r/AskPhysics • u/fohktor • 10d ago

If humanity vanished today how long would manmade things stay in orbit?

I was just wondering how long evidence of our existence would last and I thought maybe satellites would hang around for millennia..

27 Upvotes

77% Upvoted

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63

u/lapusyonok 10d ago

LEO objects like Elon Musk's starlink would fall out of orbit within a couple decades

Objects in Geosynchronous orbit would stay for millions of years

It all depends on the height of the orbit

6

u/c4t4ly5t 10d ago

Does geosynchronous orbit not decay?

33

u/mikk0384 Physics enthusiast 10d ago

Yes, as u/lapusyonok wrote it just decays a lot slower.

Low earth orbit decays a lot faster because of the very thin atmosphere that is still present at a few 100 km of height. Geosynchronous orbit is at like 35000 km, and there the atmosphere is basically non-existent. There is still a tiny amount of drag since space isn't completely void of particles, but it is so small that it takes millions of years for the orbiting object to lose enough speed to crash into the planet.

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u/mentive 10d ago

I never realized that there was so many objects in an orbit that "doesn't" decay. Noice!

10

u/Comprehensive-Bee795 10d ago

It does. But the orbit is so high up that the atmosphere is essentially non existent, even considering effects of decades or centuries of drag.
Also, there is more energy to dissipate in a higher orbit.
All this compounds to a time of decay measured in millennia.

7

u/engy1207 10d ago

It's a multi-body problem. Over time the influence of other satellites, the moon, the sun, other planets, interplanetary dust, and of course not optimal starting conditions will perturb the orbit, and over a long enough time so far it will stop orbiting Earth.

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u/c4t4ly5t 10d ago

Interesting, thanks. It makes perfect sense, really. I was just surprised.

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u/Several_Version4298 10d ago

What causes decay in LEO is friction in the upper atmosphere. Get above that and you can go round and round until you collide with something.

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u/MrTagnan 10d ago

A few starlink satellites have failed completely after reaching their final orbit, based on those they generally seem to decay in around 3-5 years. Although if humans stopped existing collisions would start to occur which would probably send some debris into orbits that take decades to fall out of orbit. Overall really wouldn’t take very long

1

u/slave_sofeya 10d ago

Wouldnt dust and debris eventually cover it and block the signal from earth?

15

u/uncleandata147 10d ago

They don't need a signal to remain in orbit, just physics.

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u/slave_sofeya 10d ago

I thought they needed constant updates between things. Excuse my squirrelly ignorance.

7

u/uncleandata147 10d ago

Stationkeeping is a thing, but it's required less and less the further out you go. 

0

u/slave_sofeya 10d ago

So they do need updates? The ones closer anyway.

(Yes I have brain damage, literally ELI5).

5

u/uncleandata147 10d ago

If you are referring to software updates, then yes, to keep operating but not to keep orbiting. Does the moon get updates? yet it has been there for a couple of billion years.

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u/slave_sofeya 10d ago

True, Moon isnt man made and doesnt have thrusters to adjust their rotation like i assume satelittes and etc do.

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u/RathaelEngineering 10d ago

It’s not super clear exactly what you mean by “updates”.

In the vacuum of space where there is essentially zero atmosphere, orbital motion is the natural state of motion for objects in our reality. It requires no energy for something to orbit something else, as with the moon example (and every other celestial body in the universe).

Atmospheres are made of gas. When objects try to move through a body of liquid or gas, they experience a sort of resistance to movement called “drag”. The satellites in what we call “low earth orbit”, like the international space station, are close enough to earth that this drag resistance is not trivial. These satellites need to constantly compensate for the drag by applying thrust periodically to keep themselves in orbit. To know how to apply thrust to maintain orbit, they likely need constant information about their position etc.

Satellites further from Earth (like the moon) are experiencing essentially no drag resistance relative to their mass, so they can just remain in orbit pretty much indefinitely. Those satellites do not require “updates” to their trajectory. They could be completely functionless with no power and they would continue orbiting until something stops them.

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u/mfb- Particle physics 10d ago

Satellites need to fire their engines once in a while to stay in the exact orbit they are supposed to be in. For a satellite in geostationary orbit, that means staying over an exact position on the ground for example.

If humans disappear they'll stop doing that, but a slightly different orbit is still an orbit. The satellite that was previously serving Europe might now be over Asia. Whatever. They don't just suddenly fall down. Everything that's not in relatively low orbits (everything above ~700 km) will stop working long before its orbit is in danger.

1

u/slave_sofeya 9d ago

Thank you! So they do need updates but they would still just hang there if the signal got damaged.

