC

I'm way out of my league, but wouldn't a speck of paint from a sat in a polar orbit be basically perpendicular?
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Re: How do satellites not crash into each other?

Postby gmalivuk » Tue Jul 12, 2016 12:52 am UTC

Neil_Boekend wrote:I'm way out of my league, but wouldn't a speck of paint from a sat in a polar orbit be basically perpendicular?

Sure, but then so is said satellite itself, which means it probably wasn't put terribly close to any existing satellite's equatorial orbit.
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Re: How do satellites not crash into each other?

Postby Neil_Boekend » Tue Jul 12, 2016 7:39 am UTC

gmalivuk wrote:
Neil_Boekend wrote:I'm way out of my league, but wouldn't a speck of paint from a sat in a polar orbit be basically perpendicular?

Sure, but then so is said satellite itself, which means it probably wasn't put terribly close to any existing satellite's equatorial orbit.

Ah, of course. It would be way above or below the geostationary orbit, right? And it would have to be a significant first impact to create a fleck of paint or piece of metal that can gain such altitude (even in an highly elliptical orbit).
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Re: How do satellites not crash into each other?

Postby Eebster the Great » Tue Jul 12, 2016 9:26 am UTC

The Wikipedia article suggests the area of greatest concern is LEO, not GSO. Can anyone speculate as to why? Is it because there are so many more satellites there on more diverse trajectories? Or because they decay more quickly? Clearly the volume of LEO is far greater than the volume of GSO.

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Re: How do satellites not crash into each other?

Postby p1t1o » Tue Jul 12, 2016 9:56 am UTC

Maybe we should stop putting so much damn paint on our spacecraft...

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Re: How do satellites not crash into each other?

Postby Tyndmyr » Tue Jul 12, 2016 5:45 pm UTC

Echo244 wrote:What about encasing them in some kind of viscous goo, so that anything that hits them (up to a certain limit), sticks? Yes, you then have the problem that that will change their orbit, but better one big sticky slightly-out-of-place agglomeration of satellites than endlessly multiplying debris, no?


Goo doesn't usually work well in space. Anything even vaguely volatile tends to offgas pretty hard.

Plus, delta-v is high.

If it's an extremely low delta-v collision, which is...improbable, then gravity would tend to stick them together regardless. So, not a lot of actual gain from stickiness.

This is significantly more difficult than putting honey on your hands in order to catch bullets.

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Re: How do satellites not crash into each other?

Postby speising » Tue Jul 12, 2016 5:46 pm UTC

p1t1o wrote:Maybe we should stop putting so much damn paint on our spacecraft...

How do you expect them to whizz around without racing stripes?

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Re: How do satellites not crash into each other?

Postby Xanthir » Tue Jul 12, 2016 6:06 pm UTC

Especially since the red ones make them go so much faster!
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Re: How do satellites not crash into each other?

Postby cyanyoshi » Tue Jul 12, 2016 8:07 pm UTC

Eebster the Great wrote:The Wikipedia article suggests the area of greatest concern is LEO, not GSO. Can anyone speculate as to why? Is it because there are so many more satellites there on more diverse trajectories? Or because they decay more quickly? Clearly the volume of LEO is far greater than the volume of GSO.

There are a few reasons why LEO might be more problematic than GSO. This demonstration suggests that there is a higher density of space junk near LEO, which makes avoiding that stuff tricky. A lower orbit also causes the nonspherical features of Earth's gravitational field to play an important role. This can make trajectories behave in strange ways that are hard to predict. The uncertain effects of drag at those altitudes also make things difficult, but that can work in our favor by making the orbits less eccentric and eventually causing the space debris to fall to Earth (which is a good thing in this case).

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Re: How do satellites not crash into each other?

Postby Cleverbeans » Tue Jul 12, 2016 9:47 pm UTC

I recall someone asking how NASA would avoid asteroids in the asteroid belt when exploring the further sections of the solar system. The way they do it just not consider it at all. The distances involved are literally astronomical so the probability of a collision is functionally zero so there is no need to even account for it. I'd be curious to know at what distance the orbit would have to be so that you could essentially guarantee no collisions would occur.
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Re: How do satellites not crash into each other?

Postby ijuin » Tue Jul 12, 2016 10:35 pm UTC

You cannot guarantee it to the degree of saying that it is utterly impossible, since there could always be some small rock out there that never showed up on radar. You can however define it statistically, calculating for example that a given flight path has a one in a million (or however many) probability of hitting anything.

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Re: How do satellites not crash into each other?

