Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V15_3/V15NO347.TXT

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Date: Tue, 27 Oct 92 05:02:44
From: Space Digest maintainer <[e-mail]>
Reply-To: [e-mail]
Subject: Space Digest V15 #347
To: Space Digest Readers
Precedence: bulk


Space Digest                Tue, 27 Oct 92       Volume 15 : Issue 347

Today's Topics:
        active planetary probes; should someone update the FAQ
                       Comet Collision (3 msgs)
                             DCX Status?
                           Dyson's Spheres
                            EM propulsion
            GEORGE BUSH: RELEASE THE APRIL GLASPIE CABLES
                          int'l halley watch
       Internal atmosphere (was Re: Ground facilities for DC-*)
                                LRDPA
                            melting Vesta
                         nasa shake up rumor?
                             Oct 31 event
                           re HRMS for ETI
                Smith-Tuttle Comet a threat to earth?
                Swift-Tuttle Comet a threat to earth?

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From: Nick Haines <[e-mail]>
Subject: active planetary probes; should someone update the FAQ
Newsgroups: sci.space

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Mea culpa, I should have gone through the FAQ before asking that. I
have now done so. It mentions the following active probes (my
summary):

Pioneer 10: flyby of Jupiter in 73, heading into deep space.
Pioneer 11: flyby of Jupiter in 74, of Saturn in 79, heading into deep space.

Voyager 1: launch 05.09.77, flyby Jupiter 05.03.79, Saturn 13.11.80,
    heading into deep space.
Voyager 2: launch 20.08.77, flyby Jupiter 07.08.79, Saturn 26.08.81,
    Uranus 24.01.86, Neptune 08.08.89, heading into deep space.

Sakigake (MS-T5): launch 08.01.85, Halley flyby 11.03.86, Earth
    swingby 08.01.92, later swingbys expected 14.06.93, 28.10.94

Galileo: launch 18.10.89, Venus flyby 09.02.90, Earth flyby 08.12.90,
    Gaspra flyby 29.10.91, Earth flyby 08.12.92, Ida flyby
    28.08.93, July 95 arrival at Jupiter, separation, tour of
    moons until October 97. 10.05.97 exploration of Jupiter's
    magnetotail. Problem w/ HGA.

Magellan: Venus radar mapping. Has mapped almost all of the surface.
    Difficulty w/ funding.

Mars Observer: launch September 92, arrival August 93, operations
    start late 93 for one Martian year.

Notice that it doesn't mention Ulysses at all (which I understand is
the `Solar Polar' mission), or Giotto, which I believe is still
active. I think that those two complete a roster of 10 probes in solar
orbits (or escape trajectories) from which we still receive signals.
Can anyone name any others? Can anyone tell me whether the Pioneers
are still alive (or give me launch dates)? Can anyone correct any of
these dates?

The FAQ also mentions the following plans for possible future
launches:

Mercury Observer : possibly 1997 launch
Mars Rover Sample Return: possible launches 1996 (orbiter), 2001 (rover)
Fire and Ice: possible launch 2001, gravity assist Jupiter, to sun &
    Pluto (reach Pluto 2016)
Cassini: launch October 97, gravity assists from Venus (21.04.98,
    20.06.99), Earth (16.08.99), Jupiter (30.12.00), arrive Saturn
    25.06.04, atmosphere probe of Titan, tour of system (35 flybys
    of Titan etc).

Are all/any of these still go? Pluto Direct (?) isn't in the FAQ.

Nick Haines [e-mail]

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

Date: 26 Oct 92 18:47:22 GMT
From: John Black <[e-mail]>
Subject: Comet Collision
Newsgroups: sci.space,alt.sci.planetary

In article <[e-mail]> [e-mail] writes:


>
>The date given in the Post is 2116, while the date given in the "SKY
>TV" notice and the date my office mate gave me were both 2016.  No
>NASA people involved in the discovery.


In "New Scientist", a reputable science weekly in the UK, the collision date is
down as 14th August 2126. The period is about 130 years according to the article. It says the chances of a collision are 1 in 400.

                John Black.

