PLEASE NOTE:
*
CCNet, 039/2000 - 24 March 2000
-------------------------------
(1) RESEARCHERS CAST DOUBT ON RECENT IMPACT RATE &
DEATH STAR THEORY
Benny J Peiser <b.j.peiser@livjm.ac.uk>
(2) PERIODICITY IN THE TERRESTRIAL RECORD
Bill Napier <wmn@star.arm.ac.uk>
(3) HAVE ASTEROID IMPACT RATES INCREASED SINCE THE
CAMBRIAN (540 M.Y. AGO)?
Andrew Glikson <andrew.glikson@anu.edu.au>
(4) MARSHALL ENGINEERS DEVELOPE METEOROID REPAIR KIT
Ron Baalke <baalke@jpl.nasa.gov>
(5) FIRST EROS FLYOVER MOVIE
Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
wrote:
(6) ASTEROID 2000 EW70
John Rogers <rogersfm@gte.net>
(7) METEOR CONFERENCE REPORT FROM GERMANY
Daniel Fischer <dfischer@astro.uni-bonn.de>
(8) SEARCHING FOR APOHELES FROM THE GROUND
David Tholen <tholen@IfA.Hawaii.Edu>
(9) DISCOVERY OF COMET SHOEMAKER-LEVY 9
Jeremy Tatum <UNIVERSE@uvvm.UVic.CA>
(10) HOW CAN WE CALCULATE THE AGE OF THE EARTH?
Bob Johnson <bob_johnson@talk21.com>
(11) AND, FINALLY: ANOTHER EXCUSE FOR A SATURDAY PARTY:
THE NEW MILLENNIUM STARTS TOMORROW!
The Guardian, 22 March 2000
===============
(1) RESEARCHERS CAST DOUBT ON RECENT IMPACT RATE &
DEATH STAR THEORY
From Benny J Peiser <b.j.peiser@livjm.ac.uk>
Two weeks ago, scientists at the University of
California-Berkeley
announced startling findings about a new research method that
appears
to prove evidence of erratic impact rates on the Moon. Should
these
results be verified, it would have significant implications for
the
the reliability of the current NASA estimates of the terrestrial
impact
hazard.
By analysing 155 minute glass spherules, derived from one gram of
lunar
soil that was thrown out by impacts (collected and returned by
the
Apollo 14 mission in 1971), Tim Culler, Tim Becker and Paul Renne
claim
to have discovered an unusual "peak of [impact] activity
that began 500
million years ago and continues today." (see CCNet, 10 March
2000)
The new research was originally suggested in 1991 by Professor
Richard
Muller who is well known for his controversial
"Nemesis" theory. Though
the dating method did not reveal any compelling evidence for
periodicity in the Moon's impact record, Professor Richard Muller
said
that "these findings fit in nicely with the Nemesis theory.
I think
most of the debris came from perturbations in the outer solar
system by
Nemesis." (CCNet, 10 March 2000)
The team of scientists claim that if their findings are correct,
it
would mean that "large impacts may have been more frequent
in the last
500 million years, creating more extinctions [...]". It
would indicate
that the impact rate on both the moon and on earth is not
constant, as
is generally believed. Hence, the impact hazard may have to be
re-assessed in view of non-uniform impact rates.
In two separate contributions to the CCNet, Bill Napier, one of
Britains leading cometary astronomers, based at Armagh
Observatory,
and Andrew Glikson, a geologist and impact expert at the
Australian
National University, have cast serious doubt on a number of
assumptions
of the Berkeley study.
Bill Napier points out that the death star theory has been
refuted
some time ago on the basis of astronomical data that is
incompatible
with Muller's particular hypothesis. While Napier rejects the
astronomical speculations of the "Nemesis" theory, he
does support the
notion of periodic impact peaks that may, in effect, be the real
trigger behind periodic mass extinctions detected in the
terrestrial
record.
Andrew Glikson, on the other hand, points to the limitations and
uncertainties that are inherent in the Berkeley teams new
method.
Clearly, this is the first attempt to use the novel dating
method,
and one gram of lunar soil should not be overinterpreted. While
Glikson applauds the highly significant information derived from
this research, he stresses nevertheless that sampling biases may
be
partly to blame for the apparent impression of an increased
impact rate.
According to Glikson, the small lunar sample contains highly
valuable
information, but it is not indicate as such an increase in the
impact rate
during the last 400 million years.
The issue of whether the terrestrial and lunar impact rates are
constant
over time or whether they are periodically punctuated by
espisodes of
increased cometary activity and sudden influx of cometary debris
into the
inner Solar System has significant ramifications for our own
view of the cosmic environment. That is the reason why the
innovative
Berkeley study is of great importance, regardless of its flaws or
eventual falsification. This is also why I would like to
encourage
further factual contributions to these enlightening and ongoing
impact
rate debates.
