Image:NASA. Weightless in the ISS — astronaut Jessica Meir.

What Does “Weight” in the International Space Station Really Mean?

Rhett Allain
7 min readJul 30


Everyone loves this short video. It shows astronaut Woody Hoburg in the International Space Station during a reboost maneuver. He states that astronauts are NOT always weightless in the space station as he slowly “falls” backwards — in the opposite direction of the acceleration of the ISS.

There’s a bunch of stuff to talk about, but I’m going to start with the thing that are here for — a video analysis of his motion.

Video Analysis

It’s possible to get data for Woody’s position in each frame of the video. To do this, I’m going to use my favorite video analysis tool — Tracker Video Analysis.

The basic idea is to mark the location of an object (like a person) in each frame of the video. If I know the distance-scale in the video, this will give me both x and y position as function of time (time comes from the video frame rate). Of course, there are two small issues to deal with in this particular video.

The first problem is the scale. I don’t actually know the size of everything — but I can make a reasonable guess. The mounted laptop near Woody is probably has around a 15 inch screen (I went with 15.5 inch). So, I can scale the video based on that. Oh, it’s not the best choice since I don’t actually know the size of the laptop. Also, the laptop is not the same distance from the camera as Woody, which gives a perspective error. But still, this should be close enough.

The second issue is easier to fix. The person recording the video doesn’t keep the camera stationary. This means that Woody could appear to move due to a movement in the camera. Tracker Video has a tool to set a coordinate axis with respect to the background and then adjust the motion of an object (the astronaut) with respect to that origin. Done.

Let’s start with a look at his y-position (up and down as seen from the camera) as a function of time.



Rhett Allain

Physics faculty, science blogger of all things geek. Technical Consultant for CBS MacGyver and MythBusters. WIRED blogger.