[meteorite-list] Giant Remnants of Cosmic Collision Found Beyond Neptune

[Sterling K. Webb]

Hi,

    Yet another piece of pre-print puffery about 
tomorrow's Nature article, but (for the first time)
it includes the details of the case: the existence of
a collisional family from the Big Whack on 2003
EL61.

    One thing that is made clear is that the often
repeated 4.5 billion year old date for the Big Whack
is nothing but prejudicial guesswork, an unsupported
assumption based on cherished desire not to have
anything going on recently in the solar system.

    The collisional families in the Asteroid Belt were
all assumed to be ancient. Every textbook and paper
for decades has said so, that they must have originated
in the first billion years of the solar system. With the
advent of truly powerful computational technology
a few brave souls suggested that the most recent "might"
be less than a billion years old, perhaps only a few
hundred million years.

    In the last decade, however, it has been demonstrated
that these collisional families that we can closely observe
are very recent, only a few million years old, Not billions,
millions. This makes perfect sense, as such families are
easily perturbed until they can only be identified with
difficulty.

    Another source of the "ancient" rationale, is the
amusing but universally held notion that the Kuiper Belt
once had 100 to 1000 times its present mass, but lost
it all. A truly whacky idea. Again, a few brave souls 
have called it a "missing mass" problem, and there are
searches underway to see if it's really so. [The first
one, a study of Sco1 Xray occultations, indicates that 
there's a lot more mass out there than was thought.]

> the distant past, perhaps as far back as 4.5 billion 
> years ago, when the Kuiper Belt was much more 
> crowded than it is now...

    So we have the paradox of being told there's nothing
there by the very people who keep finding things there.
That's basically strange.

    And finally, we have a time line on 2003 EL61 leaving
the Kuiper Belt and entering the inner solar system -- maybe
a billion years or so in the future. Since even the best
dynamic computations cannot be trusted for more 250
to 500 million years ahead, the billion year prediction 
sounds like... [politely] hot air. And we only have a 
fraction of one orbit observed of 2003 EL61 upon 
which to calculate the parameters of its orbit. Why don't 
you wait a century or so, Mike? Then we'll know what 
its orbit really is.

    Tomorrow, those with access to the journal Nature 
will get to see the actual data. Leaves me out. For now,
this piece has more information that the previous ones
posted to the List.


Sterling K. Webb
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http://www.space.com/scienceastronomy/070314_kuiper_family.html

Giant Remnants of Cosmic Collision Found Beyond Neptune 
By Ker Than
Staff Writer
posted: 14 March 2007
1:00 pm ET


Shiny, gray space boulders floating in the outskirts 
of the solar system are the remnants of an ancient 
fiery collision involving two massive objects, the 
larger of which was nearly the size of Pluto, scientists say.

This rocky goliath could one day cross the orbit of 
Neptune and become one of the biggest comets 
ever known.

The findings, detailed in the March 15 issue of the 
journal Nature, mark the first "collisional family" 
detected in the Kuiper Belt and provide new insights 
about the solar system's murky history.

The Kuiper Belt is a vast expanse of space located 
beyond the orbit of Neptune. It is littered with rocky 
and icy bodies believed to be vestiges of the 
primordial disk from which the planets formed.

A suburban family

The parent body of the new collisional family is 
thought to be 2003 EL61, one of the largest objects 
in the Kuiper Belt. It currently is football-shaped 
[image], with a diameter of about 930 miles (1,500 
kilometers), but was probably spherical and 20 
percent larger before the collision, the researchers 
say. Pluto is about 1,400 miles (2,300 km) wide.

2003 EL61 is thought to have collided with an 
object in its distant past that was roughly half its 
size and traveling at nearly 7,000 mph. The amount 
of energy generated by the blast would have 
equaled about 10 billion nuclear bombs, said 
study team member Darin Ragozzine of Caltech.

"In terms of collisions in the solar system, that's 
actually kind of mild," Ragozzine told SPACE.com.

The impact blasted large icy chunks from 2003 EL61 
into space and sent the parent body reeling, causing 
it to spin end-over-end every four hours.

"It spins so fast that it has pulled itself into the shape 
of an American football, but one that's a bit deflated 
and stepped on," said Michael Brown, a Caltech 
planetary scientist who led the study.

The impact spawned at least seven other rocky 
objects-and likely more-with diameters ranging 
from 6 to 250 miles (10 to 400 km). The researchers 
lumped the scattered objects into a family based 
on their matching gray color and evidence of 
surface water ice derived from spectral analyses.

"None of the rest of the Kuiper Belt is as shiny 
and pristine" as these objects are, Ragozzine said.
About 35 other collisional families are known, but 
they are all located in the asteroid belt, the rocky 
region of space between the orbits of Mars and 
Jupiter.

A 'milestone'
 
Alessandro Morbidelli, an astronomer at the 
Laboratoire Cassiopee in France, who was not 
involved in the study, called the discovery a 
"milestone in Kuiper Belt science."

Writing in an accompanying Nature news article, 
Morbidelli said the discovery provides a physical 
model for astronomers to test their theories about 
the kinds of large-scale collisions thought to be 
behind the birth of our Moon and the Pluto-
Charon system.

Because such large collisions are relatively rare, 
scientists think the one involving 2003 EL61 occurred 
in the distant past, perhaps as far back as 4.5 billion 
years ago, when the Kuiper Belt was much more 
crowded than it is now and objects were more likely 
to bump into one another.

If scientists can pin down when the collision occurred, 
they will have a unique glimpse into a specific time in 
the solar system's history and the evolution of the 
Kuiper Belt, Morbidelli said.

To be a comet

Some of the shards from the impact have made 
their way to the inner Solar system, the researchers say.
"Probably, there are comets that we have seen that 
came from this collision," Brown said in a telephone 
interview. "In fact, there are probably chunks of that 
collision here on the ground."

One day, EL2003 EL61 will cross the orbit of 
Neptune and become a comet itself. "That's going 
to be in about a billion years," Brown said. "It's 
a ways to wait."