As you can see, the dots are bunched around the most probable value and taper off in either direction—in the same way that the the area under a bell curve decreases away from the center. s, the distance from the blue line to Mars divided by the size of the error, is 3.7, giving a probability of 10,000:1.

Here is what the asteroid figure looked like two weeks ago:

Notice that the scale here is 500,000 km, so this is zoomed out by a factor of 5 from the 9 January picture. Two things have happened in the fortnight. First, the position of the blue line has changed a little. More importantly, the size of the error was a lot bigger two weeks ago. Back then the error was large enough so that the distance from the blue line to Mars divided by the error was only 2.2, giving a probability of 25:1.

So the probability changed from 25:1 to 10,000:1 over the last two weeks mainly because the error in the path decreased, making s increase (again, s is the distance from the blue line to Mars divided by the error, and it is also the position on the bell curve of the previous post).

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