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### How is the altitude of a satellite defined, given that the Earth is not spherical?

The altitude of a satellite is the distance between the Earth's surface and the satellite, but the Earth itself is not spherical. At the equator the Earth's radius is 21 km more than at the poles, and ...
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### Why do satellites appear to move faster when overhead and slower closer to the horizon?

I've been watching man-made satellites with the naked eye using the "Heavens Above" mobile app. They seem to move incredibly slowly near the horizon, but when they go overhead, they look like they ...
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### How do the Starlink satellites shine?

As we can see the Starlink satellites with the naked eye as they pass by. I was wondering about what makes them visible. Do they have large mirrors that reflect the sunlight making them look as if ...
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### General guidelines for modeling a low thrust ion spiral?

I've been told that for low thrust ion spirals, delta V would be the difference between speeds of departure and destination orbits. For example the delta V between a 7.7 km/s LEO and a 3.1 km/s GEO ...
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### Ratio of low-thrust slow spiral to Hohmann transfer $\Delta V$?

Going from one circular orbit to another, there are tables out there and equations that give the total $\Delta V$ needed for a two or three impulse maneuver scenario. For a very low thrust propulsion ...
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### How much Delta-V is needed for Orbital Maintenance?

My question is aimed at understanding the how much Delta-V is needed for orbital maintenance in different orbits. Obviously, there still is some residual air resistance that decreases as the orbital ...
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### What are these four “debris” objects along with the Starlink satellites?

Celestrak has an updated system of categories for retrieving lists of current TLEs, and there's already one for Starlink, yay! Currently https://www.celestrak.com/NORAD/elements/starlink.txt contains ...
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### How quickly can the Starlink spacecraft respond to an impending collision?

update: The word "refused" might be inaccurate: CNET: ESA's near collision with SpaceX Starlink satellite blamed on a 'bug' Ars Technica: SpaceX satellite was on “collision course” until ...
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### How much of the world's xenon has been used in spaceflight altogether? Is it a lot? Did it cost a lot?

This answer to Why will Starlink satellites use krypton instead of xenon for electric propulsion? says: I expect they did the math, and found that overall cost was less, even with reduced thrust/watt ...
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### How much krypton is stored on a Starlink satellite?

How much krypton (fuel) is there for the krypton-powered ion thrusters on Starlink satellites and how long does it last?
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### Replacement strategies for defective Starlink satellites

I wonder what is the approach to replace let's say: 5 defective Starlink satellites, each one in a different orbit? I guess that it can't be done with just one launch as the krypton ion thrusters ...
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### Space Debris and Megaconstellations: plans for ~1000kms altitude orbits

SpaceX's Starlink and other mega-constellation plans highlight safe de-orbiting features but does this also extend to altitudes higher than 1,000 kms? My concern is whether these smaller sats will ...
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### What orbits are Starlink satellites now deployed into? How low to do they go on their first perigee?

In my answer to Delta-V of Starlink Satellites I ballpark spherical-cow envelope-back estimated 190 m/s with 2 kg of krypton based on raising only from a 445 km circular orbit to a 550 km circular ...