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Comment by simonh | Hacker News Reader

Rifle bullet speed is a relatively small delta for orbiting craft, it goes up quite a few orders of magnitude from that. Pretty much anything you encounter that you didn't specifically try very hard to match orbits with is going to blast right through a net.

There were studies on using the Shuttle to repair satellites, but aside from the maintenance mission on Hubble those plans were shelved. The cost of a new satellite was generally a lot less than the cost of a shuttle mission dedicated to repairing it, and it would often need to be dedicated due to the requirement to match orbits.

Even if the ISS released a smallsat, which it does from time to time, that doesn't help you. By the time you need to do maintenance on it, the cubesat will have dropped into a much lower orbit due to upper atmosphere effects.

The safest, and practically perhaps the only viable way for the ISS to do maintenance on orbital instruments is if they stay attached to the ISS.

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> it goes up quite a few orders of magnitude from that

Sorry to be pedantic, but modern rifle muzzle velocities are in the 1.2km/s range. Escape velocity in LEO is 11.2km/s, so your maximal closing speed is ~22.4kms. Which is maybe 1.3 orders of magnitude difference, not "quite a few". Orders of magnitude are big.

For the rest, you are right of course: we're talking closing speeds that can easily be one order of magnitude higher than rifle bullets (or armor-piercing tank projectiles, for that matter; those are at 1.7km/s or so).

I guess it depends if you operate in base 2 or base 10.

Naturally, you should use base e, which gives about 3 orders of magnitude.

What matters in a collision isn't velocity, it's momentum. Momentum is ½mv². There's a lot of orders of magnitude just in the mass, but the velocities add up a lot faster than you'd think too. I'd say there's around 10 orders of magnitude between the momentum of a rifle bullet and the momentum of a communications satellite at escape velocity.

*Momentum* is m•v

Kinetic energy is ½m•v².

#ftfy

Ouch, how did I make that mistake?

> I'd say there's around 10 orders of magnitude between the momentum of a rifle bullet and the momentum of a communications satellite at escape velocity.

That's quite an overestimation.

Let's be generous and say a rifle bullet is 50 gram (0.05 kg) and fires at 1.1 km/s, and a communication satellite weighs 5 metric tonne (5000 kg) and escape velocity is 11 km/s. We have 5 orders of magnitude difference in mass and 1 in velocity. As momentum is m*v, total difference is 6 orders of magnitude. Kinetic energy is 1/2*m*v^2, so that gives 7 orders of magnitude difference.*

I’m not so sure that it is.

The mass of a bullet is more like 5 grams, and the mass of a satellite could be around 500,000 kg (the ISS). A quick search shows the the largest commercial communications satellite is Telstar 19V, with a mass of 7,076kg; in my first estimate I guessed 50,000kg. Finally, the closing velocity of two objects in orbit could be up to 22.4 km/s.

rifle bullet: 3×10³ J

Telstar 19V, geostationary orbit: 3.3×10¹⁰ J

Telstar 19V, geostationary transfer orbit: 3.5×10¹¹ J

ISS, LEO: 1.2×10¹³ J

hypothetical fast ISS: 1.1×10¹⁴ J

14−3 gives 11 orders of magnitude, but good luck making the ISS go that fast. Also, don’t forget to add in the kinetic energy of the net; it’s not going to be zero.

Still, even 7 orders of magnitude qualifies as ”quite a few”. You don’t need very many orders of magnitude before you have too many.

The ISS is not a communications satellite, escape velocity is lower than maximum closing velocity, and kinetic energy isn't momentum.

Of course you can make the statement true by changing everything it's about.

Forgive me for contradicting you, but I didn’t change anything; I just typed momentum when I meant energy. I first thought of the formula, not the name, and then typed the wrong name. I'm not sure how I made such a mistake, but mea culpa. Also my old friend bzbarsky mentioned closing velocity, and I stuck with it when I made my estimate.

I do agree that the ISS is not usually considered to be a communications satellite (though it does have plenty of communications gear on board, including a ham radio repeater if I recall correctly). But it is a thing that we might want to deorbit one day. Since we were considering the ridiculous closing velocity of two objects that are both on escape trajectories, I figure using the ridiculous mass of the ISS was fair game too. But like I said, I originally guessed 50t, not 400t. I was thinking more about the payload capacities of launch vehicles than the satellites themselves.

Whatever mass and closing velocity you consider, 7 or 10 or 11 orders of magnitude all qualify as “quite a few” even if the velocity itself only has 1.3 orders of magnitude of difference.

I mean you stretched the numbers as much as possible (garbage collecting the ISS? LOL!) and you still didn't get there.

And anyway, none of this matters, because the relevant OP specifically said "speed".

So the plain and obvious conclusion is: False.

> Sorry to be pedantic, but modern rifle muzzle velocities are in the 1.2km/s range

what caliber do you have in mind for that?

Good point. The source I was looking at was talking about .22 caliber rounds. I looked around some more, and that seems to be the general range in which you can achieve those velocities, though that can include discarding sabot versions of 7.62x51mm ammunition (with the projectile ending up 5.56mm or smaller).

> Pretty much anything you encounter that you didn't specifically try very hard to match orbits with is going to blast right through a net.

If a zombie satellite needed to be decomissioned, could it be shot down and left for the pieces to disintergrate in the atmosphere instead?

If the zombie sat in question is in a _very_ low orbit, this can be a reasonable approach, because the atmospheric drag can pull the pieces down faster than would've occurred on an intact satellite.

Otherwise, you risk just creating more high-v debris. At the extreme end of this, you could trigger the Kessler Syndrome (https://en.wikipedia.org/wiki/Kessler_syndrome) and make orbit practically unusable.

Yes but then instead of one trackable, piece of junk you have many small pieces some too small to track.

In fact the Chinese satellite destruction test in 2007 was a very good example of this:

“ Anti-satellite missile tests, especially ones involving kinetic kill vehicles as in this case, contribute to the formation of orbital space debris which can remain in orbit for many years and could interfere with future space activity (Kessler syndrome).[7] This event was the second largest creation of space debris in history after Project West Ford, with more than 2,000 pieces of trackable size (golf ball size and larger) officially catalogued in the immediate aftermath, and an estimated 150,000 debris particles.[24][25] As of October 2016, a total of 3,438 pieces of debris had been detected, with 571 decayed and 2,867 still in orbit nine years after the incident.[26]

More than half of the tracked debris orbits the Earth with a mean altitude above 850 kilometres (530 mi), so they would likely remain in orbit for decades or centuries.[27] Based on 2009 and 2013 calculations of solar flux, the NASA Orbital Debris Program Office estimated that around 30% of the larger-than-10-centimeter (3.9 in) debris would still be in orbit in 2035.[28]

In April 2011, debris from the Chinese test passed 6 kilometres (3.7 mi) away from the International Space Station.[29]

As of April 2019, 3000 of the 10,000 pieces of space debris routinely tracked by the US Military as a threat to the International Space Station were known to have originated from the 2007 satellite shoot down.[30]”

https://en.wikipedia.org/wiki/2007_Chinese_anti-satellite_mi...

I see. What about a huge orbiting ball of butter* with thrusters that just slowly changes its orbit and sweeps all the trash by getting the trash embedded in itself?

* not actual butter, but you get the idea, some polymer, or maybe aerogel?

Relative velocities are just a huge problem. Unless the object and the catching item are moving near the same speed, chances are the debris will just punch through the item meant to catch it and keep trucking (and maybe make more debris in the process)

Space is huge. You probably would need a very, very large sweeper to collect anything.

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