PLEASE NOTE:
*
CCNet, 21/2000 - 15 February 2000
---------------------------------
QUOTE OF THE DAY
"The images of Eros recently sent
back to Earth by the Near
spacecraft show a surface covered with
craters, some of which
are quite large and degraded relative to
Eros's size. This might
seem surprising, given the general
belief that Near-Earth
Asteroids are fragments of main belt
bodies that reached the
terrestrial planet region via injection
into powerful resonances
like the 3:1 mean-motion resonance or
the nu6 secular resonance.
Since the median lifetime of NEAs is of
order 10~My, NEAs are
generally expected to have sparsely
cratered surfaces. However,
it appears that Eros may have spent a
long time collisionally
coupled to the main belt after its
collisional birth. [...]
We favor a scenario where many main belt
asteroids slowly drift
into resonance from neighbouring
regions; this radial drift is
probably caused by "Yarkovsky"
thermal drag forces which act
over the collisonal lifetime of the
bodies [...]. In light of
these new model results, we believe that
many or perhaps most
NEAs should have an age comparable to
their collisional
lifetime, presumably explaining the
densely cratered surface of
Eros."
-- Alessandro
Morbidelli and Bill Bottke, 15 February 2000
(1) LARGE NUMBER OF CRATERS ON EROS CONFIRMS NEW
SCENARIO FOR THE ORIGIN OF NEAR-EARTH
ASTEROIDS
Alessandro Morbidelli and Bill Bottke
(2) NEAR'S HISTORIC FIRST IMAGE FROM EROS ORBIT
http://near.jhuapl.edu/iod/20000214g/index.html
(3) FIRST LIGHT FROM EROS ORBIT
Space Science News <express@spacescience.com>
(4) THE ICE MICROBES COMETH
Andrew Yee <ayee@nova.astro.utoronto.ca>
(5) BACKGROUND RISK FOR ASTEROID STRIKES
Alan Boyle <alan.boyle@MSNBC.COM>
(6) CONFUSION ON "BACKGROUND RATE"
David Morrison <dmorrison@arc.nasa.gov>
(7) PROBABILITY PROBLEMS
Konrad Ebisch <kebisch@zycor.lgc.com>
(8) IMPACT PROBABILITIES
Colin Keay <phcslk@cc.newcastle.edu.au>
(9) THE UNCERTAINTIES OF IMPACT PROBABILITY ESTIMATES
Benny J Peiser <b.j.peiser@livjm.ac.uk>
================
CCNet-NEWS, 15 February 2000
(1) LARGE NUMBER OF CRATERS ON EROS CONFIRMS NEW
SCENARIO FOR THE ORIGIN OF NEAR-EARTH ASTEROIDS
By Alessandro Morbidelli and Bill Bottke
The images of Eros recently sent back to Earth by the Near
spacecraft
show a surface covered with craters, some of which are quite
large
and degraded relative to Eros's size. This might seem surprising,
given the general belief that Near-Earth Asteroids are fragments
of
main belt bodies that reached the terrestrial planet region via
injection into powerful resonances like the 3:1 mean-motion
resonance
or the nu6 secular resonance. Since the median lifetime of NEAs
is of
order 10~My (Gladman et al., Science, 277, 197-201, 1997) NEAs
are
generally expected to have sparsely cratered surfaces. However,
it
appears that Eros may have spent a long time collisionally
coupled to
the main belt after its collisional birth.
Using a model presented at the last DPS in Abano which can
quantitatively reproduce the observed orbital and size
distribution
of NEAs, we believe we can explain this inconsistency. Our
computations show that 25-50% of the largest NEAs arrive on
terrestial planet-crossing orbits only after a slow increase of
their
orbital eccentricity (discussed in Migliorini et al., Science,
281,
2022--2024, 1998; Morbidelli and D. Nesvorny, Icarus, 139,
295-308,
1999). The rest of the NEA population should reach
planet-crossing
orbits through a "classical" fast track resonance
(e.g., 3:1 or nu6
resonance). The number of bodies that are required to pass
through
the fast- and slow-track resonances per million year, however, is
inconsistent with a dominant role of collisions in the
resonance-feeding process. For this reason, we favor a scenario
where
many main belt asteroids slowly drift into resonance from
neighbouring regions; this radial drift is probably caused by
"Yarkovsky" thermal drag forces which act over the
collisonal
lifetime of the bodies (Farinella and Vokrouhlicky, Science, 283,
1507-1510, 1998; Bottke et al. 2000, Icarus, in press), though
alternative mechanisms are under study. In light of these new
model
results, we believe that many or perhaps most NEAs should have an
age
comparable to their collisional lifetime, presumably explaining
the
densely cratered surface of Eros.