Benny J Peiser
==============
(2) PERIODICITY IN THE TERRESTRIAL RECORD
From Bill Napier <wmn@star.arm.ac.uk>
Dear Benny,
A few brief comments on the issue of periodicity in the
terrestrial
record may be in order, given recent peisergrams.
(i) Rich Muller refers to his paper with Marc Davis et al (1984)
as the
proper reference to the Nemesis theory, but readers should be
aware
that the idea of a 26 million-year solar companion was
independently
proposed by Whitmire & Jackson (1984) at essentially the same
time.
(ii) In the current Galactic environment, the half-life t_1/2 of
a body
orbiting the sun with period P million years may be shown to be
t_1/2 = 15/P**2 gigayears
approximately, due to the disruptive effect of passing molecular
clouds
(see the monograph by Bailey, Clube and Napier on The Origin of
Comets,
Pergamon 1990 for original references). Inserting P=26
million years
we find that the companion star would scarcely survive a
revolution,
let alone the age of the solar system. In fact the lifetime is so
short
that we are into the regime where the assumptions going into the
above
equation break down, and with luck Nemesis might keep going for
longer,
but the general message is clear.
Earlier assertions that the death star would survive were based
on
papers which either neglected the major perturbers (molecular
clouds)
altogether, or contained erroneous mathematics. One such paper,
widely
circulated in preprint but then withdrawn, gave a 20 gigayear
survival
time for the Oort cloud as against a probable true survival time,
for
the outer regions, of about 0.7 gigayears! The references which
Rich
invites us to look at in his home page are two 1984 papers, both
of
which neglect molecular clouds and so have long been
superseded.
Analysis aside, there is, according to Poveda (1988) and his
colleagues
in the binary star business, *empirical* evidence that strong
disrupting forces are at work on double stars. They find that any
surviving primordial remnant of the Oort cloud would by now have
only
2000-4000 AU radius. In these circumstances, the death star
(semi-major
axis A 80,000 AU, half-life varying as inverse cube of A) hasn't
a
chance.
(iii) Nemesis (the dark star, not my recent novel!) does adopt
the
central idea of a periodically disturbed Oort cloud first
proposed by
Napier & Clube in 1979. Originally, we thought in terms of
spiral arm
perturbations, before it was demonstrated that the vertical
Galactic
tide is stronger (Raup & Sepkoski's 26 Myr periodicity in the
marine
fossil record has been much debated but recent work seems to
uphold it
at a reasonable confidence level: Manley 1998). The idea that the
sun's
vertical oscillations might yield, through molecular cloud
perturbations, a cycle of the right order was proposed
independently by
Rampino & Stothers (1984) and Clube & Napier (1984).
Whether the variations in encounter rates with *individual*
molecular
clouds would in fact be sufficient to yield an observable
modulation is
doubtful, but the issue is no longer important: by 1987 it was
realized
that the smooth, continuous vertical galactic tide, due to the
smoothed-out matter of the Galactic disc, was the major force
giving
modulations. Thus (Napier 1987):
"If this tide is generated by a smooth, plane-parallel
continuum, then
it varies linearly with the effective density of local mass
perturbers.
This tidal background (Byl 1986) gives a flux of near parabolic
comets
into the planetary system directly proportional to the local
density.
The cometary flux therefore samples the instantaneous local
density as
the Sun moves up and down and, provided the `missing mass' in the
Galaxy has a half-thickness <~60 pc say, 30 Myr periodicity in
the
terrestrial record will be quite measurable. The effect of
galactic
tides on cometary orbits ... has been noted by Delsemme
(1987)."
This has been confirmed in full quantitative detail, for specific
Galactic models, by Matese et al (1995); see also Clube &
Napier
(1996), where essentially the same results were reached by
simpler
means, and the remarks in the comet monograph by Bailey et al
(1990).
Rich Muller's assertion that the variations in vertical density
are too
small to modulate the Oort cloud significantly is based on the
'individual encounter' concept and has long been refuted in the
literature. The modulation of comet flux is somewhere in the
range 2:1
to 5:1.
(iv) The predicted period from the Galactic hypothesis is very
uncertain and depends inter alia on how much dark matter one
chooses to
believe there is in the disc: it cannot simply be asserted that
the
Galactic hypothesis predicts 36 Myr. Richard Stothers (1999) now
claims
37+/-4 Myr, but a 27 Myr period cannot, in my opinion, be ruled
out. Of
course, garbage in, garbage out, and whether there is a
periodicity at
all is something which still has to be settled to everyone's
satisfaction. There are certainly mass extinctions and major
surges of
global disturbance, and these events do correlate (at the 98
percent or
so confidence level) with the incidence of large impact craters.
(v) All this may be of intrinsic interest, say through its
bearing on
catastrophism in a geological context, but is it relevant to the
here-and-now celestial hazard? I believe it is for a number of
reasons.
