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(meteorobs) Excerpts from "CCNet 58/2001 - 23 April 2001"
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From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 23 April 2001
Date: Mon, 23 Apr 2001 11:25:16 +0100
CCNet 58/2001 - 23 April 2001
-----------------------------
[...]
(2) FIREBALL STUNS SKY WATCHERS
Australia Associated Press, 20 April 2001
(3) NEAR TEAM STUDIES SMALL SCALE FEATURES ON EROS
Ron Baalke <baalke@jpl.nasadot gov>
(4) ASTEROID EROS: MOST DETAILED ANALYSIS OF UP-CLOSE IMAGES
Space.com, 19 April 2001
[...]
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(2) FIREBALL STUNS SKY WATCHERS
>From Australia Associated Press, 20 April 2001
http://news.com.au/common/story_page/0,4057,1914638%255E1702,00.html
14:22 (AEST) A BRIGHT flash in the sky captivated scores of people across
Western Australia last night, who witnessed what may have been dust shed by
a comet thousands of years ago.
Perth Observatory astronomer Jamie Biggs said the event was seen as far
north as Dandaragan, 170 kilometres north of Perth, and as far south as
Albany, 410 kilometres south of Perth.
Dr Biggs said the specific incident was what's called a fireball - a bright
shooting star.
"We're not sure, but there is a meteor stream that the earth has just
started passing through last night, and this is the dust and junk that has
been shed by a comet, possibly thousands of years ago," Dr Biggs said.
"We're ploughing through what's left of its orbit, and so we get hit by all
this dust so we get an increase in meteor activity and it looks like it's
from one point in the sky.
"This object may have been one of those but it's hard to know.
"It was a lot brighter than what these things normally are."
About 100 people rang the Perth Observatory to report the event.
) News Limited Privacy Policy
=================================================================
(3) NEAR TEAM STUDIES SMALL SCALE FEATURES ON EROS
>From Ron Baalke <baalke@jpl.nasadot gov>
For Immediate Release
April 20, 2001
Media Contacts:
Helen Worth
Johns Hopkins University Applied Physics Laboratory
(240) 228-5113
helen.worth@jhuapldot edu
Michael Buckley
Johns Hopkins University Applied Physics Laboratory
(240) 228-7536
michael.buckley@jhuapldot edu
NEAR Team Studies Small Scale Features on Eros
Concludes Surface Topography the Result of Impacts
NEAR mission science team members have concluded that the majority of the
small features that make up the surface of asteroid Eros more likely came
from an unrelenting bombardment from space debris than internal processes.
Details of the research from NASA's Near Earth Asteroid Rendezvous (NEAR)
mission were published this week in Science and are based on the NEAR
Shoemaker spacecraft's Oct. 25-26, 2000, low-altitude flyover of asteroid
Eros that brought the spacecraft to within about 3 miles of the surface of
the asteroid.
"We think that impacts to the asteroid's surface have probably been the
single-most dominant process in shaping the surface texture of the
asteroid," says NEAR Project Scientist Dr. Andrew Cheng of the Johns Hopkins
University Applied Physics Laboratory in Laurel, Md., which managed the
mission for NASA. "We saw surface details such as regolith [surface dust and
debris], craters and fields of small boulders in incredible detail. We also
saw things that confound us, but we now have a more in-depth picture of Eros
that will help us to decipher the asteroid's history."
During the flyover, simultaneous observations were taken by the spacecraft's
multispectral imager and laser rangefinder over two tracks approximately 1
mile and 2.5 miles long that showed objects the size of a doghouse at three
to four times better resolution than previously obtained. The data revealed
an inordinate number of small boulders, a saturation of large craters and a
dearth of small ones, crater "ponds," and unknown erosion processes.
A vast number of large craters, 1,630 to 3,280 feet (500 to 1,000 meters) in
diameter, have been imaged, but there is a surprising scarcity of boulders
large enough to make such impacts. There is more than 100 times the number
of 10- to 12-foot (3- to 4-meter) boulders than there are impact craters in
this region. Some angular or slab-like features were imaged that could
indicate they are composed of stronger material than rounded objects. Some
boulder clusters are thought to be fragments of a larger projectile that hit
the asteroid.
The flyover also yielded evidence of an unusually low number of smaller
craters. "There could be some unknown process, possibly something like
seismic shaking following impacts, which is more likely on a small body such
as Eros," says Dr. Joseph Veverka of Cornell University, Ithaca, N.Y., who
heads the imaging team. "Other possibilities are processes that could erode
or erase smaller craters such as micro-cratering [the pummeling of the
surface by smaller objects] or thermal creep [the erosion of surface
material through normal seasonal heating and cooling of the asteroid] that
is eroding the smaller craters."
