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CCNet DIGEST, 24 February 1999
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The fully indexed archive of the CCNet, from February 1997 on,
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          QUOTE OF THE DAY

          "But even as new threats are being found much more frequently
          than before, 'we're way behind,' said Don Yeomans, manager of
          NASA's Near Earth Object program."
          (Miami Herald, 23 February 1999)


(1) COSMIC ROCKS GANGING UP ON EARTH
    Miami Herald, 23 February 1999
    http://www.herald.com:80/usa/digdocs/024413.htm

(2) COSMICALLY MODIFIED FOOD HELPS FILL CHINA'S PADDIES
    CNN Interactive
    http://cnn.com/TECH/space/9902/22/china.space/index.html

(3) DETERMINING COLLISION PROBABILITIES BETWEEN SMALL SOLAR SYSTEM
    BODIES
    S.P. Manley et al., ARMAGH OBSERVATORY

(4) ON THE UNUSUAL ACTIVITY OF THE PERSEID METEOR SHOWER
    P. Jenniskens et al., NASA, AMES RES CTR

(5) CRATERING RATES ON THE GALILEAN SATELLITES
    K. Zahnle et al., NASA, AMES RES CTR

(6) THE STABLE KOZAI STATE FOR ASTEROIDS & COMETS
    G.F. Gronchi & A. Milani, UNIVERSITY OF PISA

(7) ON THE POSSIBLE EXTRATERRESTRIAL INFLUENCE ON EARTH'S CLIMATE & THE
    ORIGIN OF THE OCEANS
    D. Deming, UNIVERSITY OF OKLAHOMA

===================
(1) COSMIC ROCKS GANGING UP ON EARTH

From the Miami Herald, 23 February 1999
http://www.herald.com:80/usa/digdocs/024413.htm

SETH BORENSTEIN
Herald Washington Bureau

WASHINGTON -- A dangerous asteroid will whiz by Earth today in a cosmic
close call. Similar near misses are expected March 2, 18, 26 and April
1.

Astronomers are discovering potential killer asteroids at a record
pace.

The public's flirtation last year with fear of menacing space rocks --
fueled by two fictionalized movies and one widely reported threat --
has faded. But astronomers scanning the sky with new technology are
finding more asteroids than ever.

There are almost weekly additions to science's official list of
"potentially hazardous asteroids." In 1998, scientists found 55 of
the would-be killers -- more than in the previous six years combined.
Now the all-time list is at 163 and growing.

None of these rocks is expected to hit Earth directly. But they are too
close for comfort.

"It's crazy," said University of Arizona astronomer Tim Spahr,
scientist for one of three teams of sky searchers. "It's not even
active, it's just insane."

Here are a few recent examples:

- This afternoon, a half-mile-wide asteroid discovered in January will
whiz by Earth at a distance of 3.4 million miles. That rock, called
1999 BJ8, is expected to give Earth its closest call in the next 22
months, according to the ever-changing list of dangerous objects.

- On Feb. 4, a 30-foot-wide rock came incredibly close to Earth. It was
only 632,000 miles away (2 1/2 times the distance from here to the
moon), and scientists didn't notice until it had already passed and was
moving away.

- For a few brief hours Feb. 16, it looked as if a
two-thirds-of-a-mile-long asteroid discovered in January could come
even closer than that -- maybe even hit us in 2066 or 2073. But new
photographs showed that the rock with an unusual orbit would be close,
but not that  close.

The biggest reason that astronomers are finding more of these near-miss
asteroids is the redirection of Air Force technology from tracking
killer satellites to spotting killer rocks.

Massachusetts Institute of Technology's Lincoln Laboratory, a federally
funded national security research center outside Boston, changed the
asteroid tracking world last March when it began using Air Force
technology to scan the sky with a telescope based in New Mexico. The
lab's program, called LINEAR, has found 38 of the last 59 asteroids.

"We have it down to a point where we're in a groove," LINEAR chief
Grant Stokes said.

But even as new threats are being found much more frequently than
before, "we're way behind," said Don Yeomans, manager of NASA's Near
Earth Object program.

That's because last year, NASA set a goal of finding most potential
asteroid threats within 10 years. Asteroids are considered a threat if
they are bigger than half a mile wide and are going to come within five
million miles of Earth.

Astronomers guess there are 2,000 such asteroids out there, and so far
they have found about 8 percent of them. So even though the number of
asteroids being found is soaring, it's not growing fast enough, experts
said.

NASA, which spends $3.5 million a year on asteroid tracking, is
speeding up the process. A second LINEAR program goes on-line
this spring. With a second LINEAR, astronomers should find 90 percent
of those killer rocks in about 16 or 17 years.

The last time a big asteroid hit was 65 million years ago (sic). That
was the asteroid that landed in the Yucatan and is thought to have
wiped out the dinosaurs.

