From a hypothetical perspective, if one could bring a water gun into deep space and fire it, would it shoot a stream that froze and travel along or would its stream vaporize upon exiting the pressurized nozzle prior to freezing?

This has been a point of debate, especially after reading the post at http://www.madsci.org/posts/archives/may2000/959189502.Ph.r.html which states that water being ejected from a shuttle freezes. I was under the impression it freezes as an icy mist, not solid drops.

---

Actually I think the previous answer is pretty unambiguous. "An icy mist" is still composed of "solid drops" of some size. The stuff is either solid (in which case the particles are composed of more than one molecule) or gaseous (in which case the particles are one molecule in size).

On the other hand, I gather from your question that you're asking whether a stream of water would freeze into a single solid "spear" of ice, and the answer is "certainly not!" Water doesn't even do that in Antarctica, where you don't have a pressure drop to disperse the stream; it takes time for water to freeze, even when it goes from 310 K (human body temperature) to 200 K (the approximate temperature of an Antarctic winter, or low Earth orbit).

However, the water (if ejected with a particular velocity) would keep that velocity, dispersing a bit because of intermolecular and interparticulate collisions within the stream but otherwise traveling in the same direction until dispersed by collisions with other particles. These particles include the solar wind, interplanetary dust or the Earth's outer atmosphere (low Earth orbit is still within the atmosphere, you know). The water would also be subject to gravitational fields that would warp its trajectory, but this would effect the entire stream more-or-less equally and would not disperse it rapidly compared to collisions.

It is possible that having the water under pressure upon ejection into space might accelerate vaporization somewhat; but my guess is that the effect would be negligible because a pressure drop from 5 or even 50 bar to 10^-12 bar is not that different from a drop from 1 bar to 10^-12 bar. If you had the water really pressurized, maybe... perhaps 10³ to 10⁶ bar.

Anyway, a particular small set of water molecules can't simultaneously freeze and vaporize; only the bulk liquid can do it, and what we mean by that is that some of it vaporizes while other bits of it freeze. Some of the vapor might then deposit onto the ice particles before the whole mess dispersed. And eventually the ice particles might sublime... or not. But I don't think you are asking about long-term behavior.

Dan Berger

Bluffton University