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

Date: 26 Oct 92 19:08:41 GMT
From: "Richard A. Schumacher" <[e-mail]>
Subject: Comet Collision
Newsgroups: sci.space

In <[e-mail]> [e-mail] (Dillon Pyron) writes:
>They guy they interviewed was rather strange.  He kept talking about the
>tremendous threat to Earth, then says the chances of an actual collision are
>about 1:10000.

What's strange about describing this as a tremendous threat? If you knew that
on a particular day next month there was a 1 in 10,000 chance that you, your
family, all your friends and everybody you ever heard of would be boiled in
oil, wouldn't that seem to be a tremendous threat?

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

Date: 26 Oct 92 18:31:06 GMT
From: Faust <[e-mail]>
Subject: Comet Collision
Newsgroups: sci.space,alt.sci.planetary

  Here are the two reports I managed to uncover concerning the Comet
  Collision:

  "COLLISION SCENARIO SAYS WORLD TO END IN 134 YEARS"

                                 --  Tim Radford, Guardian Science Editor

  "A huge comet, due to pass close to the Sun in December, could
  on  its next trip - 134 years from now - collide with the Earth
  with catastrophic consequences, an Australian astronomer warned
  yesterday."

  "The comet, called Swift-Tuttle, was first discovered in 1862, and
  regularly crosses the Earth's orbit.  But nobody yet knows how to time
  its arrival precisely."

  "According to Dr Duncan Steel, of the Anglo-Australian Observatory,
  speaking to a space conference in Sydney, it could possibly collide
  with the Earth on August 14, 2126."

  "'It would create an impact force of 20 million megatons - or about
  1.6 million times the force of the bomb dropped on Hiroshima,' he
  said."

  "Swift-Tuttle is in the same league as the more famous Halley's
  Comet:  it could be six or ten kilometres across and as it nears the
  Sun it could be travelling at 60-70 kilometres per second.  The
  consequences of a collision with the Earth would be utterly
  devastating."

  "On the last such collision [! -ed.] - about 65 million years ago -
  much of the life on Earth, including the dinosaurs, may have perished
  in a thunder of tidal waves, acid rain and a nuclear winter.  Last
  week, Dr Brian Marsden of the Harvard Smithsonian Centre for Astro-
  physics calculated that the comet would make its next swing by the
  Sun on July 11, 2126."

  "But nobody is sure about its orbit - and if the arrival was 15 days
  later, then a collision would be possible."

  "The Earth already sails through debris left behind by the comet each
  year:  the Perseid meteor shower - a predictable celestial fireworks
  display each August - is caused by orbiting dust left by Swift-Tuttle
  on previous visits."

  "Although there is no doubt about the Earth shattering consequences of
  a cosmic splat, the odds are against it.  A comet at 70kps would take
  only three minutes to cross the Earth's orbit.  If it did so on August
  14, 2126, odds against impact would still be 400 to one."

                                     -- The Guardian, 26th October 1992

     I like the way that Tim Radford describes the dinosaur killing
     meteor strike as if it were proven fact.  I always thought that
     it was just another hypothesis.


  "COMET CLASH"

  "There is a chance that comet Swift-Tuttle could strike the Earth in
  130 years' time, but no one can be sure because astronomers cannot
  calculate an accurate orbit for all their observations of it."

  "The recently rediscovered Swift-Tuttle, which is known to intercept
  the Earth's orbit, will pass close to the Sun this December. The best
  estimate by Brian Marsden [A doctor no longer!], of the Harvard
  Smithsonian Center for Astrophysics, is that it will again pass close
  to the Sun on 11 July 2126.  But uncertainty about the orbit is large
  enough to put this encounter up to 15 days later.  In that case it
  could hit the Earth on 14 August 2126."

  "The size of the comet is unknown, but Marsden says it is probably in
  the 10-kilometre class of Halley's Comet, which could cause a
  disasterous impact.  Fortunately the chance of an impact is quite small.
  The comet only takes about 3.5 minutes to cross the path of the Earth,
  so even if it does cross the Earth's orbit on 14 August 2126 the odds
  against an impact are still about 400 to 1."

  "Marsden has called on astronomers to follow Swift-Tuttle as long as
  possible after it passes the Sun in December, to gather data that will
  allow a more precise calculation of its orbit."