P.S. A paper on this topic by Bottke, Jedicke, Morbidelli,
Gladman
and Petit is under revision for publication in Science
For further information, please contact
Alessandro Morbidelli <morby@obs-nice.fr>
Bill Bottke < bottke@astrosun.tn.cornell.edu>
==================
(2) NEAR'S HISTORIC FIRST IMAGE FROM EROS ORBIT
NEAR image of the day for 2000 Feb 14
http://near.jhuapl.edu/iod/20000214g/index.html
Today at 10:33 AM EST the NEAR spacecraft was successfully
inserted
into orbit around 433 Eros, becoming the first artificial
satellite
of an asteroid. Just over an hour later, NEAR pointed its camera
at
the asteroid and took this picture from a range of 210 miles (330
km)
above the surface. Mission navigators and operators will use this
image and others to be taken later to traingulate on landmarks on
the
asteroid's surface, precisely measuring position of the
spacecraft to
plot NEAR's course.
Features as small as a 100 feet (30 meters) across can be seen.
This
view shows the 3-mile (5-kilometer) impact crater which the
spacecraft
has spied for over a week during its approach. The two smaller
craters
superimposed on its rim are each about 1.2 miles (2 kilometers)
across.
An enormous boulder a full 170 feet (50 meters) in size sits on
the
large crater's floor. Other key features of the surface are
shallow
subsurface layering exposed near the tops of crater walls, and
shallow grooves crossing the surface and cutting the crater's
rim.
--------------------------------------------------------
Built and managed by The Johns Hopkins University Applied Physics
Laboratory, Laurel, Maryland, NEAR was the first spacecraft
launched in
NASA's Discovery Program of low-cost, small-scale planetary
missions.
See the NEAR web page at http://near.jhuapl.edu
for more details.
===================
(3) FIRST LIGHT FROM EROS ORBIT
From Space Science News <express@spacescience.com>
Space Science News for February 14, 2000
First Light from Eros Orbit: NEAR's first close-up pictures
from Eros
orbit have arrived at Earth. This story includes a beautiful
image of a
large crater on the asteroid and highlights from this afternoon's
NASA
press briefing. FULL STORY at
http://www.spacescience.com/headlines/y2000/ast14feb_1a.htm
====================
(4) THE ICE MICROBES COMETH
From Andrew Yee <ayee@nova.astro.utoronto.ca>
From NATURE, Friday, 11 February 2000
http://helix.nature.com/nsu/000217/000217-1.html
The ice microbes cometh
By PHILIP BALL
At the end of last year, microbes were found under thousands of
metres
of ice in Antarctica. The discovery not only stretched the
habitable
regions of the Earth to new extremes but also lent hope to the
idea
that life might eke out a precarious existence on other worlds.
Now new
research shows where these microbes might come from, and how they
might
survive the rigours of a life in ice.
The plucky bacteria, encased in ice many thousands of years old,
were
first reported last December by a team of US scientists[1]. They
were
found in the bottom 100 metres of a core of ice drilled 3590
metres
into the ice sheet at East Antarctica's Vostok Station.
Why are the bacteria in the lowest part of the ice, not closer to
the
surface, where they might have been deposited on wind-borne dust?
The
answer is supplied by results reported in Nature[2] by Martin
Siegert
of the University of Bristol and co-workers. Hidden beneath the
ice
sheet on which Vostok Station stands is a vast lake, called Lake
Vostok, discovered in the 1970s.
