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Comment by doikor | Hacker News Reader

The SpaceX satellites are in such a low orbit that anything that is not actively being pushed back up (well given more speed in reality but whatever) by engines will come down in a couple years due to drag from the atmosphere. Most of the Starlink satellites from the first patch have already come down on their own as planned (as have the 2 tintin proto ones)

This applies for ISS even more (it is huge and thus a lot of drag) and thus it has one of the Progress spacecraft docked or the Russian service module use its engines to boost it back into proper orbit so that it wont crash down on earth.

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> This applies for ISS even more (it is huge and thus a lot of drag)

Wouldn't drag be proportional to the surface area, hence the square of the length scale, whereas orbital energy is proportional to the mass, which is cubic in the length scale. I would expect the orbital lifetime to be roughly proportional to their quotient and thus for larger objects to stay up longer.

On the other hand, ISS has a lot more empty (well, air-filled) space inside of it, so it might be a lot less dense than a SpaceX satellite.

Drag is enough of an issue for the ISS that they actually rotate the solar panels to reduce drag when they're not in sunlight, which saves them ~1000kg of propellant per year.

https://en.wikipedia.org/wiki/Night_Glider_mode

> Wouldn't drag be proportional to the surface area, hence the square of the length scale, whereas orbital energy is proportional to the mass, which is cubic in the length scale.

Are you assuming the ISS has a similar surface-to-volume ratio as other orbiting objects?

If the ISS were built like a scaled-up satellite, you'd be right. But as you say,

> ISS has a lot more empty (well, air-filled) space

and this dominates.

It does have a lot of empty space. But I would presume it also has a lot of heavy metal structure to keep that space pressurized to a human-breathable pressure and allow entry and exit. Satellites don't need to keep much under pressure except maybe their maneuvering fuel in a small perfectly-shaped container with a single small hole.

The ISS solar panels are quite large

The ISS has a lot of doohickeys hanging off it, so the cross section is going to be much larger than if the ISS was condensed into a sphere or cube. The ISS is also a lot less dense than most satellites, so even if the drag were the same it would have more of an impact

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