I have some difficult questions about a decaying orbit, Veeky Forums

I have some difficult questions about a decaying orbit, Veeky Forums.

For plot reasons in a story I'm writing, I need pieces of a wrecked spacecraft to orbit Earth, and for the pieces to crash land bit by bit over the course of a century.

I don't know much about astrophysics, but I have a certain amount of interest and respect for it, so I'd rather not write anything outright retarded.

The debris in question is about the size of a large skyscraper, broken up into pieces not much larger than trucks or houses.

From what you told me last time, this cycle of crashes and stability would require an elliptical orbit, where the main group of debris passes close to the Earth once per orbit.

Realistically, how many days is this orbit? Can it be as long as a year, or would that force it into a new orbit around the sun?

Could it end up in an orbit around the moon, or is the earth's gravity completely dominant in that area of space?

There's also the matter of where the pieces land. As I understand it, that depends on the position of the earth, the time of day, where its axis is tilting; but the real question is: could the orbit be consistent enough that the pieces fall on the same continent in the same hemisphere?

Could it change so that the elliptical orbit shortens, making impacts more frequent?

Long term, how does this orbit come to an end?

I'd appreciate any advice you have.

Other urls found in this thread:

en.wikipedia.org/wiki/2015_TC25
minorplanetcenter.net/asteroid_explorers/show/2015 TC25
en.wikipedia.org/wiki/Comet_Shoemaker–Levy_9
youtube.com/watch?v=dpmXyJrs7iU
en.wikipedia.org/wiki/Gauss'_method
en.wikipedia.org/wiki/Meteor_shower
twitter.com/NSFWRedditGif

Bump

depends where it is in the first place. how is it in pieces?

Basically, it can be anywhere it needs to be for the orbit to be established, but let's say that it was in a stationary orbit near the Earth's atmosphere, then broke apart due to internal explosions.

as long as they pass close enough to earth to encounter the atmosphere, even just the thinnest outer layers they'll eventually decay

I started writing answers, then realized this could take a while. So here's my suggestion: download and play Kerbal Space Program. It really will give you a feel for how this works.

The gist is, with a big enough explosion and a critical enough initial orbit of the intact ship, chunks will either have gained or lost energy enough to fulfill your criteria. You will end up with an expanding cloud of debris where the center of the original mass will be in more or less the initial orbit (unless the explosion is very directed, such as... a rocket engine).
More posts for the other points coming...

P.S. The Earth's atmosphere gradually diminishes to "negligible" in terms of having an effect on orbital bodies over 700 miles, giver or take (depending on solar activity, greenhouse effect, ...)

>Realistically, how many days is this orbit? Can it be as long as a year, or would that force it into a new orbit around the sun?
This is all over the map. The longest orbit would be as nearly circular as possible, with the perigee as far as possible while still aerobraking to whatever degree you need to achieve your decay goal. Don't have numbers (too lazy), but it would for instance be farther out than the Moon (about 30 days' orbit). That means you'd have to avoid the gravitational effects of the Moon to get that large an orbit. Or a radical inclination - but I gathered this is more or less equatorial. Don't know how I got that impression.

>Could it end up in an orbit around the moon, or is the earth's gravity completely dominant in that area of space?
Think in terms of energy. An object entering a gravitational well has more energy than the well has gravity to hold it (because it comes in from outside the well). So unless there was another interaction while within the Moon's influence, no - the craft would maintain its energy and escape.

>There's also the matter of where the pieces land. As I understand it, that depends on the position of the earth, the time of day, where its axis is tilting; but the real question is: could the orbit be consistent enough that the pieces fall on the same continent in the same hemisphere?
In theory, I suppose. But the magnitude of the coincidence would be literally incredible (fancy word for "unbelievable"). Also, we're talking about pieces from an explosion - that is, random speeds, and directions). I can't think offhand how you could have a random scatter become predictable without directed intervention.

>Could it change so that the elliptical orbit shortens, making impacts more frequent?
How would it change? There has to be some force acting on the cloud. So you'd need a convenient asteroid of significant mass (because the encounters are so brief) passing by... again, incredibly precisely.

>Long term, how does this orbit come to an end?
Well, since the explosion, we're talking about *these* orbit*s*. We've mentioned aerobraking and coincidental encounters. I'm drawing a blank for other random forces happening. A huge CME maybe - at least for the smaller pieces with greater surface area.

I was thinking a tilted orbit, since I want the crashes to be mid-hemispheric.

I definitely want the crashes to be undirected, to I may have to rethink this, or just accept some big coincidences.

I mean within around a thousand miles of each other, maybe moving a thousand miles north or south with the seasons and a thousand miles east or west with the time of day.

