I'm a little puzzled about the way the word "risk" is used in this report. For instance, on page iv: "Recommendation 2 - Develop and operate a NEO search program with the goal of discovering and cataloging the potentially hazardous population sufficiently well to eliminate 90% of the risk due to sub-kilometer objects"

Now it seems to me that simply cataloguing the potential future impactors does not reduce risk, per se, it merely transfers the risk from unknown objects to known objects. These are still destined to impact the Earth, and still will cause casualties, unless a parallel programme is in place to develop ways to mitigate risk.

A true risk-reduction programme would need to include plans to evacuate target areas, hopefully identified years in advance, and to begin steps toward space interception and steering of objects to alter their trajectory and reduce impact probabilities.

In other places the report does acknowledge the dual nature of risk and for example in another piece also on page iv: "Recommendation 1 - Future goals related to searching for potential Earth-impacting objects should be stated explicitly in terms of the statistical risk eliminated (or characterised)..."

With that quibble aside, the report is an excellent summary of the sources of risk and shows that 99% of the danger lies in NEO objects and short period comets that have almost all had previous gravitational interactions with the Earth-Moon system and have become focussed on an Earth-crossing track which repeats relatively often, giving high intrinsic risk per object.

In essence, these objects are "doomed" to eventually hit the Earth or Moon, unless they hit something else first, and each close pass can result in orbit modification which will nevertheless still be Earth-crossing. Interactions with other objects can alter these orbits away from Earth crossing, but also are capable of perturbing non-Earth crossers back into the potentially hazardous population, so what you gain on the swings, you lose on the roundabout.

Only 1% of the risk lies in "new" or long-period comets that have arbitrary paths through the solar system and for which Earth is a tiny and chance target on encounters widely-spaced in time.