ZNAK SAGITE — više od fantastike — edicija, časopis, knjižara...

Unicorn, zaboravila sam da kazem par stvari ranije, a na temu teorije da prolazak Suncevog sistema
kroz ravan Galaksije izaziva/moze da izazove poremecaje i nestabilnosti orbita asteroida u
Kujperovom pojasu i kometa u Ortovom oblaku. Elem, nestabilnosti orbita pomenutih nebeskih tela moze
da izazove i bliski prolazak neke zvezde. Kako je Sunce u relativno gusto naseljenom delu Galaksije,
takvi bliski susreti nisu retki, ali nisu periodicni.

(Ovo, kao, treba da te/vas uplasi )

U ovom trenutku Sunce se krece u smeru ka severnom Galaktickom polu, davno smo presli ravan
Galaksije, negde smo na 2/3 puta ka najvecoj udaljenosti od z=0.
(Ovo, opet, treba da te/vas smiri )

Sad, pazi ovamo: osim u Kujperovom pojasu, asteroida u Suncevom sistemu ima u - asteroidnom pojasu.
Nedavno sam neletela na lepo napisanu pricu koja sledi, kljucna rec  je "haos":

~~~~~ Chaotic heavens

New Scientist vol 181 issue 2436 - 28 February 2004, page 32

IT WAS Isaac Newton who finally showed why the heavenly bodies move in predictable ways. He proved
that the planets move in response to the sun's gravitational pull, endlessly repeating their orbits
like celestial clockwork. If you know the position and velocity of a planet today, you can work out
its motion far into the future.

Or so we thought until recently. "Our research shows that for tens of millions of years, the
planets orbit the sun with the regularity of clockwork," says geophysicist Michael Ghil.
"Then, quite unexpectedly, everything goes crazy." According to Ghil, who works at the École
Normale Supérieure in Paris and the University of California at Los Angeles, this planetary
madness is all down to chaos. In chaotic systems, tiny changes in conditions can lead to huge
differences in outcome. Though you can predict what the changes will do in theory, the system is so
sensitive that you'll never get it right.

If the only consequence was a little unpredictability, that would perhaps be a shame, but not too
troubling - just a slight departure from Newton's clockwork ideal. But it's much worse than that.
According to Ghil's latest work, carried out with Ferenc Varadi and Bruce Runnegar at UCLA, the
solar system's chaotic nature can also unleash an asteroid storm, flinging massive rocks out of
their usual orbits and showering the solar system's inner planets with debris. It may even
have been what killed the dinosaurs.

We have known for a while now that the planets aren't as stable as we like to think. If the Earth
and sun were alone in space, Earth would trace out an elliptical or circular orbit. In the case of
this "two-body" system, the path can be predicted exactly. But the solar system is more
complicated. In addition to the steady gravitational pull of the sun, each planet feels smaller,
varying tugs from the other eight planets and all the various moons. Mathematicians have proved
that it is impossible to solve Newton's equations exactly even when there are as few as three bodies
present, let alone dozens. Instead, scientists have to make approximations about a planet's
position and the forces it experiences, and these errors can grow over time and send orbital
calculations wildly off beam.

In 1988 Gerry Sussman at MIT, working with Jack Wisdom at the University of California, Santa
Barbara, worked out that Pluto's orbit is chaotic. In other words, if you try to predict the shape
of Pluto's elliptical orbit around the sun in the long term, your calculation will be extremely
sensitive to the parameters you put in at the start. A year later, Wisdom and Jacques Laskar of the
Bureau des Longitudes in Paris proved that the Earth's orbit is also chaotic. They showed that an
error as small as 15 metres in measuring the position of the Earth today would make it impossible to
predict where the planet will be in its orbit in 100 million years' time.

But Ghil and his colleagues have also discovered a more disturbing way that chaos can creep in. They
were wondering what influence Jupiter and Saturn might have, and suspected that when the two
biggest planets in the solar system line up in front of the sun, in the same way that the Earth,
moon and sun align during a total solar eclipse, their gravitational pull could cause dramatic
effects. Under the right conditions, they thought, these gas giants might be able to nudge a nearby
celestial body into a more elongated orbit.

Like solar eclipses, this alignment between Jupiter, Saturn and the sun is rare. In the time it
takes Saturn to complete two orbits around the sun, Jupiter has whizzed round almost five times.
This means that the planets are only on the same side of the sun as each other every 20 years.

