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This and this answer describe the Falcon 9 fairings as being quite large, with the payload volume being roughly "the size of a bus" or simply

REALLY REALLY big. 13 meters by 4.6 meters

and being made of an aluminum honeycomb and carbon fiber.

Then this more recent answer got me thinking further.

Considering the very large area compared to mass and intrinsic strength (built for max-Q aerodynamic load and durability under launch vibrations) and the relatively low ground speed at which they are deployed, they could potentially survive re-entry and remain on the surface of the ocean.

Is this what happens, at least sometimes?

If so, their buoyancy would result in perhaps of order one meter height, making them large, but low visibility objects and potential navigation risks for smaller surface craft, both visually and perhaps radar as well.

Does it?

Would F9 fairing recovery be part 'good Earth citizenry' in addition to good economics?

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Finally can answer this definitively, yes, they do float and I can prove it!

Falcon Heavy fairing floating

Mr Steven failed to catch the fairing in its net, which looks like this:

Mr Steven in all her glory

But after the water landing it was fished out.

Half a fairing back in port

Finally the lesser half, not quite as intact.

Other Fairing half

More pics:

enter image description here

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enter image description here

enter image description here Rumour has it the second half was ok, but our little buddy was involved, so it ended poorly.

(Just sit right back and you'll hear a tale, A tale of a fateful trip That started from this tropic port Aboard this tiny fairing.)

GGGIIILLLLIIIGGGAAANNNN!!!!!! source: https://www.lyricsondemand.com/tvthemes/gilligansislandlyrics.html

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    $\begingroup$ The freeboard is 90 cm, so you need a calm sea and a nice upright landing for it to float. A bit surprising the vents on the fairing weren't letting in water. $\endgroup$ – Hobbes Feb 23 '18 at 18:35
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    $\begingroup$ They just need to mount a mast and rudder in that thing and sail it home. $\endgroup$ – Organic Marble Feb 23 '18 at 18:42
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    $\begingroup$ @OrganicMarble Added one more photo to explain why trying to sail it would be a bad idea! $\endgroup$ – geoffc Feb 23 '18 at 18:45
  • $\begingroup$ @geoffc Are you now going after a badge for the largest number of linear inches of post? :-) That said, these are really interesting, thanks! fyi I've just asked Identify this rocket and launch site from Gilligan's Island episode? $\endgroup$ – uhoh Feb 24 '18 at 1:11
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    $\begingroup$ @uhoh Is that a thing? I am in! $\endgroup$ – geoffc Feb 25 '18 at 1:18
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The fairing consists of an aluminium honeycomb sandwiched between carbon fibre sheets. The amount of air trapped in the honeycomb determines if the structure will float, but as we don't know how thick the honeycomb and the CF cheets are, there's no way to say for sure whether it'll float.

The shock of landing may damage the fairing, but we don't know its landing speed so we don't know for sure yet. The only fairing I've seen after landing was washed up on shore after being at sea for a while, so no conclusions can be drawn from that one either.
If it floats, it won't do so indefinitely. Wave action will deform the fairing until the fibre delaminates and water can get into the honeycomb. At that point, it'll sink.

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    $\begingroup$ They routinely wash up on shores. SpaceX, just as Arianespace, has people retrieve them. $\endgroup$ – Rikki-Tikki-Tavi Feb 7 '17 at 16:20
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    $\begingroup$ Until that time they will be less of a danger (aluminium, carbon fiber) than containers falling of ships ;-) $\endgroup$ – Jan Doggen Feb 7 '17 at 16:20
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    $\begingroup$ reddit.com/r/whatisthisthing/comments/2dgyiw/… $\endgroup$ – Rikki-Tikki-Tavi Feb 7 '17 at 16:22
  • $\begingroup$ @Rikki-Tikki-Tavi very helpful - I have a better view of the whole situation now. The construction is... beautiful! $\endgroup$ – uhoh Feb 8 '17 at 2:39
  • $\begingroup$ Now we know that it floats nicely, at least right after "landing" on the water :-) instagram.com/p/BfgRX-lgIt6 $\endgroup$ – Jesper Feb 22 '18 at 22:55

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