Hi Stephanie,

Predicting CMEs and how quickly they move through the solar system is one of the big goals of research into space weather.

We can't really predict the speed of a CME before it happens, although there are some general tendencies - a CMEs from outside of active regions (a.k.a. sunspot regions) tend to be on the slow end for CMEs, for instance (although they still move pretty quickly!)

Once they have erupted people make measurements of the CME close to the sun to calculate its velocity and acceleration. The speed still changes, though, as the CME moves out outwards. The CME is moving through what is called the "solar wind" - the sun's atmosphere blowing out into the solar system. The solar wind and structures in it can speed the CME up or slow it down or shift its direction. These are all things people are studying right now.
cheers,
Terry

The nice thing about science is that it is accurate no matter how you apply it. Essentially, we use speed = distance/time to calculate the CME speed. It's a slightly fancier version of that, but that is the root of it. By looking at how far a CME travels in a given time, we can work out what its speed is.

One of the big challenges is that we only have a flat, 2 dimensional image of the CME moving. So if it looks like it's moving out of the image, say directly to the east, it might actually be moving at a 45 degree angle towards the observer or away from them. This changes the speed that we can measure. If you've studied Pythagoras' Theorem then it's like needing to measure along the hypotenuse but only being able to measure *one* of the other two sides.

We can try to correct for this using stereoscopic observations from the likes of the STEREO satellites, but that's quite difficult to do.