On 1994 July 16-22, over twenty fragments of comet Shoemaker-Levy 9 collided with the planet Jupiter.
The comet, discovered the previous year by astronomers Carolyn and
Eugene Shoemaker and David Levy, was observed by astronomers at hundreds
of observatories around the world as it crashed into Jupiter's southern
hemisphere. During July, 2010 a comet or asteroid ripped another
Pacific-Ocean sized hole in Jupiter (image below). Is Jupiter a giant
protective magnet for Earth, or are these events wake-up calls similar
to Friday's meteor explosion over Russia's Ural Mountains?
Is Stephen Hawking
says, the general consensus is that any comet or asteroid greater than
20 kilometers in diameter that strikes the Earth will result in the
complete annihilation of complex life - animals and higher plants. (The
asteroid Vesta, for example, one of the destinations of the Dawn
Mission, is the size of Arizona).
Since 1941 many astronomers have thought of Jupiter as a protective
big brother for planet Earth -a celestial shield, deflecting asteroids
and comets away from the inner Solar System.
This long-standing belief that Jupiter acts as a celestial shield,
deflecting asteroids and comets away from the inner Solar System, has
been challenged by the first in a series of studies evaluating the
impact risk to the Earth posed by different groups of object.
Dr Jonathan Horner of Great Britain's Open University has studied the
impact hazard posed to Earth by the Centaurs, the parent population of
the Jupiter Family of comets. His research showed that the presence of a
Jupiter-like planet in the Solar System does not necessarily lead to a
lower impact rate at the Earth. Horner said that Jupiter's role as
guardian may have been overstated: "It seems that the idea isn't so
clear-cut."
The idea of Jupiter as protector was first proposed by planetary scientist George Wetherill
in 1941. Wetherill showed that the planet's enormous mass — more than
300 times that of the Earth — is enough to catapult comets that might
hit Earth, like a slingshot ,out of the Solar System.
Other astronomers have postulated that Jupiter's gravitational pull
would thin the crowd of dangerous asteroids and other objects, making
Earth less impact prone. Other research has suggested that, in the past,
changes in Jupiter's orbit might have actually increased the number of
objects on a collision course with earth. Until now, Horner says, little
work was done to test either idea.
The short period Jupiter Family of Comets (JFCs) are believed to
originate from the Kuiper Belt and have orbital periods of up to 20
years and low inclination controlled by Jupiter. The Kuiper Belt is a
large reservoir of small icy bodies just beyond Neptune. From collisions
or gravitational perturbations some Kuiper Belt objects escape and fall
towards the Sun. When they approach the Sun their volatile elements
will start to sublimate off the surface and we will see the object as a
comet. Because the orbit crosses that of Jupiter, the comet will have
gravitational interactions with this massive gas giant. The objects
orbit will gradually change from these interactions and eventually the
object will either be thrown out of the Solar System or collide with a
planet or the Sun.
The second class of comets, the long periods, are believed to originate from the Oort cloud. This is a vast spherical reservoir believed to exist at the edge of the Solar System. The long period comets have periods of less than 200 years and no preference in orbital inclination.
The second class of comets, the long periods, are believed to originate from the Oort cloud. This is a vast spherical reservoir believed to exist at the edge of the Solar System. The long period comets have periods of less than 200 years and no preference in orbital inclination.
"The idea that a Jupiter-like planet plays an important role in
lessening the impact risk on potentially habitable planets is a common
belief but there has only really been one study done on this in the
past, which looked at the hazard due to the Long Period Comets,"
Horner continued." We are carrying out an ongoing series of studies of
the impact risks in planetary systems, starting off by looking at our
own Solar System, since we know the most about it."
Horner and colleague Barrie Jones built several versions of the Solar
System on the Open University's computers: one with a Jupiter, one
without, and several with a gas giant that was either a quarter, half,
or three-quarters of Jupiter's mass. The system also contained 100,000 centaurs — large, icy bodies from the Solar System's Kuiper belt, within which Pluto lies.
After running their models for 10 million virtual years, Horner and
Jones found some striking results:The Earth was about 30% more likely to
be hit by a centaur in a Solar System with a life-size Jupiter than it
was in a Jupiter-less system.
"We've found that if a planet about the mass of Saturn or a bit larger occupied Jupiter's place," Horner concluded, "then the number of impacts on Earth would increase. However if nothing was there at all, there wouldn't be any difference from our current impact rate. Rather than it being a clear cut case that Jupiter acts as a shield, it seems that Jupiter almost gives with one hand and takes away with the other!"
"We've found that if a planet about the mass of Saturn or a bit larger occupied Jupiter's place," Horner concluded, "then the number of impacts on Earth would increase. However if nothing was there at all, there wouldn't be any difference from our current impact rate. Rather than it being a clear cut case that Jupiter acts as a shield, it seems that Jupiter almost gives with one hand and takes away with the other!"
The weakness of Horner's tentative conclusion is that it fails to
take into account Jupiter's ability to deflect Earth-colliding objects
from the Oort cloud, a massive cloud of comets that surrounds the Solar
System
The Open University
team is assessing the impact risk posed to the Earth by the asteroids
and will go on to study the long period comets, before examining the
role of the position of Jupiter within our system.
But back to Stephen Hawking: How many times in our galaxy alone has
life finally evolved to the equivalent of our planets and animals on
some far distant planet, he asks, only to be utterly destroyed by an
impact? Galactic history suggests it might be a common occurrence. Our
cold comfort comes from the adjective "galactic" -that's a hugely
different time perspective that our biblical three score and ten.
Source: The Daily Galaxy
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