Deflection Strategy
We may be able to deflect 100-200 m asteroid, although the technology is not validated yet, and there still may be significant problems.
1 km asteroid, on the other hand, requires 1,000 times more efforts. The only way to achieve this may be by multiple nuclear detonations.
Similarly, 10 km asteroid requires 1,000 times more efforts of deflecting a 1 km asteroid. The frequency of impact by 10 km asteroid is estimated as once in about 100 million years. However, it has happened 66 million years ago, and it will happen eventually. A small probability will become a certainty over a long period of time.
What we need is a long term strategy.
The strategy certainly needs to address the current capability gap. In addition, it has to work for a very long period of time. Asteroid impact is a risk with timeline of over thousands of years. The problem is, no organization, including governments, lasted 2,000 years*.
A long term strategy was studied and published by Thomas D. Miyano in June 2018.
- Link to the summary of the strategy
* Byzantine Empire lasted 1,058 years, or 1,480 years, if Roman Empire was included