If 2 satellites strike each other, what, if any, is the practical difference in the end result depending on the direction each was going? My assumption is any such collision would be a high speed impact (unless for some reason they were very nearly the same velocity - which is not the case I’m looking at - high speed collisions only) which would cause both to do that rapid dissemble that spacecraft are know for and disperse debris from the point of impact. Would there be a practical difference in anything that matters if let’s say the angle of impact was 10 degrees to each other compared to 180 degrees (head on)?
Not really. The energies are vast enough, and satellites built fragile enough, that whether something is shredded or SHREDDED doesn’t much change things.
Because there’s no air drag in space, whether the impact produces high or low velocity fragments doesn’t much matter. It doesn’t change how big the debris dispersion cloud gets; it only changes how long it takes the cloud to get to any particular size.
One important difference is that if they hit head on, much of the debris will be going in directions opposite of the standard direction satellites orbit in (since one of them will already be going that direction). This is very bad since the shrapnel will continue to orbit in the opposite direction, at very high velocity and odd trajectories compared to existing satellites until the pieces hit and damage or destroy them. Which would likely result in a chain reaction, a “Kessler Syndrome” resulting in near-Earth space being unusable for a long period.
Thinking about
Would a head on tend to have more debris deorbit as opposed to the 10 degree strike?
Any sort of collision will end up with an average velocity less than orbital speed. It’ll also result in pieces that are much smaller than the original satellite, which will mean relatively larger air resistance (there’s no hard cutoff for the top of the atmosphere; there are still trace amounts of atmosphere at the heights most satellites orbit at). Both of these will tend to result in most of the material deorbiting relatively quickly.
Also, head-on collisions between satellites would be extremely rare, since there’s almost never a reason to launch a satellite in a retrograde (backwards) orbit, and it would cost significantly more to do so. About the only situation where it could plausibly happen would be between two satellites in polar orbits, which most satellites aren’t.
The other point is orbits degrade, particularly in this point in the sunspot cycle heating the outer atmosphere. I presume a lot of the debris will be lower density-to-area ratio than the complete satellit(s) and deorbit faster. However, we’re still talking months to years, ad a few dense components will be a problem. Also, as their orbits decay, they fall down lower and endanger satellites below then. Plus, the direction of debris is moderately unpredictable. Even sideswipes (“fender benders”) could send debris in odd directions. Even a collission at 30° could be a significnt impact. (My ancient trig tells me, if they are going 18,000mph and orbits 30° to each other, then the collision happens at about 9,000mph - doen’t have to be a T-bone at 90°) The question is what number and size and density of fragments will result. Satellites are built as light as possible, obviously, since gravity is not a structural factor.
You can find, for example, an animation of the Starlink satellites that illustrates they are going in all sorts of orbits to create a cloud of coverage.
https://heavens-above.com/Starlink.aspx
But note too that one recent launch, due to a malfunction of the second stage, put several satellites in the launch in a position where they were expected to burn up on renetry within a few days or weeks.
ETA - Skylab, for example, deorbited after 6 years (1973-1979) due to orbital decay. OTOH, dense pieces of debris like solid batteries would be far less susceptible to drag compared to a hollow tin with sails can like Skylab.
Neither of the highlighted statements is generally true. Resultant mean orbital speed will only be reduced to “less than orbital speed” will only occur in collisions with an angle of incidence of significantly greater than 90 degrees. Were that to occur at any altitude above the lowest part of Low Earth Orbit (LEO) it would actually be a significant concern because it would essentially result in a shower of debris falling wildly through many lower orbits. In general, the collision between two satellites with an angle of incidence of less than 45 degrees will result in a spread of resulting momentum vectors that will throw some into a higher and lower orbits with a greater eccentricity and some spread of inclination, but with the other orbital elements (argument of periapsis, true anomaly, semi-major axis, right ascension of the ascending node) being close to the original body for each debris cloud.
The thermospheric and particle drag forces (what you refer to as ‘air resistance’ although there is really no continuum of air at LEO and above) of the individual components of the debris cloud do not necessarily reduce. Satellites are a lot of hollow space and while most of the structure is lightweight and will be reduced to tiny fragments by a kinetic impact at orbital speeds, many individual components have a much higher ballistic coefficient than the entire satellite, particularly components such as fasteners, connectors, valves, et cetera. Objects in the lower part of LEO will deorbit pretty quickly due to interactions with the thermosphere (about 600 km AMSL depending on the phase of the solar cycle), but above the thermosphere there is really only the charged particle environment and objects can reside for decades in orbit.
