6 ms·

> The risk comes from debris from a possible collision being ejected into a higher orbit. Is that a thing that can happen? My intuition is it would be possibl

> The risk comes from debris from a possible collision being ejected into a higher orbit. Is that a thing that can happen? My intuition is it would be possible if a rocket currently boosting to a higher orbit were to collide with something on its way there, but two objects in the same orbit colliding couldn't get enough delta-v to actually get to a higher orbit. They could maybe get "higher", but not at orbital speeds and so would rapidly decay. I know next to nothing about orbital mechanics, so maybe someone who does can provide some better insight here.

pilom 6y ago
Set a baseball on top of a basket ball and drop them both from waist high, the baseball will shoot really high into the air by stealing energy from the basketball. The same principle can occur when you start breaking pieces off of spacecraft with collisions. Also the two colliding objects might not be in "the same orbit" one could be in a polar orbit and one could be in a more equitorial orbit. One could be circular and the other highly eliptical, etc. This could boost parts of one of them into a higher orbit (though honestly it would be just as likely to slow things down).
SideburnsOfDoom 6y ago
> Ejected into a higher orbit. Is that a thing that can happen? Apparently not as such. See sibling comment here:https://news.ycombinator.com/item?id=26809297 https://news.ycombinator.com/item?id=26809297
IlliOnato 6y ago
I am no expert either, but a paper-napkin calculation tells me yes, this can happen. Basically, just based on impulse and energy conservation, and some assumptions on the size and number of fragments, a head-on collision of two satellites going opposite directions at the same orbit should send some pieces of debris into much higher orbit. How likely this is to happen in reality, and thus how big a problem this could be is a subject of modeling and much more complex calculations.
- bdamm 6y ago
  It turns out that intuitive reasoning for orbital mechanics is not easy. Two satellites having a head on collision become space junk because now you have many fragments that themselves can't control their altitude or deorbit. Fragments that are ejected from the collision cannot have more than energy than they had originally, nor can they change direction very much. If pieces are ejected vertically, they are always going to have less lateral velocity than they had going into the collision, therefore will end up in an elliptic orbit with a high probability of colliding with the atmosphere at perigree. See "can you throw a baseball from the ISS and hit the earth" (No, you can't.)
  - jholman 6y ago
    I think you're claiming that it is impossible for two colliding objects to release fragments that have more energy than either of the original objects. That's wrong. If it helps, start by imagining two satellite-like machines colliding in interstellar space (i.e. not in orbit), where they were initially moving at 1m/s in opposite directions. Even ignoring the possibility of an explosion (unspent fuel, pressurized areas, I dunno), it's still very easy for interactions to violently fling individual parts outward at speeds higher than 1m/s. Of course, my guess is that the possible amounts of additional delta-v are pretty low, like 100 m/s, and as such the resulting orbit would not be much higher than the original orbit. But that's just wild conjecture.
    - lutorm 6y ago
      If you were to collide two LEO satellites and turn them into fragments, even if you give those fragments much higher velocity than the initial satellites, you _cannot_ launch them into a higher orbit. Fragments that are launched down obviously hit the Earth. But an orbit is a closed ellipse, so the fragments that are launched up will _also_ hit the Earth -- they'll just go up steeply, turn around, and come down. The only fragments that won't initially hit the Earth are the ones that are ejected tangentially, parallel to the Earth's surface. Those will go into an elliptical orbit with a high apogee and perigee at the altitude of the collision. Which means they will _still _come down to that altitude and gradually lose energy.
Dylan16807 6y ago
The way to think about orbits is not one height but two: apoapsis and periapsis. Debris from an explosion/collision can have an unlimited apoapsis, but the periapsis is at most the altitude of the incident. So if it started in low orbit, it will still drop down regularly, and drag will still remove it over time.
robocat 6y ago
The intuition is: all the debris from a collision now has an orbit passing through the point of collision (at that point of time). Collision at 300km means all the debris must have an orbit that passes through 300km at that moment. The orbits will be elliptical: exactly how elliptical depends on the directions of the two objects hitting each other and the dispersion of the fragments (how much the fragments have sped up or slowed down compared with original objects, and change of direction). The fragments the stay in orbit the longest should be those that remain closest to a circular orbit?