                  -- The New Scientist, 24th October 1992


  Well, they are certainly an interesting contrast in journalistic style.
  The Guardian writer goes for the more sensationalistic angle on the
  story, while New Scientist presents a  sober and measured report on
  the comet collision possibility.

  As ever, reality is what you get away with...

    ********    ***      **    **   *******   ********  "Quantum Mechanics:
    **         ** **     **    **  **            **      even I don't fully
    ******    *******    **    **  *******       **      understand it."
    **       **     **   **    **        **      **              - Ian Sales
    **      **       **   ******   ********      **    [e-mail]

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

Date: 26 Oct 92 19:03:05 GMT
From: "Edward V. Wright" <[e-mail]>
Subject: DCX Status?
Newsgroups: sci.space

In <[e-mail]> [e-mail] (Saccio Vanzetti) writes:

>Fuel gauge? You know how much you started with... You know how much you
>have burned... There's gotta be some equation that will let you calculate
>the amount you have left...   :-)

Leakage?

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

Date: Mon, 26 Oct 1992 20:26:58 GMT
From: James Thomas Green <[e-mail]>
Subject: Dyson's Spheres
Newsgroups: sci.space

[e-mail] (KLINE, ERIC MICHAEL) speakith unto us:
>
>Stuff Deleted...
>
>
>>The big difference is that all the energy which normally continues on to
>>such places as Mars, Jupiter, Saturn, etc. and also to all the other stars
>>in the entire universe (eventually) would now be trapped within a sphere.
>>Think what would happen if you took all the heat required to keep a large
>>office building comfortable during the winter, and pumped it all into one
>>room.  Now multiply that by a few million times, and you're starting to
>>get a vague idea of the problem.
>
>You are ignoring the obvious solution which is to radiate excess energy
>back into space from the outer surface of the Dyson sphere.  The Dyson
>sphere ENABLES you to utilize the full energy output of a star.  It does
>not REQUIRE that you use all that energy.
>
>  Eric
>
Why not make the radius of the DS larger than 1 AU to avoid heating
up the inside too much?

/~~~(-: James T. Green :-)~~~~(-: [e-mail] :-)~~~\
| If the official Republican platform is carried out,           |
| a 13 year old girl who becomes pregnant as a result of being  |
| raped by her father, and who has an abortion, could end up    |
| in the gas chamber.                                           |

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

Date: 26 Oct 92 20:36:53 GMT
From: Charles Chung <[e-mail]>
Subject: EM propulsion
Newsgroups: sci.space

I'm grad physics student writing a term paper on the electrodynamics
of electromagnetic propulsion.  (I know, I know... term paper?  In
physics grad school?  Don't ask... <shrug>)

Anyone know of any books or papers on the basics of these devices?
Peak theoretical efficiences?  designs?  etc. etc.

Any info is appreciated.

Thanks,

-Chuck

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

Date: 26 Oct 92 13:04:54 EST
From: "John F. Woods" <[e-mail]>
Subject: GEORGE BUSH: RELEASE THE APRIL GLASPIE CABLES
Newsgroups: sci.space

[e-mail] (Tom A Baker) writes:
>In article <[e-mail]>
>   [e-mail] (Josh 'K' Hopkins) writes:
>>[e-mail] (Tom A Baker) writes:
>>[Responding to clinton position papers that don't belong here]
>>>You are in violation of network etiquette, and will give Clinton
>>>supporters a bad name.  I smell a "dirty tricks" squad?
>>In such cases, a polite note is far more effective than a post.
>No, sir.  An inappropriate post that effectively states that "Clinton
>supporters are jackasses" requires an immediate post negating that image.

However, a response that gives the impression that "Clinton supporters have
clinical paranoia" might also not be too helpful.

I too sent mail asking if they'd had a mailing-list malfunction (having
noticed that the entry-point into USENET was at ai.mit.edu), and received
a polite reply.  USENET readers should in general, remember that many
unfortunate USENET occurances are the result of honest error, and do not
call for public excoriation.  Of course, I'm posting this publically since
my fingers would get tired mailing this reminder to every reader of sci.space.
:-)

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

Date: 26 Oct 92 18:44:47 GMT
From: "Scott D. Young" <[e-mail]>
Subject: int'l halley watch
Newsgroups: sci.space,sci.astro

Perhaps the International Halley Watch could be revived for Swift-Tuttle? The
observations of this comet could prove quite valuable for meteor science,
even without the possiblilty of an impact in 2126. (Our local paper quoted
Marsden as saying it was a 1-in-400 chance. Start digging a hole...)

