If an astronaut threw a cup of coffee into space somehow, would it freeze into a block, or boil off into gas due to the zero pressure?


4 Answers 4


This was tested nearly sixty years ago. Using a very large cup filled with 95 tons of water. An empty second stage of a Saturn I under test was used. Only the first stage should be tested but with full payload of a water-filled dummy upper stage. Dummies with the tanks but without the expensive engines. The dummies should have the same shape and mass as a real second stage and the center of mass should be at the same position. The test of the first stage required dummies with water ballast anyway, only an explosive charge was needed to release the water at the highest point.

From this archived NASA media page:

enter image description here

Date: 04.25.1962
Title: Saturn I SA-2 Launch
Description: The second flight of the Saturn I vehicle, the SA-2, was successfully launched from Cape Canaveral, Florida on April 15, 1962. This vehicle had a secondary mission. After the first stage shutoff, at a 65-mile altitude, the water-filled upper stage was exploded, dumping 95 tons of water in the upper atmosphere. The resulting massive ice cloud rose to a height of 90 miles. The experiment, called Project Highwater, was intended to investigate the effects on the ionosphere of the sudden release of such a great volume of water.
ID: MSFC-6203276
Credit: NASA Marshall Space Flight Center (NASA-MSFC)

So the water would not freeze to a block but to a cloud of ice crystals. These small ice particles would not melt but turn into water vapor directly. This is called sublimation: when a solid goes directly to vapor without a liquid state between solid and gaseous.

But how does the water turn into ice? The liquid water releases some vapor and cools down by the vaporisation. So a part of the liquid goes to vapor and the rest to ice. Vaporisation of water removes a lot of heat energy.

A report of 127 pages was written.

But what about the cup of coffee? In zero gravity you can't handle liquids in cups. In the pressurized capsule, the coffee will float free, its shape will be a sphere. If you depressurize the capsule to expose the coffee to the vacuum of space, the water will boil at lower air pressure, a part will turn into vapor and the rest into ice crystals. The ice crystals will be vented together with the air into space. When pressure is so low that the hatch may be opened only very few crystals are left in the capsule.

If you want to see what happens to a small water volume in space, you need a closed and pressurized bottle to take the water out of the capsule. If you open the bottle, you will see the cloud of ice crystals.

Wikipedia: Project Highwater

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    $\begingroup$ I don't see how a huge tank of water being exploded tells us anything about what will happen to cup of coffee hand-tossed into space. Yes, there are physical processes called vaporization, sublimation, and thermal radiation, but what would actually happen in a given situation will depend a lot on initial conditions, and these conditions don't match those discussed in the question. $\endgroup$
    – uhoh
    Jun 16, 2021 at 0:05
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    $\begingroup$ @uhoh Assuming it happened in a chamber simulating the cold and vacuum of space. $\endgroup$ Jun 16, 2021 at 5:01
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    $\begingroup$ @trognanders It's important to understand that space has no temperature. Things in space do though, and that temperature depends heavily on whether they are in sunlight and how much of it they absorb. For example, the surface of the moon gets pretty hot in the sun. $\endgroup$ Jun 16, 2021 at 7:18
  • $\begingroup$ Why do you think it will be fully vaporized before the pressure reaches zero? The cup of coffee does not have enough energy to vaporize all the coffee, does it? I would expect it to start to boil, the temperature would quickly decrease and if you didn't lower the pressure too quickly, it should stay in one piece due to the surface tension and ultimately become a solid block of stronger coffee. $\endgroup$
    – JohnEye
    Jun 16, 2021 at 13:13
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    $\begingroup$ @Peter-ReinstateMonica "space has no temperature." Yes and no. For every point in space, there is a temperature such that a black body at that temperature would be at thermal equilibrium. Thus there is a sense in which that point in space has that temperature. $\endgroup$ Jun 16, 2021 at 18:06

It would not freeze into a block. It would quickly expand and boil, but not in a rolling boil. Without pressure, bubbles would form throughout the coffee and expand rapidly, causing it to spray out of the mug as soon as you released the catch that would have been necessary to hold the lid on. But evaporation causes cooling, so millions of frozen coffee crystals would form and quickly disperse. At our distance from the sun the crystals would then evaporate (that is, sublime) over time. In deep space, far from the sun, they might last indefinitely.