While satellites launched into polar and retrograde orbits are certainly not as common as in prograde orbits, they are far from unheard of, and for certain types of earth observation and weather surveillance applications where such orbits are required. They don’t necessarily cost more in terms of the cost of launch (although you don’t get the advantage of carrying the Earth’s rotational momentum with the satellite) but there are a limited number of launch sites suitable for such launches as they have to be launched westward over broad ocean area or virtually uninhabited land. There are fairly narrow windows of orbits dedicated for certain types of retrograde orbits specifically to avoid collisions as well as enhanced requirements in the Orbital Debris Mitigation Standard Practices (ODMSP) for US launch carriers. Polar orbits are typically highly elliptical and so there are only a couple of narrow regions at which they cross lower prograde orbits, and so their orbital parameters are selected to miss high traffic volumes.
Although the debris clouds will mostly follow the original trajectories of each satellite, they will spread out over time, posing an increasing hazard, A cumulative succession of impact events can result in the Kessler syndrome mentioned above, which can potentially deny entire azimuths for decades or even centuries. A broad orbital destruction zone as portrayed in the movie Gravity is unlikely from an incidental impact (certainly not on the timeframe of the film) but an intentional Kessler cascade caused by deliberate collisions or release of debris across broad azimuths and inclinations could certainly deny LEO indefinitely, and that kind of denial action is a very real concern in orbital warfare and counterspace operations.
Stranger
Yes, think of a collision as like an explosion - a cloud of pieces flies in every direction, with a preference to the original direction. (momentum of the common center of mass is conserved). Unless it’s a direct hit, the collision could also create spin, causing pieces to break off and fly in random directions… even much later after the collision.
Net result - unpredictable. Only one way to find out for sure, but we don’t want to. And the Russians or Chinese testing satellite killers by actually blowing up satellites in orbit - Bad idea.
Not unpredictable; we can model collisions and use Monte Carlo methods to tightly bound what distributions can be expected from various individual collisions. Debris does not fly out in all directions; even the errant pieces will largely retain their original portion of orbital momentum, although recollision, drag forces, and solar pressure will eventually disperse them somewhat. And the Russians and Chinese are not the only parties testing ‘satellite killers’, although at least the US has more recently advocated for a moratorium on ASAT weapon development.
Stranger
Both of the highlighted statements are always true. Trigonometry works the same way in space as it does on Earth, and cos(theta) is always going to be less than 1. And the pieces of a fragmented satellite will always have a lower sectional density, overall, than the original.
What do you think “air resistance” is? That’s just the standard name for “particle drag forces”.
Statistics is the art of torturing numbers until the tell you what you want to hear.
As I said, the net cnter of gravity will have the same continuing monentum. The full Monte will give you a good idea, but the individual particles will be chaotic; add spin to the mix from glancing collisions, and debris could fly off unpredictably in any drection.
But yes, the odds that a collision will create debris going opposite to the current orbit (i.e. from 18,000mph westward to 18,000mph eastward) is pretty much impossible. Debris going backward will, like debris pushed downward, probably re-enter quicker, leaving a cone or spray of expanding debris in the direction fo the two orbits. Debris pushed upward will likely eventually return to a lower point and reenter as part of its new orbit. (The higher angle it is pushed away from the earth, the more likely its perigee is a lower point)
Space is huge, and the odds of collision are low, but the amount of junk is steadily increasing. It’s a different set of odds - the odds your spacecraft will be hit is low, but the odds any two of the pieces up there could collide is a lot higher. Between you and n objects there is n chances of collision, whereas between n objects there are (n-1)! possible interactions.
There’s lots of ways for that not to be true if aerodynamics is considered. For an easy example, imagine a cube made from thin sheets that have various bits attached to them to give a non-uniform mass. The cube as a whole has a low sectional density since it’s hollow and there’s no way to orient it so that all the faces are edge-on. But if it breaks apart into six pieces, that’s not the case, and with the non-uniform mass the sheets will tend to fly edge-on, giving a high sectional density.
But even aside from that, the average isn’t that interesting. We care if some significant number of pieces remain in orbit. A satellite with a medium sectional density could be made from pieces with lower and higher density. Maybe the lower ones get dragged out of orbit, but that leaves the higher ones.
This does happen, incidentally. Cubesats are packed in a dispenser and are supposed to spring apart, but can bump (or even stick together) occasionally. Starlink sats are much bigger but are also designed to gently bump into each other on deployment. The rocket is put into a mild spin and the satellites are released. But obviously we’re talking about centimeters/sec velocities, not kilometers/sec.
That’s because they’re intentionally launched to a very low orbit initially. Atmospheric drag is basically exponential with altitude and at low altitudes the satellites will come down in days/weeks. Only if the satellites are alive and have working propulsion can they move to a higher orbit (though it’s still low enough that they come down in years in case of complete failure). So it avoids the “infant mortality” situation completely.