But that's not the end of the story. The orbit Saturn makes around the sun lies in a slightly
different plane to Jupiter's orbit. In other words, the planets' pathways are inclined to each
other. So every 20 years when Jupiter overtakes Saturn, the planets do not line up exactly: Saturn
usually lies above or below Jupiter. This means that the combined gravitational pull of the two gas
giants is much weaker than it would be if the planets were in perfect alignment.

If Jupiter made precisely five orbits for every two that Saturn makes, the planets would never line
up with each other perfectly. But because the ratio is not exactly 5:2, the point where they pass
slowly moves round the sun. As the planets get closer to perfect alignment, the extra tug they exert
on other planets, moons and asteroids becomes stronger. The gravitational pull is strongest when
Jupiter eventually overtakes Saturn at precisely the point where the orbits cross each other. "The
effect on other bodies in the solar system rises to a crescendo every 1000 or so years," says Ghil.

Until now, most planetary scientists have ignored this occasional extra pull from Jupiter and Saturn
in their models of the solar system because it happens so infrequently. Over long periods of time,
they reasoned, this additional tug would be of little consequence.

"We weren't sure this was right," says Ghil. And so his group set out to study its effects in
detail. To do that, they had to follow in the footsteps of 18th-century astronomers and construct an
orrery, a machine that displays how the planets move relative to one another. But this would be no
mechanical orrery. While early astronomers used cogs and wheels, modern planetary scientists use
digital orreries, computer models dedicated to simulating complex planetary motions.

Although they still use approximations in their computations, Ghil, Varadi and Runnegar have
constructed the most accurate digital orrery ever built. It crunches the numbers on finer time
intervals than any other, thereby revealing much greater detail. With this, they can start with the
positions of the planets and asteroids today and wind back the clock to see how the solar system
looked tens of millions of years ago (Icarus, vol 139, p 286).

What the team found is remarkable. Jupiter and Saturn's orbits are poised on a knife-edge: most of
the time their orbits are pretty much predictable, but the slightest disturbance can send them into
chaos - meaning they become beset by unpredictable variations. Because such systems are so complex,
it is impossible to pinpoint which aspect of a planet's orbit might go haywire. "The chaos might,
for instance, manifest itself in wild variations in the length of Jupiter's orbit, its inclination
or even its orientation," says Ghil.

Working out what could tip Jupiter and Saturn into chaotic orbits is a mammoth task, so Ghil's team
focused on just one factor. They reasoned that, over hundreds of millions of years,
non-gravitational effects such as the pressure of sunlight and particles in the solar wind, could
have affected Saturn's orbit. The team wound the clock back millions of years to a time when the
planet's furthest point from the sun - a parameter called the semi-major axis - was 0.1 per cent
less than now. "We think such a change is entirely plausible," says Ghil.

His team found that this perturbation disrupts Saturn's orbit to the point where it becomes wobbly
and "aperiodic" - each revolution around the sun takes a slightly different path from the last one.
This means that, at certain times, the orbits of Saturn and Jupiter might be in virtually the same
plane producing a much stronger gravitational pull than usual. This has the potential to unleash
havoc in the solar system. In particular, it can trigger chaotic instability in the asteroid belt
that lies between the orbits of Mars and Jupiter (see "Chaotic skies"). "I can certainly believe
that changing Saturn's semi-major axis under certain conditions may lead to an instability in the
planetary system," says Alessandro Morbidelli, a mathematician working on chaotic dynamics in the
solar system at the Côte d'Azur Observatory in Nice, France.

It's all down to the way the planets can transfer their energy to nearby objects with particular
orbits. In the same way as you can make a child's swing go higher if you push it at the right
moment, Jupiter and Saturn can push asteroids from a regular orbit into a chaotic one. In the case
of the child's swing, you transfer energy most efficiently if you shove the swing at a frequency
known as the resonant frequency. Similarly, Jupiter and Saturn have more of an effect on asteroids
whose orbital frequency around the sun forms an integer ratio with one of their orbital frequencies.
