# At what size do objects burn up in the atmosphere when falling from orbit?

As the title suggests, I am wondering the maximum size an object can be so that it will completely burn up when entering and falling through Earth's atmosphere. I'm focusing on man made objects, not asteroids or meteors. Also, how much does the make-up of the object affect its burn rate? I.e. metal vs plastic vs glass, and so on. Thanks!

• By burn, do you mean react with oxygen and turn into oxides, or get hot enough to melt/vaporize by friction with the atmosphere? The distinction will get you different answers. Jan 31, 2016 at 16:10
• I would gladly answer but it does not only depend on size, it also depends on the speed at which the object enters the atmosphere. A bigger object will have to enter faster to be disintegrated. So it's all about that ratio.
– Nico
Feb 1, 2016 at 9:34
• @Wayfaring Stranger: I'm pretty confident (though not 100% sure) that objects don't melt/vaporize due to friction. Densities are just too low for that. It's rather the heating up of the gas due to the rapid compression from the shockwave, when the object falls through the atmosphere Feb 12, 2016 at 8:55

## 1 Answer

The Earth atmosphere protects us from small impacts from both asteroids and man made objects. This is well known from meteoroids, where meteoroids as large as a few tens of meters in diameter usually fail to penetrate into the lower atmosphere because they get fragmented and dispersed at high altitude. Fragmentation height depends mainly on the meteoroid strength, only strongest irons reach the surface in one piece.

We could extrapolate from meteoroids to man made objects that even large do not have the same physical strength than a massive meteoroid body. This means than nearly all man machine objects will disintegrate before reaching earth. Heavy metallic ones (iron) will disintegrate at lower altitudes than lighter ones such glass and plastic.

Two comments:

• This is not true for objects not having reached orbit, e.g. rocket 1st or 2nd stage engines will fall without disintegrating.
• Even if they disintegrate, nuclear powered satellites will cause some level of radioactive pollution, however widely scattered in the atmosphere.

Although the risk to human beings walking on earth is next to nil, there is a real risk of impact from man made objects littering Earth orbits to other spatial vehicles.

Reference: "The Impact Hazard", David Morrison, Clark R. Chapman and Paul Slovic, in "Hazards due to Comets and Asteroids", T. Gehrels, Ed., 1994, The University of Arizona Press