- Rapid, passive disposal in the unlikely event of a failed spacecraft
- Self-cleaning debris environment in general
- Reduced fuel requirements and thruster wear
- Benign ionizing radiation environment
- Fewer NGSO operators affected by the SpaceX constellation
The first two are because there is more atmospheric drag. I believe that orbital debris in the case of a collision was something that SpaceX was struggling to mitigate (every struggles, but no one has put up this many satellites before).
The third is because originally the plan was to launch to a 400km orbit and then have the satellites lift themselves to a 1150km orbit. Now they intend to launch to a 300-350km orbit and lift themselves to 550km. They expect that the smaller amount of lifting will increase satellite lifetime by 50% even after accounting for atmospheric drag.
The fourth is apparently just "there's less radiation lower, and radiation is bad for electronics".
The fifth is just "less of the other theoretical internet constellations are at this height" as far as I can tell.
(All information sourced from the technical information attachment)
> They expect that the smaller amount of lifting will increase satellite lifetime by 50% even after accounting for atmospheric drag.
I'm curious: why will this provide a longer life? Is it the lift burn itself that affects the lifetime (so reduced lift burn is better for the satellite)?
no, because a modern satellite with ion thruster or very high efficiency (high specific impulse, low thrust measurement in newtons), once it's been ejected from the second stage of a rocket and is above 99.99% of the atmosphere, if you burned all of its stored xenon fuel immediately after launch, would end up in a 45,000 x 450 km elliptical orbit.
If you want to keep a satellite in a mostly circular orbit from 350x350km to 600x600 km you do periodic very small boost maneuvers.
On an unfortunately very theoretical (but apparently already patend-encumbered, from what I can glean from Wikipedia) level there is also the concept of air-breathing electrical propulsion, an ion engine replenished from the the same trace atmosphere that causes the drag the engine is supposed to counter. Basically a solid state propeller that can still work in very thin air.
From my layman's understanding, because the effect of aerodynamic shaping breaks down at very low pressure, exhaust speed would have to be travel speed (TAS) x (total cross section / intake cross section) to keep orbit. Assessing whether that puts the concept in the realm of feasible technology or not is beyond my skills, but at least there seem to be projects working on that question. If it does work out, it would completely change the economics of LEO use.
Basically, there's already nothing left at their new planned altitude. In either scenario they'd need to burn fuel in order to compensate for irregularities in Earth's gravitational field, but I assume they've run the numbers internally.
Station keeping doesn't only involve drag; it involves rebalancing the orbital plane when a satellite fails; you also need to provision fuel to deorbit.
The higher the orbit, the more fuel you need to deorbit.
This constellation should have lower latency over long distance than is even theoretically possible using terrestrial fiber, because the speed of light in space is substantially better than the speed of light in fiber. [0]
Even over short distance the latency (which the "Legal Narrative" pdf quotes as 15ms) is negligible for almost all applications.
- Rapid, passive disposal in the unlikely event of a failed spacecraft
- Self-cleaning debris environment in general
- Reduced fuel requirements and thruster wear
- Benign ionizing radiation environment
- Fewer NGSO operators affected by the SpaceX constellation
The first two are because there is more atmospheric drag. I believe that orbital debris in the case of a collision was something that SpaceX was struggling to mitigate (every struggles, but no one has put up this many satellites before).
The third is because originally the plan was to launch to a 400km orbit and then have the satellites lift themselves to a 1150km orbit. Now they intend to launch to a 300-350km orbit and lift themselves to 550km. They expect that the smaller amount of lifting will increase satellite lifetime by 50% even after accounting for atmospheric drag.
The fourth is apparently just "there's less radiation lower, and radiation is bad for electronics".
The fifth is just "less of the other theoretical internet constellations are at this height" as far as I can tell.
(All information sourced from the technical information attachment)