84

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?

March Ho
  • 18,688
  • 12
  • 81
  • 109
Oddthinking
  • 140,378
  • 46
  • 548
  • 638
  • 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 01 '16 at 06:23
  • 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 01 '16 at 08:09
  • 28
    The camera has more gravity than the water droplet. – PyRulez May 01 '16 at 12:27
  • 3
    @PyRulez depends how close the camera is to the needle... – Nathan Osman May 02 '16 at 16:27

1 Answers1

148

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.

March Ho
  • 18,688
  • 12
  • 81
  • 109
  • 4
    Now that I have thought about this I so wish to see a charged balloon in space with water flowing around it in a spiral :O . I mean, I get it would be hard to do, but should be possible xD . – David Mulder May 01 '16 at 14:16
  • 6
    The Twitter feed linked in the question features a tweet by @DanWeaver_ca that links to a [PDF with a paper about that experiment ("Electrostatic Model Applied to ISS Charged Water Droplet Experiment")](http://www.electrostatics.org/images/ESA_2015_I1_Stevenson.pdf). – Dubu May 01 '16 at 17:55
  • 6
    A non gravitational orbit? What a time to be alive. That's the first time I've ever said that non-sarcastically. – corsiKa May 02 '16 at 15:40
  • 1
    Some good sources of info: [The Parallel Between Gravity and Electrostatics](http://physics.bu.edu/~duffy/semester2/c02_gravity.html), [Comparison of Electrostatic & Gravitational Force](https://goforaplusplus.wordpress.com/2011/08/15/comparison-of-electrostatic-gravitational-force/) – TrinitronX May 02 '16 at 19:18