First, lunar crater counts being time-averaged, we want a model
for
their formation if we are to use them intelligently to get at
current
rates. For example, does the cratering happen in spikes when we
go
through the Galactic plane, or is it sinusoidal or flat? Is there
a
mismatch between current NEO rates and lunar ones? How does any
comet
flux modulation constrain the contribution of impactors from the
asteroid belt, the Jupiter family or the EK belt, none of which
could
yield a periodicity? To assert that we have, say, a uniform
(Poisson-distributed) rate of arrival of impactors is to adopt a
very
specific null hypothesis. And what if the model predicts
that Oort
cloud impactors, feeding their way in to the near-Earth
environment as
dark Halleys, are a major hazard? For these questions we need a
quantitative, holistic model relating the dynamical Oort cloud to
the
current celestial hazard, but at the moment it does not exist.
Best regards,
Bill Napier
=============
(3) HAVE ASTEROID IMPACT RATES INCREASED SINCE THE
CAMBRIAN (540 M.Y. AGO)?
From Andrew Glikson <andrew.glikson@anu.edu.au>
Research School of Earth Science,
Australian National University,
Canberra, ACT 0200
Shoemaker and Shoemaker [1] observed an increase by a factor of
about
x2 from a Proterozoic cratering rate of 3.8+/-1.9*10^-15 km^-2
yr^-1
(for craters with Dc >= 20 km, based on the study of
Australian
craters), to a cratering rate of 5.6+/-2.8*10^-15 km^-2 yr^-1
during
the last 120*10^6 years [2], consistent with the present-day rate
of
5.9+/-3.5*10^-15 km^-2 yr^-1 estimated from astronomical surveys.
Muller [3] and Culler et al. [4], on the basis of their
pioneering work
in laser 40Ar/39Ar age determination of impact spherules in an
Apollo
14 lunar regolith sample (sample 11199), support this suggestion,
pointing to the paucity of ages in the range of 2.0-0.4 * 10^9
years
and to an increase in age frequency since 0.4*10^9 years. Here I
consider these suggestions in view of (1) progressive elimination
of
craters through erosion and burial with time and (2)
differentiation
and mixing processes in the lunar regolith.
Due to low denudation and burial rates over much of the
Australian
interior, mean erosion and burial rates of less than 1 mm/10^6
years
are evident, enhancing crater preservation. On the other hand,
the
detection of craters in these commonly poorly exposed terrains
relies
heavily on geophysical methods - airborne magnetic, gamma ray
spectrometric, gravity, the application of which to crater search
is
still at an early stage. During the last few years several buried
Australian impact structures have been discovered onshore and
offshore
by these method, including Fohn [5], Glikson [6], Woodleigh [7],
and
new yet unreported impact craters in the McArthur Basin and South
Australia. Further discoveries are more than likely, with
consequent
updating of the cratering rate.
Given the one gram-scale of lunar regolith sample, the analysed
impact
spherules contains the signatures of a remarkable number of
impact
episodes, several of which can be correlated with terrestrial
impact
events. These include Late Imbrian impact maxima at 3.87, 3.83,
3.66,
3.53, 3.47*10^9 year, Eratosthenian impact signatures at 1.8, 1.4
(the
latter possibly representing a contribution from Copernicus),
1.08 and
1.03*10^9 year. Weaker uncertain signatures occur at 2.81, 2.53,
2.45*10^9 year. Potential correlations between age maxima in the
3.9-3.6*10^9 year range and terrestrial events are difficult to
obtain
in view of the high metamorphic grade of contemporaneous
terrestrial
suites, ie. in Greenland, North western Territory, Antarctic.
However,
sharp 3.53 and 3.47*10^9 year maxima correlate with peak magmatic
activity associated with the formation of Archaean
greenstone-granitoid
systems in the Pilbara (Western Australia), Kaapvaal (Transvaal),
Zimbabwe and India. The 1.8*10^9 year "high" (including
relatively
precise spherule ages of 1813+/-36.3*10^9 year) occurs within
error
from the 1.85 Ga age of the >250 km-diameter Sudbury impact
structure,
Ontario [8]. Two of the shallower age signatures overlap the age
of
impact spherules in the Hamersley Basin, Western Australia,
including a
2.56*10^9 year spherule marker in the Wittenoom Formation
(carbonates)
and a 2.47*10^9 year spherule unit in the Dale Gorge member
(banded
ironstones) [9]. The 1.08*10^9 year peak parallels the
emplacement of
the large layered mafic-ultramafic Giles Complex, central
Australia
[10]. Notable in their absence are spherule age peaks
corresponding to
the 2023+/-4*10^6 year 300 km-diameter Vredefort impact structure
[8].
The global c.2.7*10^9 year terrestrial greenstone-granitoid
events [11]
are not mirrored by impact spherule ages.
