Without subscribing to the paranoia associated with, e.g. EP Grondine, we need to recognise that there is a family of NEO's that has the following characteristics:-

* They have orbits highly inclined to the ecliptic plane
* At one point of crossing the ecliptic plane their orbit is essentially coincident with that of the Earth
* They do not even remotely approach the orbit of any other planet or asteroid

Examples include this newly-discovered 2003 QQ47, and our old friend 1950 DA.

Several interesting points arise:

1) How did they get into these orbits?

- It appears that a very close encounter with the Earth perturbed the orbit of a "normal" asteroid or comet out of the plane of the ecliptic. As a consequence, they are doomed to repeatedly return to that Earth-orbit intersection point.

- This former close encounter of the swooping kind probably occurred tens of millions of years ago

2) What will happen to them?

- at some point in the future, they will again experience a close approach to Earth (or the Moon). At this time, their orbit will change again, but typically only to another Earth-crossing orbit.

- a fraction of these bodies will be perturbed by such Earth encounters, or by the weak gravitational interaction with other planets or close approaches to other asteroids, such that they intersect something else.

- The remainder will, inevitably, hit the Earth (or for a small fraction of cases, the Moon). Because they have large intersection angles with the ecliptic, they have relatively large encounter velocities.

3) How long have we got?

- It is hard to predict orbits with any certainty far enough into the future, but each one of these objects seems to have about 1 chance of hitting the Earth in 100 million years, similar to the background probability of a Chixulub-scale impact.

- If there are 1,000 such objects, then the statistical expectation is that one will hit the Earth within 100,000 years

- Since most of these objects are smaller than the Chixulub impactor, there is no paradox. However, taken collectively, there is a relatively high risk that the next body to hit the Earth will be one of these special Earth-crossing objects, since their orbits doom them to close encounters of the catastrophic kind.

4) Should we panic?

- No, but we should be aware and quietly concerned. These bodies spend most of their time out of the plane of the ecliptic and require specialised search strategies to optimise their discovery.

- It is likely that we will have one or more decades of warning, but this is not long when we do not have an operational heavy lifter capable of tackling even a rudimentary diversion mission. (Not nukes, but mass drivers, or electric engines, or albedo patches, or whatever - slow, but eventually sure).

- That decade of warning will initially be Hemispheric in vagueness, but will converge on a country or region with 4 or 5 years warning. That should be ample time for an organised evacuation, although the financial and social impact of this will probably be worse than the direct risk of the impact (!).

- Without an official evacuation, a disorganised chaos will result that could lead to panic, with even worse consequences.

5) So what?

- It is inevitable that if humans survive for an extended period, our Earth will encounter a significant asteroid. On a thousand-year timescale the impact will be small. On the hundred-million year scale it will be devastating. Given those timeframes, we should continue the current program of detection & monitoring, and gradually work towards a capability for intervention and orbital mofification.

- But don't panic, we either have plenty of time, or it's already too late!