Cometary Catastrophe? Not So Fast…

by Paul Gilster on July 30, 2009

Once again we’re asked to reconsider our views about the outer Solar System. In this case, the area in question is the Oort Cloud, which begins at roughly 1000 AU and continues, by some estimates, as far as three light years from the Sun. It’s a spherical cloud of comets, probably numbering in the billions of objects, most of which will never be observed because of their distance and faint signature.

Getting comets into the inner Solar System is necessary for closer observation, but it’s also risky for living beings. At least, that’s been the prevailing belief, given that comet collisions with Earth could theoretically produce extinction events. New research, however, has begun to challenge this view. Nathan Kaib (University of Washington) and doctoral adviser Thomas Quinn have developed computer models to study how comet clouds behave.

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The simulations trace the evolution of comets over a 1.2 billion year period and allow the team to estimate the highest number of comets possible in the inner Oort Cloud. The team assumed the inner Oort Cloud as the source of long-period comets, those whose orbits take from 200 to tens of millions of years to make their way around the Sun (short-period comets originate in the Kuiper Belt — Halley’s comet is the most obvious example). 3200 long-period comets are known, with Hale-Bopp being the most recent.

Image: Comet Hale-Bopp photographed above above Val Parola Pass in Italy’s Dolomite Mountains. Credit: A. Dimai, (Col Druscie Obs.), AAC.

The outer Oort Cloud is also a source for long-period comets, but by working with the assumption that the inner Oort was the only source, Kaib and Quinn could estimate the highest possible number of comets in that inner Oort region. The results cause them to doubt the case for comets as causes of planetary extinctions.

In fact, says Quinn, no more than two or three comets would have struck the Earth during the most powerful comet ‘shower’ of the last 500 million years. Three impacts occurred nearly simultaneously about forty million years ago, resulting in a minor extinction event that may have been the result of a comet shower. That event, notes Kaib, was the most intense in the fossil record:

“That tells you that the most powerful comet showers caused minor extinctions and other showers should have been less severe, so comet showers are probably not likely causes of mass extinction events.”

The results show the power of Jupiter and Saturn’s gravitational fields, which can cause comets to be ejected into interstellar space or drawn in for collisions with the gas giants themselves. We saw evidence for the latter just last week with an evident cometary impact on Jupiter. Those comets that do make it through the gravitational screening of the gas giants do not, by this thinking, stand as the culprits for major mass extinctions.

If Kalb and Quinn are right, we can relax a bit about comets, while asteroids remain a potential threat to Earth. The team’s work also tells us something about how cometary orbits are disrupted. Long-period comets from the inner Oort, according to the simulations, can emerge in the inner system without the nudge of a neighboring star as it passes close by the Solar System. The inner Oort thus becomes a livelier place than previously imagined.

The paper is Kaib and Quinn, “Reassessing the Source of Long-Period Comets,” Science Express 29 July 2009 (abstract).

Related: Be aware of JPL’s new Asteroid Watch site, which also tracks comets, offering the latest research on near-Earth objects and news about NEO discoveries and flybys. A Twitter feed is also available, along with RSS options and a downloadable widget.

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{ 14 comments }

Adam July 30, 2009 at 16:15

Phew! Here I was in a lather over the threat from the Oort Cloud…

Lorenzo Iorio has put a paper on the eprint server which is intriguing me…

http://arxiv.org/abs/0907.4514

The perihelion precession of Saturn, planet X/Nemesis and MOND
Authors: Lorenzo Iorio
(Submitted on 27 Jul 2009)