The lunar spherule age data reflect the late Devonian impact
cluster
(Charlevoix, Quebec, 367+/-15*10^6 year, D=54 km; Siljan, Sweden,
368+/-11*10^6 year, D=52 km; Ternovka, Ukraine, 350*10^6 year,
D=15 km;
Kaluga, Russia, 380+/-10*10^6 year, D=15 km; Ilynets, Ukraine,
395+/-5*10^6 year, D=4.5 km; Elbow, Saskatchewan, 395+/-25*10^6
year,
D=8 km) and contemporaneous Frasnian-Famennian and end-Devonian
extinctions of a range of rugose coral reefs, trilobites,
ammonoids,
brachiopods and chonodont species. These events are represented
by a
precise lunar spherule age of 352.6+/-6.6*10^6 year age and a
cumulative "high" of 11 spherule ages with errors
>100 m.y.. Less well
defined correlations may be outlined by lunar spherule ages of
303*10^6
year (Carboniferous-Permian boundary), 251*10^6 year
(Permian-Triassic
boundary), 103*10^6 year, 64-71*10^6 year (Cretaceous-Tertiary
boundary), 52*10^6 year and 38-32*10^6 year (late Eocene impacts
and
extinction), although the large errors on the 40Ar/39Ar ages
allow
little confidence in this regard.
Culler et al [4] interpret the spherule age distribution pattern
in
terms of increased impact incidence since about 400 Ma relative
to the
2.0-0.4 Ga range. However, vertical stratification and lateral
movement
in the lunar regolith by cumulative contribution of ejecta and
impact
condensates, gravitational creep and impact seismic-triggered
slumping
render it unlikely any single samples contain a non-biased record
of
the lunar bombardment history. A bimodal age distribution may be
expected from a combination in the lunar spherule record of (1)
signatures of some of the major impact episodes, contributing
maximum
volumes of melt and vapour condensates, and (2) a strong imprint
of
fallout from relatively young and/or proximal impacts,
concentrated at
upper levels of the regolith stratigraphy and partly masking
older
events. A natural lunar sampling bias may therefore ensue, as may
indeed be reflected by the dominance of pre-3.0*10^9 year and
post-360*10^6 year spherules in sample 11199. The role of
vertical
stratification may be elucidated by the study of 131Xe/126Xe
exposure
ages [12].
Culler et al. [4] comment on possible relations between an
increase in
the Phanerozoic impact incidence and biological radiation. From
present
evidence the strongest radiation, represented by the
"Cambrian
explosion" about 540*10^6 year, is not known to be related
to impacts,
although such may be identified by future studies. The oldest
Phanerozoic impact episode reflected in the spherule data
correlates
with the Frasnian-Famennian and late Devonian impact cluster and
associated extinction.
In conclusion, I consider the lunar regolith sample 11199
contains
highly significant information, yet does not necessarily imply an
increase in the impact rate over the last 400 m.y. Further
40Ar/39Ar
studies of lunar samples should resolve many of the outstanding
questions.
[1] E.M. Shoemaker, C.S. Shoemaker, 1996. Aust. Geol. Surv.
Org. J.,
16/4, 379-398.
[2] R.A.F. Grieve, E.M. Shoemaker, 1994. In T.
Gehrels (ed.), The
University of Arizona Press, Tucson,
Arizona, 417-462.
[3] R.A. Muller, R.A. 1993. Tech. Report LBL-34168,
Lawrence Berkeley
National Laboratory, Berkeley, CA.
[4] T.S. Culler, T.A. Becker, R.A. Muller, P.R. Renne,
2000. Science,
287, 1785-1789.
[5] J.G. Gorter, A.Y. Glikson, Meteoritics, in press.
[6] E.M. Shoemaker, C.S. Shoemaker, 1996. Lunar
Planet. Instit.
Abstracts, Huston.
[7] R.P. Iasky, A.J. Mory, 1999. Geol. Surv. W. Aust.
Report 69.
[8] R.A.F. Grieve, 1998. Impact craters list, Crater@gsc.nrcan.gc.ca
[9] B.M. Simonson, S.W. Hassler, 1997. Aust. J. Earth
Sci., 44,
37-48.
[10] A.Y. Glikson, C.G. Ballhaus, G.C. Clarke, J.W. Sheraton,
S.S.
Sun, 1995. Aust. Geol. Surv. Org. Bull.
239, 209 p.
[11] A.Y. Glikson, 1996. Aust. Geol. Surv. Org. J. Aust Geol.
Geohys.,
16/4, 587-608.
[12] Basaltic Volcanism of the Terrestrial Planets, Pergamon, New
York,
1981.
=============
(4) MARSHALL ENGINEERS DEVELOPE METEOROID REPAIR KIT
From Ron Baalke <baalke@jpl.nasa.gov>
Steve Roy
Media Relations Department March 21, 2000
Marshall Space Flight Center
Huntsville, AL
(256) 544-0034
steve.roy@msfc.nasa.gov
http://www.msfc.nasa.gov/news
RELEASE: 00-096
Marshall Engineers Undertake Real-Life 'Mission' To Protect NASA
Spacecraft, Crews in Event of Damage
When a spacecraft in the new movie "Mission to Mars" is
caught in a
fierce meteoroid storm, the beleaguered crew rallies to patch the
damaged hull, and thrilling movie music swells over the hiss of
escaping air ...