Lake Vostok, all of which is below several kilometres of solid
ice, has
been mapped out using radar signals, which bounce back from the
ice at
the top and bottom of the lake to reveal its buried profile. At
670
metres deep and covering 14,000 square kilometres, it is the
largest
known sub-ice lake.
Siegert's group has analysed radar data from airborne
measurements
revealing that ice is being lost from the base of the sheet in
the
north and west of the lake. To the south, on the other hand, the
ice
over the lake is about 150 metres thicker on average, owing to
freezing
of the lake water.
This suggests that ice is melting over one part of the lake and
being
reformed over another. This may induce circulation in the lake
water,
just as the water in surface lakes circulates because of
convection. It
may also release rocky debris and other ice-bound impurities into
the
water, which, the researchers say, could provide nutrients for
any
organisms living in the lake.
It was precisely because of the presence of the lake that the ice
core
was drilled. The core was taken from a region where refrozen ice
had
been accreted from the lake onto the bottom of the ice, and the
drilling stopped just 120 metres short of the top of the lake.
This
meant that it penetrated about 100 metres into the 'accretion'
ice, and
it was here that bacteria were found -- some still living after
being
released from the ice core. This suggests that they may grow
within the
lake itself.
But how, asks physicist P. Buford Price of the University of
California
in the Proceedings of the National Academy of Sciences[3], could
bacteria go on living within ice several degrees below freezing
point,
at pressures four hundred times greater than the air pressure at
the
Earth's surface?
Price says that the accretion ice above Lake Vostok provides all
three
of the ingredients essential to life: water, energy and carbon.
Glacier
ice, he points out, is laced with a network of water-filled veins
between solid ice grains, in which salts accumulate, lowering the
freezing point and preventing the veins from icing up. Price
estimates
that these veins could be several thousandths of a millimetre
across in
the Vostok accretion ice -- wide enough to accommodate bacterial
cells.
The liquid veins also concentrate dissolved acids, including
organic
acids such as formic and acetic acid (the main component of
vinegar).
Price argues that chemical reactions involving these acids, which
have
been detected from the ice-core studies, could provide sufficient
energy and carbon to support the number of microbes found in the
ice
cores.
Lake Vostok is the best terrestrial analogue of Jupiter's moon
Europa.
Over the past few years, the Galileo spacecraft orbiting Jupiter
has
sniffed out strong evidence that below the crust of ice covering
Europa's surface lurks an ocean of liquid water stretching from
pole to
pole. This is the only known world in the solar system other than
Earth
on which a large body of liquid water seem likely to exist
(although it
is possible that Callisto, another of Jupiter's moons, might also
have
a subsurface ocean). If life can exist in the ice above Lake
Vostok,
thousands of metres below frozen Antarctica, who is to say that
it might
not be found also below the ice fields of Europa?
[1] Jouzel, J., Petit, J.R., Souchez, R., Barkov, N.I., Lipenkov,
V.Y.,
Raynaud, D., Stievenard, M., Vassiliev, N.I., Verbeke, V. &
Vimeux, F.
More Than 200 Meters of Lake Ice Above Subglacial Lake Vostok,
Antarctica Science 286, 2138-2141 (1999).
[2] Siegert, M.J., Kwok, R., Mayer, C. & Hubbard, B. Water
exchange
between the subglacial Lake Vostok and the overlying ice sheet
Nature
403, 643 (2000).
[3] Price, P.B. A habitat for psychrophiles in deep Antarctic
ice. PNAS
97, 1247-1251 (2000).
© Macmillan Magazines Ltd 2000 - NATURE NEWS SERVICE
=============================
* LETTERS TO THE MODERATOR *
=============================
(5) BACKGROUND RISK FOR ASTEROID STRIKES
From Alan Boyle <alan.boyle@MSNBC.COM>
dear benny:
regarding the "background risk" for an asteroid strike,
i have to say
with regret that the initial versions of the story omitted the
time
frame for the nasa estimate of "background risk" ...
once it was
called to my attention i quickly added a phrase to indicate that
this
risk applied "in a given year" -- perhaps don yeomans,
paul chodas,
ron baalke or someone else could pass along information that
would
surely be of interest relating to how that estimate was developed
(i
assume it's based on the theory that such collisions occur about
every million years, more or less, though i'm not sure
catastrophic
"deep impacts" happen that frequently).
i'm glad to hear better estimates as to the size of bf19 ... at
the
time i wrote the story, i was just aware of andrea's "much
less than
1 km" figure.
the story that i wrote also mentioned an estimate for the
probability
of a 300-meter-wide asteroid:
"University of Hawaii astronomer David Jewitt said last
month that
there was a 1 percent chance that Earth would be struck by a
1,000-foot-wide (300-meter-wide) object sometime in the next
century."
