Date: Tue, 27 Oct 92 05:02:44 From: Space Digest maintainer Reply-To: Space-request@isu.isunet.edu 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? Welcome to the Space Digest!! Please send your messages to "space@isu.isunet.edu", and (un)subscription requests of the form "Subscribe Space " to one of these addresses: listserv@uga (BITNET), rice::boyle (SPAN/NSInet), utadnx::utspan::rice::boyle (THENET), or space-REQUEST@isu.isunet.edu (Internet). ---------------------------------------------------------------------- From: Nick Haines Subject: active planetary probes; should someone update the FAQ Newsgroups: sci.space Originator: nickh@VOILA.VENARI.CS.CMU.EDU Sender: Usenet News System Nntp-Posting-Host: voila.venari.cs.cmu.edu Organization: School of Computer Science, Carnegie Mellon University Distribution: sci Date: Mon, 26 Oct 1992 19:35:50 GMT Lines: 50 Source-Info: Sender is really isu@VACATION.VENARI.CS.CMU.EDU 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 nickh@cmu.edu ------------------------------ Date: 26 Oct 92 18:47:22 GMT From: John Black Subject: Comet Collision Newsgroups: sci.space,alt.sci.planetary In article <1992Oct26.150328.22285@aio.jsc.nasa.gov> kjenks@gothamcity.jsc.nasa.gov 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" Subject: Comet Collision Newsgroups: sci.space In <1992Oct26.155841.2096@mksol.dseg.ti.com> pyron@skndiv.dseg.ti.com (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 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 ** ** ** ****** ******** ** csh019@cch.cov.ac.uk ------------------------------ Date: 26 Oct 92 19:03:05 GMT From: "Edward V. Wright" Subject: DCX Status? Newsgroups: sci.space In <1992Oct23.225027.10865@constellation.ecn.uoknor.edu> vcbowles@prowler.ecn.uoknor.edu (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 Subject: Dyson's Spheres Newsgroups: sci.space emk9267@rigel.tamu.edu (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 :-)~~~~(-: jgreen@eros.calpoly.edu :-)~~~\ | 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 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... ) 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" Subject: GEORGE BUSH: RELEASE THE APRIL GLASPIE CABLES Newsgroups: sci.space tombaker@world.std.com (Tom A Baker) writes: >In article > jbh55289@uxa.cso.uiuc.edu (Josh 'K' Hopkins) writes: >>tombaker@world.std.com (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" 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 youngs@ccu.umanitoba.ca ------------------------------ Date: 26 Oct 92 20:05:32 GMT From: Russell Olsen Subject: Internal atmosphere (was Re: Ground facilities for DC-*) Newsgroups: sci.space In article , lindsay+@cs.cmu.edu (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: David.Anderman@ofa123.fidonet.org 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: zellner@stsci.edu Subject: melting Vesta Newsgroups: sci.space > From: cain@geomag.gly.fsu.edu (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" Subject: nasa shake up rumor? Newsgroups: sci.space In article <1992Oct24.1604.15298@execnet> "tom betz" 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 | | aws@iti.org | nothing undone" | +----------------------180 DAYS TO FIRST FLIGHT OF DCX----------------------+ ------------------------------ Date: 26 Oct 92 12:17:34 From: David.Anderman@ofa123.fidonet.org 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 Subject: re HRMS for ETI Newsgroups: sci.space In article henry@zoo.toronto.edu (Henry Spencer) writes: In article steinly@topaz.ucsc.edu (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 | | steinly@lick.ucsc.edu |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 Subject: Smith-Tuttle Comet a threat to earth? Newsgroups: sci.space In article <1992Oct26.184231.1@cc.helsinki.fi> tavaila@cc.helsinki.fi writes: In article <1754@tnc.UUCP>, m0102@tnc.UUCP (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 * * steinly@lick.ucsc.edu "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: pbrown@uwovax.uwo.ca 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 Peter@canlon.physics.uwo.ca ***************************************************************************** ------------------------------ End of Space Digest Volume 15 : Issue 347 ------------------------------