Scott Young
[e-mail]

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

Date: 26 Oct 92 20:05:32 GMT
From: Russell Olsen <holmes!russ>
Subject: Internal atmosphere (was Re: Ground facilities for DC-*)
Newsgroups: sci.space

In article <[e-mail]>, [e-mail] (Donald Lindsay) writes:
|>
|>
|> Haven't there been Russian satellites that maintained an internal
|> atmosphere, at some weight penalty, so that the onboard equipment
|> would be more reliable/ require less design effort ?
|> --
|> Don        D.C.Lindsay     Carnegie Mellon Computer Science

I seem to remember that something similar was done in the US lunar orbiter
probes of the 1960's. The LO's took actual photographic pictures and then
radioed them back via some electronic scanning process. Anyway, to avoid
having to develop (no pun intended) new photographic processes, the
developing system was in a pressurized can.
--
=================================================
Opinions Expressed Are Mine, all Mine!

Russ Olsen Xerox/EDMS        607 Herndon Pkwy
uunet!xrxedds!russ        Suite 301 Herndon
                VA 22070

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

Date: 26 Oct 92  12:16:24
From: [e-mail]
Subject: LRDPA
Newsgroups: sci.space

           Lunar Resources Data Purchase Act Proposed

        With most funding for the Space Exploration Initiative
eliminated in each of the last three Federal budgets, and the
U.S. aerospace industry in need of new civil space programs to
compensate for the decline of defense outlays, it is clear that
something new must happen to push the United States to return to
the Moon. The Lunar Resources Data Purchase Act is a new
approach to jump-starting a U.S. return to the Moon.
       The widely acknowledged first step in the process of returning
to explore the Moon is a lunar polar probe that would generate a
complete geochemical map of the lunar surface and possibly
determine whether ice exists in deep craters at the poles of the
Moon.
       In the last several years, there have been several approaches
to mounting a new lunar mission: the Bush administration has
proposed a series of relatively inexpensive lunar probes
(approximate cost of $125 million per probe), and private
non-profit organizations have attempted to raise funds for their
probes via donations. There has been no success on the part of
these public and private efforts, and little prospect for future
success.
       The Lunar Resources Data Purchase Act would authorize NASA to
purchase a geochemical map of the Moon from the lowest qualified
U.S. bidder, with a cap on the bids of $24 million. There would
be few restrictions on how this data would be obtained by the
bidder, or whether a winning bidder is a for-profit or
non-profit corporation. The winner must deliver the first data
to NASA's Office of Exploration on or before December 31, 1996.
       The plan is to have this bill introduced during the early
months of the next Congress. In order to accomplish this, we
need assistance from the space activist community with the
following:
        1) The rules and specifications of the bid: should the cap on
bids be $24 million? Is this figure both low enough to re-assure
Congress, and high enough to attract qualified bidders? What
resolution should be specified in the bill?  Is December 31,
1996 a realistic cutoff date for a qualified bidder to return
data to NASA? What frequency spectra should be included in the
bid? Is NASA's discretionary fund large enough to allow Congress
to authorize the Administrator to use *no* additional taxpayer
money to fund this project?
        2) Writing the Act. This is pretty well taken care of (once the
above questions are answered), but the more help, the better.
        3) Getting congresspersons to co-sponsor the bill. This bill
will havea negligible impact on the deficit; in fact, it should
*save* taxpayer money based on earlier government proposals. We
believe that congresspersons will be happy to co-sponsor the
Act, but they won't know about the bill unless their
constituents bring it to their attention.
        Although it is expected that private companies will bid for the
contract, it is also possible that non-profit corporations such
as AMSAT, Space Studies Institute, the Experimental Satellite
Society, or universities may participate.
        The most exciting aspect of this effort is that it *does not*
require the majority of American citizens to get behind the
space program; it *does not* require a mass movement of space
activists to galvanize NASA; *the people reading this message*
are all that are needed to get America back to the Moon.
        Once this effort succeeds, future efforts are contemplated: the
Lunar Oxygen Purchase Act, the Mercury Resources Data Purchase Act,
etc. All will entail a relatively tiny Federal expenditure to
provide key knowledge about the Solar System that could provoke
the politicians to actually create a space program that gets
beyond low earth orbit.
        The last question: can space activists actually get a bill
passed through Congress? The best evidence is the Launch
Services Purchase Act, written by space activists, introduced in
Congress through the efforts of space activists, and passed by
Congress and signed by the President, largely through the
efforts of space activists. This far-reaching bill has had a
dramatic impact on the nation's space program (it prohibits NASA
from launching satellites into space, with few exceptions;
Mars Observer was launched to Mars under the auspices of the
LSPA). We have done it before, and can do it again.
        For more information on this project, please call David
Anderman at 714/524-1674.

