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Subject: SPACE Digest V10 #178

SPACE Digest                                     Volume 10 : Issue 178

Today's Topics:
         Re: Asteroids as weapons of mass de
         Re: Asteroids as weapons of mass destruction
             Re: Exhaust velocity
              Re: SPACE Digest V10 #160
            Re: Asteroids via mass drivers
            Re: Asteroids via mass drivers
          Re: Space Shuttle SRB exhaust gas makeup.
       how is Voyager gaining speed as it encounters Jupiter ?
             Re: Galileo Schedule
          Re: Space Shuttle SRB exhaust gas makeup.
             Re: Exhaust velocity
----------------------------------------------------------------------

Date: 25 Oct 89 00:12:37 GMT
From: brutus.cs.uiuc.edu!uakari.primate.wisc.edu!uwm.edu!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!uxh.cso.uiuc.edu![e-mail]
Subject: Re: Asteroids as weapons of mass de


Written  5:59 am  Oct 23, 1989 by [e-mail] (Nick Szabo):

 > 3) We were only able to detect the asteroid that came within 2e6 km because
 > it was _big_ and we were lucky.  What are the statistical estimates for
 > the number of asteroids per year between 10m and 1,000m across--which almost
 > always go undetected at our current efforts-- that come within 2e6 km of
 > Earth every year?  Probably a large number, considering there are an
 > estimated 100,000 asteroids >100m across with delta-v's closer than the
 > lunar surface.  Even an asteroid only 10m across could wipe out several
 > acres of an urban area.

Does anyone know how large an asteroid must be (before entering the Earth's
atmosphere) in order to not burn up (as do many (most?) meteors)?  How much
smaller would an asteroid (say a 100m one such as talked about in the previous
postings) be when it hit the earth's surface?  Or can this be predicted to any
accuracy?

                         Erik A. Johnson
                         [e-mail]

------------------------------

Date: 25 Oct 89 01:38:36 GMT
From: pacific.mps.ohio-state.edu!gem.mps.ohio-state.edu!ginosko!shadooby!mailrus!jarvis.csri.toronto.edu!utgpu!radio.astro!helios.physics![e-mail]  (Prof. A. Griffin)
Subject: Re: Asteroids as weapons of mass destruction

Disclaimer: I am NOT professor Griffin. If you use "F", please check the
attribution against the signature.

In article <[e-mail]> [e-mail] (Nick Szabo) writes:
>In article <[e-mail]> [e-mail] (Christopher Neufeld) writes:
>>   Well, by my calculations, an object which is in a circular orbit at a
>>distance of 1 astronomical unit from the sun will, after applying .001 km/s
>>of delta-v, be in a slightly eccentric elliptical orbit. The maximum
>>deviation of the ellipse from the original circular path is 20000 km, which
>>which the original impact parameter would be about 2 000 000 km, which is
>>roughly the margin of miss we got on that asteroid a few months ago. What's
>>the best delta-v one could put on a 100m asteroid?
>>   Based on this, I don't believe that a mass large enough to be dangerous
>>could be aimed to within 1000 km on the earth's surface
>
>1) We are talking about moving an asteroid, not the earth.  What is the
>corresponding figure for a highly elliptical asteroid?  The way you
>calculated it, the maximum deviation could be off by an order of
>magnitude.
>
   Circular orbits are the hardest to shift, but the only ones I can
calculate without having to solve the differential equations :-).
Certainly, if you hit it at the right time, around the orbit of Mars, you
would get more than a tenfold deflection in position for the an elliptical
orbit.

>2) You did not take into account the possibility of using Venus, Mars or
>the Moon for gravity assist (even gravity assists at several million km can
>provide a small delta-v).
>
   I would bet on the moon.
   A question for the experts out there: would the exquisite aiming of the
Voyager probes have been possible if the maximum maneuvering acceleration
had been only 10^-4 m/s^2, which is the highest conceivable acceleration
for an asteroid missile (100m diameter sphere, propulsion of
200000 kg m/s^2).