... i guess that can be extrapolated to a 300-meter-wide asteroid
hitting earth every 10,000 years or so (which again might sound
high
to some people, maybe low to others).
the story was also linked to an earlier one that mentioned
jewitt's
speculation about the effect of such an asteroid:
"Such an impact would deliver a withering 1,000-megaton
explosion and
cause perhaps 100,000 deaths," he said. "If the impact
occurred in or
near a densely populated region - the eastern seaboard of the
United
States, for instance, or Western Europe or coastal Asia - the
fatalities could easily rise into the tens of millions."
i hope this helps shed more light on the questions you raised (or
at
least provides more grist for the mill). if CCNet subscribers
have
any questions about anything i've written, i'd love to hear from
them... the feedback would certainly help our coverage of these
sorts
of issues.
best, alan boyle, msnbc
p.s.: i'll look forward to seeing you and other e-mail penpals at
the
aaas conference in d.c.
===================
(6) CONFUSION ON "BACKGROUND RATE"
From David Morrison <dmorrison@arc.nasa.gov>
Colin:
I don't really see your problem with the NASA news release from
Don
Yeomans.
(1) The background risk is quoted for a one-year timescale
("per
year"), which is the way we always do it. I think this
should be
clear both from context and by general usage. I agree that for a
news release it would be better to say "per year"
explicitly,
however, rather than depending on context.
(2) The background rate depends steeply on the size of the
object, and
we don't know the size of 2000 BF19, except that it is smaller
than 1
km. Thus any background rate must be a lower limit,
corresponding the
the 1 km upper limit on the diameter. It is correct to say that
this
lower limit is greater than one in a million, and to state the
result
as an inequality. Thus I believe Yeomans is correct, and
unambiguous,
to say that the background rate is higher than the estimated
probability for BF19 (all odds expressed as probability of impact
per
year). This seems straightforward to me, and it is also the
way the
Torino hazard scale is defined and used.
(3) If you want estimated background impact rates for 50 KT and
other
specific energies, please refer to the technical literature.
Published
estimates go back to the work of Shoemaker nearly two decades
ago.
See, for example, the rates given by Chapman and Morrison in
their 1994
Nature paper on the impact hazard (and references
there-in). These are
refereed papers, available through any library.
Incidentally, the
surface impact rate for 50 KT is very near zero, since these
objects
detonate at high altitude.
David Morrison
14 Feb 00
+++++++++++++++++++++++++++++++++++++++++++
David Morrison, NASA Ames Research Center
Tel 650 604 5094; Fax 650 604 1165
david.morrison@arc.nasa.gov
or dmorrison@mail.arc.nasa.gov
website: http://space.arc.nasa.gov
website: http://astrobiology.arc.nasa.gov
website: http://impact.arc.nasa.gov
=====================
(7) PROBABILITY PROBLEMS
From Konrad Ebisch <kebisch@zycor.lgc.com>
Dear Benny,
In the latest of your reports, Colin Keay pointed out that impact
probabilities without a timescale are worthless.
Your reply included this:
3) the actual impact probability for objects this size
(somewhere in the region of 1:20,000-50,000).
Please, not another estimate without a timescale.
Nor a size range. The chance of being hit with a kilometer-size
rock
(1km +/- 100m) is quite different from the chance of being hit
with a
kilometer-size rock (1km +/- 500m). If it means exactly 1
kilometer
(1km +/- 0), then the probability is zero.
Nor a mention of what bodies will be hit. I assume this is
intended for
impacts only with the Earth, but in some contexts collisions with
other
bodies may be important too.
