|MadSci Network: Astronomy|
Whether or not a passing asteroid, or other object, will be gravitationally captured by a planet is not just a matter of how close it comes to the planet but also how fast it is moving. If an asteroid passes very close to Earth but has sufficient speed it will not be captured. Its trajectory will be altered, but not necessarily enough to result in a bound orbit. For an object to escape capture very close to a planet it needs to be moving much faster than it would have to be moving farther away to avoid capture. The more massive the planet the easier it is for the planet to capture an object, as the object would need to move even faster to escape the gravitational pull. This is why Jupiter and Saturn have so many moons which are captured asteroids. Earth, however, does not have nearly as much mass as either two of these giants and it is much more difficult for it to capture asteroids which on average travel very fast when they pass near Earth. Mars did manage to capture two asteroids as moons: Phobos and Deimos. Mars is less massive than Earth, but it is much closer to the asteroid belt which increases the chances for Mars to encounter slower moving asteroids. I don't know the specifics on the asteroid you mentioned in 1989, but it was most likley moving too fast to be captured by Earth.
I hope that answers you question. If you have further questions feel free to email me.
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