Real astronauts facing actual damage to their spacecraft won't
have the
luxuries of stuntpeople, special effects or inspiring musical
crescendos to save them from the cold vacuum of space. That's why
NASA
engineer Steve Hall and a team of researchers at NASA's Marshall
Space
Flight Center in Huntsville, Ala., are hard at work on a
real-life
hull-puncture repair kit -- one that will protect lives and
vehicles as
humans venture into space for longer periods of time.
The kit, intended for use on the International Space Station, is
designed to seal punctures up to 4 inches in diameter caused by
collisions with small meteoroids or space debris. With a few
simple
tools and a couple of extra-vehicular spacewalks, crewmembers can
safely repair punctures from outside damaged modules that have
lost
atmospheric pressure.
"It pays to be prepared," Hall says. A hole as small as
1 inch in
diameter in a vehicle the size of the Space Station could bleed
off
enough air in just one hour to put the crew at risk. That doesn't
give
them much time to locate the damage and seal the leak from inside
the
station -- especially when bulky equipment and experiment racks
may
block access to many of its interior walls.
"Protecting the lives of the crew is the most important
thing," Hall
says. "The safest approach is for the crew to evacuate and
seal off the
damaged module, allow it to fully depressurize and then make
repairs
externally."
The patching operation would begin with a spacewalk to locate
damage on
the exterior of the depressurized module. The surrounding area
would be
cleaned and the hole measured with special tools, enabling the
crew to
select patch components precisely tailored to the size of the
damage.
A second spacewalk would then deliver the patch kit to the work
site.
The patch consists of a clear disk that would be solidly bolted
to the
module's metal surface, covering the crack or puncture. A strong
epoxy
adhesive then would be pumped into the hollow disk by an injector
that
looks like a double-barreled caulking gun. Once this adhesive
cures --
a process that takes two to seven days -- it forms a cast plug
that
would completely seal the hole. Then the module would be
gradually
repressurized to verify proper function of the seal.
The patch is designed to last for at least six months, Hall says,
giving the crew ample time to make permanent repairs as needed.
Development and testing of the patch kit is under way at the
Marshall
Center. It is slated for delivery to the Space Station in
September.
Note to Editors / News Directors: To interview Steve Hall, or to
obtain
photos, media representatives may contact Steve Roy of the
Marshall
Media Relations Department at (256) 544-0034. For an electronic
version
of this release, digital images or more information, visit
Marshall's
News Center on the Web at:
http://www.msfc.nasa.gov/news
[NOTE: Images supporting this release are available at
http://www1.msfc.nasa.gov/NEWSROOM/news/photos/2000/photos00-096.htm]
===============
(5) FIRST EROS FLYOVER MOVIE
From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
wrote:
NEAR image of the day for 2000 Mar 23
http://near.jhuapl.edu/iod/20000323/index.html
On March 7, 2000, the imager on the NEAR Shoemaker spacecraft
acquired
the first of several planned "flyover movies" of Eros.
This one shows
the "saddle" region from a range of 205 kilometers (127
miles). A
flyover's purpose is to show a region of the asteroid during
continually changing lighting conditions, with solar illumination
coming from a variety of directions and elevations above the
surface.
With the Sun in different positions, features with different
orientations become more evident. Also, with the Sun low to the
surface, brightness variations are dominated by the shadows cast
by
landforms. In contrast, with the Sun high in the sky, brightness
differences are dominated by the intrinsic differences in
reflectivity
of the surface materials. The combination of illuminations
maximizes
the ability to characterize landforms and to separate the effects
of
topography from differences in reflectivity.
----------------------------------------------------------------------------
Built and managed by The Johns Hopkins University Applied Physics
Laboratory, Laurel, Maryland, NEAR-Shoemaker was the first
spacecraft
launched in NASA's Discovery Program of low-cost, small-scale
planetary
missions.
===============
(6) ASTEROID 2000 EW70
From John Rogers <rogersfm@gte.net>
[as posted on the Minor Planet Mailing List, MPML, 23 March 2000]
I was able to image asteroid 2000 EW70 last night, through cirrus
clouds. I created a video for those who may be interested:
http://ourworld.compuserve.com/homepages/johnerogers/2000ew70.mov
(Apple Quick Time 356kb)
http://ourworld.compuserve.com/homepages/johnerogers/2000ew70.avi
(Windows AVI 272kb)
The closest approach to the Earth will be tonight (March 24.42
UT).