--- Maximus 2.00

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

Date: Mon, 26 Oct 1992 20:38:56 GMT
From: [e-mail]
Subject: melting Vesta
Newsgroups: sci.space

  >  From: [e-mail] (Joe Cain)
  >  Subject: electromagnetic heating
  >
  >  ... a diagram after Sonett et al giving heating of various sized
  >  bodies vs distance from the Sun postulated by induction during the
  >  T-Tauri phase of our early Sun.  It seems to explain the lava on 4
  >  Vesta which otherwise would not have enough mass/surface to generate
  >  its own ... Is this electromagnetic heating generally accepted for
  >  4 Vesta, and was it supposed to have been a major factor in liquifying
  >  the early Moon (top layers only?)

I believe the Moon is large enough to melt without exotic mechanisms.  But
since the inception of physical studies of asteroids nearly 20 years ago it
has been recognized as a major puzzle that Vesta at least partially melted
to make basalts on its surface, while Ceres, much larger and only a little
further from the sun, apparently did not.  Similarly we seem to have lots
of differentiated materials (E, M, and probably S-type asteroids)in the inner
parts of the main belt, but little to none in the outer parts.

Sonett's mechanism is about the only theory available that can melt small
objects out to a sharply limited heliocentric distance.  If people are a
little nervous about it, I think that's only because it's a positive-feedback
mechanism in which a little melting is necessary to get more melting started,
like trying to light a big log with a small match.

Along those lines, Vesta is well-known to be unique among the larger
asteroids, but at the DPS meeting in Munich a couple of weeks ago Rick
Binzel of MIT reported spectroscopic studies of small asteroids (around
5 km diameter) in the newly recognized dynamical family associated with
Vesta.  They all look like Vesta, with the individual variations as
expected for various types of basaltic achondrites.  They are clearly
chips off its surface.

It has long been known that many asteroid families show compositional
similarities, but this is the best case yet of clear geochemical relation-
ships.  It also shows that it's possible for sub-catastrophic impacts
to eject surprisingly large chunks of intact material.  Maybe there
is a piece of EARTH out there somewhere, in orbit for the last 63 million
years?  Will Mexico claim it?       (Only joking!)

Cheers, Ben

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

Date: 26 Oct 92 19:31:42 GMT
From: "Allen W. Sherzer" <[e-mail]>
Subject: nasa shake up rumor?
Newsgroups: sci.space

In article <1992Oct24.1604.15298@execnet> "tom betz" <[e-mail]> writes:

>To feed the rumor mill still further, rumor has it that Richard Truly is
>preparing, once Al Gore takes over for Dan Quayle, to take a White House
>position directing NASA.

Which just goes to show that Gore has no interest in an effective space
program.

  Allen

--
+---------------------------------------------------------------------------+
| Allen W. Sherzer | "A great man is one who does nothing but leaves        |
|  [e-mail]     |  nothing undone"                                       |
+----------------------180 DAYS TO FIRST FLIGHT OF DCX----------------------+

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

Date: 26 Oct 92  12:17:34
From: [e-mail]
Subject: Oct 31 event
Newsgroups: sci.space

       -Campaign for Space- Rally for Congressman George Brown

On Saturday, October 31, at 10 am, space activists will stage
a rally in support of the re-election campaign of Congressman
George Brown (D-Colton). The rally will be held at 495 W. Valley Pkwy
in Colton in San Bernardino county.

