4 ms·

> 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, w

by kmm 6y ago

> 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.

knute 6y ago
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 https://en.wikipedia.org/wiki/Night_Glider_mode
webmaven 6y ago
> 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?
beervirus 6y ago
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.
- ufmace 6y ago
  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.
barkingcat 6y ago
The ISS solar panels are quite large
Rebelgecko 6y ago
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

by kmm 6y ago

knute 6y ago

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

webmaven 6y ago

> 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?

beervirus 6y ago

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.

ufmace 6y ago
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.

barkingcat 6y ago

The ISS solar panels are quite large

Rebelgecko 6y ago

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