Retrograde orbits are extremely rare, but sun-synchronous orbits are common and before Starlink was actually the most common orbit (>1/3 of all sats, and >1/2 of all LEO sats). And most of those have low eccentricity. Given the high density at the polar regions, a head-on strike is a real risk.
ETA: Technically, sun-synchronous is slightly retrograde. But that’s not what most think of as a true retrograde orbit.
Referencing “trigonometry” without actually addressing the physics of the problem is not a persuasive argument. A collision between two bodies in orbit with an angle of incidence of less than 90 degrees will result in an exchange of momentum which will impact the resulting distribution of lateral velocities, but unless one has significantly less radial velocity than the debris will remain in orbit with most differences being in eccentricity and inclination, as described above. In fact, unless the specific orbital energy of a piece of debris falls below that to remain above the horizon, it will continue in orbit until drag forces reduce the speed. And, as described above, while some fragments, particularly lightweight structure will produce lightweight debris, many discrete components such as fasteners, electrical connectors, valves, are quite dense and will have higher overall ballistic coefficient even in their broadest aspect than the satellite overall. In fact, most satellites and launch vehicle stages, if sealed, would be buoyant in water at Earth’s surface, which is actually a problem when considering expended stages or a loss of mission event, where it now becomes a sea hazard (and in the case of classified payloads, a security risk). When a satellite or expended stage is reduced to debris by a kinetic impact, nearly all of that empty space is eliminated, and all but the lightest debris is more dense than the body overall, notwithstanding what deploying solar arrays, radiator panels, and sun shades does to ‘sectional density’ in their broad aspect.
‘Air resistance’ assumes a continuum fluid, which includes numerous forms of laminar and turbulent drag which greatly increases kinetic losses beyond just the mass in direct contact with the body at the boundary layer. “Particle drag forces” is just direct momentum exchange between the particle and a body without reference to any other particle-to-particle interactions because the mean free path of a rebounding particle essentially precludes it ever recontacting or interacting with another particle that will touch the body.
This is nonsense. There are physical limits on what momentum change even the most extremely unlikely interaction of a piece of debris of a given mass can experience, and overall the distribution of the debris clouds will be quite predictable within specified bounds. This is not a case of “torturing numbers until the tell you what you want to hear” but of the basic laws of Gaussian distributions, which a sufficiently diverse cloud of debris will closely fall into.
Stranger
Some friends of mine were part of the project that resulted in the first ever in-orbit cubesat docking. It’s not what they were planning to do, but they still did it.
Sometimes the best you can do is serve as a lesson for others 
. We were told (by Andrew Kalman, IIRC) to please not put a giant magnet on ours. Passive magnotorqueing works just fine with a small magnet. Don’t be those guys.
True, but it can be extreme- imagine a glancing blow that creates a significant spin resulting in a piece flying off at an odd angle. It’s one thing to say “this is the statistical distribution” and another thing to predict the biggest outlier not only by momentum but also by direction.
As I said, the good thing about orbital dynamics is that stuff thrown significantly vertically (up or down) from the current orbit, or retrograde, will likely have a lower perigee and decay faster. This is scant comfort when possibly measured in multple years.
Again outliers are the problem. As mentioned, fasteners and washers, etc. are small but dense items with much smaller cross-sections. Imagine too a component like a battery, a fairly heftly device with a small aerodynamic cross-section packed into a fairly hollow box of components. The big flat solar panels or shell of the box will decay slowly, the dense battery, not so much. How well tied down inside the satellite is the battery when every ounce of fastener and strapping is critical? No point in planning for thousand-mph collisions, since they (a) are rare and (b) you can’t practically make the satellite proof against those.
Except for launches from Israel.
Just…no. Momentum is conserved, and even a piece that acquires a significant amount of angular momentum (“spin”) is still going to continue traveling closely along the path of the original body, with some spread dictated by the distribution of mass of the components of the debris field and the total momentum of each of the bodies prior to collision. Absent of some other source of impulse, i.e. some large tank of energetic monopropellant within the body detonating because of the shock of impact, pieces do not fly off at any random “odd angle” but instead fall into generally narrow fans of debris field that are predictable within definable statistical bounds.
The widest distribution would be two bodies impacting each other at orthogonal azimuths (which is also the least likely impact event), and even then you’ll very rarely see debris running at 45 degrees between the azimuths of the two original bodies, but instead most debris will continue on close to the original trajectory of the body (at least initially). Any component that would have its trajectory radically altered in direction has also likely lost enough total energy that it will promptly fall out of orbit, but unlike the claim above, an impact does not result in an average velocity for the entire field of “less than orbital speed” unless it is a head-on impact.
Stranger