Orbital mechanics

So excuse the randomness - I’m at work, but without a flight at the moment, and was perusing some news when I came across the tidbit of news about the DirecTV satellite that is reportedly (by Boeing) in danger of exploding if its batteries are used after something damaged them. What I found interesting while reading the article as that they plan to move it up and into a higher “graveyard” orbit far above the geo-stationary orbit it is currently in.

“Barring technical failures, the FCC requires licensed satellite operators to vent onboard propellant before moving satellites into the graveyard orbit.”

So I was thinking to myself…well, certainly it would be better to bring it down and let the atmosphere take care of it…so a reentry type burn. So I Googled “Graveyard orbit” and was surprised about this fact:

“A graveyard orbit is used when the change in velocity required to perform a de-orbit maneuver is too large. De-orbiting a geostationary satellite requires a delta-v of about 1,500 metres per second (4,900 ft/s), whereas re-orbiting it to a graveyard orbit only requires about 11 metres per second (36 ft/s).”

I thought that was pretty nifty…and I’m sure I don’t understand it, but I also don’t understand why my cat sniffs the other cat’s butt every time he comes in the house. :man_shrugging:


You need to spend some time with a Kerbal Space Program. A few hours with that and Orbital Mechanics will make a lot more sense. Plus, you can make big explosions :sunglasses:.


The fact that it is a DirecTV satellite brought this scene from Spies like Us to mind.

Spies Like Us: MTV Satellite



The relevant XKCD:

I had read about free return trajectories and gravity assists etc…but none of it really clicked until my little green guys were almost stranded far from home (after some dreadful miscalculations on my part…and quite possibly an explosion or two) with far too little fuel and I really wanted to make every last unit of delta-V work :slight_smile:

I mean, half the time I’d still get them killed…but sometimes it would all work out because of maths and it was wonderful when it did :smiley:


What the other guys said. +1000
I cannot really stress it enough. KSP teaches you, and it does so gloriously simple and fun.

Long story short for this case:
Geostationary orbit is pretty far out (~36000km for Earth). Coming back from that far out to deorbit requires energy of the same order of magnitude than putting it up there. Energy the satellite doesn’t have of course. Changing its altitude by a few (even a few hundred) kilometers to get to its graveyard orbit is nothing in contrast to that.

In fact, throwing the satellite onto the Moon or into a solar orbit would be easier from there. :smiley:


As I understand it, that satellite is likely in an orbit this is geostationary - so it completes 1 orbit when the earth completes 1 rotation (1 day). Going slightly faster puts it up into an orbit that is slower than a day (1 satellite orbit will be longer than a day) and takes it out of the standard geostationary orbit that most satellites of it’s type want to occupy. Coming back down the gravity well means slowing enough to change from a nice circular orbit to one that arcs down and into the atmosphere enough to capture it and not bounce it or slingshot it away. The probably don’t want that to take many orbits (ie: it uses the atmosphere to slowly decelerate over months) because that would cross the paths of many lower satellites and increase the risk of a collision. So to either arc down in only a few orbits, or to circularize an orbit just at the top of the atmosphere (where atmospheric braking will bring it down) is a large speed change.

As I understand it, anyway.

They are just checking up on the health of the other cat :slight_smile:

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Well what happens when the sniffing can’t catches a whiff worthy of a concern? Does he say to the other cat, “You might want to have your prostate checked!”? Or does he just avoid the other cat so that he does not get the same bug? I hate shaking hands. Yes I know this puts me on the Howard Hughs spectrum somewhere. But I just don’t see the point of touching another human being that I am not planning to have immediate intimate relations with. I mean, really it’s worse than butt sniffing which at least doesn’t involve the transfer of sweat and bacteria.

Oh and what everyone else says about KSP. After a few hours of serious play, questions like yours are as easily understood as why it is harder to walk up a hill than down one.