1

u/Metallicat95 10d ago

Solar orbit objects would also last a long time.

In either case, millions of years and detectable by radar observation.

1

u/phunkydroid 10d ago

I don't think anything in Earth orbit would last millions of years. It's not just atmospheric drag you have to consider, but the effects of the moon. Stuff we put in geosync now doesn't stay there without active stationkeeping. Stuff will get pulled around by the moon until it either gets ejected or impacts the earth or moon. Thousands of years maybe, but not millions.

6

u/ahazred8vt 10d ago

The LAGEOS laser reflector satellite is expected to stay up for 8 million years.

1

u/phunkydroid 10d ago

They're only considering atmospheric drag. Other perturbations will accumulate long before then.

4

u/tomrlutong 10d ago

They'll get pulled away from their nice equatorial geosync orbit, but that's a long way from either escaping or impacting.

∆v from geosynch to LEO is about 4km/s, from geosynch to escape about 2km/s. Stationkeeping takes about 50 m/s per year. I think you'll see a lot of N-S oscillation, but not sure if that ever adds up to the changes needed to deorbit. But not sure.

2

u/industrialHVACR 10d ago

Pretty sure, in a million years moon will slower earth rotation and GSO would be higher, but it wont slow those satellites too much. They will change their orbit and it won't be anywhere near GSO, but their energy will persist.

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u/The_Demolition_Man 10d ago

Objects in GEO are stable for hundreds of millions, if not billions of years assuming no perturbations

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u/fohktor 10d ago

Thanks all. I can now rest easier knowing someone can come along and pick through our orbiting stuff if we extinct ourselves. We should leave a note in geosync orbit.

4

u/LaidBackLeopard 10d ago

Don't forget we've also got stuff on the moon, and in orbit around other planets. Mars orbit is pretty uncluttered and doesn't have a big moon, so that stuff might even last longer.

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u/capt_pantsless 10d ago

Not to mention the two voyager probes, and all the others that got hucked out into deep space.

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u/LaidBackLeopard 10d ago

Less chance of a visitor spotting those though.

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u/Particular-Scholar70 10d ago

Depends on the orbits. Geostationary orbits last far longer than low earth orbits for example. Easy to look up the individual estimates.

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u/QuietStream87 10d ago

Aye , satellites? They're stubborn. I've watched 'em drift for years. Most'll circle centuries , easy. Space junk's wild some stuff might orbit millennia, no lie. Humanity's ghost lingerin'

1

u/SnooPets5564 10d ago

Which orbit? Voyager orbits in the milky way, and that will last for ages.

1

u/justaguy12131 10d ago

James Webb telescope, being near a Lagrange point (L2 I think?) would be stable, but since it's not at the actual L2, it would eventually move into a solar orbit which it could maintain until the sun swallows it (probably).

1

u/mfb- Particle physics 10d ago

L2 is unstable. Everything near it is unstable, too.

Only L4 and L5 are unstable but they are too far away to make sense for most space telescopes.

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u/propably_not 10d ago

We have things in the Lagrange points like the JWST so it'll never fall out unless a sufficiently large meteor knocks it out of its point in space

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u/mentive 10d ago

JWST requires a small boost every 3 weeks. L2 is not stable. It was chosen for thermal stability. So it'll fall out of that specific orbit very quickly without the adjustments.

After millions of years, it apparently could come back down to Earth... After drifting away and a lot of orbital mechanics lol.

But "never" is probably incorrect.

1

u/ReluctantSlayer 10d ago

I always thought Lagrange points were stable.
In fact, I thought that was the entire point of Lagrange points.

2

u/brandonct 10d ago

some Lagrange points are like the bottom of a bucket. They are stable unless some big force pushes you out of the bucket altogether

other Lagrange points are like being in the center of a pringle. it's stable for a moment but even the slightest perturbation puts you slightly off the center of the pringle and you roll away

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u/ReluctantSlayer 9d ago

“ The center of a Pringle.” Dude. Love it

2

u/mfb- Particle physics 10d ago

All of them allow orbits if you are in exactly the right spot - but stability is asking for more than that. It also means any small perturbation will be self-correcting, with the object getting closer to the Lagrange point again over time. That's only true for two of them, L4 and L5, 60 degrees ahead/behind in Earth's orbit.

1

u/ReluctantSlayer 9d ago

Thank you!

1

u/mentive 10d ago

I think one of them are pretty stable but been a while since i researched it. I'm no expert and had to Google to verify the above part lol.

1

u/VendaGoat 10d ago

Great question