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    $\begingroup$ So, essentially - the coffee would turn into snow. $\endgroup$
    – SF.
    Jun 15, 2021 at 9:59
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    $\begingroup$ @SF. Coffee Snow sounds like a name for a delicious icecream. $\endgroup$
    – T. Sar
    Jun 15, 2021 at 13:58
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    $\begingroup$ I am assuming there would be enough particulates in the coffee that, with the boiling point now so much lower than the temperature, nucleation would happen in lots of places. But possibly bubbles rapidly forming against the mug surface would be the main ejector; I'm not sure it would look noticeably different. $\endgroup$ Jun 15, 2021 at 14:52
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    $\begingroup$ Frozen coffee bubbles sounds more like a frozen cappuccino. What would the Italians and Australians say? $\endgroup$
    – Fred
    Jun 15, 2021 at 15:54
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    $\begingroup$ @uhoh: If you start with hot coffee, the rapid expansion is quite likely. If you start with coffee near freezing temperature, the expansion would be way more moderate, as latent heat of evaporation of water is quite high, so after quickly dropping the water temperature to zero through the initial boil-off it would take quite a while for the remainder to acquire enough heat to create more bubbles. If you start off with ice latte, you're likely to see only slightly frothy lump of ice. $\endgroup$
    – SF.
    Jun 16, 2021 at 10:48

While the existing answers appear mostly right to me, especially on the "part will turn into vapor and the rest into ice crystals" fact, I think they omit one interesting explanation. Namely, why does evaporation cause cooling - understanding which seems to suggest the exact effect will depend on how violently you throw the cup.

While I've never conducted such an experiment in 0g, I have done it repeatedly in 1g, to explain this particular phenomenon. So, I'll start with the part I have seen (using water and a vacuum pump).

What is evaporation? Is it random which part of a liquid evaporates and which remains? Not quite. On the microscopic level it is, somewhat self-explanatory, the molecules which break past the surface that form the vapour. Now, in the liquid, as elsewhere, the molecules have a distribution of different kinetic energy values. The ones most likely to escape are those with the highest kinetic energy. When these are gone, the ones which stayed have, on average, lower kinetic energy. This energy is precisely the temperature - getting lower.

So, you'll have the higher-energy molecules turn to vapour, and the low ones - into ice. In 1g, those stay together at the bottom of the vessel. For a bit of 0-g speculation: depending on how big the cup is, and how violently it is thrown, I'd think these low-energy remnants can form larger or smaller ice crystals. Most will be vapour either way.


Assuming total darkness and a coffee temperature below the boiling point in space The coffee can only cool down by emitting radiation. If the coffee is thrown out hot or cold has the only difference that it takes a bit longer for the hot coffee to freeze than the cold coffee. If the space is dark and the coffee is given a lot of velocities it will get torn into small droplets and it will cool down and freeze. If the droplets are very small, almost immediately all heat will be radiated away. So if you can throw like spraying it will be a nice sight.
The radiation loss of the coffee will be the highest for large sheets of connected coffee. A flat sheet of coffee will radiate its heat away the fastest. So if you manage to throw a sheet of coffee out of the cup, you will see the sheet turning solid. The thinner the sheet the faster it freezes.
The frozen coffee will end up in thermodynamic equilibrium with the vacuum of space, reaching a shivering temperature of about 2.7(K).

The temperature of the water is below the boiling temperature on Earth. But in space this boiling temperature is lower. So if it has the boiling temperature of space it will be boiled to small droplets, causing a spray. No matter how you throw it. For lower temperatures, the above scenario suffices.

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    $\begingroup$ "coffee temperature below the boiling point in space" Not going to happen. $\endgroup$ Jun 16, 2021 at 19:25
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    $\begingroup$ What is the boiling point of water in a vacuum? $\endgroup$ Jun 16, 2021 at 19:45
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    $\begingroup$ en.m.wikipedia.org/wiki/Triple_point $\endgroup$ Jun 17, 2021 at 4:29
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    $\begingroup$ @Methadont: I don't think the problem is that the water won't boil, I think the problem is that the water will always boil in a vacuum. Even steel sublimates in a vacuum (slowly) $\endgroup$ Jun 17, 2021 at 15:33
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    $\begingroup$ @Methadont Water can't be liquid at pressures below the triple point, and as such it doesn't literally boil. As Mooing Duck correctly points out, it does sublimate, which is basically the same thing but without being a liquid. It's what dry ice does. $\endgroup$
    – Fax
    Jun 17, 2021 at 16:03

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