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I recently observed someone being worried about small space debris (e.g. discarded gloves (?), paint chips) hitting them on the ground. (For these cases, the worry is of course unwarranted, since the object would disintegrate into tiny pieces, and eventually dust or gas, leaving no solid impactor). The object must be large, and if not designed to survive, larger still.

It then occurred to me, what would be the smallest possible object to survive a re-entry, alone?

Specifically, you get one object in a circular LEO (eventually; it will re-enter due to weak air resistance). At least part of it must survive the re-entry in a macroscopic form traveling at dangerous speed.

  • How small (volume) can you make this object?
  • How small (mass) can you make this object?
  • In both cases, what is it made of?

EDIT: this article puts the limits much lower than I would have thought:

Bits and pieces of trash constantly fall from the sky, but nearly everything larger than 4 inches (10 cm) survives in some form, likely in smaller fragments.

To salvage this question, let's focus on the danger aspect. The smaller objects reported are probably harmless dust specks by the time they reach the ground, or maybe pebble-sized nodules. So, how big (by mass, by volume) would an object have to begin to injure someone on the ground (culminating in, say, maybe a golf-ball-sized object falling at terminal velocity)?

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  • $\begingroup$ related: space.stackexchange.com/q/9464/4660 $\endgroup$
    – kim holder
    Commented Jun 13, 2015 at 3:06
  • $\begingroup$ You want some limitations on the third point right? I'm pretty sure using something like solid diamond would break this thought experiment ;) $\endgroup$
    – Vedant Chandra
    Commented Jun 13, 2015 at 7:43
  • $\begingroup$ @VedantChandra nope; I want to know what the limits are. For example, diamond is brittle; I think the thermal shock would pulverize it. My bet is on a tungsten block: dense and durable. See also my edit. $\endgroup$
    – geometrian
    Commented Jun 13, 2015 at 8:10
  • $\begingroup$ Doesn't it depend on the reentry speed? Low density objects may slow enough in air and survive reentry conditions. $\endgroup$
    – mins
    Commented Jun 15, 2015 at 7:00
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    $\begingroup$ So why isn't the answer a Xenon atom? $\endgroup$
    – Joshua
    Commented Jun 15, 2015 at 20:21

2 Answers 2

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We know that tiny dust particles can survive even hyperbolic entry. This is a piece of interplanetary dust collected by an aircraft in the stratosphere:

several micron dust particle

It probably looks pretty much like it did before entry. It experienced very high acceleration with relatively little heating.

Cosmic rays survive as well, and they are much, much smaller.

As for dangerous things, it doesn't really matter if it came from space. All that matters is the mass and its terminal velocity in the atmosphere. Any and all size and mass objects can result from a satellite reentry and breakup. The question really comes down to what kinds of falling objects can kill you if they hit you in the head. That's not a space question.

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  • $\begingroup$ Fascinating. Do you have a reference for further reading on interplanetary dust recovered from aircraft? $\endgroup$
    – pericynthion
    Commented Jul 21, 2015 at 22:09
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The most likely thing to survive that I'm aware of is a Tungsten block. I'm aware of a space mission that used tungsten blocks weighing a few kg, and there was serious concern that one of them could survive reentry in the event of an issue, to the point where it would cause damage. Tungsten is dense and heat resistant, and would probably survive with a relatively small block (A few inches on each side.) Your golf ball of tungsten would probably survive reentry and potentially hurt someone or something.

Usually materials are selected to not survive re-entry. Aluminum is a great choice for such a material. With proper material selection, this is not usually a problem.

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