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# Satellites will still be visible.

A couple of characteristics make human satellites easy to see.  They are fast and bright.  Slow objects are difficult to pick out from the background star field.  Faint objects don't emit enough light to be seen by the human eye.

## Fast
Let's see how orbital period compares with orbital decay. 

| Satellite Altitude	 | Lifetime | Orbital Period (hh:mm:ss)	| 
| -- | -- |  -- | 
| 200 km |1 day | 01:28:29.65 |
| 300 km | 1 month |01:30:31.18 |
| 400 km | 1 year | 01:32:33.63 |
| 500 km | 10 years | 01:34:36.98 |
| 700 km | 100 years | 01:38:46.38 |
| 900 km | 1000 years | 01:42:59.33 |
| 5000 km | -- | 03:21:18.64 | 
| 10,000 km | -- | 05:47:39.69 | 
| 20,000 km | -- | 11:50:36.07	| 
| 30,000 km | -- | 19:10:51.27 | 
| 36,000 km | -- | 24:07:00.8 |
[Decay Source](http://www.spaceacademy.net.au/watch/debris/orblife.htm)
[Orbital Period Source](https://keisan.casio.com/exec/system/1224665242)
The difference in orbital periods between 200km and 900km is only 15%. These speeds should be easy to pick out against the background star field.  If humans have regressed to where light pollution isn't a problem anymore, then it should be even easier.


## Bright
Brightness will depend on the satellite's inclination toward the viewer, altitude, physical size and orbital position.  The larger and lower it is, the brighter it will be.

Lumosity is calculated by $I = C / d^2$ [source](cosmology.berkeley.edu/Education/Projects/Desktop_Stars/DTS/ISQ/Discussion.html)
 where _I_ is lumosity, _C_ is the source brightness and _d_ is the distance.  For our purposes, we only care about the effects of _d_.

At 200km, the lumosity of the satellite is reduced by a factor of 40000.  At 900km, it's reduced by a factor of 810000 or 20x dimmer than a satellite at 200km.

## The Sweet Spot
Satellites between 500km and 900km seem to have the best chance.  Their orbital decay is measured in hundreds or thousands of years.  They are also low enough that brightness shouldn't be too dim because of distance.

There's plenty of wiggle room on the brightness calculations to plausibly assert that there are lots of visible satellites or only a few.