Do asteroids hit the Sun like they hit the planets and moons?
No asteroids have ever been observed to hit the Sun, but that doesn't mean that they don't! Asteroids are normally content to stay in the asteroid belt between Mars and Jupiter, but occasionally something nudges them out of their original orbits, and they come careening into the inner solar system. The "something" that changes the asteroid orbits is often thought to be the Yarkovsky effect (illustrated here). It is known that Jupiter has a strong effect on the asteroid belt. Jupiter's gravity interacts with the Belt to form the Kirkwood gaps. Orbits within in a Kirkwood gap are not stable, and any asteroid whose orbit wanders into such a region will eventually get pulled into a different orbit, which may take it into the inner solar system. Therefore the Kirkwood gaps have almost no asteroids. In addition to Jupiter's influence, occasional random impacts within the belt probably send asteroid pieces flying in toward the inner solar system.
Once they are on their way in toward the Sun, you might think that they should be guaranteed to hit the Sun, but that's not the case! It is actually difficult for something that is orbiting to fall all the way into the Sun. This is because of a property of orbiting objects called angular momentum. Angular momentum is a sort of measure of how much something is rotating around a central point. The reason that this is important is that one of the fundamental principles of physics is that angular momentum must be conserved. For something to fall into the Sun, it has to lose nearly all of its angular momentum somehow, so that it is falling straight at the Sun. If it is off just slightly, instead of falling in, the asteroid will just fall very close, and then slingshot back out far from the Sun. It is probably quite rare for an asteroid to lose all of its angular momentum and fall straight into the Sun. However, there might be quite a few that lose enough to get close to the Sun and vaporize.
As I mentioned, we have never seen an asteroid come close to the Sun and vaporize. That's because asteroids are small rocks or pieces of metal, and even when they are being vaporized they are hard to see. Comets, on the other hand, give off huge glowing plumes of gas when they get close to the Sun, making them very easy to spot. The SOHO satellite has detected more than 1100 comets known as "sun grazers." These are comets that get close enough to the Sun to glow very brightly and show up in the SOHO images. Some of them disintegrate while others survive the close call and sail back out to the outer solar system until their next orbit brings them back. Check the SOHO Comets website for more information. You can even help discover new sun-grazers by studying the data yourself!
This page was last updated by Sean Marshall, on July 18, 2015.