"We do know there is a substantial amount of regolith from erosion and
impacts that is covering blocks [boulders] and craters possibly to a depth
of several meters. So it could be that many smaller craters do exist but
they're buried under the regolith," says Veverka. "A thick covering of fine
dust that prevents us from seeing what lies beneath might also be part of
the answer to why the asteroid has little color variation. It is possible
that parts of Eros are covered in regolith as deep as a 10-story building."
The data also revealed ponds - flat surfaces at the bottom of craters -
formed by regolith deposits. These ponds are intriguing science team members
because of their extremely smooth surfaces. "The smoothness indicates that
there is an efficient process on Eros which is able to sort out the finest
component of the regolith from the coarser, more blocky portion and
concentrate this fine material into some low-lying areas such as crater
bottoms," Veverka says.
Moreover, the laser altimeter found that ponded deposits are not only smooth
but also extremely horizontal - level relative to local gravity - as if
formed by fluid-like motions. "It is astonishing that the total dry regolith
of an asteroid like Eros can apparently be mobilized like a fluid," says
Cheng. "There is no water on Eros, and there has not been any water, for
billions of years. However, seismic shaking caused by impacts may be able to
produce fluidized movement of regolith."
"Aprons" of debris at the base of some of the larger boulders indicate
another phenomenon the researchers are studying: efficient erosion or
disintegration of ejecta boulders (boulders forced out of a crater as the
result of an impact) after they have landed on the surface. But scientists
say they need to study higher resolution images to more definitively
interpret the various forms of regolith that the low-altitude images have
provided. "What causes this efficient disintegration remains a mystery,"
Veverka says. "But one we hope to solve over the coming months by studying
the wealth of data that the NEAR mission has provided."
More information on the NEAR mission can be found at the NEAR mission Web
site: http://near.jhuapldot edu.
=================================================================
(4) ASTEROID EROS: MOST DETAILED ANALYSIS OF UP-CLOSE IMAGES
>From Space.com, 19 April 2001
http://www.space.com/missionlaunches/missions/eros_wrap_010419.html
By Robert Roy Britt
Senior Science Writer
If like a talented basketball player you had a strong vertical leap (say 36
inches, or just under 1 meter) you could jump a mile (1.6 kilometers) high
on Asteroid 433 Eros. But be careful, because you might also launch yourself
into orbit around the distant space rock.
But if you keep your footing, you'd have a clear view of a strange and
surprising world with house-sized boulders, inexplicable smooth spots and
very near horizons that in some directions loom like mountains on Earth --
all on a fairly firm but incredibly uneven and rocky surface coated with a
thin layer of dust.
If, by chance, you were also an asteroid expert, you'd be very surprised at
the lack of small impact craters on the otherwise heavily scarred, vaguely
shoe-shaped asteroid.
That's the picture painted by Cornell University researcher Peter Thomas,
who in an interview explained what's been learned by the most recent study
of detailed photographs sent back by the NEAR Shoemaker spacecraft.
NEAR landed on the asteroid Feb. 12, with the mission ending 16 days later.
All told, the craft returned more than 160,000 detailed pictures, and Thomas
said researchers will study them for many years.
But Thomas and a host of other scientists have now produced the first
comprehensive peer-reviewed analysis of Eros' small-scale features -- with
details shown down to 3 feet (1 meter). The study will be published in the
April 20 issue of the journal Science.
Lack of small craters
Eons of impacts have created at least 100,000 craters on Eros, which is
about 21 miles (33 kilometers) long, 8 miles (13 kilometers) wide and 8
miles thick.
But one puzzle revealed by the new study is a paucity of small impact
craters -- fewer in a given amount of surface area than are observed on the
Moon, Thomas said.
The lack of tiny divots is not due to a shortage of things to run into --
the "Asteroid Belt" between Mars and Jupiter where Eros roams is loaded with
millions of smaller rocks prone to taking pot shots at each other.
Something must be covering up or eroding small craters, researchers say. One
possible explanation is a phenomenon called seismic shaking. It goes like
this:
When another space rock slams into Eros, it sends shock waves, or sound
waves, reverberating through the asteroid, Thomas explains. This seismic
shaking causes small crater walls to crumble and settle, much like a mound
of flour in a bowl would level out upon shaking.
On an asteroid, seismic shaking would be enhanced by the low gravity, which
would allow small rocks and other debris to move around more freely than on
a more massive object like the Moon.
Larger craters would not be affected as significantly by seismic shaking.