Clear Skies,
John
================
(7) METEOR CONFERENCE REPORT FROM GERMANY
From Daniel Fischer <dfischer@astro.uni-bonn.de>
http://www.geocities.com/skyweek/mirror/182.html
Story 3:
New discoveries about the Leonids show amateur astronomy at its
best
The systematic observation of meteors with the naked eye,
photographic and especially image-intensified video
cameras has
become one of the rare fields in astronomy in which
amateurs can not
only contribute to science - but where the science
produced from the
amateur data can be crucial to advance the whole field.
This has
become clear again at the annual meeting of the German
Working
Group for Meteors (AKM) at the hospitable Sternwarte
Radebeul on
March 17-19, where both new insights into the workings of
the
Leonids were revealed but also the high state of 'routine'
observations
these days.
Surprising fine structure in the ZHR
The main discoveries about the Leonids, as derived from a
torrent of
data from the 1999 storm presented at Radebeul were:
There is an enormous
fine structure in the activity profile,
i.e. the rate of
meteors seen as a function of time, during the
hour-long storm - but
it becomes evident only when one looks
at observations
(visual and esp. by video) from specific locations
in the world. If one
adds up the profiles from all places (Tenerife
to Jordan), the
details average out. The video data from the
Jordan camp in
particular reveal a strong 'early' peak of activity
around 1:45 UTC, 20
minutes before the sharp main peak, plus
enhanced activity
around 2:30 UTC - all these features are
considered significant
now. Confirmation by other (non-visual)
methods could be
forthcoming.
Since observers at
other sites (Spain was covered particularly
well) saw and recorded
a rather different profile than Jordan or
France, it is even
possible to generate a 'tomographic
picture' of the dust
trail(s) that made the meteor rate explode.
The 1:45 UTC peak,
e.g. was probably due to Earth's distant
encounter with a dust
trail from Tempel-Tuttle's 1932
perihelion passage,
though a significant effect on the meteor rate
had not been
predicted. The main peak has resulted from the
1899 dust trail, of
course, confirming brilliantly the model
calculations by D.
Asher and R. McNaught.
Other surprises were
the lack of faint meteors - video
cameras with better
limiting magnitudes but smaller fields of
view saw far fewer
meteors than those with worse sensitivity
but larger fields -
and a possible breakdown of the
geometrical ZHR
correction formula. Since decades the
influence of the
elevation (h) of the radiant on the number of
meteors seen has been
corrected geometrically into the Zenithal
Hourly Rate (ZHR),
dividing the seen number of meteors by
sin(h). (Other
corrections, such as for obstructions in the field of
view and the sky
quality, apply as well.) The data from the 1999
Leonid storm cast a
doubt on that simple formula: Those with a
low h got ZHRs of only
2000-3000 for the peak despite the
correction formula,
while those with the highest h got 5000 as
the peak rate - it
seems that the sin(h) effect must be replaced
by a (sin(h))**gamma
correction, with gamma other than one.
Given the success of the Asher/McNaught approach in
predicting the
time of the storm (a feat hailed by the IMO as equal in
importance to
the basic understanding of how meteors work that came
after the 1833
storm; Rendtel in WGN 28 [Feb. 2000] 1), there is great
optimism now
that there will be even bigger storms in 2001 and 2002.
The AKM
which had gone to Mongolia in 1998 and fielded teams to
Tenerife and
Spain in 1999 has now started preparing two expeditions
for 2001: One
will probably return to Mongolia (shudder!), the other go
to Northern
Australia.
There is life beyond the meteor storms, too
Routine meteor observing can be a tough job, especially
under bad sky
conditions and when no major meteor streams are active:
Only a
handful of super-dedicated observers have spent more than
1000
hours gazing at the sky (with J|rgen Rendtel's breaking of
the 4000
hour mark in 1999 an epic exception) - but video comes to
the rescue.
The image-intensified video cameras have by now been
automated to
such a degree that a couple of them watches the (mostly
poor...)
German sky every night, feeding the signal directly into a
PC where all
meteors are detected and logged.
Since not only numbers but also (very rough) brightness
values, the
direction and angular speed are recorded, many advanced
studies can
be done on the basis of these data - especially checking
the reality of
'new' weak meteor streams that visual observers believe to
have
discovered now and then. Thanks to such video coverage in
January
and February 2000 its was possible, e.g., to dismiss the
existence of the
'Xi Bootids' while discovering possible other radiants in
that region of
the sky. Within 3 to 5 years there could be enough video
cameras at
work that all meteor activity in the sky is monitored all
the time and
from anywhere in the world.
=============================
* LETTERS TO THE MODERATOR *
=============================
(8) SEARCHING FOR APOHELES FROM THE GROUND
From David Tholen <tholen@IfA.Hawaii.Edu>
Benny,
Regarding CCNet, 17 March 2000:
OBJECTS INTERIOR TO EARTH'S ORBIT PART OF NEO-PROBLEM
> From Alberto Cellino <cellino@otoax4.to.astro.it>
>
> Hello Benny,
>
> this is the first time I am sending a short contribution to
the CCNet
> discussion, since I must admit that I am only a desultory
reader. I
> write in order to express my appreciation of the letter by
Duncan Steel
> (CCNet, 16 March). I like his approach, and I agree with
most of what
> he said, mainly the general concept that nothing is
straightforward in
> the problem of NEO detection and appreciation of the impact
risk.
