Friday, June 1, 2007

NASA Report on Asteroid Deflection

Ok, based on my previous post, I decided to read the NASA report on asteroid deflection (warning: link is to a large .pdf file). Previously, I had commented that there could be some relationship between setting up a space-SOSUS network & asteroid tracking. My purpose in reading this report was to determine if I was right.

The short, short, short version (original report was 271 pages):

Facts & Terms I did not know:
1) Atens & IEOs – neither type of asteroid is easy to find because they either never cross Earth’s orbit (IEO are Inner-Earth Orbits, always inside Earth’s orbit, so detecting them in the presence of the sun is difficult) or they criss-cross it relatively quickly (have very eccentric orbits, like a stretched out oval).

2) Radar has a limited range of 0.3 AU (~45 million miles, which sounds like a lot until you realize the Solar System is 100+ AU across) so it can’t be very useful to find asteroids and can only be helpful for characterizing asteroids if they approach close to Earth (where the best radar detection equipment is).
3) Precovery – When an asteroid is discovered, astronomers go back to scan old data to see if it was recorded but not identified before. The interesting part is that amateur astronomers most often perform this service thanks to several shared databases.

Good News:
We can theoretically stop most types of potential impacts. The report studied 4 scenarios & performed analysis using the variety of impact prevention (called “mitigation”) techniques. The results demonstrated that is it physically possible & technically feasible for us to stop most impacts (see “Terrifying News” for exception). The catch is getting enough warning to design & deploy the mitigation equipment/material in a timely manner.

Once this survey is complete, we should be able to predict asteroid orbits for up to a century barring interaction with other bodies. This means two things (1) if an asteroid is a danger we had more time to do something about it and (2) if something happens to change orbit of a NEO, we will be able to identify it quicker. In terms of solar system surveillance, this is a step in the right direction.

The most effective means of asteroid deflection require the least information about the asteroid. Without question, nuclear explosives were the most effective means of asteroid deflection (as in orders of magnitude more effective). This is good news because this is the technology we have the most experience in and considering, we may only get one shot at deflection, reliability & development time are big factors in success. It also means we don’t need to do too much characterization (ie – send probes to find out info about possible threat). Once we know the target’s size & mass, mitigation is possible. More info is good & could help reduce the need for multiple mitigation attempts but it is not strictly necessary.

Not-so-Good News:
Some of the results, including the feasibility analysis, are dependent on hardware not currently in use such as the proposed Ares V rocket. There are also several mentions of in-orbit assembly of systems beyond the lift capacity of even the Ares. Unfortunately, NASA has a bad habit of counting on equipment or processes that either don’t get built or don’t perform as well as they planned (see Space Shuttle). I don’t particularly like the idea that the fate of the planet could depend on NASA’s sluggish & over-constrained design processes.

Bad News:
We need a lot more mission reliability in order to pull off mitigation for all but easiest scenarios. Most of the 4 scenarios mention multiple mitigation attempts as a worst-case (ie – things don’t work as we expect them, always a good bet in space travel). There is a very real possibility that initial mitigation attempts will not be successful. Currently, interplanetary missions have a 10% failure rate. In order to be competent at mitigation (ie – be able to deflect all different types of threats), we need a <1% failure rate. Very little is mentioned about how to go about getting from point A to B.

Terrifying News:
We can’t do anything about long-period (LP) comets. Now, don’t panic. They are pretty rare. Unfortunately, they are also nature’s own “worst-case scenario”. The reason they are difficult to mitigate is that long-period have unpredictable (long-period means they rarely show up in the inner solar system) routes, fast velocities (longer to “fall” into the gravity well) and, most dangerously, are difficult to detect because the volatiles on the surface would not render the comet visible until it was a mere 9-24 months from impact. The only slightly good thing here is that LP comets are a tiny fraction of all possible impact scenarios. But again, they are also likely to be among the worst situations: little warning, large size & speed (meaning more required to change its course), the effect of composition more important to mitigation (more likely to fragment and/or require multiple, complicated mitigation strategies).

Interesting / Thought-Provoking News:
Appendix O: Cross-Over potential with DoD. This appendix was apparently written by DoD personnel as a classified report. They did this for the appendix on nuclear explosive design as well. I didn’t even know this was an option although in retrospect I’m not surprised. The resulting consensus from the DoD states that there is not much cross-over potential between DoD plans & asteroid survey/mitigation due to different constraints imposed by the different agencies. Obviously, they didn’t go into too much detail. But the opinion does not lead much credence to my relating asteroid detection to a possible space-SOSUS.

After the NEO survey is complete, the improved tracking assets could be re-tasked to find Kuiper Belt Objects (KBOs). One particularly compelling estimate states that if just 10% of the “Dedicated LSST” survey option is re-tasked to search for KBOs, they predict it could find up to 100,000! I don’t know how to judge that number but the report lists several reasons why studying the Kuiper belt could be useful, most of these sound worthwhile.

The Bottom Line:
Space-SOSUS is a LOOONNGG way off which is not really a big surprise. However, this report did clarify for me as to why. Constraints are everything in a design. If we have nowhere near enough even for situations like asteroid detection and deflection that we are able to define and quantify well than designing a system to detect possible alien contact it not currently feasible (damn, it really sounds weird when I say it like that). The technical issues revolving around these missions predict future and more complex problems with space-SOSUS. Also, the budgetary issues are significant. Generally speaking, the less specific you are, the most your design will cost. I still consider preparing & countering asteroid impacts a good “test” run for multiple reasons but the pay-off is far in the future. Until there is a justification, will remain nothing more than a potential for future good.

The results of the study clearly show what people on sci-fi space battle sites have long theorized. Nothing is as effective or versatile against space targets, in this case asteroids & comets, as nuclear explosives. However, deploying said explosives to prevent an impact could be a violation of the current UN Space treaties or possibly the Nuclear Test Ban treaties. Not to mention the issues involved in testing such devices. This report explicitly points this out in several places. How that situation gets resolved on a policy level will probably be a very interesting learning experience all-round.

Also, although the long-period comets remain a fairly hopeless situation, the work we do surveying & mitigating other threats will greatly increase our ability to do something about this particular scenario. While this will always be worst-case for impacts, in time, I believe we can be more ready for it.

Looking at it though, I honestly cannot see the US* spending the kind of money needed to create this system without at least credible evidence of a threat first. The original SOSUS network was only created after Soviet & American nuclear submarines were around for several years. And, the Soviets had achieved enough technical prowess with nuclear submarines for them to be a credible threat to NATO. If such a system would ever be worth the money & effort of creating it, there would have to be significantly more justification than is currently available.

*Why not anyone else? Well, who built the original SOSUS? Who spends the most on Defense research? Who has the largest space-military infrastructure? If it’s done at all, it’ll be by the US, US plus allies or nobody.

1 comment:

Anonymous said...

The way to get from "point A to B" regarding reliability is to use mutiple independent designs and methods (e.g. kinetic impactor + nuclear standoff) and to launch / provide effects in sequence.

Doesn't require heavy lift, just requires lots of launches without it. Can all be done incrementally, with enough time.

Good summary, thanks. Find anything wrong in your read through? Any obvious conspiracies or government stupidities? (cue evil music)