That's not exactly true, small changes in the sun's shape, gravitational field, solar wind, and a variety of other factors can make it hard to predict the exact path of a near-earth asteroid.
And to reply to LE.Skiing. Asteroids hit the Earth all the time, but most are too small to matter much. Here's a quote from wikipedia that gives a rough esitmate of asteroid-earth collisions,"Asteroids with a 1 kilometre diameter hit the Earth a few times in each million year interval. Large collisions with 5 kilometre objects happen approximately once every ten million years. Small collisions occur a few times each month." Here's a couple of the larger asteroids that came very close to the Earth:
"On March 23, 1989 the 300 metre (1,000-foot) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 kilometres (400,000 miles) passing through the exact position where the earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history." (Wikipedia)
"On March 18, 2004, LINEAR announced a 30 metre asteroid 2004 FH which would pass the Earth that day at only 42,600 km (26,500 miles), about one-tenth the distance to the moon, and the closest miss ever noticed. They estimated that similar sized asteroids come as close about every two years." (Wikipedia)
Additionally the comet Shoemaker-Levy 9 collied with Jupiter on July 16, 1994 emphising that such an event can occur.
As a "documentary" on ABC is hardly a quality source of information I recommend those interested look at the following pages at wikipedia.
A cataclysmic asteroid impact event is possible, however, there is no reason to panic and try to estimate the end of the world. Anyways I just wanted to clear the waters on the issue.