>
> In this context, I would like to point out that a paper has
been
> recently published by Icarus (the authors being P. Michel,
V. Zappala`,
> P. Tanga and myself, Icarus 143, 421-424), that shows that a
class of
> NEOs with orbits completely interior to Earth's must exist,
and should
> be fairly abundant. This conclusion follows from numerical
integrations
> of a large number of NEOs of different orbital classes. The
> integrations show that these objects spend a significant
fraction of
> their lifetime in orbits having aphelion distances smaller
than the
> perihelion distances of the Earth. These objects have been
called
> preliminarily IEOs (objects Interior to Earth's Orbit) and
should be as
> populous as at least 60-70% of the current Aten population.
These
> objects are important and should be taken into account when
assessing
> the NEO impact risk and when planning discovery surveys.
>
> The major problem with IEOs, however, is that they are
hardly
> observable from the ground, since they never reach large
solar
> elongation angles. Therefore, a space-based facility seems
most
> appropriate and needed if we want to find these objects.
I have been addressing the issue described above for the better
part of
a decade now. My concern arose at the time that the Spaceguard
Survey
report was written (1991; I was a member of the committee), in
which
search strategies were described that were based on the known
population of NEOs. I was concerned that the observational biases
in
the known population could lead to an observational strategy that
would
maintain those biases. I identified two key problems, the second
a
consequence of the first. At small solar elongations, objects
close to
the Earth exhibit high phase angles and are therefore fainter
than an
identical object at an identical distance from the Earth, but
positioned near the opposition point where the phase angle is low
(it's
like the difference between looking at a crescent Moon and a
nearly
full Moon). On average, such objects would be 2 magnitudes
fainter when
seen at small solar elongations. To compensate for that, one
needs to
use a larger telescope. The second problem is that larger
telescopes
tend to have smaller fields of view, making it difficult to image
large
portions of the sky, and as LINEAR has demonstrated, the key to
finding
NEOs is to cover large amounts of sky.
I was finally in a position to do something about the issue in
1996,
when the world's first mosaic CCD camera became available on our
2.24-m
telescope on Mauna Kea, providing a 19 arcmin field of
view. With that
camera, we estimated that we had a reasonable chance of finding
NEOs at
small solar elongation and initiated a pilot project to
demonstrate the
feasibility of doing such observations from the ground.
I will be the first to admit that there are significant
advantages to
using a space-based facility. One doesn't need to worry
about the
weather turning bad on you, and such a facility would almost
certainly
be dedicated to the task, unlike our 2.24-m telescope, which must
be
shared with many other faculty members, postdocs, graduate
students,
and visiting astronomers. But over the last four years, my
graduate
student (R. J. Whiteley) and I have demonstrated that objects at
small
solar elongation are observable from the ground. We discovered
the
Apollo asteroid 1997 QK1 (a PHA) at a solar elongation of 84 deg.
We
discovered the Apollo asteroid 1998 DV9 (another PHA) at a solar
elongation of 78 deg. We discovered 1998 DK36, what is likely to
be the
first "IEO" (we have been using the term
"Apohele", a Hawaiian word
meaning "orbit", which follows the alliterate scheme of
Amor, Apollo,
and Aten) at a solar elongation of 74 deg. We discovered the
large (4.6
km!) Apollo asteroid 1999 OW3 at a solar elongation of 72 deg. As
you
can tell, as we've gained experience with these kinds of
observations,
we've been pushing to smaller solar elongations. Our most recent
discovery, the Amor asteroid 2000 AB246, was made at a solar
elongation
of 66 deg. Our discovery rate is about one object per 20
square
degrees of sky (and in a wonderful example of how statistics can
be
skewed in your favor, that is the highest discovery rate of any
NEO
search effort, on a per square degree of sky coverage basis; the
problem, of course, is that it takes us quite a while to cover
that
much sky).
So in conclusion, while there are advantages to doing the job
from
space, the big disadvantage is cost. A space-based mission would
dwarf
the current funding for NEO search efforts. It seems prudent to
utilize
our ground-based resources to the fullest extent possible first.
To
that end, I have been attempting to secure funding to expand on
our
current minimal efforts. It's time to switch from a feasibility
demonstration to monthly observing.
Dave Tholen
=================
(9) DISCOVERY OF COMET SHOEMAKER-LEVY 9
From Jeremy Tatum < UNIVERSE@uvvm.UVic.CA
>
Re Ron Baalke's item on NEAR-Shoemaker, in which he said that
Comet
Shoemaker-Levy 9 was discovered by Gene Shoemaker and David Levy,
I am
sure that Gene and David would be the first to point out that
they were
but participants in the team that discovered it, and that they
weren't
actually the first to see it.