>4) The terrorists can arrange for a delta-v greater than .001 km/s, if
>they really need to, with larger nuclear warheads.
>
>Now you might ask, why go to all the trouble of launching nukes way out
>in space when a city could just be nuked directly?
>
>a) Unlike smuggling nukes across borders, there is no risk of getting caught
>in foreign territory.  Once you do it, you can ask for ransom without any
>possibility of the bomb being sniffed out and diffused.
>
>b) ditto if you launch an ICBM directly; it will be tracked, its source
>determined, and the perpetrators punished.
>
   Put the bombs on the asteroid interceptor, fire it at the moon,
slingshot it around the moon for a straight-in approach to your target.
Easy to aim, lots of maneuvering acceleration, and the origin of the
missile is unknown. If you want to wait longer between launch and impact,
have it enter a parking orbit around the moon for a while.

>c) For the larger asteroids (100m+) the potential destruction is much
>greater.  Entire continents could be wiped out, or even the whole planet,
>depending on how crazy the terrorists are.
>
   I can't believe that terrorists who wanted to destroy the world could
get the technical support. The backers who provide thrusters for small
asteroids to bomb their enemies would balk at providing thrusters powerful
enough to throw a planetbuster.

>d) In the future, when we start asteroid mining, the temptation for mine
>workers, disgruntled about wages, conditions, or whatever, will be very great.
>
   Only if they're self-sufficient. Blowing up your only source of food
isn't a great union negotiating tactic. If we had self-sufficient space
colonies, I really believe we would also have the early-warning ability to
see it coming.

>>   All right, I've been making sweeping statements on a topic about which I
>>know very little.
>
>Same here.  Same for 90% of the other stuff on the net.  We're the first
>two to admit it.  :-)
>
>I would be highly grateful for more rigorous calculations on this subject--
>it is important, but I'm afraid my math is lacking.  :-(
>
   Same here. I'm just recalling my undergraduate classical mechanics
stuff, since my field (condensed matter) doesn't give me much practice at
this.

>--
>--------------------------------------------
>Nick Szabo
>uunet!ibmsupt!szabonj
>These opinions are not related to Big Blue's

   Well, I agree that asteroid diversion as a weapon is possible, but I
think it is unlikely. It presupposes equipment which could do as good a job
at lower risk of failure without bringing the asteroid into it. For
instance, it would be easier to put a base on the moon to throw rocks at
your target. Not only would you have a shorter interval between launch and
impact, and better targetting, but it would be reusable.  I just hope no
bunch of environmentalists tries to ban asteroid diversion on the
grounds that it could be a "dangerous science".
--
 Christopher Neufeld....Just a graduate student  |  "Scotty..now _would_
 [e-mail]               | be a good time!"
 [e-mail]              |   - Pavel Chekov
 "Don't edit reality for the sake of simplicity" |

------------------------------

Date: 25 Oct 89 02:01:04 GMT
From: cs.utexas.edu!ginosko!shadooby!mailrus!jarvis.csri.toronto.edu!utgpu!radio.astro!helios.physics![e-mail]  (Prof. A. Griffin)
Subject: Re: Exhaust velocity

Disclaimer: I am NOT professor Griffin. If you use "F", please check the
attribution against the signature.

In article <[e-mail]> [e-mail] (Conor O'Neill) writes:
>In article <[e-mail]> [e-mail] (Henry Spencer) writes:
>>In general, correct.  For one thing, it's easier to build solid motors
>>in large sizes (i.e. high thrusts).  For another, the average molecular
>>weight of the exhaust is higher, which is bad for getting maximum velocity
>>but good for getting maximum thrust.
>
>
>I've seen this said before (many times) but never with a simple explanation.
>My school physics seemed to imply that it is exhaust momentum which matters,
>not simply velocity nor molecular weight. Could someone please elaborate.
>
   All right, here's a high-school physics description. We assume that the
reason the gas is moving so quickly out the back of the rocket is that the
gas is hot, and that it has thermalized according to a Maxwell-Boltzmann
distribution. Fancy talk which means that the average velocity (careful,
not the root mean square velocity) is sqrt( 8 k T /(pi m) ) where m is the
molecular weight of the particle which has been thermalized, and k is the
Boltzmann constant. So, if you're going to throw one kilogram of gas heated
to T degrees out the back of your rocket, you would do best to choose one
with a low molecular weight, because of the m^-1/2 dependence.
   By the way, about that equation. I've been known to slip a decimal on
occasion, as history will show, but the m^-1/2 dependence is as described
in a book, and we all know that textbooks are never wrong :-)

