An asteroid is approaching Earth

[Logan24]

Hi, I am writing a sci-fi comic and would need an info, which is very similar to this thread:

https://www.physicsforums.com/showthread.php?p=4724396#post4724396

My question is, how much would it take for an asteroid to reach Earth after detection? A cca 'from to' range would be sufficient, but I don't know for this info the following factors, therefore I don't even have a rough estimation (years? months?):

- what is the distance of asteroids that we are monitoring currently for possible impact analysis?
- what is the average speed of an asteroid?

The first one is the tougher, I digged for the info on Nasa sources (their asteroid monitoring program), but found nothing. As I plan it to be a realistic story, if anyone can give me any additional details on this topic (asteroid monitoring details) it would also be awesome. Thanks!

 

[Janus]

There is no one answer to your question. Let's say the we detect a distant asteroid. Just detecting it is not enough to tell us whether or not it is a danger to Earth. We have to watch it for awhile to determine its orbit. And even then, how accurate the predicted orbit is depends on how long we observe it. At first we just get a rough estimate, with a wide variety of possible orbits. Earth may or may not be within this range of trajectories. The longer we watch the asteroid, the tighter we can narrow down its possible path. As long as Earth remains in that range, it is a possible threat.
How long we have to watch the asteroid in order to say that it has a high probability of hitting the Earth depends on a number of factors: What type of orbit is it in? Does it pass close enough to any other bodies that could deflect its path, etc.

As far as the average speed of an asteroid, that depends on what type of orbit it is in. For example, if it has an orbit that takes it far out into the outer solar system, it will be moving faster when it reaches Earth orbit than it would if its orbit doesn't take it out so far.

 

[Logan24]

Thanks for the info!