# how many satellites burn up on re-entry every day?

Carl Sagan says “2 or 3 every day”.*

Sagan doesn’t specify how many satellites are in orbit**, but it just seems weird that so many are destroyed every day.
If 2 or 3 are lost every day, that implies that 2 or 3 new satellites are being launched every day.

Sagan wrote this in 1996. And with today’s profusion of GPS, cell phones, satellite radio,military systems,etc, I would assume that there are more satellites today than 12 years ago.

So how many launches take place on earth every day? And from where?
*cite: Sagan’s book “Demon-Haunted Worlds”, in which he fights ignorance by applying the scientific method to superstitions and memes in pop culture.
Specificly, in the chapter on UFO’s he writes: " there are so many artificial satellites up there that they’re always making garish displays somewhere in the world.Two or three decay every day in the earth’s atmosphere, the flaming debris often visible to the naked eye."

** (but I assume it is fewer than “billions and billions” )

Does he make the distinction between natural satellites, various space manmade space debris in orbit that is big enough to be tracked, and the large functioning satellites you are thinking of?

Only if the system has achieved steady state.
If we were to stop launching tomorrow, it would be some time before the number of reentrys per day declined to zero.
That said, I don’t know how many deorbit each day, but suspect Sagan made a mistake, or is including flecks of paint and other space junk in his estimate.

Something sounds wrong there. This guy’s blog shows a graph of the number of launches annually, which sounds about right:

Peaking in the 1980s at about 130/year, now more like 60/year. Some of those launches may have had multiple payloads. The number of satellites seems to be single digit thousands in most estimates (with 2 or 3 times as many bits of junk). The numbers don’t seem to support “2 or 3 every day”, unless the numbers are decreasing rapidly, or Sagan was counting significant sized pieces of debris as well as actual “satellites”, as suggested.

It depends, like so many things, on how you define the relevant term: satellite.

If you’re talking about an object in orbit around a planet, and then add Sagan’s modifier of “artificial”, then you can include a whole lot more than just discrete apparatus. Any introduced space junk large enough to become visible during re-entry could count.

Reading Wikipedia, I made the inference that a meteoroid would need to be the size of a grain of sand or larger to be visible to the naked eye during re-entry. That would leave us with tens of thousands, if not hundreds of thousands, of bits. Some of them have stable orbits, but I suspect that the majority will fall back into the atmosphere at some point.

Wikipedia states:

“There are more than 600,000 objects larger than 1 cm in orbit (according to the ESA Meteoroid and Space Debris Terrestrial Environment Reference, the MASTER-2005 model).”

However, this doesn’t differentiate between natural and artificial debris, and it doesn’t include meteoroids larger than a grain of sand and smaller than 1 cm, presumably because they don’t pose as much of a threat to spacecraft.

A bit of data:

Celestrak keeps total stats on things in orbit:

``````
On Orbit 	Decayed 	Total 	On Orbit 	Decayed 	Total 	 	On Orbit  	Decayed 	Total
3,298	 	2,776	 	6,074	9,691	 	 17,643	 	27,334	 	12,989	 	20,419	 	33,408

``````

According to the Heavens Above database, there were 262 satellite launches in 2007. 57 of those satellites have reentered the atmosphere.

We aren’t launching fast enough to sustain Sagan’s reentry number, unless debris is included.

Not necessarily.
With the advances in electronics, a single modern communications satellite can do functions that required several previously. And given that there are a limited number of spaces available in the prime (geo-stationary) locations, they are probably replacing old ones with more modern ones as soon as feasible.

Isn’t there a completely predictable lifespan for these things depending on their power supply and their position relative to the Earth? I would imagine that more modern satellites have a longer orbit lifespan than older ones, let alone being more capable of handling multiple duties.

Telcom companies aren’t going to spend millions on launching a piece of junk that falls back to the Earth every couple years.

Not counting military satellites, of course, of which there are invariably many.

Oh yeah, a question: is a random object in a decaying orbit that’s only about 1cm in size really visible to the naked eye upon reentry? Really?

The object itself is not visible, but the light & flames generated as it burns up is visible.

I think it’s more because of the limitations of existing optical and radar space surveillance systems. I was once told that the Air Force could detect and track anything larger than a baseball, and I assume things have improved since then. A very small piece of debris can have a devastating amount of kinetic energy. One small screw could easily destroy a satellite if it hit it in the right place.

DOH! That’s what I meant to say, was something that small in size/mass visible upon entry into the atmosphere. Still pretty impressive that it is, though.

That’s a really good find. If we add up all decayed materials there’s enough for two per day for almost thirty years, so it’s likely those were the same numbers Sagan used.

Cold War’s over.