I thought the force keeping it in motion might decline, kind of like how swirling water can stop moving. Doesn't really apply in a zero friction environment, I suppose.

>I was thinking a tilted orbit
It will be all over the place. Exploding objects can fly off to all kinds of orbits, the range depending on the energy of the explosion. As long as you have enough energy, you may even end up with pieces that have enough speed to break the escape velocity and so chunks that pass by once every year or more, like comets.

>explosions
An explosion throwing enough crap off in the right direction (note than a typical explosion would throw crap off in ALL directions, but many of them would decay quickly) could boost the apogee and make it an elliptical orbit. Perigee would be the original LEO of the space station or whatever. It would really have to start at LEO to begin with so that it has a chance to decay at all, otherwise it will just stay up there for decades being nuisance space junk.

Then it would need to fragment a bit but not too much, so that there's a cloud of juink that goes back to the original LEO altitude for decay time.

At least that makes more sense than that movie a few years ago where some chick flew between space stations that had completely different orbital inclinations.

As to how much time between perigees, probably on the order of days at most, unless it gets a miraculously aimed boost. Remember that LEO is on the order of 90 minutes per orbit, and that the Moon's orbit is about 28 days.

Perhaps it could be something that was trying to boost to escape orbit, and broke up in a path just enough for Earth to recapture it into a really, really elliptical orbit.

I think your fundamental problem is going to be that it has to come in really close to decay, but has to go really high to happen less than once a month.

You mentioned a year, maybe some kind of co-orbit with earth, but it would have to be something big and with serious velocity to get that elliptical, not escape, and have enough bits left to be a problem.

en.wikipedia.org/wiki/2015_TC25
>Orbital period: 382 days

This seems to be very close to what I'm looking for, except its orbit is too circular to have much of a crash chance.

minorplanetcenter.net/asteroid_explorers/show/2015 TC25
>1-in-7,700 Impact Probability of Hitting Earth in the Future

Here's another interesting case, but it seems to have played out within a few decades and crashed all at once.
en.wikipedia.org/wiki/Comet_Shoemaker–Levy_9
>captured in Jupiter's orbit in the 1960s
>broke apart 1992
>impacted 1994

What I was initially imagining was a cloud of debris that has a single orbit, but what would you say about hundreds of 2015-TC25 type orbits, some very elliptical, others very stable?

I think the odds of one or two pieces crashing per decade are non-ridiculous when there are hundreds of orbits, and it's not essential for all the pieces to land or be in an unstable orbit.

The actual breakup of the spacecraft is in the distant backstory of the novel and isn't really a focus. In other words, I can change it any way I like in order to make the main events more plausible. It could explode, it could be sabotaged by strategic detonations, it could have modules decoupled like rail cares. It's ancient history, so it can be anything that fits the main event.

In this case the main event is a roughly predictable crash of spacecraft debris every decade or so. I was hoping predictable annual crashes would be feasible, but it looks like that would be crazily convenient.

I suppose the key is: can a 2015-TC25 type asteroid form an elliptical, decaying orbit after many decades in a circular orbit? Say as a result of a collision with another of the pieces of debris that follows a slightly different orbit.

How would a large number of stable orbiting asteroids (in this case spacecraft parts) appear from the Earth's surface? Could they be observed with pre-modern telescopes?

>but what would you say about hundreds of 2015-TC25 type orbits, some very elliptical, others very stable?
Hundreds of orbits would quickly get out of sync, and would end up with something in decay range at almost random invervals. It also breaks up your initial mass into a hundred pieces, of which only a small portion could be lost each time, or else there would be nothing left before long.

Really, a break-up in orbit is only dangerous to other things in orbit, and might not even be a curiosity to people on the ground. So it depends on what you want the bits to mean when they re-enter.

I remember an episode of Max Headroom (yes, from the '80s) where (if I recall correctly) there was an annual mass re-entry of obsolete satellites happening, but it was more like going out to watch fireworks.

Changing the debris' orbit to a different one would take a tremendous amount of energy, and I dont think a simple collision with a satellite or rocket could change its trajectory(if I'm reading your story right I think).

Definitely could not be seen with a pre-modern telescope

Stable orbits? Unstable orbits? Unstable orbits require three interacting bodies. We're leaving the Moon out of this for simplicity, so all these orbits are stable, regardless of circularity.

>I think the odds of one or two pieces crashing per decade are non-ridiculous
Agreed. Having them coincidentally hit within a target area however, ...

>In this case the main event is a roughly predictable crash of spacecraft debris every decade or so.

