Business · Wired
How Can Astronauts Tell How Fast They’re Going?
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Weirdly, spaceships have no direct way to gauge their own speed.
Key facts
- Speed vs. Velocity First, a word about words: Speed is how far you go in how much time—like 50 miles an hour
- The team can still take the change in vector position ( Δr ), or displacement, and divide by the change in time ( Δt = 10 seconds)
- But there’s no trees or cows, no air, not even a GPS signal to help you out
- Here on Earth, it’s easy to know how fast you’re driving
Summary
Here on Earth, it’s easy to know how fast you’re driving. You get a good sense of it by seeing trees and cows pass by. If there’s any wind, this won’t be the same as your speed relative to the ground, but you can get that by using GPS location data from orbiting satellites. But there’s no trees or cows, no air, not even a GPS signal to help you out. So how do you know your rate of travel? Speed vs. Velocity First, a word about words: Speed is how far you go in how much time—like 50 miles an hour. For an airplane using GPS coordinates, it’s easy to calculate: take the distance between two locations and divide by the time it took to get from point A to point B. It doesn’t work at all for a bumblebee, whose path more resembles that of a drunken sailor.
Even if the bee flies at a constant speed, its velocity is always changing. To map the bee’s path, the reporter drew an xy coordinate plane on the scene above. (For simplicity, the reporter is keeping it two-dimensional.) Someone looks at their watch and records a time of 1:00:05 (five seconds after 1 o’clock); at that moment the bee is at a position defined by vector r 1 . At 1:00:15, it’s position vector is r 2 .