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Comment by ars | Hacker News Reader

The article claims "one proposed communications network using the technology could result in hundreds of metric tons of additional mercury"

And searching for mercury released by coal burning comes up with 50 tons in the US, and around 250 tons/year worldwide from all sources.

I have a hard time believing that figure for the satellite, but if true, it's actually significant.

Edit: Seems like an ion engine uses around 1 ton of propellant. So, yah, I guess banning mercury for this, really is significant.

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Maybe mercury is different (though I doubt it), but current electric propulsion sats use nothing like that much mass. A Boeing 702 HP (a big GEO comsat) Xenon propulsion system has a capacity for 266kg of Xenon; uses about 5kg per year for station keeping. Constellation sats are usually smaller; a Starlink sat is about 260kg total. Dunno how much krypton they carry, but probably in the low 10s of kg each.

I got it from Wikipedia: "The Dawn spacecraft broke the record, with a velocity change of 11.5 km/s (41,000 km/h), though it was only half as efficient, requiring 425 kg (937 lb) of xenon."

I rounded up to one ton, since we're only trying to figure out order of magnitude.

Dawn is an interplanetary probe, though, and requires orders of magnitude more fuel than satellite constellations. For a more realistic figure, the Starlink satellites are fueled with about 5 kg of krypton.

Starlink alone has launched more than 2,000 satellites already and plans a total of at least 12,000. That's a lot of fuel - and that's just one constellation.

> That's a lot of fuel - and that's just one constellation.

Is that a lot of fuel? 5kg times 12000 is about 60 tonne. That's the weight of two big truck approximately. In which sense is that a lot of fuel?

And besides krypton is a noble gas, it doesn't react with anything.

60 tons of Krypton, eh. 60 tons of Mercury is huge.

I agree with you.

I think the concern was that some organizations were considering big low-altitude constellations. Once you go below 400km or so the drag increases quite a bit so the idea would be to use near-constant thrust similar to GOCE or SLATS.

And there was at least one vendor (Apollo Fusion) proposing a mercury thruster, although I think they have backed off on that. It would have had about 3x the impulse per volume and 2x the impulse per mass of xenon.

I think banning it now is wise, if any small operators had started using it it would have been hard to block it for a megaconstellation and it was very clear that anything release in VLEO would be coming back into the atmosphere. Right now there are no users so you aren't really penalizing anyone.

It mentions communications _networks_. If you have 10,000 satellites with 10kg propellant each, there's your hundred tonnes of propellant.

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