>--
>Conor O'Neill, Software Group, INMOS Ltd., UK.
>UK: [e-mail]        US: [e-mail]
>"It's state-of-the-art" "But it doesn't work!" "That is the state-of-the-art".
--
 Christopher Neufeld....Just a graduate student  |  "Scotty..now _would_
 [e-mail]               | be a good time!"
 [e-mail]              |   - Pavel Chekov
 "Don't edit reality for the sake of simplicity" |

------------------------------

Date: Tue, 24 Oct 89 00:26:47 PDT
From: Ken Harrenstien <[e-mail]>
Subject: Re: SPACE Digest V10 #160
Cc: [e-mail]

Why such horrible delays in turning around contributions to the SPACE
digest?  I thought it was basically automated by now.  Getting things
on Oct 21 that were sent Oct 10 pretty much renders useless all of the
time-critical information that people are volunteering...
-------

------------------------------

Date: 24 Oct 89 07:25:08 GMT
From: crash!orbit!pnet51![e-mail]  (S Schaper)
Subject: Re: Asteroids via mass drivers

I like mass drivers, and would love to be caught up on other means of advanced
propulsion techniques, my last detailed update was an Omni article a couple of
years ago.
  BUT
    Mass drivers as asteroid moving engines would possibly clutter the solar
system with high-velocity meteors, and hence, would be a hazard to future
navigation. I might guess that using such to get an ice moon from Saturn to
terraform Mars might be ok, but I don't think that they would be the best to
use for many missions.

  On Asteroid weapons, what if Brazil decided to knock off the northern
Hemisphere via impact produced dust cloud and climatological effects, such as
to say the least, crop failures (they are already hurting American farmers
badly with their soybean production :-) ).

UUCP: {amdahl!bungia, uunet!rosevax, chinet, killer}!orbit!pnet51!schaper
ARPA: crash!orbit!pnet51![e-mail]
INET: [e-mail]

------------------------------

Date: 24 Oct 89 22:52:08 GMT
From: mailrus!jarvis.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Asteroids via mass drivers

In article <[e-mail]> [e-mail] (S Schaper) writes:
>    Mass drivers as asteroid moving engines would possibly clutter the solar
>system with high-velocity meteors...

Sigh.  This has been a solved problem since the days when mass-drivers were
only sketches on paper.  In the long run, you run mass-driver rockets on
liquid oxygen, which evaporates into space after being used as propellant.
In the short term, you use rock dust, and spray an electrostatic charge on
it as it leaves the mass-driver, so it will disperse.  (There are many
megatons of natural rock dust already at large in the inner solar system.)
--
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry [e-mail]

------------------------------

Date: 25 Oct 89 02:20:19 GMT
From: cs.utexas.edu!mailrus!jarvis.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Space Shuttle SRB exhaust gas makeup.

In article <[e-mail]> [e-mail] (George William Herbert) writes:
>    A _dash_ of aluminum???  The majority of the thrust is achived
>by the aluminum component of the fuel !
>    The actual materials are Ammonium Perchorate, Aluminum powder, PVC
>binder and extra bits to stabilize.  It's about 99% those three.

If we're being picky, the numbers (according to NASA) are 69.83% ammonium
perchlorate, 16% aluminum, 12% rubber (it's not PVC, it's a polybutadiene
acrylic acid acrylonitrile polymer), 2% curing agent, and more or less
0.17% iron oxide catalyst.

