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> But higher orbits are just higher speeds and when two objects collide and break apart some smaller fragments might fly off with speeds far greater than those

> But higher orbits are just higher speeds and when two objects collide and break apart some smaller fragments might fly off with speeds far greater than those of colliding objects. Objects in higher orbits are slower[1] than objects at lower altitudes, and debris from a hypothetical collision between two high-orbiting objects would not accelerate to a speed "far greater" than the originating objects, because that increase in speed would require some form of acceleration. One satellite smashing into another would not add sufficient energy to the debris to alter its speed significantly. Any change to the speed of the resulting debris, if it were to occur, would also necessarily alter its orbit. The object's speed and its orbital altitude depend on each other. > So failure at very low orbit might add a lot of debris to higher orbits that might trigger Kessler syndrome. It should be noted that "failure" does not mean "exploding into a thousand pieces", so a malfunctioning satellite by itself is not that much of a problem. Low altitude satellites such as Starlink are actually better from a space junk perspective — their altitude means the effect of atmospheric drag on the spacecraft will eventually (within months, sometimes) cause it to re-enter the atmosphere instead of just hanging around in Earth orbit forever, like dead satellites do at higher orbits. Kessler syndrome would be more likely to occur with satellites in medium and high Earth orbit, since there is no drag to bring the dead spacecraft or debris back down to Earth. Anti-satellite weapons are particularly dangerous here, because they're likely to target high-orbiting satellites and leave debris fields behind. --- [1] As pointed below, in terms of pure kinetic energy this is not correct. I was referring to apparent speed vs an object in low-Earth orbit.

scotty79 4y ago
> Objects in higher orbits are slower than objects at lower altitudes You are right. My point still stands. On collision debris might have significantly different velocities than the original objects thus have different orbits. > One satellite smashing into another would not add sufficient energy to the debris to alter its speed significantly. I think that's completely dependant on the mass of the specific particle of debris. The smaller the particle, the larger possible speed change attainable in a messy collision. I'd really prefer that there was some scientific study simulating possible results of impact between two SpaceX satellites. Because there's non-zero chance of that happening. For example if there are some errors during the deployment of new batch.
- gatonegro 4y ago
  Yes, sorry, my phrasing is somewhat confusing. In terms of energy they are faster, since more speed is required for higher orbits. They appear to be slower in a relative sense when observed from Earth — the Space Station or a Starlink sat will appear to be moving significantly faster than a satellite in medium or high-Earth orbit. It also depends on the geometry of the orbit — an object in a circular orbit will move at a constant speed, while an object in an elliptical orbit will move faster or slower depending on which point of the ellipse it is. I understood the parent comment to be focused on an Earth-centred perspective, since they were talking about Kessler syndrome and Earth-orbiting debris. I was trying to address that point from an Earth-centred perspective too, but I appreciate how that can be confusing.
  - scotty79 4y ago
    No, no. You were right about the speeds and I was wrong. Higher circular orbits do have lower speeds. Starlink satellites have speeds over 7km/s while satellites on much higher geostationary orbit have little over 3km/s. I haven't built my intuition about why it's true but it is. Orbital speed is proportional to inverse square root of orbit radius.

scotty79 4y ago

> Objects in higher orbits are slower than objects at lower altitudes You are right. My point still stands. On collision debris might have significantly different velocities than the original objects thus have different orbits. > One satellite smashing into another would not add sufficient energy to the debris to alter its speed significantly. I think that's completely dependant on the mass of the specific particle of debris. The smaller the particle, the larger possible speed change attainable in a messy collision. I'd really prefer that there was some scientific study simulating possible results of impact between two SpaceX satellites. Because there's non-zero chance of that happening. For example if there are some errors during the deployment of new batch.

gatonegro 4y ago
Yes, sorry, my phrasing is somewhat confusing. In terms of energy they are faster, since more speed is required for higher orbits. They appear to be slower in a relative sense when observed from Earth — the Space Station or a Starlink sat will appear to be moving significantly faster than a satellite in medium or high-Earth orbit. It also depends on the geometry of the orbit — an object in a circular orbit will move at a constant speed, while an object in an elliptical orbit will move faster or slower depending on which point of the ellipse it is. I understood the parent comment to be focused on an Earth-centred perspective, since they were talking about Kessler syndrome and Earth-orbiting debris. I was trying to address that point from an Earth-centred perspective too, but I appreciate how that can be confusing.
- scotty79 4y ago
  No, no. You were right about the speeds and I was wrong. Higher circular orbits do have lower speeds. Starlink satellites have speeds over 7km/s while satellites on much higher geostationary orbit have little over 3km/s. I haven't built my intuition about why it's true but it is. Orbital speed is proportional to inverse square root of orbit radius.