This is quoted From: "http://www.asterism.org/tutorials/tut25-1.htm"

"When comets pass close to a massive body like the Sun or Jupiter, they
may break up due, at least in part, to the tidal forces encountered.
Recall that tidal forces occur from differential gravity forces created
on an object because of the difference in distance on either side of
say, a comet from a planet or Sun. Several examples come to mind. In
1846, Biele's comet split in two while passing close to the Sun. Comet
XIV passed within 10 million miles of the Sun in 1947 and also split in
two. In 1976 Comet West broke into four pieces near the Sun. More
recently, Shoemaker-Levy 9 disintegrated into a 20+ fragments after
passing too close to Jupiter, and returned the insult by spectacularly
plunging into its aggressor.
.... If a satellite or comet that is held together solely by its
gravitational force (no tensile strength) passes within the planet's
Roche limit, it will break apart. A non-rotating liquid satellite
without surface tension is an example. Note also that the limit depends
on the relative densities of the two objects as well as the planet's
size. The Roche limit for Earth is approximately 20,000 km above the
surface. Earth's Roche limit for comets is 34,700 km or 5.43 radii and
for asteroids, 19,000 km or 2.98 radii.
.... Let's now imagine the inevitable. A million years from now, a comet
may head directly towards Earth at 20 km/s. As it approaches our Roche
limit (approximately 28,300 km from the surface), it begins to break up
23.6 minutes before impact. At about 120 km above the ground the pieces
encounter a tenuous atmosphere. In the next few seconds before impact
many more pieces are created from atmospheric shock. The impact area
would cover many square kilometers, destroying everything within
hundreds of miles. Hopefully, by then, we'll have early detection and
avoidance technology to prevent such a catastrophe."

More info: http://www.cyberspaceorbit.com/
 

- Yikes!