- First NASA is looking into that problem, as well as some talented amateurs, but not enough people are looking into the problem.
- Second an asteroid physically hits the earth about once ever 4 hours. These asteroids are the size of a football or less though and those are just the ones that reach the ground. We have a lot of astronomical trash (asteroids and comets) hitting our earth almost all the time.
- Third if one is on course to hit us you can’t stop it and right now no one else can either. The chances of one hitting us is pretty small though.
What you are worried about is if a BIG ASTERIOD hits the earth say one 1/2 mile or larger in size. This has happened, it will happen again and there is no way to predict it.
The program is called Sky Watch, it was first put forth by Arthur C. Clarke, the inventor of the communication satellite. It wasn't started though until the work of L. Shoemaker who proved that asteroids are hitting us and big ones are hitting us frequently. Then the theory of the death of the dinosaurs, thanks to the Iridium layer on the KT boundary, gained acceptance. Finally comet Shoemaker Levy 9 hit Jupiter, and after breaking up in to pieces it still impacted with the force of several mega tons of TNT. All of this raised public awareness and caused people, mostly amateur astronomers to start looking in the sky.
The problem is that only a very few people are doing this and the solar system has a whole lot of junk in it. Currently there is an asteroid on the way to hit the planet earth in the year 2029. There is only a 1:350,000 chance of Apophis actually passing through a small window of time space and swinging around in the year 2039 for an impact.
The problem is that we don’t see all this junk until often it is too late. Comets are not normally detected until they start glowing with a tail, usually after they pass the orbit of the earth. Asteroids have a low abode (reflected light) so they are hard to detect. If an asteroid came out of the direction of the sun on course for an earth impact then we wouldn’t detect it until it was too late.
Hollywood has got it wrong an atomic bomb in the billions of megatons would be required to destroy an asteroid. No nation has one or would allow one to be put into orbit. Other plans range from painting the asteroid, putting up a huge Mylar solar sail, to just putting a space craft near by. The MADMAN program (http://www.sei.aero/downloads/SEI_LOEM_30March2004.pdf) will use mass drivers to hurl rocks from the asteroids to propel it off a dangerous course. The problem is that all these programs are theoretical, need work before then can be used, and work only by pushing the asteroid a few degrees off course so it will be a close miss. We need a lot of time to do this. A few pounds of thrust over a few years could push a huge asteroid way out off an impact course. The problem is finding the asteroid and then getting something out there in time to make a difference.
Your concern is a real one, but one you can’t do anything about; unless you buy a telescope and join Sky Watch. Huge telescopes like the Hubble or the ones that reach deep into the Universe are useless for this kind of work. A small telescope that searches the night sky looking for tiny points of light that move over a few days are what does the work. Stars move slowly, but an asteroid or comet moves much faster, and the closer it gets to the sun the fast it will move. So we can detect these objects with minor telescopes.
Here is NASA’s take on it: http://impact.arc.nasa.gov/
The NEO (Near Earth Orbiting Catalog) lists the known dangerous asteroids and comets.
Here is a Sky Watch computer program to track your own asteroids: http://www.clearskyinstitute.com/psc/intro.html
Here you can figure out the size of an impact: http://janus.astro.umd.edu/astro/impact/
I just read your additional notes.
There are a lot of things to worry about and Global Warming, to Asteroid Impacts are only a few. The advice to worry about what you can change is a good one, but at 11 years old these kinds of worries can set a course for your life; for example if you get a telescope and start looking for yourself to find asteroids you will be joining a few hundred others and probably on your way to becoming an astronomer, at least an amateur one. Worry gives you ulcers and nightmares so don’t worry. Action gives you piece of mind and maybe even a career. If you are a night owl who doesn’t mind the cold of high altitudes you have the natural makings of an astronomer. If you specialize in math, science and physics then you are on the course to possibly become one.
Any way an interest in math, science and physics will let you understand and might even let you contribute to some solutions for a lot of similar problems. I encourage it we don’t have enough scientists. But right now I would calm down this issue at least is being worked on and is not a major one; pollution is more pressing. But, don’t ridicule yourself for your interest. You are were I was at your age and I turned out to be pretty smart (just read my other answers to see if you agree).