Jeremy Tatum
MODERATOR'S NOTE: Once again, Jeremy is quite correct: Comet SH9
was discovered on photographic films in 1993 by Carolyn and the
late
Eugene Shoemaker and David Levy. It was first observed by their
colleague Jim Scotti (Spacewatch) shortly after its discovery.
===============
(10) HOW CAN WE CALCULATE THE AGE OF THE EARTH?
From Bob Johnson < bob_johnson@talk21.com
>
Benny,
Andrew Glikson's erudition is formidable - I only wish I
understood
half of his jargon.
Nevertheless, I am puzzled as to the derivation of his enormous
timescales - what is the basis for the age of the earth at some
4.5*10^9 years? Compare, for example, recent comments on <50
years old
formations that geologists have stated that they would have
considered
to have taken x*10^6s years to form had they not known the truth.
Best regards, Bob Johnson
================
( 11) AND, FINALLY: ANOTHER EXCUSE FOR A SATURDAY PARTY:
THE NEW MILLENNIUM
STARTS TOMORROW!
From The Guardian, 22 March 2000
http://www.newsunlimited.co.uk/science/story/0,3605,149964,00.html
Another excuse for a party comes on Saturday - it's new year's
day,
writes Duncan Steel
When does (or did) the new millennium start? On January 2000, or
2001?
The answer is neither. The full 2000 years are up this Saturday,
March
25.
Nowadays it's difficult to imagine the year beginning with any
date but
January 1. In the modern world, though, that's quite a new
convention.
France started it in 1564. Various other European nations soon
followed, previously using events such as Christmas or Easter.
Another
much used was September 1, the start of a tax cycle introduced by
Constantine the Great in AD 312. That was employed in the Holy
Roman
Empire until Napoleon abolished it in 1806.
Britain used March 25 until 1752. That's why the income tax year
begins
with April 6. When Britain reformed its calendar, coming in line
with
the Continent, eleven days were deleted. The non-appearing dates
were
September 3 to 13 in 1752, making that year just 355 days long,
and
1751 was even shorter, at 282 days, running only from March 25 to
December 31.
To keep the tax years of equal length, the date of reckoning was
postponed by 11 days, to April 6. Over the decades that's caused
much
argument. Count off the days from March 25 and you get 12, not
11. Why?
Although March 25 was the start of the legal year, it was the
final day
of the financial quarter.
But why March 25? It all goes back to the origin of our year
numbers.
The monk Dionysius Exiguus was charged by the pope in AD 525 to
calculate a new set of Easter tables for the subsequent century.
In
doing so he developed a framework for past year numbers, that
being the
system we have inherited.
Actually he got it wrong, and it seems likely that Jesus was born
in 5
BC. The error originated in a misinterpretation of Augustus's
reign:
the monk took that to count from 27 BC, when the emperor took
that
moniker, rather than 31 BC when, under the name Octavian, he
defeated
Mark Antony and Cleopatra to seize power.
Dionysius needed to derive not the year starts, but the dates of
Easter. That church festival is celebrated as the first Sunday
after
the first full moon after the spring equinox (although the
"moon" there
is the ecclesiastical moon, not the astronomical moon, and the
equinox
comes from the ecclesiastical sun, rather than the sun in the
sky). So
the equinox was vital in his calculations.
Traditionally the Incarnation, or Annunciation, when the
Archangel
Gabriel appeared to Mary, occurred at the spring equinox, taken
to be
March 25. It was that date, in the year we call 1 BC, which
Dionysius
adopted as the basis for his year count, because Easter was his
concern. The annual numbering he termed (in Latin) as the Year of
the
Incarnation. And that means that the two full millennia will have
elapsed come March 25 in the year 2000. This Saturday, in
fact.
So why do we use January 1? In part because the Romans earlier
employed
that date, but its eventual ascendancy in the Christian calendar
stems
from a remarkable pair of coincidences. The human gestation
period of
nine months leads to a skip forward from the Annunciation on
March 25
to December 25 (the traditional winter solstice). The latter was
a
pagan festival that had already been purloined as Christmas by
the era
of Dionysius.
The second coincidence is this. If Jesus was born on December 25,
then
as a Jew he would be circumcised on the eighth day, which is
January 1.
Hence our New Year, and the fact that it is celebrated as the
Feast of
the Circumcision in the liturgical calendar.
And here's the final item of confusion. Britain used March 25 as
New
Year until 1752, but that year count was wrong. In effect it
counted
from that date in AD 1, rather than the correct 1 BC. If we had
not
changed the calendar in the 18th century then we'd just be coming
to
the end of 1999.
When does the new millennium dawn? On Saturday March 25, if one
really
wants to count from the year dot. That's when the full 2000 years
are
up.
* Duncan Steel teaches physics at the university of Salford. His
book on
calendar matters, Marking Time, is published by Wiley, New
York.
Copyright Guardian Media Group plc.
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