|Image credit: SPACE.com.|
All of the calculations are made by doing some careful comparisons with the “known” distances of stars near the asteroid. As stars move very slowly, they can be fairly reliable space-scale distance markers, but the passage of time still has to be taken into consideration upon each new measurement.
If it does not smash into earth in 2029, it should pass by again in 2036 and 2068. The pass in 2029 will likely alter its path, too, making it slide ever so much closer to earth. The pass in 2029, in fact, will most definitely be closer to earth than a lot of communications satellites are! You can expect to see Apophis without the aid of a telescope or even binoculars, weather permitting. Until then, all we can do is sit, measure, and crunch some more numbers.