Expert: Ed - 3/2/2009

Question
Hello!

I have got two questions concerning comets.

I would like to know

1. how fast in general do comets fly?

2. - as the space is monitored intensely and I can recon with early discovery - how long will it take a comet to impact after calculations show that it most probably will collide with the earth?

Thank you very much for your answers!

Best regards

Ted

Answer
Ted,
Hello!  Sorry I'm a little late.  I have been going crazy worrying about exactly the kind of thing you are asking about.  As you probably heard, we had a pretty close call with an asteroid this week.  That's an asteroid, not a comet, but still...

Before I forget, I bet you would like this NASA website:
http://neat.jpl.nasa.gov/neofaq.html

Now, your questions.  Comets move relatively slowly when they are floating waaayyyy out in space, beyond the orbit of Neptune.  But as their long, elliptical orbits bring them closer to the sun, they speed up a LOT.  Their speed when they are closest to the sun could be 50 or 60 times faster than their speed at the other end of their orbit.  Astronomers use the expresion "slingshot" to describe this phenomenon.  A comet gets ALMOST sucked in by the sun's enormous gravity, and the incredible speed it reaches by being pulled at the sun gives it a lot of momentum, so it flies very, very, very far out in the solar system after it slingshots around the sun.

So you see, the speed of comets varies tremendously, but when they pass the Earth they are often travelling at somewhere between 15 and 40 miles per second.  Halley's comet is considered very fast, and it passes us at about 40 or 45 miles per second.

As for your second question, I just don't know.  Hopefully, there would be lots and lots of warning.  Sometimes, we know what is coming for many years before the object gets close.  But I would be a total idiot to say that you could depend on adequate warning, because just two days ago we had a very near miss by an asteroid, and no one was aware of it until the very end.
http://news.nationalgeographic.com/news/2009/03/090302-asteroid-earth.html

I hope this helps.  
--Ed