But most aren't. Lulin certainly isn't. Space is vast, immense, and the Earth is tiny by comparison.
Probably the most dangerous comet known at this time is Swift-Tuttle:
http://en.wikipedia.org/wiki/Comet_Swift-Tuttlehttp://en.wikipedia.org/wiki/Comet_Swift-Tuttle
discovered by Lewis Swift and Horace Parnell Tuttle. This is the parent body of the Perseid meteor shower, an annual event:
http://en.wikipedia.org/wiki/Perseidshttp://en.wikipedia.org/wiki/Perseids
These are sometimes known as the "Tears of St. Lawrence", since he died on August 10 and the meteor shower hits between Aug. 9 and Aug.14. This shower has been reported for about 2000 years.
The main body had some concern during it's 1992 apparition when it was seen that the then current predicted perihelion passage was off by 17 days. Somebody calculated that if the August 14, 2126 passage was off by 15 days, the comet could hit the Earth. As a result of these observations an intensive tracking and computing system was set up and it was discovered that the comet's orbit was actually pretty stable and it would not hit the Earth on that day.
Comet Swift-Tuttle is locked into what's called a "resonance"; it completes one orbit around the Sun in the same amount of time that Jupiter completes 11. So it is locked into it's orbit by Jupiter's gravity. It's orbit is retrograde like Lulin's. It's not green. It's farthest distance from the Sun is 51.225 Astronomical Units, about four and a half billion miles, so it will keep coming back.
It's orbit was calculated thousands of years into the future. In September of the year 4479, there is about a million to one chance that the comet might hit the Earth. This is plenty of time to design and build a rocket to move it.
I wouldn't sweat it.
-Michael C. Emmert