pilom 6y ago

Set a baseball on top of a basket ball and drop them both from waist high, the baseball will shoot really high into the air by stealing energy from the basketball. The same principle can occur when you start breaking pieces off of spacecraft with collisions. Also the two colliding objects might not be in "the same orbit" one could be in a polar orbit and one could be in a more equitorial orbit. One could be circular and the other highly eliptical, etc. This could boost parts of one of them into a higher orbit (though honestly it would be just as likely to slow things down).

SideburnsOfDoom 6y ago

> Ejected into a higher orbit. Is that a thing that can happen? Apparently not as such. See sibling comment here:https://news.ycombinator.com/item?id=26809297 https://news.ycombinator.com/item?id=26809297

IlliOnato 6y ago

I am no expert either, but a paper-napkin calculation tells me yes, this can happen. Basically, just based on impulse and energy conservation, and some assumptions on the size and number of fragments, a head-on collision of two satellites going opposite directions at the same orbit should send some pieces of debris into much higher orbit. How likely this is to happen in reality, and thus how big a problem this could be is a subject of modeling and much more complex calculations.

bdamm 6y ago
It turns out that intuitive reasoning for orbital mechanics is not easy. Two satellites having a head on collision become space junk because now you have many fragments that themselves can't control their altitude or deorbit. Fragments that are ejected from the collision cannot have more than energy than they had originally, nor can they change direction very much. If pieces are ejected vertically, they are always going to have less lateral velocity than they had going into the collision, therefore will end up in an elliptic orbit with a high probability of colliding with the atmosphere at perigree. See "can you throw a baseball from the ISS and hit the earth" (No, you can't.)
- jholman 6y ago
  I think you're claiming that it is impossible for two colliding objects to release fragments that have more energy than either of the original objects. That's wrong. If it helps, start by imagining two satellite-like machines colliding in interstellar space (i.e. not in orbit), where they were initially moving at 1m/s in opposite directions. Even ignoring the possibility of an explosion (unspent fuel, pressurized areas, I dunno), it's still very easy for interactions to violently fling individual parts outward at speeds higher than 1m/s. Of course, my guess is that the possible amounts of additional delta-v are pretty low, like 100 m/s, and as such the resulting orbit would not be much higher than the original orbit. But that's just wild conjecture.
  - lutorm 6y ago
    If you were to collide two LEO satellites and turn them into fragments, even if you give those fragments much higher velocity than the initial satellites, you _cannot_ launch them into a higher orbit. Fragments that are launched down obviously hit the Earth. But an orbit is a closed ellipse, so the fragments that are launched up will _also_ hit the Earth -- they'll just go up steeply, turn around, and come down. The only fragments that won't initially hit the Earth are the ones that are ejected tangentially, parallel to the Earth's surface. Those will go into an elliptical orbit with a high apogee and perigee at the altitude of the collision. Which means they will _still _come down to that altitude and gradually lose energy.

Dylan16807 6y ago

The way to think about orbits is not one height but two: apoapsis and periapsis. Debris from an explosion/collision can have an unlimited apoapsis, but the periapsis is at most the altitude of the incident. So if it started in low orbit, it will still drop down regularly, and drag will still remove it over time.

robocat 6y ago

The intuition is: all the debris from a collision now has an orbit passing through the point of collision (at that point of time). Collision at 300km means all the debris must have an orbit that passes through 300km at that moment. The orbits will be elliptical: exactly how elliptical depends on the directions of the two objects hitting each other and the dispersion of the fragments (how much the fragments have sped up or slowed down compared with original objects, and change of direction). The fragments the stay in orbit the longest should be those that remain closest to a circular orbit?