Does it have to be a spacecraft? Perseid meteor shower is a great example of material having come off a comet nucleus forming debris that tracks the comet's path around the Sun. Kinda why meteor shower strength is unpredictable - you don't know how much material is there until the Earth hits it. Thing is, the comet spews material every pass, so the particles are resupplied. Your ship has limited material.

>Could they be observed with pre-modern telescopes?
Possibly. If the pieces are large and reflective enough. Look up "Iridium flares."

>I suppose the key is: can a 2015-TC25 type asteroid
You *were* talking about orbiting the Earth. Asteroids all orbit the Sun.

I remember you from the last time. I proposed expelling parts in order to maintain orbit, parts that wold fall down on Earth.

Did you look into this possibility?

Part of the plot involves people tracking crashes. I'm still trying to figure out how to make this plausible with Victorian-level technology, and my first guess was that if there was a predictable pattern of crashes, they could predict future crashes based on that.

Crashes moving steadily southeast over the years, for example. But even a basic understanding of the rotation of the earth makes that implausible, so that's definitely out.

I suppose I could just handwave in some alien technology that they use to find more alien technology (the crash sites,) but I'd prefer the characters to use their brains.

>Look up "Iridium flares."
Excellent. Shiny silver spaceship it is.

>Asteroids all orbit the Sun.
I'm learning quite a bit ITT.

I'm coming around to an idea like this. The largest component could be in a stable orbit, with a cycle like Halley's Comet (minus the tail.) But regular expulsions of parts doesn't really fit with the unplanned, accidental nature of the crashes.

Chelyabinsk Event, 2013. Not an impact, but interesting.
youtube.com/watch?v=dpmXyJrs7iU

I'll answer a few of these

>could the orbit be consistent enough that the pieces fall on the same continent in the same hemisphere?
Something called a Molniya orbit achieves this, which is the type of orbit in your image on your post.
Wikipedia it they aren't too complicated

>Could it change so that the elliptical orbit shortens, making impacts more frequent?
>Long term, how does this orbit come to an end?

You can never really escape drag in space since the space is never perfect. Over time almost all satellites decay in their orbits and will fall down, this happens slower the further out from the Earth

Actually I feel like explaining why the Molniya orbit works for this

Say you want the pieces to always land at 30deg south, 0 deg east

An elliptical orbit inclined to an angle of 30 degs so that the low point of its orbit is 30deg south, then the high point of its orbit is what height it need to be so that the orbit takes the same amount of time as the length of a day (or any integer multiple) so that every day when the debris arrives at the perigee its always over the same degree east/west.
If the orbit only took a third of a day then the first orbit would have the perigee over say 0 degree east, then the next orbit would be at 120 degree east, then at 240 deg, then back around to 360/0 degree.

Soviets used these types of orbit since their apogees (high points) are over their inclination altitude for a long time. So they could have satellites that stayed over theirs or the enemies territory for a long time to either broadcast TV or do some spying

I should add that because the Earth is not perfectly round these orbits rotate with respect to the Earth such that even if the satellites orbit was perfectly matched with the length of a day over time they will still drift so that after a long time the perigee might now have drifted from 0deg to 10deg. To stop this either boosters need to constantly maintain the orbit or put it at a certain inclination. The inclination at which this doesn't happen is at 63.4 deg which is the specific example of a Molniya orbit

>I'm still trying to figure out how to make this plausible with Victorian-level technology, and my first guess was that if there was a predictable pattern of crashes, they could predict future crashes based on that.

IRL Viccies didn't have a huge drive to get gud at orbital mechanics but they knew about gravity and there definitely were people who knew how to calculate orbits. They could get their telescopes, follow the orbit of whatever the source of the crashing stuff is and work out what their orbit is using Guass' method, then make predictions from that.

en.wikipedia.org/wiki/Gauss'_method

It sounds a lot like a meteor shower kind of scenario as well, except instead of meteors its whatever the alien tech is. Meteor showers normally are only consistent in how often they occur and from what direction in space they occur, not where they land. But for the sake of the plot their source could maybe be fine tuned to fix that

en.wikipedia.org/wiki/Meteor_shower

It's more complicated. OP wants debris to fall over decades. That means aerobraking and orbital decay. Your Molniya orbit is designed to *not* decay, at least within the operational lifespan of the satellite.

Basically, the explosion needs to be big, the orbit needs to be big, and that means lots of perturbation and randomness.

But the perigee could be low enough that a cloud of debris following it would aerobrake a bit each time, although that would probably make the whole cloud come in at once when it does decay rather than it shrink but say 5% each time so you get the crashes spread out over a long time. But maybe if the cloud was sufficiently big such that different sections of the cloud were following slightly different orbits some that may decay years before others

This can happen, but... that means after one or two passes, it's no longer a Molniya (synchronized) orbit.