And don't sneer at the idea of doing without the aluminum, because the
rubber is also a fuel; similar combinations without aluminum have almost
as high a specific impulse.  (For that matter, ammonium perchlorate by
itself is apparently non-trivial as a monopropellant, although it's not
used that way because its physical properties aren't right.)
--
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry [e-mail]

------------------------------

Date: 24 Oct 89 14:46:33 GMT
From: mcsun!inria!irisa!sigle.irisa.fr![e-mail]  (Henry Thomas)
Subject: how is Voyager gaining speed as it encounters Jupiter ?

Could someone explains me spacecraft like Voyager or Galileo can gain speed
when they encounters Jupiter (resp. Venus)? I suspect it is linked to the
loss of propellant at perihelie, but I need a clear explanation. Thank you.

                    Henry.


Henry Thomas                | Phone : [phone] extension 549
Equipe API                | Fax   : [phone]
IRISA                    | Telex : UNIRISA 950 473F
Campus Universitaire de Beaulieu    | E-mail: [e-mail]

------------------------------

Date: 24 Oct 89 16:23:04 GMT
From: cs.utexas.edu!mailrus!jarvis.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Galileo Schedule

In article <[e-mail]> [e-mail] (Rod Stephens) writes:
>What is "Venus Data Playback?" Why is it so long after the Venus flyby?

Galileo can't deploy its high-gain antenna until it's far enough out from
the Sun to get rid of its sunshades.  (Remember that Galileo was designed
to go straight Earth->Jupiter, and wasn't built to go in closer to the Sun.)
And the data rate from the low-gain antennas isn't anything wonderful at
long range.  So the Venus data is played back later, when conditions are
more favorable (I forget whether they wait for high-gain antenna deployment
or just wait until the range closes a bit).
--
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry [e-mail]

------------------------------

Date: 25 Oct 89 02:45:33 GMT
From: cs.utexas.edu!mailrus!jarvis.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Space Shuttle SRB exhaust gas makeup.

In article <[e-mail]> [e-mail] (Stephen Milton) writes:
>>>     Does it contain hydrogen oxide?
>>I.e., water?  Yes...
>
>technically, water is not hydrogen oxide, its dihydrogen oxide...

No, technically water is hydrogen oxide (ref: Hdbk of Chemistry & Physics,
table of inorganic compounds).  It is normal to omit numeric prefixes when
no ambiguity is present (e.g. N2O4 is nitrogen tetroxide, not dinitrogen
tetroxide), and also common to refer to the most common or "normal"
oxide of XXX as "XXX oxide" (e.g. Fe2O3 is iron oxide even though FeO
also exists; if one is being fussy one uses "ferric" and "ferrous" or
"iron (III)" and "iron (II)" to distinguish).  Chemical nomenclature
in practice is nowhere near as formalized and pedantic as it could be
in theory.
--
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry [e-mail]

------------------------------

Date: 25 Oct 89 03:04:57 GMT
From: mailrus!jarvis.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Exhaust velocity

In article <[e-mail]> [e-mail] (Conor O'Neill) writes:
>>... the average molecular
>>weight of the exhaust is higher, which is bad for getting maximum velocity
>>but good for getting maximum thrust.

To get one picky issue out of the way here:  I goofed slightly.  To a first
approximation, molecular weight is not an issue for maximizing thrust.
(Should have looked it up rather than trying to figure it out myself...)

>I've seen this said before (many times) but never with a simple explanation.
>My school physics seemed to imply that it is exhaust momentum which matters,
>not simply velocity nor molecular weight...

If all you want is thrust, then momentum is the significant number, and
that is determined basically by pressures and areas in the engine.  But
for many applications propellant consumption also matters, which puts a
premium on the highest possible exhaust velocity, to get a given amount
of exhaust momentum with the minimum exhaust mass.

In a thermal rocket, where exhaust velocity is achieved by heating gases
to high temperatures and letting them expand out a nozzle, to a first
approximation exhaust velocity is set by chamber temperature and
exhaust molecular weight.  The significance of molecular weight is that
at a given temperature, the average *energy* of gas molecules is constant
regardless of their mass, so the ones with the lowest molecular weight
move fastest.
--
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry [e-mail]

------------------------------

End of SPACE Digest V10 #178
*******************