Abstract: We show that the retrograde perihelion precession of Saturn \Delta\dot\varpi, recently estimated by different teams of astronomers by processing ranging data from the Cassini spacecraft and amounting to some milliarcseconds per century, can be explained in terms of a localized, distant body X, not yet directly discovered. From the determination of its tidal parameter K = GM_X/r_X^3 as a function of its ecliptic longitude \lambda_X and latitude \beta_X, we calculate the distance at which X may exist for different values of its mass, ranging from the size of Mars to that of the Sun. The minimum distance would occur for X located perpendicularly to the ecliptic, while the maximum distance is for X lying in the ecliptic. We find for rock-ice planets of the size of Mars and the Earth that they would be at about 80-150 au, respectively, while a Jupiter-sized gaseous giant would be at approximately 1 kau. A typical brown dwarf would be located at about 4 kau, while an object with the mass of the Sun would be at approximately 10 kau, so that it could not be Nemesis for which a solar mass and a heliocentric distance of about 88 kau are predicted. If X was directed towards a specific direction, i.e. that of the Galactic Center, it would mimick the action of a recently proposed form of the External Field Effect (EFE) in the framework of the MOdified Newtonian Dynamics (MOND).

Ron S July 30, 2009 at 16:16

I have a question (is the paper available?). The Oort cloud is predicted (not yet observed) to be a sphere, therefore not constrained to the plane of the solar system. If so, the likelihood interacting with the outer planets, first, before making it to the inner planets is not a consideration. That is, if a comet from the cloud is headed toward Earth, it is most likely on an orbit without passing any other planet.

Administrator July 30, 2009 at 16:42

Ron, the paper is to run on Science Express but has not yet appeared there. I suspect it will be up by tomorrow, and I’ll link to it then.

Denver July 30, 2009 at 18:26

Hi,

Thanks for the website.

“In fact, says Quinn, no more than two or three comets would have struck the Earth during the most powerful comet ’shower’ of the last 500 million years. Three impacts occurred nearly simultaneously about forty million years ago, resulting in a minor extinction event that may have been the result of a comet shower. That event, notes Kaib, was the most intense in the fossil record:

“That tells you that the most powerful comet showers caused minor extinctions and other showers should have been less severe, so comet showers are probably not likely causes of mass extinction events.””

I’m not being critical. I’d like to point out the only “impact triplet” that I have seen conjectured is Chesapeake Bay, Popigai and Tom’s Canyon. These appear to have occurred within a geologic instant of each other around 35.4 mya.

I’ve always wondered how three medium (large?), impacts with diameters of 100km, 90km and 22km, failed to have any more environment impact than has been noted.

Mark July 30, 2009 at 22:41

Three light years radius? Brings up some questions:
1) Are they all in orbits about the sun?
1a) If so, how fast would the outermost bodies be travelling in their orbits?
2) Is there likely to be an equivalent of the Oort cloud around Alpha Centauri?
2a) If so and assuming it was of the same outer diameter as our Oort cloud, that would mean there would be a region of space between our system and Alpha Centauri a full two light years across in which the Oort clouds overlapped. Could we expect their Oort comets to collide with our Oort comets and produce an observable flash as they vaporize each other?
3) Would the Oort cloud pose a significant threat to outbound or inbound interstellar craft?

Doug M. July 31, 2009 at 3:01

Who says the Oort cloud goes out to 3 LY? That seems pretty extreme. ISTR estimates of around 0.5 LY as the top end.

Note that stars regularly pass within 3 LY of the Sun — Alpha Centauri and Barnard’s Star are both getting closer to us, and will be around 3 LY away within the next 50,000 years. Passes of as close as 1 LY occur about once every million years or so — IMS there’s one due around 400,000 AD, when some red dwarf or other will get within about 0.45 LY or about 30,000 AU.

Doug M.

Administrator July 31, 2009 at 8:15

Doug M., I don’t disagree, but was using the 3 light year figure because it’s the one Kaib and Quinn are working with.

Re Mark’s question:

If so and assuming it was of the same outer diameter as our Oort cloud, that would mean there would be a region of space between our system and Alpha Centauri a full two light years across in which the Oort clouds overlapped. Could we expect their Oort comets to collide with our Oort comets and produce an observable flash as they vaporize each other?

