[meteorite-list] Comets vs. asteroids

From: E.P. Grondine <epgrondine_at_meteoritecentral.com>
Date: Tue, 13 Jan 2009 18:31:34 -0800 (PST)
Message-ID: <579647.7188.qm_at_web36905.mail.mud.yahoo.com>

Hi Bob -

There were so many falacies in your post that with my stroke damage I let some major howlers get through in my reply. So....

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

and you wrote:
> True; then again, Roman Candles burn nice and slow.
> Cryogenic liquid tanks explode. Choose your poison. --Rob

Actually, turbine, combustion chamber and propellant line failures give some warning, and the ahutdown/abort systems can be brought into play.
Not so with solids, which have sudden catastrophic failure modes - though the Direct team has re-sensored the SRBs to deal with this as best as can be done.

Why Mike resized the CEV so that it exceeded EELV capablities and required a large solid launcher is a great question. Given your work with Griffin in SDIO, I would ask about the need for large solid launchers for defense purposes, but then this is a public forum. I assume Garver, Ladwig, and Obama already know, they''ll share want they want to with us sometime next week, or within the next few months.

> Well, where do you draw the line on the expense of your
> "insurance policy"

Ask the Chinese. Their national emblem is a dragon commemorating a comet;
their first emperor was killed in an impact event; they lost nearly all their commercial shipping fleet to impact mega-tsunami around 1431 CE, which left them open to foreign attack and centuries of suffering.

I haven't broken CAPS out into CZ5 launches yet, to come up with remin costs. Whatever the cost, the value returned by CAPS far exceeds the value of flying a few men to Mars for a few days. In any case, it is highly unlikely that China will bear this cost by itself, but quite likely that other nations will want to participate with them in CAPS by the 2020's.

E.P. Grondine
Man and Impact in the Americas

--- On Tue, 1/13/09, Matson, Robert D. <ROBERT.D.MATSON at saic.com> wrote:

> From: Matson, Robert D. <ROBERT.D.MATSON at saic.com>
> Subject: RE: Comets vs. asteroids
> To: epgrondine at yahoo.com, meteorite-list at meteoritecentral.com
> Date: Tuesday, January 13, 2009, 2:02 AM
> 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 09:31:34 PM PST