Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V15_3/V15NO358.TXT

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Date: Fri, 30 Oct 92 05:03:55
From: Space Digest maintainer <[e-mail]>
Reply-To: [e-mail]
Subject: Space Digest V15 #358
To: Space Digest Readers
Precedence: bulk


Space Digest                Fri, 30 Oct 92       Volume 15 : Issue 358

Today's Topics:
                        A career in Aerospace?
                           Comet Collision
                      Galileo High Gain Antenna
                   Galileo High Gain Antenna Update
                        HRMS for ETI (2 msgs)
                Launching Radioactive Waste to the Sun
                            mini-asteroids
                  Need help FAST! Looking for answer
               Re:Swift-Tuttle Comet a threat to earth?
                   REPOST!  UN Space/Moon Treaty?!
               Scenario of comet hitting Earth (3 msgs)
                Smith-Tuttle Comet a threat to earth?
      Surveyor landings (was Re: QUESTIONS: Apollo, Earth, Moon)
             Two-Line Orbital Element Set:  Space Shuttle
                        UN Space/Moon Treaty?
              What does the "P/" mean in P/Swift-Tuttle?

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

Date: Thu, 29 Oct 1992 21:29:00 GMT
From: "E. V. Bell, II - NSSDC/HSTX/GSFC/NASA - (301" <[e-mail]>
Subject: A career in Aerospace?
Newsgroups: sci.space

In article <[e-mail]>, [e-mail] (John Judge) writes...
>I am a college freshman considering a career in aerospace.  What are the
>chances that the job market will improve by the time I graduate?
>                                        John Judge
>

You caught me on a pessimistic day, so I'd say pretty poor. Your best
chances are for a fairly broad-based background (computers,
engineering, physical science.....choose two or more). Also, be
willing to adapt to the job market. It may take a few years to get to
do what you "want" to do or were "trained" to do. Then again, you may
find it impossible to get back on track or decide you don't want to.

All of this is another way of saying, be flexible and versatile. Often
companies will train you to do exactly what they want done anyway. If
you're hoping for government (i.e., civil servant) or academic
positions, my opinion is that they will only get tougher to get.

+------------------------------------------------------------------------------+
|  Dr. Edwin V. Bell, II    |  E-mail:                       |
|  Mail Code 633.9        |    (SPAN) NCF::Bell               |
|  National Space Science    |    or   NSSDC::Bell               |
|    Data Center        |    or  NSSDCA::Bell               |
|  NASA                |    or  NSSDCB::Bell               |
|  Goddard Space Flight Center    |   (Internet) [e-mail]       |
|  Greenbelt, MD   20771    |                           |
|  [phone]        |                           |
+------------------------------------------------------------------------------+

P.S. This time I'm adding a disclaimer. My opinions are no one else's.
If anyone else has this opinion, than they are probably as crazed as I.
If anyone has a contrary opinion, then that's their problem.

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

Date: 29 Oct 92 21:49:03 GMT
From: John Black <[e-mail]>
Subject: Comet Collision
Newsgroups: sci.space,alt.sci.planetary

Maybe a near comet approach could be a good thing. I did a rough calculation
and estimated that there must be something of the order of 10 to the power 11
metric tonnes of water. Maybe in 130 years time somewhere on the Earth could do
with some water, eg the interior of large continents affected by drought. I
know that one of the effects of the greenhouse effect is to make sea levels
rise, but that is salt water, no good for crops, and still would be hundreds of
miles from a continental desert region. The comet presumably is almost pure
water and therefore would be ideal for crops, drinking etc.

So if in 130 years technology is sufficiently advanced, it could be possible
to alter the orbit of the comet so that it goes into a stable orbit around the
Earth. The trick then would be to "chip" bits off the comet and "land' them
in the appropriate region of the Earth. This would be the most difficult bit
since viz the probable cause of the Tunguska event (see previous article on
comet hitting Earth) you would end up doing the equivalent of nuking the region
The comet could also be an almost infinite supply of water for lunar bases if
there are any by then.

Maybe this is all too much "pie in the sky" (no pun intended :-), 130 years is
much too short to evolve technologies to do this sort of stuff, witness  what
could be viewed as comparativly slow progress in maned spaceflight in the last
20 years (still that's probably a function of the spending). So this sort of
stuff is more like 500 years off in the future.

