Our Solar system is full of debris; these are the Meteorites, Asteroids, and Comets. The debris consists of planets that never formed, planets that were destroyed by collisions, comets that are vaporized, etc. In most cases, these are small (by planetary standards) objects of a few km across, although few can grow up to hundreds of kilometers. But don't be deceived by their size; some of the most spectacular collisions witnessed in the Solar System have involved these "miniature" objects. Indeed, we now believe that dinosaurs, which roamed the planet 65 million years ago, became extinct by the impact of a relatively small 10 km object. The impact crater, was recently discovered.

Chicxulub Crater, Mexico, and the Cretaceous-Tertiary (KT) boundary. This is a 3-dimensional gravity anomaly map over the Chicxulub crater. Proof of the existence of the crater gave new force to the theory that the extinction of the dinosaurs 65 million years ago was caused by a large meteorite, comet, or
asteroid colliding with the Earth.

Meteorites:

Meteorites are rocks from space that have fallen on Earth. They are fragments of other bodies in the solar system: asteroids, comets, the Moon, and planets. These ancient rocks give us clues to the origin and history of the solar system. Meteorites are sometimes called the "poor man's space probe" because they arrived on Earth free of charge. Antarctica is a very special place to collect meteorites, due to the special conditions in Antarctica. Meteorites which have fallen on Antarctic ice are well preserved for long periods of time. Indeed, based on a sophisticated examination
of an ancient Martian meteorite, labeled Allan Hills or ALH 84001, that landed on Earth about 13,000 years ago and found in Antarctica in 1984, NASA administrators announced that a primitive form of microscopic life may have existed on Mars about 4 billion years ago. Meteor showers are relatively common and occur when the orbit of comets gets disturbed as it passes near the Earth. Some of these showers can be seen with the naked eye on a clear moonless night.

Many of the 1998 Leonid meteors were so bright that they could be seen in day-lit skies, like the one shown here in an early morning photo from Hong Hong.
 
 

Asteroids:

Asteroids are small bodies that are believed to be left over from the beginning of the solar system 4.6 billion years ago. They are rocky objects with round or irregular shapes up to several hundred km across, but most are much smaller. More than 100,000 asteroids lie in a belt between Mars and Jupiter. These asteroids lie in a location in the solar system where there seems to be a jump in the spacing between the planets; we now know that the inner and outer planets have  different properties. Scientists think that this debris may be the remains of an early planet, which broke up early in the solar system. Several thousand of the largest asteroids in this belt have been given names.
 
 

Comets:

Not long ago, many people thought that comets were a portent that something bad was about to happen to them. Since people didn't understand yet about the objects in the solar system and how they moved, the sight of a comet must have been very disturbing. There are many historical records and art works that record the appearance of comets and associate them with terrible events such as wars or plagues. Now we know that comets are lumps of ice and rock that periodically come into the center of the solar system from its outer reaches, and that some comets make repeated trips. The solid part or nucleus of comets can be oddly shaped and only a few miles across. When comets get close enough to the Sun, heat makes the nucleus evaporate. A cloud of gas and dust called the coma forms which can reach the diameter of a giant planet. Jets of gas and dust form long tails that we can see from Earth. These tails can sometimes be millions of miles long. In 1985 a spacecraft called Giotto visited the most famous comet, Halley, on its most recent visit to the inner solar system. In 1996 and 1997 we saw comet Hyakutake, and comet Hale-Bopp. Hale-Bopp was one of the brightest comets ever seen from Earth.

     When comets are near the Sun they display several distinct parts:

Nucleus	Relatively solid and stable, mostly ice and gas
Coma	Dense cloud of mostly water and carbon dioxide
Hydrogen Cloud	Huge (millions of km) but sparse envelope of hydrogen
Dust Tail	Up to 10 million km long composed by dust particles;   this is the most prominent part of a comet to the naked eye.
Ion Tail	Up to 100 million km long composed by charged particles.

If you think this is Hollywood at its best or pure science fiction, think again. Comet Shoemaker-Levy 9 - a comet discovered by Gene Shoemaker, his wife Carolyn, and partner David Levy - plunged directly into Jupiter in 1994.
 

Closer to home - but in the far past, 65 million years ago- a relatively large 10 km meteorite or comet may have altered the atmosphere and climate on Earth so dramatically that dinosaurs and 75% of the species then on Earth became extinct. Geologists have now located a crater at least 150 km in diameter centered near Chicxulub, in the northern part of Yucatan, one of the southern states in my country of Mexico. The chance of another asteroid colliding with Earth are very small! But some do come close, like Hermes with a closest approach of 777,000 km (the distance to the moon is 384,400 km).

Finally, there is still the prospect of the Doomsday Asteroid. On March 10, 1998, Brian Marsden, Director of the International Astronomical Union's central telegram bureau in Cambridge, Mass., announced that celestial object 1997XF, a 1 to 2 mile asteroid, could pass within 30,000 miles of Earth in October of the year 2028. One day later, scientists from NASA's JPL announced that the asteroid would instead miss Earth by 500,000 to 600,000 miles. JPL's Paul Chodas said that the chance of it impacting the Earth was "...so unlikely as not to worry about." Essentially, zero. Yet, with more than 10,000 days until "impact" ... one never knows.