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Comment by trymas | Hacker News Reader

I always had this question, though maybe there's a simple answer why nobody is doing this.

Why it's not mandatory to have some smallish engine attached, which at the end of satellite's life would lower the orbit enough until atmosphere picks it up and it will slow down significantly on it's own and burn up?

Is it because most satellites will not fully burn and actually hit the ground, i.e. it's liability?

Is it because of too great of a risk of crossing and colliding with a satellite in another orbit, i.e. liability again?

Is it because "attaching smallish engine" which will fire at satellite's end life is actually really hard thing to do?

Something else?

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The engine is not the problem. It's the fuel. Large orbit changes take a lot. Lowering orbit is not easier than raising orbit, it takes the same delta-v.

IMO what should happen is we should ban putting satellites in high orbits. Satellites in low orbit decay naturally within a few years due to atmospheric drag. Satellites in high orbit will stay there essentially forever. More importantly, any collision in high orbit creates a permanent debris cloud which will spread over time and pollute orbit forever, being essentially impossible to clean up even with sci-fi technology. A collision in low orbit creates a similar debris cloud but it will be naturally cleaned up in a few years or less.

Putting satellites in high orbits made sense back when it was incredibly expensive to launch each satellite, because satellites last longer in high orbit and you don't need as many to cover an area. Also, stationary satellite dishes only work with geostationary satellites, and geostationary orbit is a very high orbit. But today we can use phased arrays to communicate with moving satellites without physically moving a dish, and SpaceX is about to drop launch prices through the floor with Starship, making it feasible to launch enough satellites to cover the Earth even in low orbit and replace them frequently. So to me, the space debris pollution risk of high orbit satellites can no longer be justified.

> Lowering orbit is not easier than raising orbit, it takes the same delta-v.

Why is this? From a layman's perspective it seems like gravity would be a massive form of help here and therefore lowering orbit should require much less fuel.

Orbit is unintuitive. Objects in orbit are not just floating up there. They are constantly falling under the influence of gravity, just as objects here on Earth. The reason they don't hit the Earth when they fall is that they are traveling sideways at 25,000+ km/h. This is so fast that they miss hitting the Earth, and simply fall forever.

When a rocket launches to orbit, it only goes up a little bit, just to get out of the atmosphere, and then spends most of its time/fuel on going sideways to reach orbital velocity. If you watch a rocket launch you can see that the rocket starts to tip over and go sideways soon after leaving the pad. This is also why launching from a plane doesn't help you very much, because going up is the easy part of getting to orbit. A plane can't help you with the hard part of getting to 25,000 km/h sideways.

For an object in orbit to stop missing the Earth as it falls, it must slow down that sideways velocity, and gravity doesn't help with that.

Gravity is already helping all it wants to. Gravity is constantly redirecting the velocity in the circular orbit. A velocity change will push it into a new orbit, that will be slightly offset from the old one. You need so much velocity change to offset the orbit so much, and then you are still orbiting - still in something of a a steady state trajectory, just a slightly different one. It’s symmetric in fact

Gravity 'helps' too much, by making you go faster as you get lower, and then you slingshot right back up to your original height.

That's why a lopsided orbit is stable. It wouldn't be if down was easier than up.

Think of raising an orbit like moving an item from one side of your desk to the other.

You have to put energy into doing it, but the end result is just as much in balance with gravity as the starting point. So if you want it back where it was, you need to spent the same amount of energy to move it in exactly the same way in the opposite direction.

Well there are other reasons for debris besides just satellites going out of commission. For example in 2007 China deliberately blew up the Fenyun-1C satellite for some kind of research purpose accounting for probably thousands of current pieces of debris up there.

“Research” - they were testing their orbital targeting capabilities.

It depends on the orbit.

The satellites we put in orbit will naturally have orbits which will decay in a matter of days to hundreds of millions of years. End of life is indeed a consideration for launch approval and many satellites do accelerate their decay with onboard thrust.

Some satellites can’t, would require too much thrust to get back to earth. Some push in to higher orbits to get out of the way for replacement satellites.

Some satellites break in orbit and can’t be controlled.

The FCC _is_ working on making orbital cleanup mandatory: https://www.fcc.gov/document/mitigating-orbital-debris-new-s...

Yes but the FCC is going the wrong direction with that. Firstly it treats constellations differently than individual satellites which makes no sense from a statistical and mathematical perspective and it would also completely kill off the smallest of satellites that students learn with that are primarily launched by universities.

> For purposes of calculating the probability of successful post-mission disposal, we define successful post-mission disposal for spacecraft in LEO as re-entry into the Earth’s atmosphere within 25 years or less following completion of the spacecraft mission.

I don't see the problem.

They do.

https://en.m.wikipedia.org/wiki/Graveyard_orbit

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