Should man-made satellites feature an intelligent inverted "cow catcher"?

Question

Disclaimer: I have no formal education in science. Also, I'm a dummy!

Locomotives usually feature a shield that deflects objects on a train's track.

My understanding is that since satellites gradually succumb to gravity their path is constantly changing to a lower and lower altitude relative to earth. This suggest to me that the first contact with debris that a satellite (for example, the ISS) would be likely have would be on that bottom edge.

It seems to me then, that by having an inverted cow catcher on the bottom of the station it might be possible to deflect debris toward the earth where it can be burned up on entry into the atmosphere.

The intelligent part might be that it would include a detector that would scan ahead for incoming debris and, taking into account other vulnerable satellites in the area it would adjust the angle of the cow catcher to deflect earthward.

The catcher could be coupled in a way that would allow it to absorb impact gradually by continuous springs.

The effect then would be that the satellites, particularly the ISS, of course, would pre-sweep what will be their subsequent path due to gravitation so when they descend they know that they are not in for any surprises.

The cow catcher would catch any cows who happen to be in the area, jumping over the moon. :)

^{Obstruction clearing device ("Cow catcher") on narrow gauge locomotive LWR6, Jokioinen Museum Railway (WikiMedia}

@Ruminator Nooooooooooo! We need a visualization, please add the cow catching device image! :) — varun, Apr 21 '19 at 13:31
I like it. I'm starting to appreciate the term "not a rocket scientist"! — Ruminator, Apr 21 '19 at 14:31
Obligatory xkcd: Space isn't about being high up, it's about going fast — Daniel Jour, Apr 21 '19 at 18:40
Thanks. I kind of knew that already but I had an image from a Sandra Bullock movie or something in my imagination where that info was supposed to be stored! — Ruminator, Apr 21 '19 at 18:49
Try playing some Kerbal Space Program (you can get it cheaply on Steam) to get a feel for orbital mechanics. It is not at all like the kind of motion we are used to on earth. — Mike Wise, Apr 21 '19 at 20:48
@Ruminator Omg thanks, but I have to admit, my imagination as to what a "cow catcher" would look like deceived me! :P — varun, Apr 22 '19 at 04:48
I really like question from people not biased by space engineering background :) . — Manu H, Apr 22 '19 at 11:03
Well tell me the truth, @ManuH, if it weren't for the expense, you would want a cow catcher on the ISS, you engineering types really would want a cow catcher on the ISS because it would look so darn cool, right?! — Ruminator, Apr 22 '19 at 13:11
And really, it isn't that the cow catcher wouldn't work, but it would require so darn much duck tape! — Ruminator, Apr 22 '19 at 13:12
Another factor--even if things were pretty much as you envision the reality is the debris is smaller than the stuff we are protecting--and smaller stuff comes down faster. Your cow catcher would be on the top, not the bottom! — Loren Pechtel, Apr 22 '19 at 15:19
@LorenPechtel In which case we'll have to provide the ISS with a Stetson: https://www.hatcountry.com/stetson-bozeman-soft-wool-cowboy-hat.html?fee=10&fep=99076 — Ruminator, Apr 22 '19 at 15:26
@Ruminator But since we don't have any moon-jumping cows about wouldn't that be a case of all hat, no cattle? — Loren Pechtel, Apr 22 '19 at 16:58
@Lauren are you really suggesting that there are no cows jumping over the moon anymore? — Ruminator, Apr 22 '19 at 17:01
There has to be a connection between OP's user name and "ruminant", the family cows belong to... — JPhi1618, Apr 22 '19 at 20:09
As Joey Tribbiani of Friends fame would say, it looks like my plan for a cow catcher is "moo"! — Ruminator, Apr 22 '19 at 20:12

score 36 · Accepted Answer · edited Jun 17 '20 at 08:54

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...adjust the angle of the cow catcher to deflect earthward

The catcher could be coupled in a way that would allow it to absorb impact gradually by continuous springs.

The problem with this is that it's not possible to deflect debris. Things in orbit are moving around at 10 km/s (20,000 mph!) and when they collide, the impact is so energetic they basically vaporise. This is known as hypervelocity. Any debris striking a cow-catcher will just punch a hole through.

Currently the way spacecraft are protected from debris is through the use of Whipple shields. The principle of operation is there is a relatively thin aluminium sheet on the outside of the spacecraft, separated from the main wall by a gap. When a piece of debris strikes the outer shield, it and a portion of the thin sheet vaporise and keep going towards the main wall. However, in this process, they spread out, so the pressure of the impact on the main wall is reduced and (hopefully) they don't penetrate the main wall.

...their path is constantly changing to a lower altitude. This suggest to me that the first contact with debris would be likely have would be on that bottom edge.