            John Black.

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

Date: Fri, 30 Oct 1992 00:16:42 GMT
From: [e-mail]
Subject: Galileo High Gain Antenna
Newsgroups: sci.space,sci.astro

In article <[e-mail]>, [e-mail] (Jim Klavetter) writes:
>
>  So the question 1) does HST have the capability of imaging Galileo
>  when it gets close to earth?

No.  If Galileo were close enough to resolve (remember one arcsecond is
about one meter at 200 km distance), the relative velocities would be
MUCH too high.

>  and 2) would this help the engineers at all?

Very doubtful.  They know what the problem is, they just can't find any
way to fix it.

Ben

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

Date: 29 Oct 92 16:21:07 GMT
From: Jim Klavetter <[e-mail]>
Subject: Galileo High Gain Antenna Update
Newsgroups: sci.space,sci.astro,alt.sci.planetary

I've had a question and I don't know who to ask, so I'll do it here.
This question really doesn't require an answer to me, I'm just
wondering if the High Gain Antenna problem could be helped...

So the question 1) does HST have the capability of imaging Galileo
when it gets close to earth?  and 2) would this help the engineers at
all?

[e-mail] also for Athabasca and Reudi
Jim Klavetter
Astronomy
UMD
College Park, MD  20742

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

Date: 30 Oct 92 01:09:50 GMT
From: Stanley Friesen <[e-mail]>
Subject: HRMS for ETI
Newsgroups: sci.space,sci.bio

In article <[e-mail]> [e-mail] (Paul Dietz) writes:
|In article <[e-mail]> [e-mail] (David M. Palmer) writes:
|>It turns out that life occurred just about as soon as the Earth was
|>tolerable, like within 100 million years (this number is approximate, I
|...
|>accreted.)  This is very near the start of the window, which is roughly
|>5 billion years long as a rough number.
|
|The last statement cannot be supported, since we don't know what
|conditions are required for life to originate.  It could be that they
|would exist only soon after the formation of a planet.  For example,
|perhaps deposition of organic material from space is required.

This last is an extremely unlikely requirement.  The basic upshot of the
Urey experiment and its follow-ups is that organics form spontaneously
under most non-oxidizing 'medium-energy' conditions.  The presence of
organics in interstellar dust clouds just confirms this basic ubiquity.

Now it is true that under current conditions life might have a problem
developing, but since these oxidizing conditions were *caused* by life,
this is not a particularly significant issue.

Now, it is true that we do *not* know the exact range of temperatures
and pressures under which the ubiquitous organics are able to generate
life forms, but this is a much smaller level of uncertainty than you suggest.

|Another thing one can do is to look at how long it took humans to
|appear -- about 4.5 billion years.  The earth will not be habitable
|much longer (due to the increasing brightness of the sun with time),
|perhaps as little as a few hundred million years.

Perhaps, as another poster has pointed out this is not exactly certain.
Many confounding factors exist, including, perhaps, biotic feedback that
could stabilize the temperature of the Earth, at least until the Sun goes
red giant (which will heat the Earth beyond any possbility of rescue).



Now, I am *not* saying I believe there are other thinking beings out there.
There may be, or there may not.  The *only* way to tell is to look.
Speculation, even well thought out speculation, is still not conclusive.
Only substantive evidence is really usable.

Thus, while I give the current SETI program a small chance of finding
anything, I support it, since it is the *scientific* way of approaching
the problem.  (My main technical objection to SETI is that it assumes
a technology too similar to ours - considering how quickly we have moved
from primitive radios to mass media, I am skeptical that our current
communication technology is the last word in long range linkage).
--
[e-mail]            (formerly tdatirv!sarima)
  or
[e-mail]

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

Date: Fri, 30 Oct 1992 02:35:42 GMT
From: Paul Dietz <[e-mail]>
Subject: HRMS for ETI
Newsgroups: sci.space,sci.bio

In article <[e-mail]> [e-mail] (Stanley Friesen) writes:

> This last is an extremely unlikely requirement.  The basic upshot of the
> Urey experiment and its follow-ups is that organics form spontaneously
> under most non-oxidizing 'medium-energy' conditions.  The presence of
> organics in interstellar dust clouds just confirms this basic ubiquity.

