Near Earth Objects

Information posted in July 2012

What is the precise nature of this global risk?

Two types of objects comprise the near Earth object risk: asteroids and comets.

What is the present status of information and information-gathering on this risk?

Our reading of the literature suggests that of the two risks, the asteroid risk is now well understood. This has arisen from the leadership shown by the US Congress over the last 20 years in  supporting  increased and enhanced telescopy to carry out extensive scans seeking near-Earth objects. In this, for asteroids, the task has been simplified by the fact that while a very small proportion of space, near Earth objects have, to speak colloquially, “nowhere to hide”. The second reason simplifying the management of the near Earth asteroid risk is that the asteroids follow very steady orbits. These orbits do change, but again in a very predictable fashion. These properties have enabled scientists to project asteroid orbits into the future for the next 200 years. This crucial exercise has shown, happily, that no asteroid orbit is predicted to intersect that of Earth at a point when Earth is at the same point in space is the asteroid.

But comet risk is different from the asteroid risk. The same scanning process as described above suggested that comet risk is only 1% of the asteroid risk. That said, comet orbits are not so predictable as asteroid orbits because comet orbits are both highly elliptical and what is termed long period.  This means that while many asteroid orbits last two or three years, orbits for comets can last hundreds of years. The longtime and elliptical nature of the orbits therefore can make the arrival of a comet  seem to be “from out of nowhere”.

There is a further problem concerning the comet risk. This is that, while there is considerable consensus worldwide among experts concerning the nature of the population of near Earth asteroids, this is not the case for comets. Here they are basically two schools: one suggesting that historical earth impacts arising from comets are rare; the other that they are surprisingly frequent. Happily, a major advance in the resolution of this question may shortly be possible: a much more sensitive and extensive telescopic space survey has just been completed by NASA. A prediction held by the more frequent impact group is that a large number of so-called dark comets should exist in near Earth orbits: the present NEOWISE survey should show these if they exist. In further good news, the full results for the NEOWISE survey were fully published by NASA in March this year. Global risk progress has written to both NASA and to the more frequent impact group asking if there are current plans to directly assess the dark comet question from the data.

Which are the main agencies involved in building capacity to address this risk?

Because of the budget it is actually deploying, there is only one agency to consider in terms of impact prevention capacity and that is NASA. Here the news is good: the most recent NASA strategic plan reallocates its  focus to interaction with asteroids.

As from January 2012 the European Space Agency is conducting some fresh analysis.  However its budget is small in terms of the budget size of NASA and its capability to project force.

Where does a reading of the literature suggest these agencies need to be in terms of addressing this risk?

From a risk prevention perspective, the gap in risk management capability concerns the comet risk. Also from a risk analysis prevention perspective of the relative certainty of the comet risk in comparison with the asteroid risk — the issue which is at the heart of the difference between the NASA and British led groups — is ultimately immaterial. If there is a nonzero risk, and especially where what is at stake is civilisation, it is prudent to develop the capacity to combat that risk, and have that capacity standing by.

With this background the news again becomes encouraging. There are reasonably well worked out studies in the literature depicting scenarios for the interception and rendering harmless of even an unexpected comet or comet debris stream. This type of comet risk falls into two classes: the aforementioned dark comets; and standard comets freshly entering the inner solar system. The dark comets are those bodies which originated as comets but which are now behaving like asteroids — albeit ones visible only to infrared telescopes. These, seen by an adequate infrared telescope detection system can be prevented from impact with Earth by a standard anti-asteroid techniques. A new standard comet has the problematic features of a much shorter pre-impact warning time. This comes about because of both a first appearance and then a typically threefold faster speed compared with at an asteroid.

Fortunately propulsion systems exist which can enable an intersecting spacecraft to travel quickly enough to intercept even comets of the above type. Further, a number of these systems exist in relatively well proven prototype form.

One in particular – the VASIMR  propulsion system  – is being strongly supported and funded for further development by NASA at present.


In conclusion, the literature suggests the worst-case scenarios for planet Earth are twofold. The first is intersection with a stream of debris from a comet which has broken up. This would require the capacity to intercept up to 100 objects. The good news is that these objects would be traveling in a stream like a shotgun blast. Hence it would appear that a smaller number than 100 intercepting spacecraft would be required, these spacecraft having the ability to target the full hundred objects and deflect or pulverise them into objects small enough to be neutralised by impact with the Earth’s atmosphere.

The second scenario again involves a comet. This time the comet is large-scale and intact. Here a scenario exists for the breaking up of the large comet by a nuclear explosive. These broken up objects would as discussed above continue on their current trajectory and hence be available for further breakdown by further nuclear explosive impacts.

All the ingredients — the nuclear explosives and the VASIMR rocket type — are presently available, or feasible at the scale required with further thoroughgoing workup. There would be considerable value in this workup being adequately funded starting with the next occurring budgetary round.


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