Nor what is meant by "objects this size". All
objects this size? An
average probability for any object with perihelion less than 1.2
AU?
There is another thing in the NASA statement that is
unclear:
"... asteroid that has not yet been discovered."
Not yet discovered as of 2000 February 7? 2022 January 1? Or not
yet
discovered when it reaches the Earth's atmosphere?
But let's not be too hard on this Don Yeomans' NASA press
release. For
those familiar with the context, I think that it is fairly clear
that
they mean the time span to be the year 2022. Yes, it should have
specified.
A major problem of the English language (and others) is that it
is
often difficult to be precise and specific, and more so when
speaking
to an audience that is not intimately familiar with the
context.
Yes, we all need to be more careful, but let's not condemn each
other
for things that are sometimes unavoidable.
Konrad Ebisch
===============
(8) IMPACT PROBABILITIES
From Colin Keay <phcslk@cc.newcastle.edu.au>
Dear Benny:
I thank my friends who responded to the point I raised. In the
absence
of a time period for an estimate, "a year" is what I
would have assumed,
as David Morrison points out. But members of the general public
interested in matters of potential collisions would interpret it
otherwise, usually taking a "lifetime" as the
applicable period, i.e.
almost two orders of magnitude different. This is reinforced by
frequent statements comparing the chance of a person dying in
a plane crash with perishing due to an NEO impact.
I will follow up David Morrison's suggestion that I should
consult his
and Clark Chapman's 1994 NATURE paper, although I feel the
numbers will
have changed a little since then, and certainly changed since the
classic Shoemaker rates.
My suggesting 50 kT as an energy for which I'd like a probability
estimate was from the standpoint of ground destruction - large
enough
to wipe out a city but small enough for the probility to be
significantly higher than for a 1 MT blast. Whether or not a
colliding
object yielding 50 kT dumps it high in the atmosphere or closer
to
ground zero must surely depend on its composition, a factor which
could
be taken into account in the probability estimate using density
distribution data such as Ceplecha's.
If my query leads to a reduction in the number of woolly risk
estimates
reaching the general public it will have been well worth while.
After
all, scientists should not leave it to the scary news stories or
block-buster movies to convey essential warnings to the world.
Cheers ..... Colin Keay, Physics Department, University of
Newcastle,
NSW, Australia (whose government couldn't care less).
Dr Colin Keay
::::::: ~
~ To achieve anything really *
Physics Dept
~
:::::
~ worthwhile in research it is *
Newcastle Univ
~ :::\ | /
~ necessary to go against the *
NSW, AUSTRALIA 2308 ~
~ - o
- opinions of one's fellows.
*
phcslk@cc.Newcastle.edu.au
/ | \ ~
"Where the Wind Blows" *
www2.hunterlink.net.au/~ddcsk
~
~ ~ - Fred
Hoyle *
===============
(9) THE UNCERTAINTIES OF IMPACT PROBABILITY ESTIMATES
From Benny J Peiser <b.j.peiser@livjm.ac.uk>
I should perhaps clarify my comment in yesterday's CCNet: Our
knowledge of the size of 2000 BF19 has not really improved. In
fact,
the size estimates of asteroid 2000 BF19 are still pure
guesswork. I
understand that there are no colour observations of the object.
The
absolute magnitude of this asteroid has been revised from 19.6 to
19.2 and it seems likely that 2000 BF19 might be smaller than 1
km
across. But given the possibility that the magnitude may still
change, in addition to the fact that we have no idea what the
albedo
is, even this vague size estimate may still have to be revised
significantly.
If we take all of these limitations into account, it is obvious
that
impact probability calculations for 2000 BF19 still are rather
vague.
I, for one, would like to know, for instance, exactly how Andrea
Milani calculated his 1 a million chance of impact for an unknown
date in 2022.
The problems with the *annual* background impact risk for similar
objects of an unknown size between, say, 500-1000 m are actually
even
more problematic: they vary from somewhere between 1:20,000 -
1:1,000,000! It is this level of inherent uncertainty we have to
live
with for the time being. That's why it is so important to
eliminate
"virtual impactors" - whenever possible.
Benny J Peiser
----------------------------------------
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