The Miller-Urey experiment is widely acknowledged to be irrelevant,
as it assumed an atmosphere that likely bears little resemblance
to the early earth.  Specifically, methane and ammonia are unlikely
to have been abundant in the early atmosphere, as they are destroyed
within centuries by UV light.

When performed with more reasonable gas mixtures, the experiments have
much lower yields.  And, in any case, all that these experiments
indicate is that some of the basic molecules of life are rather stable
(and, importantly, that others are not), not that a particular source
is where they first came from.


> Now it is true that under current conditions life might have a problem
> developing, but since these oxidizing conditions were *caused* by life,
> this is not a particularly significant issue.

The unstated assumption here is that life would *not* have had a hard
time originating back then.  We simply don't know enough about how
life started to say this.


>Now, it is true that we do *not* know the exact range of temperatures
>and pressures under which the ubiquitous organics are able to generate
>life forms, but this is a much smaller level of uncertainty than you suggest.

Au contraire!  Our ignorance about origin of life is profound, and
very little progress has been made in dispelling it.  There is no
really plausible model for the origin of life.  We're missing at least
one, and possibly several, fundamental ideas.

    Paul F. Dietz
    [e-mail]

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

Date: Thu, 29 Oct 1992 19:31:16 GMT
From: "Hoffman Eric J.SDO 5186 " <[e-mail]>
Subject: Launching Radioactive Waste to the Sun
Newsgroups: sci.space

   A while back there was a discussion of various ways to dispose of
high-level radioactive waste, e.g., launching it into the Sun.  Ignoring
for a moment the ecological, political, and moral issues, does anyone
recall the relative launch energy costs of--

    a) direct ascent from Earth to Sun

    b) into Sun via inner planet gravity assist

    c) into Sun via outer planet gravity assist

    d) direct ascent to Moon

    e) direct ascent to Venus

    f) into solar system escape trajectory?

   As I recall, (e) and (f) were favored, but what is the rank order?
Thanks.      --Eric Hoffman

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

Date: Fri, 30 Oct 1992 00:08:06 GMT
From: [e-mail]
Subject: mini-asteroids
Newsgroups: sci.space

  > From: [e-mail] (Paul Dietz)
  >
  > Science (10/16/92, page 403), reports that Gehrels and colleagues in
  > the Spacewatch program have detected 8 very near earth asteroids over
  > the last two years with sizes from 5 to 100 meters ... This figure is
  > some 100 times larger than had been inferred from observations of the
  > number of larger bodies.

That result is controversial, since it depends on statistics of small
numbers (though not as small as they were, and rapidly growing), plus
large corrections for selection effects.  However David Rabinowitz of
Gehrels' group defended it stoutly at the DPS meeting a couple of weeks
ago.  As to what they are compositionally, let's not guess, let's look
at them!  Unfortunately that takes fairly ready access to a large telescope,
which few people have.

Also, someone at the DPS suggested that the Tunguska event was caused by
the pressure-induced explosion of a a stony object, not a comet.  I believe
his main point was that an icy body would have exploded high in the
atmosphere, and the Tunguska explosion was low.  Stony meteorites often
do make strewn-fields, whereas irons come down intact.  He didn't say
where the Tunguska fragments might be, but I suppose they could be widely
scattered and hard to find in a remote, heavily forested location, and
some of them can weather away rather quickly.

Ben

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

Date: Thu, 29 Oct 1992 21:30:56 GMT
From: "jodi.weber" <[e-mail]>
Subject: Need help FAST! Looking for answer
Newsgroups: sci.space

Help! I need a fast answer to the following questions:

What are the Leonids?
What did the Leonids do in 1799 and 1833 that was spectacular?

Please e-mail me or post any response ASAP!  Thank you!!

Jodi Weber
[e-mail]   or   [e-mail]

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

Date: Thu, 29 Oct 1992 20:04:56 GMT
From: Robert Sheaffer <[e-mail]>
Subject: Re:Swift-Tuttle Comet a threat to earth?
Newsgroups: sci.astro,sci.space

I should think that, given that a comet consists largely of ice and
other volatiles, even one good-sized H-bomb placed directly into the
nucleus would pretty much eliminate any "threat to earth". Seems it
would be mostly vaporized and scattered.



