You mean like this sort of Asteroid that is heading for us now?
A bit from an article:
"It will be visible enough to be studied by radar in 2006, and the next time that condition will come about will be 2013."
And this bit:
"At first they were scared enough to believe that it could hit the earth in 2029. Then they did some more fine-tuning of their computer data and decided that it would come close to the earth in 2029, but wouldn't be on a possible collision course until 2036.They're worried. As one of the conferees said: "It's question of when- not if - a near-Earth object collides with Earth."
Full article found here:
http://www.unexplained-mysteries.com/viewnews.php?id=58797I am not sure if Aliens would "help us" - or rather if they would want to interfere with the "course" of things on this planet.