Postby cyanyoshi » Tue Jul 12, 2016 10:43 pm UTC

It always comes down to a risk/benefit analysis and a judgement call. Having a 1% chance of having a broken spacecraft could be worth it if it would cost 10% more to keep it a not-broken spacecraft, but I think it's fair to say that we're past the point where you can't launch a satellite into LEO without first considering the space junk in its vicinity.

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Re: How do satellites not crash into each other?

Postby DaBigCheez » Wed Jul 13, 2016 1:13 am UTC

Eebster the Great wrote:The Wikipedia article suggests the area of greatest concern is LEO, not GSO. Can anyone speculate as to why? Is it because there are so many more satellites there on more diverse trajectories? Or because they decay more quickly? Clearly the volume of LEO is far greater than the volume of GSO.

My guesses include "wider variety of orbits = higher likely velocity on impact" and...the denial of the claim that "the volume of LEO is far greater than the volume of GSO". When you're working with the surface of a sphere with less than a quarter the radius, things are going to be a bit more concentrated even if you allow for a greater shell thickness. (Especially given that unpowered satellites will tend to drift slightly out of their assigned orbits over time, so "abandoned" GSO satellites aren't restricted as closely to either equatorial orbits or the tighter distance band, widening the available volume considerably.)

Given that GSO is 22,400 mi above surface level and LEO is often 100 or so...yeah. Lot more space to work with.
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Re: How do satellites not crash into each other?

Postby Eebster the Great » Wed Jul 13, 2016 6:50 am UTC

Yeah it's a significantly larger radius, but there is no way the volume is greater. There is practically no thickness to GSO, as all circular orbits have a radius of 42,164 km, whereas low Earth orbits stretch from 160 km all the way to 2,000 km. I know GSO includes elliptical orbits, but those can't be geostationary (which many satellites are), and even then they are usually not so eccentric, and they pass through the geostationary altitude.

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Re: How do satellites not crash into each other?

Postby Soupspoon » Wed Jul 13, 2016 11:34 am UTC

Eebster the Great wrote:Yeah it's a significantly larger radius, but there is no way the volume is greater. There is practically no thickness to GSO, as all circular orbits have a radius of 42,164 km, whereas low Earth orbits stretch from 160 km all the way to 2,000 km. I know GSO includes elliptical orbits, but those can't be geostationary (which many satellites are), and even then they are usually not so eccentric, and they pass through the geostationary altitude.

GSO is "Geosynchronous", and contains a wide variety of orbits that typically trace out (small) figure-8 paths around a 'stationary' point as seen from the ground. "Geostationary" is the special case where no (significant, FCVO...) drifting around the point occurs, thanks to a precise (or perhaps intermittently continual) station-keeping thrust, upon its final insertion.

GEO is 'thin' insofar as functionally there's no reason to be drifting at GEO±(a small value), unless you're thete to maintain/attack any number of GEO satellites and are transitting. Given the difficulty of getting there, there's not much reason to send up anything not intended to be upon that narrow (but 'volumatic', even given the restructions) equatorial orbit. (Sun Synchronous Polar Orbits, for earth observations where shadows/heating from the sun are desired to be consistant between each pass over a point, exploit the precessive qualities of orbits slightly retrograde of 90°, at heights of a mere few hundred miles. Also means more surface can be seen, and nearer, than from a 'geosychronous polar' orbit, which sounds... not so useful.)

LEO is far easy(/ier) to get to, without so much precision needed even if you're interested in accuracy of position once your package is there. While its vertical seperation is greater, its horizontal area is far smaller and there's a lot more 'stuff' whizzing in (and potentially through) that volume of space, without even an expectation of a known common relative separation between any two arbitrary occupants.

But I'd have to peruse and perhaps spreadsheet a decently complete list of orbital items to be sure of.this, my somewhat instinctive grok.

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Re: How do satellites not crash into each other?

Postby p1t1o » Wed Jul 13, 2016 12:12 pm UTC

Just for reference, I banged out a few volume calculations:

Volume of a 1km thick shell at GSO = ~2.30e19m^3
Volume of 200-1000km LEO shell = ~4.89e20m^3
Volume of 200-300km LEO shell = ~1.70e20m^3
Volume of 200-250km LEO shell = ~2.373e19m^3
Volume of 200-240km LEO shell = ~2.18e19m^3

So as a general rule of thumb, GSO has very roughly the same volume as a 50km deep shell of low orbit.

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Re: How do satellites not crash into each other?

Postby Neil_Boekend » Wed Jul 13, 2016 2:33 pm UTC

Correct me if I am wrong, but why would GSO be a shell? I'd expect more of a donut shape with a primary diameter of 42,164 km and a secondary diameter of 1km. This results in a torus volume of 8.3*1014 m3. You'd need only a tiny slice of LEO to make that volume.
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