What's 'top speed' for a 'giant comet', anyway?
Speed of light?
You do remember Einstein, don't you? As you try to move an object faster and faster toward the speed of light, one of the things that happens it takes more and more energy, but the bottom line is you can NEVER get to really close to the speed of light.
From a web site about a previous comet (Hale-Bopp):
http://www.algonet.se/~sirius/eaae/newsl4/hb.htm
Dooms-Day Science Exercise.
A comet speed will increase even further, when entering the inner parts of our Solar System.
Comet Hale Bopp will reach a speed around 200 000 km pr hour, while being closest to the Sun (slightly below 1 AU, April 1997).
Convert this speed to the unit meters pr second.
Let the Comet Nucleus have a diameter of say 20 km, let us assume the mass density is equivalent to Ice (0.9 ton/m3).
Now estimate the kinetic Energy of this object.
1 million ton (Megaton) of Plastic TNT explosive releases 1 250 000 000 kWh. 1kWh (kiloWatt hour) is equal to 3 600 000 Joule.
How many Joule is one Megaton TNT?
How many Megatons TNT would a comet impact correspond to?
It is estimated, that the total amount of atomic weapons technically available during a Global Atomic War equals 10 000 Megatons.
How many atomic Wars would such an impact correspond to?
As the figures above show, such an impact would have a most dramatic consequence. Chemical changes may occur, model calculations by NASA suggest that the Earth atmosphere Nitrogen, Oxygen may combine to a kind of smog , which together with water moisture may give acid rain (pH between 0 and 1, corresponding to the strong acid, you find in car batteries).
In addition, dust may shield the sunlight for a year, implying temperature fall to -40 deg.
Luckily, these space-accidents occur seldom; such a global event is only likely to happen once during typically
50 000 000 to 100 000 000 years.
However, on a geological time scale, this kind of impact may have been important. At the end of the K-T period, 65 million years ago, 70-80% of all life on Earth died probably due to such a dramatic impact.
In the very early start of our solar system, these impacts may have been quite frequent, isotopic investigations indicate that most of the water in our oceans may be due to comet impacts.
Comet Hale-Bopp is followed by students and amateurs across all the world as a part of the Astronomy On Line Programme:
http://www.eso.org/astronomyonline/.