# Question about space junk in orbit

I did not know where else to post this question but put it here since it is more related to mechanics than to astronomy.

I am curious as to why small bits of space junk (bolts for example) are traveling so fast around Earth that they could poke holes in the International Space Station. Are different pieces of junk moving around Earth at different velocities even though they are located in similar (or the same) orbit? When I read news stories about space junk, the same number is mentioned as the speed of orbiting debris in space, approximately 17,000 mi/hr.

Is it because although the objects are the same distance away from the surface of Earth, they might be traveling in different directions?

Last edited:

mgb_phys
Homework Helper
If they are in the same orbit then there is no problem, they are moving at huge speeds (nearer 17,000 mph - your value is about 10% c)
And since most spacecraft are launched west-east to get a boost from the Earth's spin - most are moving in the same direction.

The problem is in low Earth orbit - there are good reasons to have satelites in a polar orbit.
So like a busy intersection you have spacecraft going west-east and others going north-south. Unfortunately many of these spacecraft are from California and so ignore stop signs.

If they are in the same orbit then there is no problem, they are moving at huge speeds (nearer 17,000 mph - your value is about 10% c)
And since most spacecraft are launched west-east to get a boost from the Earth's spin - most are moving in the same direction.

The problem is in low Earth orbit - there are good reasons to have satelites in a polar orbit.
So like a busy intersection you have spacecraft going west-east and others going north-south. Unfortunately many of these spacecraft are from California and so ignore stop signs.

oops I meant 17,000 mi/hr not second.

Thanks for the reply, it makes sense that the reason the small objects are a hazard to the space station is that not all the objects are traveling in the same direction and the orbits cross one another.

rcgldr
Homework Helper
The space station and other low orbit objects are still in the outer fringes of the atmoshpere, so without an occasional boost, they will even eventually slow down and hopefully burn up in the upper atmosphere. The issue of collisions would be stuff with a higher energy but somewhat elliptical orbit. I'm not sure how space junk would end up with an elliptical orbit enough to result in large speed differences if there is a collision.

D H
Staff Emeritus
The issue of collisions would be stuff with a higher energy but somewhat elliptical orbit. I'm not sure how space junk would end up with an elliptical orbit enough to result in large speed differences if there is a collision.
Most pieces of space junk are very small and thus will have a fairly small ballistic coefficients. Orbits of things with a small ballistic coefficient circularize rather quickly. The issue of collisions is not so much eccentricity as it is inclination (which can be anywhere from -20 degrees or so to 100 degrees) and right ascension (pretty much uniformly distributed over 0 to 2*pi).

For example, suppose two orbiting objects collide. If both are in circular orbits at the same LEO altitude, both have a 30 degree inclination, but the right ascensions of ascending node differ by 180 degrees, the relative velocity at collision will be about 17,000 mph (2*sin(30 degrees)*17,000 mph).

rcgldr
Homework Helper
For example, suppose two orbiting objects collide. If both are in circular orbits at the same LEO altitude, both have a 30 degree inclination, but the right ascensions of ascending node differ by 180 degrees, the relative velocity at collision will be about 17,000 mph (2*sin(30 degrees)*17,000 mph).
But the idea behind the low orbit of the space station within the outer fringe of the atmosphere is that any old space junk at the same orbital energy, especially if circular, should have been slowed down by the atmosphere, burnt up, and no longer a threat.

D H
Staff Emeritus
But the idea behind the low orbit of the space station within the outer fringe of the atmosphere is that any old space junk at the same orbital energy, especially if circular, should have been slowed down by the atmosphere, burnt up, and no longer a threat.
Where ever did you get that idea? The only reason the ISS operates in as low an orbit as it does is because of resupply constraints. The Soyuz spacecraft can only get to 425 km or so.

rcgldr
Homework Helper
But the idea behind the low orbit of the space station within the outer fringe of the atmosphere is that any old space junk at the same orbital energy, especially if circular, should have been slowed down by the atmosphere, burnt up, and no longer a threat.

Where ever did you get that idea?
As the ISS constantly loses altitude because of slight atmospheric drag, it needs to be boosted to a higher altitude several times each year.
From wiki:
ISS_altitude_control.htm

Skylab did ultimately slow down and burn up.

DaveC426913
Gold Member
As the ISS constantly loses altitude because of slight atmospheric drag, it needs to be boosted to a higher altitude several times each year.
Yes, but correlation does not equal causation. That isn't why the ISS is in a low orbit, as you seem to intimate.

rcgldr
Homework Helper
That isn't why the ISS is in a low orbit, as you seem to intimate.
True, energy is the main reason, but it is a beneficial side of effect of low earth orbits in the atmospheric fringes. If the object loses power, it eventually slows down and burns up, a method of self disposal.

On the other hand Vangard 1, launched in March 17 1958 (happy belated 50th birthday), lost communications in 1964, but it's still up there orbiting for over 50 years now. Expected life time is 240 years.

http://en.wikipedia.org/wiki/Vanguard_1

Yes, but correlation does not equal causation. That isn't why the ISS is in a low orbit, as you seem to intimate.

Yeah! I mean those NASA guys would never have taken the LEO advantages for space junk into consideration when they designed ISS's shape and shielding and so forth. I mean after all, they're not rocket scien... uh, nevermind.

mgb_phys
Homework Helper
The ISS is in a terrible orbit because of political compromises - it is at a very high inclination so it can be reached from Baikanur which limits the payoad of a shuttle trying to reach it.

russ_watters
Mentor
One thing about some space junk like bolts is they have a very high density and low surface area, which reduces the average effect of atmospheric drag, compared to the ISS which is basically a big sailboat.

D H
Staff Emeritus