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Comment by londons_explore | Hacker News Reader

No - I'm suggesting that a satellite already at the end of its life, and heading to the graveyard orbit, go push it.

Damage is unlikely - collision speeds would be measured in millimeters per second, and forces in millinewtons.

For position accuracy, it would probably require a camera onboard - but most satellites already have a star tracker that includes a capable camera.

The main risk is that the satellites touch at the 'wrong angle' or blocking an attitude control thruster. That would immobilize both of them.

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Your suggestion does not comport with reality.

> collision speeds would be measured in millimeters per second, and forces in millinewtons.

What satellite ending their end of life, and already in orbit do you suggest are capable of such delicate manoeuvring? This is a very delicate task you are describing, and most satellites are simply not designed for it.

> The main risk is that the satellites touch at the 'wrong angle' or blocking an attitude control thruster.

If someone tries this, the most likely outcome is that the satelites wizz by each other without ever touching. If you somehow manage to aim the "pushing" satellite accurately enough at the "stranded" satellite they would most likely collide at anything from a few hundred meter/s to a few of km/s depending on the orbital arrangements.

I don't think you understand what geostationary orbit is. They all have to be going the same direction (with the Earth's rotation) or else it isn't stationary (not moving) over the geo (earth).

As a result I think you're misunderstanding what the orbital dynamics would be.

> I don't think you understand what geostationary orbit is.

I do understand what geostationary orbit is.

The commenter I was responding to said "any other satellite who is travelling from one orbit to the other can stop off on-route for no fuel or time cost". If they wanted to only talk about geo sats they should have said that.

But sure, let's think about using a geo sat to fly this intercept. Let's just pick Echostar-8 as a concrete example. It is at 6.4 degree inclination while Echostart-7 is at 1.5 degree. So you will at best have a glancing blow between these two.

This[1] is the typical satellites thruster:

https://www.satnow.com/products/thrusters/space-electric-thr...

It has a thrust of ~20 milli Newtons.

Power that up for 10 seconds on a satellite weighing 500 kg, and you will move an extra 0.4 millimeters per second.

So how exactly is precise maneuvering hard?

GEO satellites typically don't use hall effect thrusters for much. Primary propulsion for large GEO satellites is typically a hypergolic system. Ion thrusters, if present, are typically a station keeping tool.

For example, the satellite bus used by EchoStar-7, which is the satellite in question here (https://en.m.wikipedia.org/wiki/Lockheed_Martin_A2100) uses Hydrazine and NTO for primary propulsion, with hydrazine monoprop for atittude control, and the monoprop thrusters and ion thrusters for station keeping.

While the new version of that bus has the option for all-electric propulsion, it really isn't clear how popular that option has actually been. There aren't really many details about it.

> This[1] is the typical satellites thruster:

Define "typical" because it's not being used on any geostationary satellites.

> Power that up for 10 seconds on a satellite weighing 500 kg

500kg? Hilarious. These satellites are the size of a bus and their weights are measured in tons.

Even assuming that it would be practical to match the inoperative's satellite orbit to the required precision to have "collision speeds measured in mm/s", the contact interface would need to be perfectly on the center of mass - otherwise any further propulsive maneuvers would cause it to tip to one side. The exterior faces of satellites are really unsuitable for this unless they are specifically designed for this.

> satellites already have a star tracker that includes a capable camera

Which is focused at infinity. Neither the hardware nor the software is capable of tracking a big object nearby and providing feedback to align itself how you're proposing, without significant changes.

Edit: just to give an idea of how difficult docking in space is in general, take a look at the International Docking System Standard (used on the ISS): https://www.internationaldockingstandard.com/

Table 3.3.1.1-2 tells you the conditions for docking; velocities of 50-100mm/s are expected. And look at how much hardware is needed to dampen the energy. It has 6 DoF actuators, hydraulic components, etc. etc. And that's for the ISS - all spacecraft going there are specifically designed to be able to accurately approach and dock with the ISS. There's just no way you can get that level of precision with a satellite bus designed for communications and station keeping.

There are other big risks involved.

For instance, if the satellite is dead or has lost propulsion, it will likely have started to spin and will have no way to stop that. Reaction wheels can only do so much and can get saturated, requiring other attitude control systems to be used to desaturate them first. Approaching it with an unspecialized satellite will then definitely generate smaller debris, which is far more problematic.

This also amplifies the risk you mention about ending up immobilized for other reasons. You'd have two pieces of junk literally on top of each other, it would take very little for them to damage each other and start producing much smaller debris.

Further, you can't just repurpose a star tracking camera to look at a satellite instead, they're dealing with completely different kinds of requirements, and it isn't even a given that you would have the processing capability on the tracker to handle the kind of computer vision needed. Nor is it likely that the tracking camera feed can be transmitted down to Earth for manual control.

The only cases we have of a satellite contacting another satellite when it wasn't initially designed with that in mind, involved dedicated extension vehicles designed and launched to dock to the engine nozzle of satellites which were still fully functional and without the extension would've still been able to move themselves into a graveyard orbit.

I'd say the biggest risk is that upon one of the pushes of one satellite toward the other, the propulsion of an old satellite fails, keeps accelerating and causes a crash.

They don't have that much precision in controlling velocity. Solar panels are fragile. It's a recipe for getting two broken apart satellites in that orbit.

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