--

        Robert Sheaffer - Scepticus Maximus - [e-mail]

 Past Chairman, The Bay Area Skeptics - for whom I speak only when authorized!

       "Beware when the great God lets loose a thinker on this planet.
        Then all things are at risk. It is as when a conflagration has
        broken out in a great city, and no man knows what is safe, or
        where it will end."
                               - Emerson: Essay, "Circles"

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

Date: 29 Oct 92 20:41:47 GMT
From: Robert Nychka <[e-mail]>
Subject: REPOST!  UN Space/Moon Treaty?!
Newsgroups: sci.space

Please reply!

I am looking for information concerning the "Moon Treaty" or some sort
of treaty drafted by the UN dealing with "...the Use of the Moon
and Other Celestial Bodies".

Specifically I'm looking for limitations on private exploration and
exploitation of space, and if there are - if they are enforcable, and by
whom?

Thanx in advance.
ROb Nychka.

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

Date: 29 Oct 92 18:16:02 GMT
From: Henry Spencer <[e-mail]>
Subject: Scenario of comet hitting Earth
Newsgroups: sci.astro,sci.space

In article <[e-mail]> [e-mail] writes:
>Just because a nuclear explosion could 'destroy' a comet, does not
>mean that the material making up the comet would thereby _disappear_
>(conservation of matter), nor would the rubble necessarily not
>continue on the same trajectory as it did before being nuked...

What one would hope for in such an attack would be to disperse the cloud
of rubble widely, so that Earth would be struck by relatively small amounts
fairly widely spread, minimizing damage.  However, it makes much more sense
to try to divert it rather than destroying it.  It's hard to guarantee
disrupting it to the point of leaving no large pieces.  Diverting it, by
exploding bombs to one side of it, should be easier.  If you can do it a
good distance out, you don't need to alter its velocity very much.

>(This is one of the SDI falacies of 'shooting down' ballistic missles.
>They be coming down no matter what...

Bad analogy, actually.  A nuclear warhead is precision machinery; even
modest damage to it will probably prevent it from exploding.  And it's
wrapped in a heatshield that gets a severe workout during descent, so
putting even a pinhole in that heatshield will destroy the warhead.
Damage from debris, while not zero, would be trivial by comparison.
Debris is only an issue if someone is stupid enough to use ballistic
missiles with conventional-explosive warheads -- a singularly costly
and ineffective weapon, as witness the V-2 campaign in WW2.

However, while the reasoning is fallacious, the conclusion is correct:
the debris from a destroyed comet/asteroid could be nearly as dangerous
as the original object.  This is why diversion wins over destruction,
and also why you'd prefer to start early, so you have a chance to deal
with mistakes and accidents.

>... just so it misses.  And a small, steady thrust over
>a long period of time could be just as effective, if not as spectacular,
>as a last-minute explosion.

Please compute fuel requirements for that "small, steady thrust".  Remember,
a comet head or an asteroid is *heavy*... and in the case of comets, gas
and dust boiling off the surface *already* create small, semi-steady
thrusts in various random directions.

Bombs are actually a relatively good way to move asteroids.  Comets
are a difficult case, because they are probably fragile and because they
probably have a thin crust with concentrated volatiles underneath.  A
small nearby nuclear explosion would blow off the crust on one side, and
the result would be a tremendous spill of gas from the comet itself.
If you knew what you were doing, and did it carefully, this could permit
steering a comet with far less effort than the brute-force approach.
--
MS-DOS is the OS/360 of the 1980s.      | Henry Spencer @ U of Toronto Zoology
              -Hal W. Hardenbergh (1985)|  [e-mail]  utzoo!henry

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

Date: Thu, 29 Oct 1992 21:17:56 GMT
From: Jim Larson <[e-mail]>
Subject: Scenario of comet hitting Earth
Newsgroups: sci.astro,sci.space

In article <[e-mail]> [e-mail] (Henry Spencer) writes:

>In article <[e-mail]> [e-mail] writes:
>>... just so it misses.  And a small, steady thrust over
>>a long period of time could be just as effective, if not as spectacular,
>>as a last-minute explosion.
>
>Please compute fuel requirements for that "small, steady thrust".  Remember,
>a comet head or an asteroid is *heavy*... and in the case of comets, gas
>and dust boiling off the surface *already* create small, semi-steady
>thrusts in various random directions.
>
>Bombs are actually a relatively good way to move asteroids.  Comets
>are a difficult case, because they are probably fragile and because they
>probably have a thin crust with concentrated volatiles underneath.  A
>small nearby nuclear explosion would blow off the crust on one side, and
>the result would be a tremendous spill of gas from the comet itself.
>If you knew what you were doing, and did it carefully, this could permit
>steering a comet with far less effort than the brute-force approach.

