Lifted from another web site (WARNING: reading this can lead to brain shriveling!):

"The University of Maryland proposed space mission to penetrate deep into the nucleus of a comet and uncover secrets about the origin of the solar system has won approval by NASA. The $240 million mission -- which was conceived by University of Maryland astronomy professor Michael A'Hearn -- will be the first to study the interior of a comet, which astronomers believe contains material unchanged since the formation of the solar system.

It has been suggested that one effective technique for deflecting a small comet or asteroid that is on an Earth threatening trajectory would be to run into it with a massive spacecraft at high velocity several years prior to its predicted Earth encounter."

NASA launched the Deep Impact space probe in January 2004 to meet with the comet Tempel 1.
see deepimpact.jpl.nasa.gov/science/tempel1-orbitalhist.html

Deep Impact's penetrator is a 320kg hunk of metal and it will be fired at the comet on July 4 with a relative impact speed of 10.2 km/sec.

What is a Comet made of?
It is thought comets are made of dirty water ice. Because water ice and hydrogen peroxide are diamagnetic, cosmic bodies made of these compounds are perturbed by extraneous magnetic fields in unusual ways causing them to change orbits. It is thought that short-periodic comets in our Solar System reside in the Oort Cloud and Kuiper Belt and when pushed out of orbit, they wander the Universe.

Water is unstable in space, the only stable form of hydrogen and oxygen is HYDROGEN PEROXIDE ice.

If the comet is made of hydrogen peroxide it may explode under chemical decomposition if it is hit..... and break into many pieces that can and will go ANYWHERE. The forward momentum of Tempel 1 is Earthward.

A frozen hunk of H2O2 hitting the ground on Earth will explode,... like an atomic bomb, leaving no trace except vitrified matter...

Comets can hit the Earth. Comets have hit the Earth in the past (The Tunguska explosion of 1908 is a possible candidate for a cometary impact).

"About the time of the Hale-Bopp comet appearance in 2213 BC, there seems to have been a widespread collapse of ancient civilization, followed by an apparent lengthy dark age.

Not only did two great ancient civilizations suddenly collapse and enter into chaos for over a century, but there was a serious sudden temporary climate change, which appears to have taken place at the onset of the collapse.

Humankind has observed the impact of a comet into Earth, and the superstition that comets bring death and destruction is well foundered.

If comets are hunks of dirty ice, we have little to worry about.

HOWEVER

"We have largely confirmed that the chemical balance ( referring to hydrogen peroxide) of the Mars atmosphere is determined by the products of the photolysis of water vapor, without the need for special or unknown changes to current theory." (www.spaceref.com/news/viewpr.html?pid=13756)

Hydrogen peroxide has been found in the upper atmosphere and on the surface of Mars, indicating that the chemical environment at the Mars surface is similar to space. Hydrogen peroxide is detected in the upper atmosphere of Earth.

Hydrogen peroxide is more stable in a cosmic environment than water is and any water left in space will rapidly be converted to hydrogen peroxide by UV radiation.

The atmosphere of Mars has some oxygen and water vapour in its atmosphere, presumably from the breakdown of hydrogen peroxide by dust particles in the atmosphere.

Earth witnessed the Shoemaker-Levy 9 comet in 1994 smashing Into Jupiter,.. the comet broke up (exploded) into at least 15 individual fragments on a previous near encounter with Jupiter. The sum mass of the fragments appeared to be of less mass than the mass of the original comet.

The fragments impacted into the planet with an energy output many magnitudes greater than what was expected. However no water vapor was detected in the plumes thrown up by the impacts of the fragments.

The biggest fragment (G) was about 2 km diameter and it struck Jupiter with an estimated energy equivalent to 6,000,000 megatons of TNT (about 600 times the estimated arsenal of the world). The fireball from fragment G rose about 3000 km above the Jovian cloud tops and was observed by many observatories (mostly in infrared).

The diameter of Tempel 1 is about 5 km which is sixteen times bigger in volume than Shoemaker-Levy's biggest fragment.

There have been many mass extinctions of life forms on Earth in the past. These could have been caused by a comet-hit which ignited the organic matter resulting in a depletion of atmospheric oxygen killing larger animals.

This could happen again. Deep Impact could be a mistake that could result in the extinction of human life some short time (one year or less) into the future.

A glancing blow by Deep Impactor's bullet could just set the whole comet or a fragment or fragments on a fatal path to Earth. The comet is just outside Mar's orbit and travelling in the same direction as the Earth.

Cosmic Chemistry

In a space environment there is low pressure and radiation. Water vapour in this environment is photolysed by UV radiation into H and OH. (hydrogen and hydroxyl ions)

Two hydroxyl ions in a suitable environment form H2O2. Indeed irradiating water vapour with UV light is a standard laboratory procedure to prepare hydrogen peroxide.

Normally, around energy emitting bodies there exists a plasma where all matter is in its atomic state, so hydrogen and oxygen can be represented as H and O ions.

Hydrogen H and oxygen O ions form stable hydroxyl ions OH, so in the first instance water is not formed. Any water that does exist in space is a breakdown product of hydrogen peroxide.

Crystalline hydrogen peroxide is transparent to UV radiation while crystalline water is more opaque making solid hydrogen peroxide more stable in a space environment. Water is more stable than hydrogen peroxide to IR (infra red) radiation. Thus when a comet passes near the Sun the intense IR radiation will break the solid hydrogen peroxide on the surface of the nucleus into a water vapour shroud. The liberated water in the comet's tail would quickly be converted back into hydroxyl ions, reforming into solid hydrogen peroxide crystals.

When Hydrogen Peroxide decomposes it releases water H2O
and oxygen O2 and a large amount of energy. The energy released is not only impact energy, it is decomposition energy, and over and above that massive energy release, the liberated oxygen would precipitate massive fires that could deplete the level of the Earth's free oxygen killing off all large animals.

Comets are not dumb meteors. These objects are BOMBS, leaving no trace except vitrified matter. The blast force they produce is from a chemical reaction and not just from impact forces alone.

A minute chance that human kind could be made extinct is still a chance, and after that there are no more chances."