For instance, an asteroid that goes round the sun three times for every two turns by Jupiter is said
to be in a 3:2 resonance. And if an asteroid is in a resonant orbit, chaos in Jupiter's orbit can
trigger a large and unpredictable change in the asteroid's path.

As yet, says Ghil, it is impossible to tell how long the chaotic effect on the asteroid belt
persists, but they have calculated the likely outcome. "We find an incredible wealth of effects," he
says. Some asteroids swing from their usual orbits into ones that are closer to the sun, while
others fly out to larger, elongated orbits. "A whole population of asteroids can drift back and
forth through a succession of resonant orbits," says Ghil.

Most of these asteroids stay within the asteroid belt, but crucially, Ghil has found that the
gravitational pull of other planets can yank some rocky bodies free of the asteroid belt altogether.
"Some are even catapulted into orbits that cross the Earth's," he says.

This is exactly what Ghil and his colleagues' digital orrery says happened 65 million years
ago. As if some mischievous god had reached in and stirred things up, the solar system suddenly
became a hornets' nest of activity. Chaos in the Jupiter-Saturn system caused a flurry of
Earth-crossing asteroids, and among them may have been the one thought to have had the dinosaurs'
name written on it. Something certainly crashed off the Yucatan coast in Mexico at the end of the
Cretaceous period, just when the dinosaurs died out. "We can't say we've actually caught the culprit
that fell into the Yucatan peninsula yet but we're on its trail," says Ghil. "The dinosaurs might
have been the victims of an event hard-wired into the dynamics of the solar system."

Ghil and his colleagues first suggested a link between chaos and the Cretaceous-Tertiary boundary in
2001. Since then, their digital orrery has been working hard to refine the calculations, and they
are now confident that the last burst of chaos in the skies did indeed occur at that significant
point 65 million years ago (Astrophysical Journal, vol 592 p 620).

And last year, Ghil's team calculated that another burst of planetary chaos occurred about
250 million years ago, give or take 10 million years. They speculate that it may be linked to
the catastrophic impact event that many believe was responsible for wiping out 95 per cent of
species at the end of the Permian period, 251 million years ago. Ghil is not worried about the 10
million year uncertainty. He points out that simulating the solar system with his digital orrery
becomes difficult beyond 70 million years. As you look further back in time, the accuracy of the
calculations deteriorates due to uncertainties in the current position of the planets. The timing of
this burst of chaos is at the very limit of the digital orrery's precision, so it may be possible to
link it with the Permian-Triassic boundary.

Such suggestions are contentious, of course. Indeed, not all palaeontologists are convinced that the
mass extinction at the end of the Permian was due to an asteroid impact. Many believe that a massive
and prolonged volcanic eruption may have been the real culprit (New Scientist, 26 April 2003, p 38).

Finding evidence to corroborate the chaos theory will be difficult. Some of the asteroids stirred up
at this time will have been flung into orbits crossing Mars and Venus, so other planets and moons
should be scarred with 65 million-year-old impact craters. But strong winds and other weather
effects erode ancient craters on planets, making them difficult to date. And although our moon has
no atmosphere to weather its craters, past geological activity makes it extremely difficult to date
impacts accurately.

But whatever the truth about the dinosaurs, we are still gaining a new understanding of Newton's
"clockwork" heavens. Ghil's work is one of several watershed discoveries that is changing our view
of the solar system, says Thomas Quinn at the University of Washington in Seattle. Though our solar
system evolves quietly and sensibly for tens of millions of years, it also goes through periods of
madness, and what has happened in the distant past will happen again. "Our simulations last year
show that another burst of chaos is due in 30 million years time," says Ghil.

Should we be worried? Well, when the dinosaurs met their untimely end, seemingly insignificant
animals ended up inheriting the Earth. Look around you today. Our successors may already be waiting
in the wings.

http://order.ph.utexas.edu/clock/

~~~~~

(Ovo, naravno, treba da te/vas navede da se smrznes/te   )

u je.

Sto? Tebi malo 30 miliona godina?!

manje nego da se cita onoliki poust.

Stani, bre, malo lepoto devojko! Ne rece li ti meni onamo da treba i da citam tekstove, a ne samo da gledam slike?

da. na papiru.