How do astronauts know how fast they are traveling?

Let’s use our car again, but this time we’ll get real numbers from our smartphone’s accelerometer. Suppose we start at a red light and accelerate at 2 m/s² (meters per square second) five seconds. From the above equation, Δv₁ will be 2 x 5 = 10 m/s, which is our speed. Now, after cruising for a while, we accelerate again at 1 m/s² Five more seconds. Δv₂ Then 1 x 5 = 5 m/s. After adding these two changes, our speed is now 15 m/s. etc.

The only problem is that inertial measurements are less accurate than the Doppler method over long periods of time because small errors accumulate. This means you need to use other methods to recalibrate your system periodically.

optical navigation

On Earth, people have long relied on the stars to navigate. In the Northern Hemisphere, just find Polaris. It’s called Polaris because the Earth’s axis of rotation points exactly toward it. That’s why it appears to be stationary while the other stars appear to surround it. If you point your finger at Polaris, you’ll be pointing north, and you can use this direction to go in any direction you want.

Now, if you could measure the angle of Polaris above the horizon, you would also know your latitude. If the angle is 30 degrees, your latitude is 30 degrees. See, it’s easy. Once you can measure position, you only need to do it twice and record the time interval to find your speed.

But celestial navigation works because we know how the Earth spins, and that doesn’t help a spacecraft. Oh well, can we use the stars like the cows on the roadside? No. The stars are so far away that astronauts would have to travel many, many generations to detect any changes in their positions. Just like an airplane flying across the ocean, you appear to be stationary even when traveling at 25,000 mph.

But we can still use the basic idea. For optical navigation in space, spacecraft can locate other objects in the solar system. By knowing the precise location of these objects (over time) and their position relative to an observer, position can be triangulated. Likewise, by taking multiple position measurements over time, you can calculate speed.

Finally, even if spacecraft don’t have a speedometer, their speed can be tracked indirectly through a little physics. But this is just another example Flying in space is indeed completely different— and much more complex than driving or flying on Earth.