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I keep hearing astronomers saying there are observatories scanning the skies all day and night looking for near-Earth objects, extrapolating their trajectories.

My questions are

  1. If we were to get the news of an on-coming object, how early we will find out or how much time we will have?

Currently, people don't even think about it; Governments are cutting spending in their space exploration programs, let alone deflecting an impact-er.

Under certain circumstances, would the notice from the watchdogs be enough for us to build something that would prevent the loss?

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  • $\begingroup$ Your main question "How early will we find out or how much time will we have?" is on topic, but "would the notice from the watchdogs be enough for us to build something that would prevent the loss?" is out of scope for Astronomy. $\endgroup$ – called2voyage May 23 '16 at 19:14
  • $\begingroup$ There's no shortage of info on this subject available for quick internet searches. If you have a more specific question, I'm sure it can be answered, but you inquired about the entire subject. Generally speaking, we see major natural disasters on earth pretty much every year. A meteor impact having large natural disaster consequences is a rare event, maybe once every 5-10,000 years. Yes, it can happen, but statistically it's a low risk event AND efforts are being made to see it coming. To go over every aspect that you covered would get long. Try worldbuilding perhaps? $\endgroup$ – userLTK May 24 '16 at 3:05
  • $\begingroup$ And for perspective: 1931 China flood - over 1 million deaths 1887 yellow river flood - over 900,000 1556 Shaanxi earthquake - over 800,000 2004 Indian Ocean Tsunami - 300,000 Known deaths caused by meteor impacts last 500 years - 2. $\endgroup$ – userLTK May 24 '16 at 3:11
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The short answer is "not very, but we're getting better".

In the case of particularly large Earth orbit intersecting bodies, kilometers across and bigger, they're fairly well known and tracked. We'd probably get months to years of warning. Technologically, we've probably got the capability of diverting such an object if we have enough warning, but it'd likely be the most complex human endeavor ever. Blowing it up is unlikely to help, we'd need a nice long shove from as far away as possible to divert it.

The bigger problem is smaller bodies in the tens to hundreds of metres. Some of these are known about and tracked, but plenty more aren't. The Chelyabinsk meteor was about 20 metres across and, in terms of size/density/speed/impact angle wasn't that far off being a city killer. We never saw it coming. (Ironically, a very similar but unrelated 30m object was being tracked through a near-miss that same day.)

I'd suggest that a city-killer impact with little or no warning is still quite possible. Something capable of taking out, say, a small country we'd probably have some warning of, but probably not enough. A mass-extinction event we'd probably have lots of warning for, but doing anything about it would be a very big challenge.

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