Of course, there are millions of others that could cause major
disasters, like a 150-foot-wide one that exploded over Tunguska,
Siberia, in 1908 and leveled thousands of square miles of forests.

"If you were going to find every object that's going to threaten the
Earth with a Tunguska, you're talking millions, not thousands," said
researcher Jeff Larsen of the University of Arizona's Spacewatch
tracking program.

Copyright 1999, Miami Herald

==============
(2) COSMICALLY MODIFIED FOOD HELPS FILL CHINA'S PADDIES

From CNN Interactive
http://cnn.com/TECH/space/9902/22/china.space/index.html

February 22, 1999

BEIJING (Reuters) -- Ever on the lookout for innovative ways to feed
its 1.2 billion people, China has turned to conditioning rice and
wheat seeds in space to lift crop yields, state news media said late on
Saturday.

Seven times in the past 11 years China has launched seed-laden
satellites and balloons into the upper reaches of the Earth's
atmosphere to expose them to strong doses of solar radiation, Xinhua
news agency said.

The result was a minimum 10 percent boost over normal crop yields when
the irradiated seeds were planted back on Earth.

"Seeds subjected to radiation at an altitude of between 20-400 km and
other forms of special treatment offer greatly improved and much higher
yields," it quoted Li Jinguo, a genetics expert at the Chinese Academy
of Sciences, as saying.

The conditioned seeds have been planted across 70,000 hectares (175,000
acres) in Heilongjiang province in the northeast, Jiangxi and Shandong
provinces in the east, and Hunan province in central China, Xinhua
said.

Copyright 1999 Reuters Limited. All rights reserved.

==================
(3) DETERMINING COLLISION PROBABILITIES BETWEEN SMALL SOLAR SYSTEM
    BODIES

S.P. Manley*), F. Migliorini, M.E. Bailey: An algorithm for determining
collision probabilities between small solar system bodies. ASTRONOMY &
ASTROPHYSICS SUPPLEMENT SERIES, 1998, Vol.133, No.3, pp.437-444

*) ARMAGH OBSERVATORY, COLL HILL BT61 9DG,NORTH IRELAND

The introduction of surveys (e.g. Spacewatch, OCA-DLR) dedicated to the
discovery of asteroids and other small bodies is likely to increase the
number of known objects to many times the current figure of roughly 30
000. Previous methods for determining collision probabilities amongst
these objects (e.g. those due to Opik, Wetherill, Greenberg and
Kessler) all have idiosyncrasies which make them inappropriate for
analyses of interactions between large numbers of solar system bodies.
Here we present an adaptation of the Wetherill and Greenberg methods,
which avoids approximations made by Opik but which remains accurate and
fast enough in its implementation to allow the direct analysis of the
collision probabilities and impact velocities of thousands of
potentially colliding objects. Copyright 1999, Institute for Scientific
Information Inc.

==================
(4) ON THE UNUSUAL ACTIVITY OF THE PERSEID METEOR SHOWER

P. Jenniskens*), H. Betlem, M. deLignie, C. ter Kuile, M.C.A. van
Vliet, J. vant Leven, M. Koop, E. Morales, T. Rice: On the unusual
activity of the Perseid meteor shower (1989-96) and the dust trail of
comet 109P/Swift-Tuttle. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL
SOCIETY, 1998, Vol.301, No.4, pp.941-954

*) NASA,AMES RES CTR,MAIL STOP 239-4,MOFFETT FIELD,CA,94035

We present the first measurements of the radiant and orbit of
meteoroids that are part of the unusual Perseid activity called the
'Perseid Filament'. This filament was encountered by Earth in the years
before and after the return of the comet to perihelion in December of
1992. Between 1989 and 1996, there were brief meteor outbursts of
near-constant duration with a symmetric activity profile. In 1993,
however, rates increased more gradually to the peak. That gradual
increase is identified here as a separate dust component, which we call
the 'Nodal Blanket'. We find that the Nodal Blanket has a very small
radiant dispersion. On the other hand, the Perseid Filament has a
radiant that is significantly dispersed and systematically displaced by
0.3 degrees. This dispersion implies that unusually high ejection
velocities or planetary perturbations must have had time to disperse
the stream. In both cases, one would expect a rapid dispersion of
matter along the comet orbit. In order to explain the concentration of
dust near the comet position, we propose a novel scenario involving
long-term accumulation in combination with protection of the region
near the comet against close encounters with Jupiter due to librations
of the comet orbit around the 1:11 mean-motion resonance. Copyright
1999, Institute for Scientific Information Inc.