Yes, it’s not at all inconceivable that there’s an area where debris from the Centauri A/B system mixes with Oort Cloud material. We don’t know yet but this is obviously an area of great interest for future missions to study, as is the question of how much debris is out there. This goes to your point four, about whether or not there is enough to pose dangers to interstellar craft — I doubt this seriously given the spacing, but there again, we have to learn more and assume less about the Oort’s comets.

Doug M. July 31, 2009 at 11:18

There’s a 1994 paper — don’t have the cite at hand — that says it’s possible Alpha Centauri could perturb our Oort Cloud a bit at its closest approach in 50,000 AD or thereabouts. Presumably it would work the other way, too.

But overlapping Oort clouds would do nothing. The comets are spread out over an ENORMOUS volume of space. So, even though there may be billions of comets, it’s stil 99.99999999999% emptiness. It’s like, a truckload of sand contains billions of sand grains. But if you dissolve that truckload in the Pacific Ocean? no grain of sand is ever going to meet another again.

So, two colliding Oort Clouds would just pass through each other; the odds of even one collision would be very, very small. And flying a spaceship through an Oort Cloud… well, you’re in more danger of being struck by lightning in the next ten seconds.

On the other hand, it is possible that we occasionally swap some comets with other stars passing nearby. Again, there was a recent paper that suggested a particular comet had a composition so unusual as to suggest an extrasolar origin… [googles] yah, here it is: http://www.newscientist.com/article/dn16184-has-an-alien-comet-infiltrated-the-solar-system.html.

cheers,

Doug M.

Doug M.

philw1776 July 31, 2009 at 12:26

Great news! Now as an amateur astronomer I can once again enjoy the sheer beauty of comet observing without wearing my 70s SkyLab helmut outside for protection.

Administrator July 31, 2009 at 15:01

Doug M:

Again, there was a recent paper that suggested a particular comet had a composition so unusual as to suggest an extrasolar origin…

Also discussed here:

http://www.centauri-dreams.org/?p=4739

ljk July 31, 2009 at 16:48

July 30, 2009

Sub-surface Oceans In Early Comets Suggest Possible Origin of Life

Written by Brian Ventrudo

A view of NASA’s Deep Impact probe colliding with comet Tempel 1, captured by the Deep Impact flyby spacecraft’s high-resolution instrument.

A new study claims early comets contained vast interior oceans of liquid water that may have provided the ideal conditions for early life to form.

In a paper published in the International Journal of Astrobiology, Professor Chandra Wickramasinghe and his colleagues at the Cardiff Centre for Astrobiology suggest the watery environment of early comets, together with the vast quantity of organics already discovered in comets, would have provided ideal conditions for primitive bacteria to grow and multiply during the first 1 million years of a comet’s life.

The Cardiff team has calculated the thermal history of comets after they formed from interstellar and interplanetary dust approximately 4.5 billion years ago. The formation of the solar system itself is thought to have been triggered by shock waves that emanated from the explosion of a nearby supernova. The supernova injected radioactive material such as Aluminium-26 into the primordial solar system and some became incorporated in the comets.

Professor Chandra Wickramasinghe together with Drs Janaki Wickramasinghe and Max Wallis claim that the heat emitted from radioactivity warms initially frozen material of comets to produce subsurface oceans that persist in a liquid condition for a million years.

Professor Wickramasinghe said: “These calculations, which are more exhaustive than any done before, leaves little doubt that a large fraction of the 100 billion comets in our solar system did indeed have liquid interiors in the past.”

Full article here:

http://www.universetoday.com/2009/07/30/sub-surface-oceans-in-early-comets-suggest-possible-origin-of-life/

ljk August 3, 2009 at 10:26

http://www.technologyreview.com/blog/arxiv/23878/

Monday, August 03, 2009

Lunar Crater Stats Indicate Hidden Population of Asteroids

The asymmetric distribution of craters on the Moon may have been caused by an undiscovered population of near Earth asteroids

Many moons are locked in synchronous rotation with their mother planets. Examples include the Galilean moons of Jupiter, Neptune’s moon Triton and our own Moon.