Has anyone studied the possibility of using reflective materials to
divert comets?  If you wrap half of a comet nucleus in a reflective
covering (solar sail material?) you could control the natural trust
from evaporating ice.  An auxilairy reflector could concentrate more
sunlight on the exposed surface.

--
Jim Larson                      Navigation Software Development
[e-mail]           Jet Propulsion Laboratory

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

Date: 29 Oct 92 22:26:10 GMT
From: Steve Linton <[e-mail]>
Subject: Scenario of comet hitting Earth
Newsgroups: alt.sci.planetary,sci.astro,sci.space,talk.origins

|>
|> Upon reaching the atmosphere, the comet would begin heating up as do
|> meteoroids and space craft. Like the Tunguska object of 1908, the
|> outer layers of the comet would begin heating up and a large trail
|> of dust would begin forming behind the nucleus and hanging in the
|> sky. If at night, this dust trail would exceed the Milky Way in
|> brightness, while in daylight the trail would appear grayish against
|> the blue sky.
|>
|> Somewhere around 20-40 miles above the surface of Earth, the comet
|> nucleus would explode. At this point, the Tunguska object, which has
|> been theorized as perhaps 100 feet across, may have been completely
|> vaporized and a shock wave circled the Earth twice. On the ground,
|> a forest was hit by the downward moving shockwave and was leveled for
|> a distance of 50 miles in every direction. In other words, a 100-mile
|> circle of trees was laid flat. The evidence leading to the belief
|> that the Tunguska object was vaporized was the lack of any apparent
|> craters. The shock wave from P/Swift-Tuttle should be tremendously
|> larger, though I have no means of computing exactly how large.

Surely this is wrong. The square-cube law prevents you just scaling up the
atmospheric interaction like this.  The comet hits atmosphere at say 150 km
altitude at 50kms^-1. We have no way of predicting the angle of impact, but it's
unlikely to be very flat. Thus the comet will almost certainly be in the
atmosphere for less than ten seconds. In this time th outer layers will
undoubtedly heat up, and volatiles near the surface will boil and explode, but
the bulk of the 10km diameter nucleus would almost certainly survive intact, not
having had time to notice the atmosphere much before it hits the ground.

On diverting it:

In their book, posisbly called `comet' or maybe not, Greg Benford and David Brin
discuss two methods of steering a comet:

1) gentle thrust at aphelion, where a delta-v of a few dozen ms^-1 can make a huge
difference. This would require us to get out to rendezvous at aphelion in 2059 or
so.

2) controlling outgassing at perihelion where huge forces are at work. Does
anyone know whether P/S-T meets Earth before or after perihelion?

Someone has already suggested a more dramatic variant of 2, using nuclear
explosions to create additional outgassing to order. This strikes me as the best
idea so far. Of course the earlier the better.

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

Date: 29 Oct 92 21:05:34 GMT
From: FRANK NEY <tnc!m0102>
Subject: Smith-Tuttle Comet a threat to earth?
Newsgroups: sci.space

What I'd like to know is where folks are getting all these neat
orbital prediction sofware.  I have Gravitator-Pro for my DOS machine,
but it doesn't appear to use the orbital elements the astronomer types
use.

Rather than attempt to translate (and get it wrong, most likely), I'd
like to get real software.

Suggestions?

Frank Ney N4ZHG EMT-A  LPVa NRA ILA GOA CCRTKBA "M-O-U-S-E"
Commandant and Acting President, Northern Virginia Free Militia
Send e-mail for an application and more information
----------------------------------------------------------------
Smith-Tuttle in 2116: "I die, you die, everyone dies." (Hvy Mtl rules!)
--
The Next Challenge - Public Access Unix in Northern Va. - Washington D.C.
703-803-0391 To log in for trial and account info.