==================
(5) CRATERING RATES ON THE GALILEAN SATELLITES

K. Zahnle*), L. Dones, H.F. Levison: Cratering rates on the Galilean
satellites. ICARUS, 1998, Vol.136, No.2, pp.202-222

*) NASA, AMES RES CTR,MS 245-3,MOFFETT FIELD,CA,94035

We exploit recent theoretical advances toward the origin and orbital
evolution of comets and asteroids to obtain revised estimates for
cratering rates in the jovian system. We find that most, probably more
than 90%, of the craters on the Galilean satellites are caused by the
impact of Jupiter-family comets (JFCs). These are comets with short
periods, in generally low-inclination orbits, whose dynamics are
dominated by Jupiter. Nearly isotropic comets (long period and Halley-
type) contribute at the 1-10% level. Trojan asteroids might also be
important at the 1-10% level; if they are important, they would be
especially important for smaller craters. Main belt asteroids are
currently unimportant, as each 20-km crater made on Ganymede implies
the disruption of a 200-km diameter parental asteroid, a destruction
rate far beyond the resources of today's asteroid belt.
Twenty-kilometer diameter craters are made by kilometer-size impacters;
such events occur on a Galilean satellite about once in a million
years. The paucity of 20-km craters on Europa indicates that its
surface is of order 10 Ma. Lightly cratered surfaces on Ganymede are
nominally of order 0.5-1.0 Ga. The uncertainty in these estimates is
about a factor of five. Callisto is old, probably more than 4 Ga. It is
too heavily cratered to be accounted for by the current flux of JFCs.
The lack of pronounced apex-antapex asymmetries on Ganymede may be
compatible with crater equilibrium, but it is more easily
understood as evidence for nonsynchronous rotation of an icy
carapace. (C) 1998 Academic Press.

===================
(6) THE STABLE KOZAI STATE FOR ASTEROIDS & COMETS

G.F. Gronchi & A. Milani: The stable Kozai state for asteroids and
comets - With arbitrary semimajor axis and inclination. ASTRONOMY AND
ASTROPHYSICS, 1999, Vol.341, No.3, pp.928-935

UNIVERSITY OF PISA, DIPARTIMENTO MATEMAT,VIA BUONARROTI 2,I-56127
PISA, ITALY

Semianalytical averaging is used to compute secular perturbations on
the orbits of asteroids and comets; the method is applicable even for
planet-crossing orbits. We prove that for every value of the
asteroid/comet semimajor axis, and for an arbitrary number of
perturbing planets, there is a stable region of orbits free from node
crossings; it corresponds to either circulation or libration of the
argument of perihelion. This has implications on the possibility of
collisions with the planets and also, when encounters are possible, on
the algorithms to compute the probability of collision.
Copyright 1999, Institute for Scientific Information Inc.

====================
(7) ON THE POSSIBLE EXTRATERRESTRIAL INFLUENCE ON EARTH'S CLIMATE & THE
    ORIGIN OF THE OCEANS

D. Deming: On the possible influence of extraterrestrial volatiles on
Earth's climate and the origin of the oceans. PALAEOGEOGRAPHY
PALAEOCLIMATOLOGY PALAEOECOLOGY, 1999, Vol.146, No.1-4, pp.33-51

UNIVERSITY OF OKLAHOMA,SCH GEOL & GEOPHYS,NORMAN,OK,73019

A consideration of observational and circumstantial evidence suggests
that Earth may be subject to high influx rates (10(11)-10(12) kg/yr) of
extraterrestrial-sourced volatile elements (carbon, hydrogen, oxygen,
nitrogen) derived from comets or other primitive solar-system material.
The total extraterrestrial influx rate may be four to five orders of
magnitude greater than previously thought, large enough to account for
today's total near-surface inventories of water and carbon. The
possibility of high rates of extraterrestrial volatile-accretion
suggests a new climatic paradigm wherein Earth's surface temperature is
influenced by conflicting internal and external processes. A variable
influx of volatile elements tends to warm the Earth, while terrestrial
processes cool the planet by absorbing these gasses at a more uniform
rate. Variations in extraterrestrial influx rates may explain the
variation of sea level and mean global temperature over geologic time,
as well as some types of climate change, the occurrence of the
Pleistocene ice ages, and the asymmetry of the Phanerozoic climate
record (sudden warmings, slow coolings). The extraterrestrial influx
rate may also act as the pacemaker of terrestrial evolution, at times
leading to mass extinctions through climatic shifts induced by changes
in accretion rates with concomitant disruptions of the carbon and
nitrogen cycles. Life on Earth may be balanced precariously between
cosmic processes which deliver an intermittent stream of
life-sustaining volatiles from the outer solar system or beyond, and
biological and tectonic processes which remove these same volatiles
from the atmosphere by sequestering water and carbon in the crust and
mantle. (C) 1999 Elsevier Science B.V. All rights reserved.

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