Satellites' orbits do decay to tenuous atmospheric drag; however, the rate of change in altitude is insignificant compared to the orbital velocity (10 km/s) so on average debris will just strike the front and sides of the spacecraft. (When debris collides with a spacecraft, remember that it's 2 orbital trajectories crossing)

edited Jun 17 '20 at 08:54

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answered Apr 21 '19 at 11:52

binaryfunt

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So there is no debris just floating motionless? I see. I was picturing nuts an bolts just hanging there. But as I think about it, they would also be descending anyway! Thanks for the excellent answer. – Ruminator Apr 21 '19 at 12:03
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@Ruminator It's tempting to think things just float in space. But you need to remember everything up there is orbiting, otherwise they would indeed straight down to Earth. If e.g. an astronaut on a spacewalk lets go of a bolt, it might float away from them at a leisurely pace, but they were orbiting when they let go, so the bolt is orbiting too – binaryfunt Apr 21 '19 at 12:06
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Okay, right, and their orbit is likely tangential to that of our satellites. I see it now. Thanks again. – Ruminator Apr 21 '19 at 12:11
@MrLister Well, drag sort of slows down objects, but as they lower in altitude they actually speed up, counter-intuitively – binaryfunt Apr 21 '19 at 15:55
Things in orbit may be moving at ~10 km/sec with respect to the Earth, but they are seldom moving that fast WRT each other. (As a limiting case, consider a mission docking with the ISS.) Most things in low Earth orbit are launched into prograde, nearly equatorial orbits, so relative velocites are much less than orbital velocity. – jamesqf Apr 21 '19 at 16:07
@jamesqf 10 km/s was an order-of-magnitude figure, and in fact is pretty accurate for the average impact velocity, according to (Kessler 1978). Debris in inclined orbits can cross paths with satellites at significant angles of incidence, so the impact velocity will be about that high – binaryfunt Apr 21 '19 at 16:29
@binaryfunt: I suppose you mean the angular velocity increases, but that is not actually speeding up. – Ben Voigt Apr 22 '19 at 02:29
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@binaryfunt: Sure, things CAN impact at high relative velocities, like something in a polar orbit impacting something in an equatorial orbit. Or even one of the rare satellites that are launched into retrograde orbits, or N-S vs S-N polar orbits. So statistically you have a range of velocities from near zero - that bolt dropped by an astronaut working on the ISS - to ~20 km/sec for a head-on collision. – jamesqf Apr 22 '19 at 04:25
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@BenVoigt From the vis-viva equation, and if we assume the debris takes a shrinking circular orbit, $v=\sqrt{\mu/r}$ so actually the lower $r$ is, the higher the velocity. One simplified way I have of looking at this is that drag imparts a negative delta-v which serves to reduce the altitude on the opposite side, then on the opposite side it's moving at higher speed and again drag imparts a negative delta-v which lowers the apogee etc. – binaryfunt Apr 22 '19 at 10:07
@jamesqf Yes that's technically true, but on the other hand it's not the debris that's moving slowly relative to your satellite that you need to worry about. And the flux of debris from directions where the debris is travelling faster towards it would be higher for the same density of debris – binaryfunt Apr 22 '19 at 10:36
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Nearly nothing in LEO is launched in near-equatorial orbit. It's expensive and not very useful. GEO satellites are sure in equatorial orbit but they are just a small subset. And if a satellite in LEO in 45 degree inclined orbit crossing the equator north to south hits a piece of debris in 45 degree inclined orbit crossing the equator south to north, their relative speed will be of order of 8km/s. For more inclined orbits - even more. – SF. Apr 22 '19 at 13:08
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@Ruminator You're probably thinking of the movie Gravity--the debris appears to hang around because everything is moving at basically the same speed. If the camera weren't traveling with the debris you would get a very different picture (not that you could even see the debris, it would be going too fast.) – Loren Pechtel Apr 22 '19 at 15:24
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@Ruminator actually the movie Gravity is a pretty good example of why this wouldn't work. The opening scene shows an alright example of the dangers of orbital space junk. The debris comes in at an oblique angle and moves at an extreme velocity, punching straight through several large pieces of metal. I wouldn't regard pretty much anything else from that movie as accurate, but that one scene is most of the way there. – bendl Apr 23 '19 at 12:13

score 6 · Answer 2 · answered Apr 22 '19 at 08:34

binaryfunt explained the problem with speeds and energies, but I'll comment on one your assumptions:

My understanding is that since satellites gradually succumb to gravity their path is constantly changing to a lower and lower altitude relative to earth. This suggest to me that the first contact with debris that a satellite (for example, the ISS) would be likely have would be on that bottom edge.

Anything in an orbit has already "succumbed to gravity". If a space station was held still relative to earth and then released, it would indeed start falling towards the ground, but that's not how things start orbiting. When something is in orbit, the gravitational pull is exactly the same as when they would be falling down, but the difference is that they had some initial velocity which results them falling past Earth. That is precisely how, for example, the Moon stays in the sky: it's always pulled toward Earth in the same fashion, but when the system of celestial bodies formed, it already had some speed.

Classically, something in orbit stays in orbit forever unless something drastically lowers its kinetic energy. Gravity all by itself doesn't pull things closer by time. A collision might do this, but that would probably be destructive already, the athmospheric drag is more significant as mentioned, but still a small factor.

That being said, there is a way for orbiters to gradually fall closer and closer to the source of gravitation – in the same way accelerating charges send out electromagnetic radiation, so do masses in orbit also send out gravitational waves and lose part of their energy. But gravitation is a very, very weak interaction compared to electromagnetism and the energy lost to this process is miniscule in human timescales and will not the cause for any satellite to crash. :)

Thanks. That makes all the sense in the world - and beyond. – Ruminator Apr 22 '19 at 08:40 — Ruminator, Apr 22 '19 at 08:40

Should man-made satellites feature an intelligent inverted "cow catcher"?

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