In Leo S's comment to the question, Do astronauts develop the ability to regularly place an object at rest inside the ISS after extended periods in microgravity?, he said "forces due to circulation of air inside the ISS....would have the largest effect" "that will move [a] 'stationary' object."

Is it known what is the most massive object that moved (perceivable to the unaided human eye) by the circulating air in the ISS?

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    $\begingroup$ It's apparently hard to find real examples on internet. In the meantime, I've performed back of the envelop calculations. Some sources state air flow velocity in ISS are in the range of 0.05 to 0.4 m/s. A very rough calculation shows that an average apple (50mm in diameter with mass of 100g), after being placed with 0 velocity in this airflow would travel 100 mm within dozens of seconds to few minutes. There are, of course, deviations from the average airflow velocity. In front of the blower it would be the highest, and at periphery, in vicinity of the walls it would be the lowest. $\endgroup$ – Sergiy Lenzion Jan 22 at 13:09
  • $\begingroup$ Momentum is conserved. Any object not locked down will drift in the presence of airflow. $\endgroup$ – Carl Witthoft Jan 22 at 13:26
  • $\begingroup$ @CarlWitthoft Yes, any object will drift, but will the movement of massive objects be fast enough for a human to detect the drift? $\endgroup$ – Bob516 Jan 22 at 15:21
  • $\begingroup$ @Bob516 I came across this really nice virtual tour of ISS and thought it might be interesting to you. Pretty much everything is attached to the walls. maps.app.goo.gl/TnHarf81GCqburBE6 $\endgroup$ – Sergiy Lenzion May 20 at 9:25
  • NASA astronaut Clayton C. Anderson In his Quora answer describes displaced (by the air flow) chain/medal:

    In a micro-gravity environment, things can get lost. I had a chain/St. Christopher's medal, given to me by Sunita Williams' mother before I launched, that I lost while "showering" one morning. Turns out I found it near air vent (inlet) where the air flow had sucked it up against the grate of the vent. Yea! Air vents sometimes serve as our "lost and found" location. Many things ... turn up there.

  • Document The ISS: Operating an Outpost in the New Frontier, on page 340 mentions that items as large as "small tools" and a fork would be carried by the air flow all the way to the vent grate.

    in microgravity, objects collect on the inlet of vents and not on the floor. This process also acts as a handy way to find missing items—e.g., lost screws, misplaced washers, small tools, or even a pack of gum or a fork.

  • NASA astronaut Sunita Williams describes the case of displaced bag of coffee:

    Cowing: ... do you guys spend a little bit too much time looking for stuff?... Williams: ... the worst thing about space is also stuff floating. And you are right - you can't just put something down because a couple of seconds later it is going to float away - sort of like this microphone. Just yesterday I lost a bag of coffee - if you can believe it. I just thought I velcroed it down and it was gone. What typically happens is that you find it not too long afterwards. You get to know the airflow patterns in the space station because the air is flowing in and out of the vents and through the vestibules. So you sort of get an idea of where something is.

  • This website shows picture of an ISS vent with sucked glasses: http://www.projectrho.com/public_html/rocket/images/lifesupport/lostAndFound.jpg

But amongst all the reported cases I was able to find (and if I haven't misinterpreted the exact phrases used or a context in which they were used), the largest and most massive displaced "objects" were not objects, but humans.

The sources don't explain over what period of time this happened (perhaps a long time), but the ultimate change in position was obvious enough to notice the difference:

  • The article by Michael D'Estries in the section "Ventilation is a constant necessity" discusses air circulation on ISS and mentions the following:

    For those who decide to sneak in an untethered nap outside the sleep pod, astronaut Mike Fincke wouldn't recommend it. "We were sitting around the table drinking some tea, and I just fell asleep," he shared in a video. "I started floating away."

  • NASA astronaut Scott Kelly describes his floating experience whilst in his sleep (although it is unclear why this has happened, as many sources claim that sleeping bags are typically strapped to a wall for this very reason: to not float away from the vent located near the head, otherwise CO2 concentration would build up around the face and this would give headaches to the astronaut; maybe he just wasn't using a sleeping bag?) :

    I had been on the station for a week, and was getting better at knowing where I was when I first woke up. If I had a headache, I knew it was because I had drifted too far from the vent blowing clean air at my face.

Emphasis added.

P.S. In addition to the above, with the purpose of supporting the claim I made in the mentioned comment, I've performed some rough calculations.

The result is that an average apple (50mm in size with mass of 100g), after being placed with 0 velocity in 0.1 m/s airflow would travel 100 mm within the order of minutes.

If we increase the apple size 10 times (to get a hypothetical giant apple of 0.5m in size), its mass increases 1000 times, i.e. the apple mass now becomes 100kg.

The calculation shows it would take only 3.2 times longer for this giant apple (being exposed to identical conditions) to travel the same distance of 100mm. Below is the details of the calculation:

Some sources (such as this) state the air flow velocity inside ISS are in the range of 0.05 to 0.4 m/s.

For the calculations I took the figure of 0.1m/s to estimate how long it would take for an average apple (50mm in diameter with mass of 100g) to travel 100mm in the airflow stream from stationary (relative to ISS' walls) position.

Dynamic pressure P that acts on the apple surface:

P=0.5*Rho*(v^2), where

Rho - air density; v - airflow velocity.

Approximating the apple with a cube of 50×50×50mm (for the ease of area calculation), we obtain the force F acting on one of its surfaces (assuming the surface is always perpendicular to the airflow velocity):


where P - the pressure, A- surface (50x50mm).

Using second Newton's law we obtain apple acceleration:


where F - the force acting on apple, m - the apple mass

Then I applied iterative process:

After a time t1=0+dt apple velocity grows from 0 to v1=0+a1*t1, travelled distance is d1=0+v1*t1.

For the next step velocity in the formula for dynamic pressure is (0.1m/s - v1), then we obtain new figure for acceleration a2. For the time t2=t1+dt, we calculate new velocity v2=v1+a2*dt, travelled distance is d2=d1+v2*dt,

and so on, until travelled distance reaches 100mm.

So, for 50mm cubic shape "apple" with 0.1kg mass, with one of its sides being always perpendicular to the airflow direction, it takes approximately 33 seconds to travel 100 mm. As the real apple is near-spherical, the applied force (and therefore acceleration) would be lower than calculated, hence the time to travel 100mm would be longer. That's why I stated "an order of few minutes".

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