Minimum possible warning for deadly comet/asteroid

[bland]

What is the minimum possible warning that we on Earth can get for a very destructive space object from striking the Earth, say something about a kilometre in size (I'm guessing that would be big enough to be noticed all over the Earth). By minimum possible, I mean with the way the skies are monitored is there some way by an extreme angle or whatever, that a large object can evade detection for the maximum time maybe even by being recently thrown into the path of Earth by Jupiter or one of the outer planets.

I have no idea if it would be years, months, or weeks, I suppose that it could not be days.

 

[rootone]

It not really possible to pin it down exactly, but probably all the really large near Earth objects have been found by now.
More moderate size objects which are nevertheless capable of causing major damage can arrive completely unexpectedly.
The Chelyabinsk meteor of a couple of years back was unexpected, and astronomers were actually engaged in tracking a different object at the time, (which did not hit the Earth, but as predicted passed by at a distance closer than the moon.)
The two objects were unrelated, and the meteor which did hit Earth was unnoticed because from the Earth's point of view it was coming from an angle close to the Sun.

There is always a possibility of new large comets being discovered which currently are not near Earth, but could start heading this way.
Anything as large as several Km in size would probably become noticeable around the distance of Jupiter's orbit, from there we are talking approximately of several months to a few years to reach Earth, but it could be more or less depending on the object's exact speed and trajectory.
Fortunately for us though, Jupiter's large gravity does quite a good job of getting rid of such gatecrashers.

 

[Chalnoth]

What is the minimum possible warning that we on Earth can get for a very destructive space object from striking the Earth, say something about a kilometre in size (I'm guessing that would be big enough to be noticed all over the Earth). By minimum possible, I mean with the way the skies are monitored is there some way by an extreme angle or whatever, that a large object can evade detection for the maximum time maybe even by being recently thrown into the path of Earth by Jupiter or one of the outer planets.

I have no idea if it would be years, months, or weeks, I suppose that it could not be days.

Some objects that have made very close flybys have not been detected until they have passed the Earth. So there really isn't a minimum distance for detection.

It's really difficult to detect an object that is heading straight for us (or close to it), because the way we detect asteroids is through the fact that they move compared to the stars in the background. But if it's heading right for us, then it doesn't move compared to the stars in the background.

We think we've detected more than 90% of the near-Earth asteroids greater than 1km in size, but there's still the outside possibility of the large asteroid or comet that we haven't yet detected.

 

[newjerseyrunner]

Comets tend to have a great deal of warning time because they get really bright as they near the sun and start producing a coma. Asteroids can be almost completely black, so it's possible that we wouldn't see it until it was right on top of us.

 

[russ_watters]

I would think with a few months' warning we could probably knock a 1km object off course.

The problem with that is that we don't detect objects at a few months' notice. We either detect them long before (when they cross Earth's path) or not until they are right upon us, as the others noted. For example: http://www.science20.com/astro_watch/blog/will_asteroid_2012_tc4_hit_earth_in_october_2017-154930

 

[Chronos]

A large body approaching us from the direction of the sun may offer hardly any warning before it struck. Fortunately, bodies like comets do not achieve an intercept course near enough to remain obscured by the sun for very long. So we could expect to see a comet headed our way several or more months before impact. Not so much for smaller, like km size bodies. They could get quite close without being noticed. That adds incentive to locate and track such bodies before they settle into a collision course. We are still pretty much defenseless against sub kilometer size bodies like the chelyabinsk meteor.

 

[Chalnoth]

I would think with a few months' warning we could probably knock a 1km object off course.

In principle, maybe. In practice, I imagine it would be monstrously difficult.

The problem is: if the mission goes wrong, and the object causes serious damage anyway, people are going to want to place blame. While many scientists may be perfectly comfortable taking that risk, a lot of politicians probably wouldn't be. So sadly, I think there's a really good chance that political paralysis will prevent any action against an oncoming asteroid or comet.

 

[bland]

OK thanks all those replies pretty well answer all my questions including the ones I implied.

 

[Chronos]

Yes, scientists want to do something even if its wrong. Politicians prefer to take sides, argue, and hope for the best in the midst of doing nothing. It's called bayoneting the wounded after the battle.