But this shaking, or some other process, might also have created another
curious feature: smooth, flat areas at the bottom of several craters, large
and small. The flat spots, ranging from about 1 to 16 yards (1 to 15 meters)
across, appear to be made of the same material as the crater walls within
which they sit. This would rule out a process known to create similar flat
features on the Moon, when an impact carves out surface soil, exposing
different and often more solid material below.
House-sized boulders
Small boulders, up to the size of a house, are plentiful on Eros. In certain
areas, 25 boulders larger than 26 feet (8 meters) across sit on every square
kilometer, the study found. Researchers estimate there are about a million
such rocks total.
Clamoring up on one of these larger boulders, a visitor could in some
regions get a view of all horizons, which would fall away quickly on the
small rock. Researchers are nearly certain that dust or small pebbles coat
parts or all of the asteroid, but images are not of high enough resolution
to determine this with certainty. A decade ago, no one expected an asteroid
of this size to have enough gravity to retain small particles.
Smaller boulders, likely distributed over the eons during many collisions
with other space rocks, are everywhere. These boulders have been spotted
before, but the new study shows their prevalence.
Boulders 3 feet (1 meter) across or bigger are 500 times as plentiful as the
house-sized boulders.
In general, boulders on Eros appear to be buried in the asteroid's regolith,
or soil, to different depths. Researchers expect that older rocks are buried
deeper, and boulders that have been kicked up by more recent collisions sit
more fully on the surface.
Pile of rubble
Scientists have long wondered whether asteroids were solid rocks or large
collections of rubble. Figuring out how asteroids are structured has
potentially pragmatic uses: Properly destroying or deflecting an incoming
space rock, were one ever determined to be headed our way, would require
knowledge of the asteroid's composition and how well it is bound together.
Researchers have identified more than 350 near-Earth asteroids larger than 1
kilometer (about 0.6 miles) in diameter -- big enough to cause significant
regional or global damage in an impact. They estimate that the total is
likely between 500 and 1,000, and may be higher.
None are known to be on a collision course with Earth, but most experts
agree that an impact is inevitable, eventually.
Building on data published last September, the new analysis further shows
that the heart of Eros is solid, but the surface is clearly a scattered
collection of material. Eros' mass should be enough to create a solid core,
Thomas said, but closer to the surface this would give way to "progressively
fragmented rubble."
A solid core indicates that an asteroid has never been entirely broken up by
a collision, which means that its core, at least, has been around since the
early days of the solar system's formation.
Long grooves, previously spotted in lower-resolution images, have now been
found to be up to 33 feet (10 meters) wide and 82 feet (25 meters) deep.
These grooves appear to be created by cracks in solid material deeper within
the asteroid, and also show signs of collapsed edges that leave piles of
rubble within the grooves.
Beyond Eros
The images of Eros collectively represent by far the most detailed look at
an asteroid ever obtained. In fact, researchers say we have gotten a better
look at much of Eros than at many parts of Mars or the Moon.
And the new study did not examine the images taken at closest range.
Instead, because of the constraints required to publish a study, the
pictures reviewed were taken when NEAR zoomed down to within 4 miles (6.4
kilometers) of Eros in October 2000. Many more images were created later on
other close flybys and even as the spacecraft descended for landing.
One challenge researchers now face is simply absorbing and analyzing the
vast quantity of data and images.
"We solved mysteries, we unveiled more mysteries," said NEAR project
scientist Andrew Cheng shortly after the mission was completed in February.
"Now we're sharing the amazing amount of data that we collected with
scientists all over the world, to sort through and debate and hopefully to
help us discover facts about Eros and our solar system that no one knows
today."
Cheng, of Johns Hopkins, and other researchers contributed to the new
understanding of Eros with an analysis of laser data that is published in a
second paper in Science.
A map, and a benchmark
Thomas said efforts are underway to create a detailed "old-fashioned" global
map of Eros to pinpoint the many dips, mounds, protrusions and holes, as
well as understand how each feature was created.
He also said Eros would now serve as a benchmark against which researchers
could compare data on other asteroids, including ground-based observations.
The detail known about surface features could allow useful interpretations
to be made from other, less-detailed observations.
But Eros is just one asteroid, and researchers suspect others have different
makeups and histories.
"This is a small asteroid," Thomas said. "What a lot of us would like to do
is see a big asteroid up close and in detail."
Thomas said a favorite candidate is Ceres, which at about 620 miles (1,000
kilometers) wide was the first asteroid ever discovered. While other
asteroid missions have been proposed to NASA, currently none are firmly on
the drawing boards.
The NEAR spacecraft was operated by the Applied Physics Laboratory at Johns
Hopkins University, in cooperation with NASA. Those involved in the new
studies included researchers from these and several other institutions.
Copyright 2001, Space.com
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