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

Date: Fri, 30 Oct 1992 09:39:08 GMT
From: Bill Higgins-- Beam Jockey <[e-mail]>
Subject: Surveyor landings (was Re: QUESTIONS: Apollo, Earth, Moon)
Newsgroups: sci.astro,sci.space

In article <[e-mail]>, [e-mail] (Henry Spencer) writes:
> In article <[e-mail]> [e-mail] (Wan Ngai Wayne Lee) writes:
>> [lengthy list of detailed questions deleted]
>  [tremendously impressive "Answer Man" performance-- but no less
>    than we've come to expect from Henry-- deleted, except for
>    discussion of Apollo lunar module:]
> Without the descent stage, landing was not possible.  The descent stage
> carried the landing gear.  It also had the throttlable engine needed for
> a soft landing.  (Making a rocket engine throttlable is not simple and
> is done only when necessary; the ascent-stage engine was not throttlable.)
> Even if you could land with the ascent stage, you wouldn't have the fuel
> to return thereafter.

I've played Lunar Lander (being a relic of the compter dark ages
beforre Flight Simulator and Sim City), so I can appreciate the
usefulness of a throttlable engine. So how did Surveyor manage to
land?  I recall a big solid-fuel motor and small liquid-fuel "vernier"
engines.  Was there a computer playing Lunar Lander on board?  Was
there a radar altimeter?  Or did they just get the probe to
*approximately* zero velocity with the big motor, and build it
extra-sturdy to survive a drop?  How does this compare with the scheme
Luna 9 used to land, which I understand was rougher?

(I recall that one of the Surveyors was moved after landing with the
verniers.)

Bill Higgins, Beam Jockey              |
Fermi National Accelerator Laboratory  | "Get the dinosaurs in, Martha,
Bitnet:           [e-mail]  |  they're predicting comets."
Internet:       [e-mail]  |      --Dr. Barry D. Gehm
SPAN/Hepnet:           43011::HIGGINS  |

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

Date: Fri, 30 Oct 1992 05:48:47 GMT
From: TS Kelso <[e-mail]>
Subject: Two-Line Orbital Element Set:  Space Shuttle
Newsgroups: sci.space

The most current orbital elements from the NORAD two-line element sets are
carried on the Celestial BBS, [phone], and are updated daily (when
possible).  Documentation and tracking software are also available on this
system.  As a service to the satellite user community, the most current
elements for the current shuttle mission are provided below.  The Celestial
BBS may be accessed 24 hours/day at 300, 1200, 2400, 4800, or 9600 bps using
8 data bits, 1 stop bit, no parity.

Element sets (also updated daily), shuttle elements, and some documentation
and software are also available via anonymous ftp from archive.afit.af.mil
(129.92.1.66) in the directory pub/space.

STS 52
1 22194U 92 70  A 92303.13168981  .00103889  00000-0  25599-3 0   207
2 22194  28.4677  72.4829 0006704 340.3576 262.5102 15.94459338  1023
--
Dr TS Kelso                           Assistant Professor of Space Operations
[e-mail]                    Air Force Institute of Technology

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

Date: 29 Oct 92 23:34:20 GMT
From: "Marc A. Boschma" <[e-mail]>
Subject: UN Space/Moon Treaty?
Newsgroups: sci.space

Robert,
    Your mail address [e-mail] bounces, do you have another ?
(X.400 perhaps, though internet prefered).

--------------------------------------------------------------------------------
Founding member of APANA and :
Marc A. Boschma                                ACSnet: [e-mail]
img Consultants,                       [e-mail]
GPO Box 3304GG, Melbourne 3001            [e-mail]
Australia                                       Phone: [phone]    (work)
--------------------------------------------------------------------------------
        Reflections are images of tarnished aspirations
                    Racter (raconteur).

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

Date: 30 Oct 92 04:48:00 GMT
From: BRIAN BURTT <[e-mail]>
Subject: What does the "P/" mean in P/Swift-Tuttle?
Newsgroups: sci.astro,sci.space

In article <[e-mail]>, [e-mail] (John Black) writes...
>I've been wondering why the names of comets are prefixed by 'P/'. What does the
>'P/' signify?
>
>            John Black.
Periodic.  (i. e. a comet for which a definate period has been established.)
--Brian Burtt

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

End of Space Digest Volume 15 : Issue 358
------------------------------