[meteorite-list] FW: Comets vs. asteroids

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Tue, 13 Jan 2009 00:39:40 -0800
Message-ID: <GOEDJOCBMMEHLEFDHGMMAEKGDKAA.mojave_meteorites_at_cox.net>

Hi E.P.,

> What you have to weigh that high cost against is the fact that
> mankind nearly went the way of the dinosaur several times over
> the last six million years, and several mt DNA groups disappeared
> more recently than that, and several nations disappeared more
> recently than that.

Well, where do you draw the line on the expense of your "insurance
policy", when there is no way to cash it in if you're right?
We're finding the planet killers and continent killers at a pretty
fast clip. Launching a satellite or two to join in the search
is a bit like confiscating shampoo bottles from airline passengers:
it's "security theater". (No one is in the least bit safer on
jet airlines, btw -- just more inconvenienced and irritated.)
Within a few years we will have found and ruled out all of the
potential state-sized killers. Beyond that, you're money is
probably better spent elsewhere.

> Of course, the difference between you and myself is in our
> estimates of the risk. Mine is built on historical and geological
> data... yours on hopes and Morrison's theoretical models.

My threat estimate is based on my own math and understanding of
solar system dynamics.

> Speaking of money, how many tens of millions has NASA wasted looking
> for Nemesis?

Seems to me you're simply looking for your own version of Nemesis.
What makes your Moby Dick comet any more urgent or probable in the
next five centuries?

>> The risk, while real, is puny compared to more mundane threats.

> While we certainly have a lot of "mundane" threats, risk equals
> probability of occurrence versus loss per occurrence.

Knowledge of the threat doesn't mean a lot when you have no means
of preventing it. We ~barely~ have the technology to prevent an
impact that is, say, a decade out. The threats that worry you we
have absolutely zero ability to prevent, any more than a supervolcano
eruption in Yellowstone. The smart money is spent on those threats
that we CAN do something about.

>> The odds of a 75-meter impactor (of any flavor) are indeed
>> close to one, but only if you're willing to wait long enough. But you
>> can't say the odds of being blind-sided by one are unity

> With NASA's current and planned detectors, yes I can.

Okay, you can say it, but that doesn't make it true. ;-)

>>-- we have space-based sensors operating 24/7

> Now that's news there - are your IR detectors capable of finding
> 75 m objects with the luminence of a chunk of charcoal at several
> lunar distances?

Surely you must know that most catalogued asteroids are about as
dark as charcoal. Typical reflectivities are 6-10%. You can find
them just fine in the visible; even better in the IR.

>> and dozens of highly capable ground-based instruments
>> scattered around the globe, so there is at least some
>> chance of spotting such an interloper before impact.
>> (Don't forget the 3-meter object that Catalina Sky Survey
>> spotted about a day before impact in Sudan.)

> Specious rationalization of the worst sort, Bob

Hardly -- it demonstrates that current ground-based technology
is perfectly capable of detecting threats smaller than you care
about. It's not the brightness that's the problem, it's the
revisit rate -- a classic surveillance problem. Adding one
satellite helps incrementally, but it's no magic bullet.

> - you mention 3 meters, but you do not mention luminence

I chose a physical dimension because the radiance units (watts/cm^2-sr)
would likely be of no help to you.

> Did you work the Columbia foam impact by any chance?

I have no idea where you're going with such an unrelated topic.
If you insist on employing sarcasm, slamming NASA, or both, that's
you're business, but I don't see the relevance. I don't work for
NASA.

> Once again, it was a comet that killed the dinosaurs, not an asteroid.
>> What's the difference?

> Don't you know the difference between a comet and an asteroid?

I do. I'm asking for relevance to the problem at hand. I doubt the
city about to be wiped off the face of the planet cares whether the
rock they get hit by happens to have a little ice mixed in.

>> In terms of detection, there is no difference between an earth-crossing
>> asteroid and a short period comet.

> Oh really?

Yes -- unless of course you'd like that comet to be active, in which case
the comet is far easier to detect than the asteroid of comparable mass.

>> If you're arguing that the main threat is a long-period comet, then
>> fine. But a space-based sensor won't help you in that case.

> Really?

Yes, because the more sensitive ground-based instruments would most
likely find it first.

I asked:

>> How much aperture are you talking about putting up in orbit?

You replied:

> You put it on the Moon - see the CAPS study, if NASA has not destroyed
> all their copies of it.

Well, now you're REALLY talking about some money. I doubt anyone
could do it for under ten billion and in less than 10 years. That's
a pretty expensive insurance policy, in return for a very small
incremental benefit. It's not going to pass a Congressional cost-
benefit analysis.

> One more time, it wasn't an asteroid that killed the dinosaurs, it was
> a comet.

So we should stop looking for asteroids? What's your point? Repeating
the same phrase over and over adds nothing to the discussion. Context?
Relevance?

> NASA impact risks from comets and asteroids are published, and they're
> defective.

Some astronomers have argued the opposite -- that they're overly
alarmist.

> Wernher von Braun said it a long time ago: solids lack abort modes.

True; then again, Roman Candles burn nice and slow. Cryogenic liquid
tanks explode. Choose your poison. --Rob
Received on Tue 13 Jan 2009 03:39:40 AM PST