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
Britain’s 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 team’s 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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