In the 1980s and 1990s astronomers noticed that the distribution of craters on these objects was asymmetric: They were more heavily cratered on their leading hemispheres which makes sense since it seems obvious that these areas should be struck more often.

It wasn’t until 2003, however, that the same asymmetric crater distribution was measured on our Moon. Now Takashi Ito at the National Astronomical Observatory in Japan and Renu Malhotra at the University of Arizona have asked an interesting question. of the data. Can the asymmetric distribution of craters on the Moon be explained by the known distribution of near Earth asteroids that are thought to have caused them? Their answer is a cautious “no”.

To properly explain the crater distribution, Ito and Malhotra say some other factor must have been involved. One possibility is that we simply haven’t seen all the craters yet: The ongoing lunar mapping missions may help on that score.

Another idea is that the Earth’s tidal forces tear Earth-crossing asteroids apart, creating a higher number of impacts than might otherwise be expected.

But the most exciting and potentially worrying possibility is that there exists a previously unseen population of near Earth asteroids that orbit the Sun at approximately the same distance as the Earth. These have gone unnoticed because they are smaller or darker than other asteroids, say Ito and Malhotra.

“More complete observational surveys of the near-Earth asteroids can test our prediction,” they say.

And let’s not waste too much time about it. By some reckonings, asteroid impacts represent the greatest threat to humankind that we are able to calculate.

Ref: http://arxiv.org/abs/0907.3010: Asymmetric Impacts of Near-Earth Asteroids on The Moon

ljk August 5, 2009 at 11:17

http://www.technologyreview.com/blog/arxiv/23942/

Wednesday, August 05, 2009

The Puzzle Of The Half Comet-Half Asteroid

A mysterious object that ejects dust like a comet but orbits like an asteroid could be a new class of object in the Solar System

In 1996, astronomers identified an extraordinary object orbiting the Sun between Mars and Jupiter in the region best known for its asteroids. And yet this body, called 133P, defied description: it had the orbit of an asteroid and yet was emitting dust like a comet.

Clearly, this is a rare object. After centuries of observation, not a single other object in the asteroid belt has burped gas and dust in the same way.

So how could this have got there? According to Henry Hsieh at Queen’s University, Belfast in Northern Ireland, there can be only two explanations. The first is that 133P is a comet that has somehow recently become trapped in an asteroid-like orbit. This would have required a hugely unlikely combination of gravitational kicks from other planets as the comet travelled into the solar system from the Kuiper Belt or Oort cloud.

Hsieh says this is so fantastically unlikely that it is almost certainly a one off event. So there’s almost no chance that we’d see another comet-like object in this kind of orbit.

The second explanation is that 133P is an asteroid formed partly of ice and that this is being released, perhaps by a collision with another asteroid. If this were the case, there would almost certainly be other asteroids with a similar make up which ought be releasing dust. These we ought to be able to see.

So Hsieh set out to find one, making some 657 observations of 599 asteroids in the asteroid belt.

The big news is that he has found one other object called 176P/LINEAR which is also emitting dust.

So it looks as if the mystery is solved. That more or less rules out the possibility that 133P is a captured comet. Instead, 133P and 176P are a new class of comet-like asteroids made up partly of ice which is ejected whenever these bdoes are struck in the occasional unavoidable collision.

That’s an interesting new addition to the asteroid menagerie. The only question now is what to call these beasts that are half comet and half asteroid. Comsteroids? Asteromets? Hsiehroids?

Ref: http://arxiv.org/abs/0907.5505: The Hawaii Trails Project: Comet-Hunting in the Main Asteroid Belt

ljk August 17, 2009 at 9:27

Ironic if the lack of a few dollars is what really dooms humanity and maybe
all life on Earth:

http://www.universetoday.com/2009/08/13/nasa-doesnt-receive-enough-money-for-mandated-asteroid-search/

Another data point for the Fermi Paradox?

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