Congressman George Brown is the current Chair of the House Committee
on Space and Science, and one of the nation's most powerful voices
in support of the space program. He is a life-long supporter of
space development, and was an early and important co-sponsor of
the Launch Services Purchase Act.

During the campaign, he has responded to a questionnaire submitted
by NSS members, with detailed answers on his views on the future of
the space program. His opponent in the race did not respond to the
questionnaire. The questionnaire and Congressman Brown's responses
are available upon request.

All space activists in southern California are asked to show their
support for space development by attending the October 31 rally, which
is free of charge, and open to the general public.

For more information, please contact David Anderman at 714/524-1674.


--- Maximus 2.00

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

Date: 26 Oct 92 12:35:02
From: Steinn Sigurdsson <[e-mail]>
Subject: re HRMS for ETI
Newsgroups: sci.space

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

   In article <[e-mail]> [e-mail] (Steinn Sigurdsson) writes:
   >   ... the Drake Equation (which says there ought to be lots
   >   of communicating civilizations out there)
   >
   >Well, not quite, there is a several order of magnitude uncertainty
   >in the Drake equation...

   Indeed so.  But there are so *many* stars in the galaxy that you have to
   stack the deck pretty thoroughly to make civilizations really rare.  The
   middle-of-the-road estimates say "probably plenty of them".

Yah, the Fermi paradox is undeniably the more interesting issue
right now, especially as opinion is converging on planetary systems
being common and plenty of planets per.
There is of course the question of evolution of intelligence,
but I think the dominant factor is lifetime of _communicating_
civilization - interestingly our single number statistic puts
the lifetime of order 100 yrs which implies one per galaxy at
any one time :-(
But then the inevitable problem is that with that many samples one
of them should make it to many mean lifetimes and take over
and we recover the paradox. Curioser and curioser.

|  Steinn Sigurdsson    |I saw two shooting stars last night        |
|  Lick Observatory    |I wished on them but they were only satellites    |
| [e-mail] |Is it wrong to wish on space hardware?        |
| "standard disclaimer"    |I wish, I wish, I wish you'd care - B.B. 1983    |

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

Date: 26 Oct 92 12:58:54
From: Steinn Sigurdsson <[e-mail]>
Subject: Smith-Tuttle Comet a threat to earth?
Newsgroups: sci.space

In article <[e-mail]> [e-mail] writes:

   In article <[e-mail]>, [e-mail] (FRANK NEY) writes:
   >
   > On the radio news this morning, I heard a report that an Australian
   > astronomer by the name of Duncan Steele predicted that the
   > Smith-Tuttle comet will strike the earth sometime in the 22nd century.
   >
   > The news report gave an exact date, but I couldn't write fast enough
   > to catch up.

   Somebody has indeed calculated, that this Comet should be very near to Earth
   sonetime in the future (I should think, that the date was set to next cenury,
   but please, don't take my word for it).  However comets are known to change
   their orbits due to reaction effects caused by ejection of gas and matter
   close to perihelion.  Therefore calculating cometary orbits is still
   far from the accuracy required to make such predictions.  What's more
   the uncertainties of the parameters of comets present orbit are so large
   that even the best of calculations could only tell, that comet will pass
   Earth at a distance, less than 1.5 million kilometers - about four times
   the radius of Moon's orbit.  We've had much closer calls already, so though
   one can't say, that this piece of news is entirely false it is certainly
   misleading.

The reference is IAUC 5636. The orbital solution is due to Marsden and
Nakano (working independently I believe)

comet Swift-Tuttle is as noted the parent of the Perseids meteor
shower, it is in a 130+ year high inclination orbit which intersects
Earth orbit. The comet is still approaching perihelion and orbit
determination is not complete, it will have to be tracked as far as
possible past perihelion to get a good orbit. Diameter estimates
are order 10 km.

The current best solution has it missing the Earth in July 2126,
but a 15 day delay places it on collision on August 14th.
As a matter of interest, the best orbit fit for its return once
it was realised which returning comet it was off by 17 days.

