Andrew Rader, an author and engineeer, tweeted a GIF video showing a droplet of water rotating around some sort of probe. enter image description here

Everything has gravity - here's a droplet of water orbiting a needle in 0G.

No source is provided. The scale is unclear. The result is counter-intuitive, and I am skeptical.

I realize that there is a stitching point somewhere to make the GIF repeat. I'm not challenging that aspect.

Does this video show a water droplet in (gravitational) orbit around a needle?

  • 2
    I'm curious now quite how slow a water droplet would have to travel to actually orbit a needle (assuming you could actually set this system up far enough from any other masses for it to work at all.) – reirab May 1 '16 at 6:23
  • 7
    @reirab For a rough approximation let's take a much more massive needle (say, M = 0.1 kg), also join all its mass in a ball (the mass of a needle is mostly "far away" and thus pulls less). And let the droplet orbit at a distance of just 1 mm. Then the orbital speed for a circular orbit is approximately sqrt(G M / r) or about 80 micrometers per second (i.e., during a full day it moves by just about half a centimeter) – Hagen von Eitzen May 1 '16 at 8:09
  • 28
    The camera has more gravity than the water droplet. – PyRulez May 1 '16 at 12:27
  • 3
    @PyRulez depends how close the camera is to the needle... – Nathan Osman May 2 '16 at 16:27

The GIF was sourced from the International Space Station video by astronaut Don Pettit. The video depicts small water droplets orbiting a polyethylene knitting needle which was charged by rubbing it with a piece of paper. Don Petitt states that the blue knitting needles are "8mm in diameter".

In the video, Don Pettit clearly says that the attractions are electrostatic, and not gravitational in nature.

A transcript of the relevant section for those who can't watch the video (starting around 0:50, emphasis mine)

This is like a little satellite going around a cylindrical shaped planet , except the physics here is a little different, it's not about gravitation, it's about charge forces, and of course charges can exert a potential field as we call it, so it can exert a force at a distance with no tangible connection.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .