Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V15_4/V15NO412.TXT

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Date: Thu, 12 Nov 92 05:04:30
From: Space Digest maintainer <[e-mail]>
Reply-To: [e-mail]
Subject: Space Digest V15 #412
To: Space Digest Readers
Precedence: bulk


Space Digest                Thu, 12 Nov 92       Volume 15 : Issue 412

Today's Topics:
                             Apollo fire
                Lunar "colony" reality check (4 msgs)
        Lunar "colony" reality check and Apollo fire (2 msgs)
                           Man in space...
                           Man in the loop
                           NASA is too big
         O2 spacecraft (was Re: Lunar "colony" reality check)
                     Obscure Help Needed (3 msgs)
                          oxygen atmospheres
                     Pioneer 6 Update - 11/11/92
Sikhote-Aline-Meteorite (was Re: "Earth Gains a Retinue of Mini-Ast.")
                           The Big Picture
              What kind of computers are in the shuttle?
                Where are Pioneer and Voyager Headed?

    Welcome to the Space Digest!!  Please send your messages to
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----------------------------------------------------------------------

Date: Wed, 11 Nov 92 18:37:53 GMT
From: "John P. Mechalas" <[e-mail]>
Subject: Apollo fire
Newsgroups: sci.space,alt.sci.planetary

In article <[e-mail]> [e-mail] (Michael K. Heney) writes:
>Finally, the image of being killed by Velcro is mind-boggling. (Lunatic
>Loop-strips!  Hook-strips from Hell!)  Toxic fumes in the smoke were the
>cause of death - burning velcro could have been one source, but wiring,
>insulation, etc also contributed.

The burn rate of the velcro and nylon was so rapid that they were the most
likely contributors to the toxic fumes that led to the deaths outright.  Kinda
makes you lose faith in synthetics, doesn't it?  :)
   What's also sad about the whole affair, was that Harrison Storms was turned
into the scapegoat at the end.  He wasn't even allowed on the launch site of
the Apollo 12 mission, and he had been basically the centerpiece of the whole
Apollo project...


--
John Mechalas                                          "I'm not an actor, but
[e-mail]                                 I play one on TV."
Aero Engineering, Purdue University                     #include disclaimer.h

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

Date: Wed, 11 Nov 92 16:39:40 GMT
From: "John P. Mechalas" <[e-mail]>
Subject: Lunar "colony" reality check
Newsgroups: sci.space,alt.sci.planetary

In article <[e-mail]> [e-mail] (John D. Boggs) writes:

>From article <[e-mail]>, by [e-mail] (John P. Mechalas):
>>
>> This is true for the most part, but at high O2 pressure, normally inflammable
>> objects burn quite well.  So in that respect, it is indirectly a fire
>> hazard...sort of.  :)
>>
>
>Actually, normally inflammable objects burn quite well at sea-level pressure
>of pure *air*.  However, you need the high pressure to get normally
>*non*flammable objects to burn.       [high pressure O2]

I guess that explains why I'm not an English major, huh?  :)  I'll get this
"in-" "non-" thing straigthened out one of these days...

--
John Mechalas                                          "I'm not an actor, but
[e-mail]                                 I play one on TV."
Aero Engineering, Purdue University                     #include disclaimer.h

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

Date: Wed, 11 Nov 1992 17:01:05 GMT
From: "Phil G. Fraering" <[e-mail]>
Subject: Lunar "colony" reality check
Newsgroups: sci.space,alt.sci.planetary

In article <[e-mail]> [e-mail] (John D. Boggs) writes:

>From article <[e-mail]>, by jenkins@fritz (Steve Jenkins):
>>
>> Oxygen, like many gases, has narcotic effects at very high pressures,
>> such as in deep-sea diving.  It can cause blindness in newborns
>>

>Yes to blindness in newborns, but it is the *nitrogen* that has the narcotic
>effect in deep sea diving -- hence the use of helium for the really really
>deep dives.

>Trust me, I'm a librarian.

_MAYBE_ to blindness in newborns. There are anti-blindness drugs
that apparently sometimes work and have been used on newborns who
are born with lung disorders and need pure oxygen in their first
weeks of life.

I personally ;-) know of _one_ case...

--
Phil Fraering
In the country of the blind....
60 minutes doesn't run stories about people trying to ban hearing
implants!

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

Date: 11 Nov 92 17:52:42 GMT
From: Carl J Lydick <[e-mail]>
Subject: Lunar "colony" reality check
Newsgroups: sci.space,alt.sci.planetary

In article <[e-mail]>, [e-mail] (John D. Boggs) writes:
=From article <[e-mail]>, by jenkins@fritz (Steve Jenkins):
=>
=> Oxygen, like many gases, has narcotic effects at very high pressures,
=> such as in deep-sea diving.  It can cause blindness in newborns
=>
=
=Yes to blindness in newborns, but it is the *nitrogen* that has the narcotic
=effect in deep sea diving -- hence the use of helium for the really really
=deep dives.

If oxygen at high pressures DIDN'T cause the bad effects, why bother mixing it
with helium?  Yes, nitrogen has narcotic effects at high pressure, but so does
oxygen.
--------------------------------------------------------------------------------
Carl J Lydick | INTERnet: [e-mail] | NSI/HEPnet: SOL1::CARL

Disclaimer:  Hey, I understand VAXen and VMS.  That's what I get paid for.  My
understanding of astronomy is purely at the amateur level (or below).  So
unless what I'm saying is directly related to VAX/VMS, don't hold me or my
organization responsible for it.  If it IS related to VAX/VMS, you can try to
hold me responsible for it, but my organization had nothing to do with it.

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

Date: Wed, 11 Nov 1992 18:46:37 GMT
From: "Thomas E. Smith [LORAL]" <gothamcity!tes>
Subject: Lunar "colony" reality check
Newsgroups: sci.space,alt.sci.planetary

>>
>> This is true for the most part, but at high O2 pressure, normally inflammable
>> objects burn quite well.  So in that respect, it is indirectly a fire
>> hazard...sort of.  :)
>>
>
>Actually, normally inflammable objects burn quite well at sea-level pressure
>of pure *air*.  However, you need the high pressure to get normally
>*non*flammable objects to burn.       [high pressure O2]

This is alt.sci.planetary isn't it?

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

Date: Wed, 11 Nov 92 16:36:43 GMT
From: "John P. Mechalas" <[e-mail]>
Subject: Lunar "colony" reality check and Apollo fire
Newsgroups: sci.space,alt.sci.planetary

In article <[e-mail]> soc1070                          writes:
>>   The fire ignited the velcro under high-pressure, and the resultant toxic
>>fumes killed the astronauts within seconds.
>
>I beg to differ. It was in fact Apollo 1, as many have pointed out. If you
>check your Apollo history, you will see that 8 circled the moon, 9 did
>LEO tests, 10 tested the LM in lunar orbit, 11 landed, 12 landed next to
>the Surveyor, 13 blew an oxygen tank on the way to the moon, etc.

You are right, of course...  Rechecking my refs., I should have said
"Spacecraft 12" and not "Apollo 12".  Damn naming conventions.  :)
Apollo 1 was the mission number, and Spacecraft 12 was the spacecraft
name...Pardon my confusion.

>Secondly, the cause of the fire was never traced to any specific source.
>According to Micheal Collens in _Carrying The Fire_, the inside of the
>craft was so fried that a single cause could not be found; rather there
>were several *probable* causes that resulted in an almost complete redesign
>of the inside of the capsule.

Correct.

>You were right that almost anything will burn at 16psi pure O2, even
>stainless steel. Unfortunatly for Grissom, White, and Chaffee, they didn't
>die of toxic fumes. It was the fire that did them.

I beg to differ on this point.  The burns weren't fatal.  The astronauts had
suffocated.  The velcro and nylon netting in the spacecraft burned very *very*
rapidly under 16psi of pure O2( later tests showed that the velcro burned at
a rate of almost 3 inches per second), and filled the entire cabin with gas.
The fire itself was very selective...a flight manual inside the craft (where
White was located) was almost untouched.

--
John Mechalas                                          "I'm not an actor, but
[e-mail]                                 I play one on TV."
Aero Engineering, Purdue University                     #include disclaimer.h

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

Date: 11 Nov 92 18:30:34 GMT
From: Henry Spencer <[e-mail]>
Subject: Lunar "colony" reality check and Apollo fire
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (John P. Mechalas) writes:
>You are right, of course...  Rechecking my refs., I should have said
>"Spacecraft 12" and not "Apollo 12".  Damn naming conventions.  :)
>Apollo 1 was the mission number, and Spacecraft 12 was the spacecraft
>name...Pardon my confusion.

Actually, the mission code at the time of the fire was AS-204, aka
Apollo 204.  There was no official name; the astronauts were calling it
Apollo 1 but the booster people were calling it Apollo 4 (since three
unmanned tests had preceded it), and NASA HQ had not issued a decision.
The astronauts' wishes were complied with posthumously, at the request
of their widows, and the Apollo 1 name was made official.
--
MS-DOS is the OS/360 of the 1980s.      | Henry Spencer @ U of Toronto Zoology
              -Hal W. Hardenbergh (1985)|  [e-mail]  utzoo!henry

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

Date: 11 Nov 92 17:24:10 GMT
From: Steve Jenkins <jenkins@fritz>
Subject: Man in space...
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (John Roberts) writes:
>The world record for voluntarily remaining underwater (and conscious) is
>something like 13 minutes. The fellow hyperventilated pure (1 Atm) oxygen
>(don't try this at home!) ahead of time. Does this fit in with your model?
>(I realize his lungs weren't exposed to vacuum, but he didn't have any way
>of getting more oxygen except perhaps very slow diffusion from the water.)

First of all, it's not *my* model, it's standard first-year med school
physiology.  So none of this should be controversial.  :-)  I'm
assuming you haven't yet seen my reply to Henry, so I'll try not to
repeat too much.  It's well known that hyperventilation can extend the
limit of voluntary breathholding.  It's also well known that this is
extremely dangerous.  Last night I was checking up on myself by
reading some reference material (I was right :->) and found a couple
of good paragraphs about this.  I'll type them in tonight.

Now, hyperventilation doesn't add any appreciable amount to the stores
of oxygen outside the lung, but it does reduce the CO2 stimulus to
breathe.  Holding a big breath of O2, however, provides at
least 5 liters of O2.  Normal resting O2 consumption is about 250
ml/min, so 5 l gives you plenty of O2 to hold your breath for 13
minutes.

The point of all this is that breath-holding is not a good model for
depressurization.  When you hold your breath, you have a few minutes's
worth of O2 in the lungs so you can maintain arterial oxygen partial
pressure.  In zero ambient pressure, your arterial PO2 will be zero,
and it's lights out.

>What's the mechanism by which the hemoglobin gives up its oxygen where
>it's needed? And is there a 100% oxygen turnover on each pass of the
>blood through the lungs?

Oxygen moves from the lungs into arterial blood purely by
diffusion--it moves from a region of higher to lower PO2.  The magic
of hemoglobin is that it binds O2 such that the PO2 on the blood side
rises more slowly than the O2 content.  (If O2 were merely dissolved in
blood, the relationship would be linear.)  The consequence is that
more O2 can be loaded into the blood before partial pressure
equilibrium is reached.  (In normal humans equilibrium is always
reached before a red blood cell leaves the lung.)  At the tissues, the
reverse process occurs.  The tissue PO2 is lower than the blood,
so O2 diffuses into the tissue.  This time, hemoglobin holds the PO2
"artificially" high so that more O2 unloads before equilibrium.

There are also secondary effects, in which the low arterial PCO2
assists O2 loading into blood, and the higher tissue PCO2 assists
unloading O2 into tissue.  O2 and CO2 have this mutual relationship;
one is called the Bohr effect, the other is the Haldane effect.
pH also shifts the dissociation curve.

The PO2 of mixed venous blood is about 40 mmHg, so there is not
complete unloading in the tissues.  However, as O2 delivery to tissue
is by diffusion, the low partial pressure prevents effective uptake.
This is especially problematic in the brain, whose O2 demands are very
high.

>People seem to do "reasonably" well breathing air at around half normal
>pressure, which would imply an oxygen partial pressure of 80mm Hg or not
>much over. What's the lowest pressure of pure oxygen that people can
>manage on? And what is the air pressure on the top of Mt. Everest? (It
>has been climbed (strenuous physical activity) without supplementary
>oxygen. It's very dangerous, but it has been done.)

I don't believe the limits are well understood.  *Slow* adaptation to
low PO2 allows certain compensatory mechanisms (in particular renal
adjustment of bicarbonate and left shifts in the hemoglobin
dissociation curve).  If you were to helicopter directly from sea
level to Everest, you'd be unconscious long before the summit.  On the
other hand, it's clearly possible to ascend it one step at a time
without supplementary oxygen.  There is evidence to indicate that such
feats engender brain damage.  A standing joke among climbers is that
you'd have to be brain-damaged to climb Everest anyway, so what the
hell!

>There's an article in a recent Scientific American on altitude sickness -
>I haven't had time to more than scan it. Apparently it's not really healthful
>to breathe air at any pressure noticeably less than sea level pressure, and
>some people are much more susceptible than others to altitude-related
>health problems.

We're getting a little beyond my knowledge here.  Altitude sickness is
a complex of phenomena, at least some of which is related to low PO2,
not merely low ambient pressure.  It's certainly true that there is
wide individual variation in response to altitude, and no good
predictors of response that I'm aware of.

--
Steve Jenkins                  [e-mail]
Caltech/Jet Propulsion Laboratory    [phone]

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

Date: Wed, 11 Nov 1992 15:50:04 -0500
From: Lawrence Curcio <[e-mail]>
Subject: Man in the loop
Newsgroups: sci.space

Well it's strange, ain't it. Whenever there's a slow decision to be
made, the responsibility in a human-machine system is given to the
human, figuring the machine will make a mistake. (That's why we have
decision *support* systems instead of decision *making* systems.)
Whenever there's a fast decision to be made, as in fly-by-wire and
nuclear power plant control, the responsibility is given to the machine,
figuring the human will make a mistake.

It's true that the human is always given the last word, but then if you
look at nuclear power plant accidents, they seem to occur most when the
human intervenes.

Bottom line: Never trust a human, and never trust a machine.

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

Date: Wed, 11 Nov 1992 17:01:30 GMT
From: "I am a terminator." <[e-mail]>
Subject: NASA is too big
Newsgroups: sci.space

I was reading Feynman's account of his work investigating the Challenger
and he was complaining about how NASA is so backward. Now the shuttles
are flying again, what is NASA's situation?

Henry Choy
[e-mail]

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

Date: Wed, 11 Nov 1992 13:54:43 -0500
From: Kevin William Ryan <[e-mail]>
Subject: O2 spacecraft (was Re: Lunar "colony" reality check)
Newsgroups: alt.sci.planetary,sci.space

roelle@uars_mag.jhuapl.edu (Curtis Roelle)
>>Suffice it to say that Mercury, Gemini, and Apollo astronauts worked in a
>>5 psi pure O2 environment.  This simplified the environmental control system
>>and eliminated the need to purge N2 prior to EVA.
>
>Are you certain that Apollo astronauts breathed pure O2?  I thought that
>after the fatal Apollo 1 fire, which killed astronauts Grissom, White, and
>Chaffee on January 27, 1967, pure O2 was no longer used because it was a
>proven fire hazard.  Or did NASA simply reduce the cabin pressure as
>suggested by Carl Lydick?

    People can withstand a long term pure O2 environment, as long as the
O2 pressure is on the order of the partial pressure of O2 in standard
air. Since oxygen is about 20% of the atmosphere, about 3psi of oxygen
would suffice. Somewhat higher pressure (the 5psi referred to) would be
preferable simply for reasons of sound and temperature conduction. 15psi
of oxygen is manageble for a time but not a good idea for the long run,
due to problems with flammability, and I believe to problems related to
hyperventilating (I could be wrong on the last point).

    The fatal Apollo fire happened with 15psi oxygen in the cabin during
a ground test. That was an intermediate step leading to 5psi oxygen for
orbital conditions, where the standard atmosphere was flushed. Following
this the flushdown procedure was changed, as well as the Apollo hatch.
The original hatch design sealed under positive pressure and opened
inward: the new design, although heavier, opened owtward and therefore
could be opened under a high internal pressure, such as that caused by a
fire. The cabin atmosphere expansion of the fire effectively sealed the
cabin until far too late for the crew.

    Incidentally, a number of the spacesuit designs have specified
internal pressures on the order of 3psi in order to simplify design,
including the Space Activity Suit of the early 70's, which was basically
a multilayer leotard with a air tight neck seal. The air bag for the
head and chest was at 200 torr (760 torr in one atmosphere) with
pressure tapering to ~150 torr at the extremities, supplied by the
stretch of the suit. (Sorry, pet project of mine, had to throw it in...)

                                                    kwr

Internet: [e-mail]

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

Date: Wed, 11 Nov 1992 17:06:02 GMT
From: "Phil G. Fraering" <[e-mail]>
Subject: Obscure Help  Needed
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (James Davis Nicoll) writes:

       An embarrassing request: my  HP died and I discover I cannot
   remember how to  do logs with a slide-rule,  nor  can I find my sr-related
   texts. Clearly, I am  senile. Anyone out  there recall how  the damn  things
   work?

Hell, I would like to find out where to get a slide rule for when
my calculator breaks...




--
Phil Fraering
In the country of the blind....
60 minutes doesn't run stories about people trying to ban hearing
implants!

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

Date: 11 Nov 1992 17:07:10 GMT
From: "David M. Palmer" <[e-mail]>
Subject: Obscure Help Needed
Newsgroups: sci.space

[e-mail] (James Davis Nicoll) writes:

>    An embarrassing request: my  HP died and I discover I cannot
>remember how to  do logs with a slide-rule,  nor  can I find my sr-related
>texts. Clearly, I am  senile. Anyone out  there recall how  the damn  things
>work?

>                            James Nicoll

Sorry to post this to the net, but mail to [e-mail]
bounces.

Look for linear scales.  The linear scale which goes from 0 to 1.0
is your base-10 log scale: put the cursor on the number you want
the log of on the "A" scale, and read off the mantissa(? the fractional
part of the log) on the linear scale.  The characteristic(? the integer
part) you get trivally by remembering where your decimal point is.

For natural logs, there is a stack of linear scales, one for each of
a range of decades.

Hope this helps.  If all else fails, you can use tables, or see if
you can figure it out on an abacus.  There may be computers on the
net which can calculate logs as well.  Followups to comp.sys.super,
expecially if you are going to talk about logs in ARBITRARY BASES!!!
(it's possible).

--
        David Palmer         [e-mail]

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

Date: 11 Nov 92 17:28:31 GMT
From: James Davis Nicoll <[e-mail]>
Subject: Obscure Help  Needed
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Dave Jones) writes:
>James Davis Nicoll ([e-mail]) displaying brain-death wrote:
>>
>>     An embarrassing request: my  HP died and I discover I cannot
>> remember how to  do logs with a slide-rule,  nor  can I find my sr-related
>> texts. Clearly, I am  senile. Anyone out  there recall how  the damn  things
>> work?
>>
>You're serious, right?  I mean praying to the netgods is easier than dashing
>down to the university bookstore with a $10 bill in hand?  OK, maybe you'd
>need $20 for a calculator these days.
>
    Advice for the slide-rule-challenged  deleted

    Yes, it was a stupid request. At least I didn't
ask for  the time...

    I still find it hard to believe that I could so completely
forget a skill I used every day not so  long ago. It's also hard to
believe no book store seems to stock books on the subject anymore.
Are there no slide-rule makers left?

    At least sliderules never run out of power, nor do they suddenly
decide to display '0.00' only, just  when the user is calculating mass
ratios.

>Now when is my 1965 Faber-Castell going to become a collectors' item?

    I think it may be already...

                            James Nicoll

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

Date: Wed, 11 Nov 1992 16:59:16 GMT
From: Pat <[e-mail]>
Subject: oxygen atmospheres
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (John Roberts) writes:
>
>-From: [e-mail] (Henry Spencer)
>-Subject: oxygen atmospheres
>-Date: 10 Nov 92 21:29:43 GMT
>
>-Neither.  In space, the Apollo spacecraft used the atmosphere it was
>-designed for:  low-pressure pure oxygen.  It wasn't feasible to run that
>-way on the pad, though, because the spacecraft wasn't built to stand an
>-external pressure exceeding internal pressure.  After some attempts to
>-fireproof the interior for 1atm of oxygen -- abandoned as impossibly
>-difficult -- they switched to using a mixed-gas atmosphere before and
>-during launch, with switchover to low-pressure pure oxygen on the way up.
>

Why didn't the SHuttle do this?  i would have thought following apollo's
lead, of  reducing cabin pressure and going to pure O2, you could
reduce the difficulty of  EVA  which is a primary shuttle mission.

what win do they get out of having a 1 atm cabin at 80/20 N2/O2?

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

Date: Thu, 12 Nov 1992 01:08:36 GMT
From: Ron Baalke <[e-mail]>
Subject: Pioneer 6 Update - 11/11/92
Newsgroups: sci.space,sci.astro

                            PIONEER 6 STATUS REPORT
                              November 11, 1992

     Yesterday, the Deep Space Network's 70 meter antenna in Goldstone,
California supported a tracking pass of the Pioneer 6 spacecraft.  Receiver
lock was attained, but we were unable to attain telemetry lock because no
subcarrier was detected.  The expected bit rate was 16 bps uncoded telemetry.
The AGC (Automatic Gain Control) was initially at -175.3 dbm, but was seen to
fluctuate causing the receiver to toggle in and out-of-lock.  The problem is
under investigation.

     Pioneer 6 was launched on December 16, 1965 and is the oldest surviving
spacecraft.  Pioneer 6 was last supported on December 16, 1990, on the
25th anniversary of its launch, and no problems were encountered then.
     ___    _____     ___
    /_ /|  /____/ \  /_ /|     Ron Baalke         | [e-mail]
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
 ___| | | | |__) |/  | | |__   M/S 525-3684 Telos | Give people a second
/___| | | |  ___/    | |/__ /| Pasadena, CA 91109 | chance, but not a third.
|_____|/  |_|/       |_____|/                     |

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

Date: Tuesday, 10 Nov 1992 10:46:23 CET
From: [e-mail]
Subject: Sikhote-Aline-Meteorite (was Re: "Earth Gains a Retinue of Mini-Ast.")
Newsgroups: sci.astro,sci.space

I suppose Sikhote-Aline was the largest space-rock ever OBSERVED to come down
Although it had probably around 200t before it entered the atmosphere, "only"
23t were collected. (By the way, 47 grams finally landed in my home...)
Sure enough, thousands of small pieces (less than a few grams) and probably
some large masses were not found in the dense forest, bringing the total
weight well up to 50t.
But Sikhote-Aline is NOT a Stony-Iron-Meteorite: It's a iron-meteorite
(coarsest Octahedrite, chemical group IIB).

Keep looking up|
  Herbert

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

Date: Wed, 11 Nov 92 14:06:22 GMT
From: Daniel Briggs <[e-mail]>
Subject: The Big Picture
Newsgroups: sci.space

In article <[e-mail]>
  [e-mail] (Philip Young) writes:
>Given our propensity to fling hardware into the heavens, and our desire
>to get a good handle on what's very old and far away, has anybody done
>any serious investigation of the possibility of tacking astronomical eyes
>on craft headed for interstellar space which would be suitable for
>verrrrrrrrrrrry long baseline interferometry?  We're not just talking
>Earth orbit here. Seems to me we have the clocks, the computers, the
>comms.  What would be the shortest frequency we could realistically
>deal with?  Could costs be contained with a standardized, shrink-wrapped
>observatory package?  What might we discover with a (radio?) telescope
>whose effective diameter increases 10E+4 km/sec or more for the forseeable
>future?

Well, I don't know of any serious investigation into this, but I can think
of at least a few problems that are going to be very tough to beat.
Basically, sensitivity is going to be the limiting factor.  First of all,
the existing space VLBI projects are designed around 10m class antennas,
and they are none too sensitive.  With antenna, receivers, electronics and
maybe a maser, you're talking about a full sized mission right there.  It's
not anything you're going to tack onto a mission that happens to be going
in that direction.  (If you want to timeshare the main antenna between
earth communications and observing, you'll need an onboard maser and a very
high density tape recorder to record the astronomical signals.  If you send
up the "local" oscillator from earth, you'll need two big antennas on the
spacecraft.)  And of course, there's the question of bandwidth on the comm
link, never mind the LO.  Normal earthbound VLBI stations typically record
the order of hundreds of kilobits per second or more, and it's this
bandwidth that is typically the limiting factor in such systems.  This
isn't the sort of data you're going to send over a 300 baud link to a
Voyager style spacecraft out beyond Pluto.  There's also the fact that we
don't know that there is going to be anything to see on au scale baselines.
Essentially, interferometers are blind to emission distributed on a scale
larger than lambda/baseline.  So as the baseline gets longer, more and more
of the flux resolves out.  One of the OVLBI missions currently in the works
plans to extend the orbit to 10 earth radii.  It may or may not see
anything on those size scales, assuming that it ever flies in the first
place.  Multiple au baselines are definitely dicey!  Finally, you'll
eventually run up against scattering disk caused by propagation thought the
interstellar medium.  It's a direct analog to visual seeing.  We can
already see this effect looking at maser spots towards the Galactic center.
Out of the plane of the galaxy the FWHM of the seeing disk is .0015 l^2 /
sqrt(|sin(b)|) milliarcseconds, where l is the wavelength in cm, and b is
the galactic latitude.  At the longer wavelengths, the existing OVLBI
missions will be flirting with these limits.  At 100 GHz, which is fairly
tough even from the ground at the moment, you're limited to about
1 microarcsecond, which is only a baseline of .003 au, or 80 earth radii.

So depressing as the thought may be, we couldn't even begin to mount an
au scale VLBI mission at the moment, and if we could, it wouldn't tell us
a whole lot since we can't see through the ISM.

--
| Daniel Briggs  ([e-mail])                      |  USPA B-14993
| New Mexico Tech / National Radio Astronomy Observatory |      DoD #387
| P.O. Box O / Socorro, NM 87801   [phone]        |
Support the League for Programming Freedom   (info from [e-mail])

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

Date: Wed, 11 Nov 1992 17:53:36 GMT
From: "I am a terminator." <[e-mail]>
Subject: What kind of computers are in the shuttle?
Newsgroups: sci.space

The computers used to control the shuttle use very complex programs
that people don't want to rewrite for newer computers. Has any
upgrades been done or are the computers still the same old beasts?

Henry Choy
[e-mail]

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Date: Wed, 11 Nov 1992 17:38:58 GMT
From: [e-mail]
Subject: Where are Pioneer and Voyager Headed?
Newsgroups: sci.space

A student asked me what direction the Pioneer and
Voyager spacecraft are headed.  I had no idea but
I said I'd try to find out.  Does anyone have an
approximate RA and Dec for the asymptote of the
trajectories of any of the four spacecraft that
are beyond Pluto's orbit?

            Thank you,

            Matt

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End of Space Digest Volume 15 : Issue 412
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