Speed relative to Earth is 50 km/s so a 100 sec error gives a miss,
with aphelion at 50 AU we'll lose it on its way out somewhere
between 3 and 10 AU out (depending on scopes allocated) and
not reacquire it until a year or two out on its return (barring
an amazing improvement in large space based telescopes...).
The further out it's tacked the better the orbit, but comets
have been known to have delayed outgassing at large solar distances
which can significantly perturb the orbit.

Now, original estimates for collision probability were 1:10,000,
I've heard recent numers as low as 1:1000, which are starting to be
very poor odds considering the potential impact, this time next year
the odds will be much better determined. However, Swift-Tuttle is
virtually certain to hit the Earth at some point during the next
few million years, it is self-selected for close Earth approaches and
unless a flyby scatters it into a solar escape orbit (which is quite
improbable, would need a very close approach on certain trajectories,
or multiple more distant approaches - Jupiter is not a strong
influence on Swift Tuttle if I have the right orbit). As it is a high
inclination orbit it is very difficult to remove it from Earth
crossing, it can be perturbed away from any one collision, but as is
we'd have no hope of moving the orbit far enough to avoid it random
walking across Earth orbit again. As is we can't even rendezvous with
the thing, would be nice to put a transceiver on it for future
reference ;-)

*  Steinn Sigurdsson               Lick Observatory          *
*  [e-mail]        "standard disclaimer"      *
*  The laws of gravity are very,very strict            *
*  And you're just bending them for your own benefit - B.B. 1988*

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

Date: Mon, 26 Oct 1992 21:00:11 GMT
From: [e-mail]
Subject: Swift-Tuttle Comet a threat to earth?
Newsgroups: sci.astro,sci.space

Having noted the commotion the P/Swift-Tuttle news has made I thought I
might add a few notes to the discussion.
First off, I am rather surprised it has taken so long for the media to
catch up with the story - the original IAUC announcing the possible
collision was out 10 days ago.
The comet in question is the parent of the Perseid meteors which produce
a good celestial fireworks show every August. P/Swift-Tuttle was last
recovered in 1862, it had an orbital period of 120 years at that time and
many people (including Marsden, the fellow who made the collision
prediction) thought it most likely the comet would return in 1981-1983.
Marsden, in a 1973 paper in the Astronomical Journal also noted that the
comet seen in 1737 by Kegler (a Jesuit missionary in China) was similar in
brightness and general position to what might be expected from P/Swift-Tuttle.
Marsden went on to indicate that there existed a slight chance that the comet
would return in 1992 and predicted a perihelion date for the comet which turned
out to be only about a week off the true valu if the 1737 comet was ST.
Now, to make the 1737-1862-1992 orbital solution work one requires VERY large
non-gravatational forces (due to reaction forces from the expanding cometary
gas); in fact the values obtained for the force in Marsden's 1973 paper are
close to an order of magnitude greater than for any other known comet! What
is more, to make the best fit solution the forces have to act in unusual
directions.
The current excitement results from the most recent set of solutions based
on the new 1992 astrometry of P/Swift-Tuttle; a slight change in the non-
gravitational forces that are assumed could result in a direct impact on
August 14, 2126, during the comets next return. Considering the huge
uncertainty in the outgassing forces and the rather bizarre orbital
behaviour of Swift-Tuttle it seems reasonable to assume that somewhere along
the line we are missing some big details of the comet (such as
non-gravitational forces out of the cometary plane). When all this is put together it should
become obvious that everyone batting around odds is simply guessing - the
truth is we won't know what will happen in 2126 until P/Swift-Tuttle
"shuts down" (that is stops sublimating gases) on its outward leg. Then its
future orbit can be determined with all the precision that Newtonian mechanics
has to offer without worry for these non-gravitational forces. I would liken
these impact announcements to someone spreading news in the media that they
have picked a 6 digit number and may win the lottery (ie. I think the comet
has 1 in a million chance of hitting the Earth ;) ).
BTW, Duncan Steel is involved with the Spaceguard project and it is thus not
surprising that he is promoting the impact possiblity perhaps as a means
to secure additional funding for planetary science types (GO Duncan!)

Peter Brown
[e-mail]
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End of Space Digest Volume 15 : Issue 347
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