Sounds a bit Klingon...
The European Space Agency is launching a new research project to study satellite collisions in space. “We want to understand what happens when two satellites collide,” said Tiziana Cardone, an ESA structural engineer leading the project, on Tuesday. “Up until now a lot of assumptions have been made about how the very high …
Which could arise from my ignorant presumptions.
1- satellites orbit in the same direction
1a - 'circum-equatorial' and 'circum-polar' orbits are at different altitudes (like air lanes) to avoid dangerous crossing points.
2 - objects in the same orbit are travelling at the same speed (otherwise they would quickly diverge into separate orbits).
So, where, how and at what relative speeds do satellites collide, please?
I can understand that the debris will be scattered widely into different orbital paths and so be moving at very large speeds relative to anything they encounter in these new orbits but not clear on the initial collision conditions.
@Alister, As TechnicalBen put it; your rendezvous closing speeds will be relatively low.
I believe that rendezvous manoeuvres can include changing orbits for the 'catch-up' or 'waiting-around' part of the process but then changing speed to get back into the correct orbit for the actual contact.
"I believe that rendezvous manoeuvres can include changing orbits for the 'catch-up' or 'waiting-around' part of the process but then changing speed to get back into the correct orbit for the actual contact."
Yup, pretty much; orbital velocity decreases as you move further out, so if you want to catch up with something that's ahead of you then you stay in, or drop down to, a lower and faster orbit whereas, if you want to rendezvous with something behind you, you'd move further out to a higher and slower orbit.
The basic principle in orbital dynamics is: speeding up takes you out and slows you down; slowing down takes you in and speeds you up. But note, assuming an initial circular orbit, that you need two burns for each change; a single burn will just put you in an elliptical orbit, where you will cycle between moving out and slowing down (at apogee) then moving back in and speeding up (at perigee).
Satellites don't all traipse round in neat circular equatorial orbits. Most are inclined to the equator and many have elliptical orbits, some very elliptical indeed. Imagine a three-dimensional motorway with traffic continally swapping lanes; sooner or later someone's going to get side-swiped.
That's useful information (I'm now wondering what the value is of satellites in highly elliptical orbits but that's for another day) and it does explain why satellites in elliptical orbits near perigee would be travelling much faster than satellites at a comparable altitude in a circular orbit and therefore create a large debris field.
But, sorry, couldn't the perigee of the highly elliptically orbiting satellites be kept at a slightly higher altitude than the satellites in roughly circular orbits? I know there are orbits other than LEO but as you move out from the Earth you do get the property of space being big making collisions less likely - and presumably fewer satellites being out there as well.
And again, presumably there is some mandated separation between satellites orbiting at different inclinations to the equator (as I posited for circum-equatorial versus circum-polar orbits).
It just seems to me (from the afore-mentioned position of ignorance) that this should be much better ordered and regular than "a three-dimensional motorway with traffic continually swapping lanes".
Thanks for the reply.
A lot of what is in orbit is junk from old launches. Only a fraction of it is currently working and controllable satellites. Think about explosive bolts used to separate stages, empty upper stages that blew up, or just wandered off, dead satellites whose orbits wander over time, and so on. Anything not active that isn't in a stable Lagrange point will drift from irregularities in the earth's mass distribution, effects of the moon and other planets, atmospheric drag, and even uneven solar heating/illumination.
"1- satellites orbit in the same direction"
Initially yes, but it doesn't take long for them to start crossing paths at anything from "nearly parallel" to "head on" paths.
2: Impact speed is dependent on approach angle and varies from 2 * orbital velocity (head on collision) to almost zero (matching orbits)
It's not just collisions either. Several second stages launched in the early days spontaneously exploded after some several years in orbit, leading to changes in mission policies that all valves were opened and every thing vented at the end of the operation and since concerns about obiting junk were raised in the 80s to more modern policies that stuff is deorbited quickly (Skylab's booster took several months to come down and wasn't controlled. NASA and the USAF took a close interest in where/when it came down as they felt it was a good proxy for how Skylab would eventually deorbit.)
Bear in mind that only the "big stuff" is being tracked and counted. Depending on whose figures you believe, nothing smaller than 5cm or 10cm is trackable. It only took a fleck of paint to gouge a chunk out of a space shuttle window.
>1- satellites orbit in the same direction
Where did this assumption come from?
Most satellites are launched, from near-equatorial launchpads, to use the rotation of the earth to give a little free boost to escape velocity. So, they tend to round thataway (unless they go polar or inclined).
The experiment could be in space, and still not cause permanent debris. Simply have two satellites, aimed carefully at a convenient very-low-orbit crashpoint over the Pacific Spacecraft Graveyard. The immediate debris would make a lovely display, which could be examined for trajectory, brightness and spectrum. That'd give a rough idea of what came out of the crash, size and material and all. Of course there would be things that headed out - but it's an orbit. They'll be back, to very-low-orbit. The whole mess would decay rapidly into the atmosphere, and we'd know where to watch for it.
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