PLEASE NOTE:
*
CCNet DIGEST 4 January 1999
---------------------------
(1) AUSTRALIAN MINISTER FOR DEFENCE REFUSES TO SUPPORT SPACEGUARD
Michael Paine <mpaine@tpgi.com.au>
(2) THE BIBLIOGRAPHIC METEOR DATABASE
Rainer Arlt <rarlt@aip.de>
(3) LATEST NEAR NEWS
http://near.jhuapl.edu/news/flash/99jan03_1.html
(4) QUADRANTIDS
Mark Davis <MeteorObs@charleston.net>
(5) LIFE IN EXTREME ENVIRONMENTS
Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
6) LOUIS FRANK REFUSES TO BACK DOWN: 'SMALL COMETS ARE
REAL'
Andrew Yee <ayee@nova.astro.utoronto.ca>
(7) JANUARY'S CHILLY METEORS
NASA Science News <expressnews@sslab.msfc.nasa.gov>
(8) ROSETTA PASSES IMPORTANT DESIGN REVIEW
Andrew Yee <ayee@nova.astro.utoronto.ca>
==================
(1) AUSTRALIAN MINISTER FOR DEFENCE REFUSES TO SUPPORT SPACEGUARD
From Michael Paine <mpaine@tpgi.com.au>
Dear Benny,
Disappointing news for the end of 1998. I have received a reply
to a
letter I wrote to Bruce Scott, the new "Minister Assisting
the Minister
for Defence" in October. The reply was from the "Chief
of Staff, Office
of the Minister for Defence" and, predictably from a
tunnel-visioned
bureaucrat, it dismissed the issue with the sentence
"Asteroid detection
is currently not a priority for Defence" (if it was I
wouldn't have
raised the issue!!).
On the positive side the Shadow (Opposition) Minister for
Science,
Martyn Evans, has been supportive and has indicated he plans to
raise
it as a budget issue in the Senate.
See http://www1.tpgi.com.au/users/tps-seti/spacegd3.html#scott
for details.
Best wishes for 1999
Michael Paine mailto:mpaine@tpgi.com.au
New South Wales Coordinator
The Planetary Society Australian Volunteer Coordinators
=====================
(2) THE BIBLIOGRAPHIC METEOR DATABASE
From Rainer Arlt <rarlt@aip.de>
The BIBLIOGRAPHIC METEOR DATABASE is vailable on the WWW now at
http://www.imo.net/bib/bibindex.html.
Almost 10000 articles, books, and notes about meteors and related
topics are stored in the database, dating from 1794 to 1998. The
web
page allows you to search through the files by a tree of
keywords, an
authors index and an input mask for direct character string
searches in
titles, authors, and keywords. Note that the topics you search
for may
not be present in the title; this is why a tree of keywords
was
created in
http://www.imo.net/bib/bibindex.html,
which satisfies your query despite the topic not being present in
the
title of the article.
The Bibliographic Meteor Database was created in 1982 by Paul
Roggemans
(Belgium), the first computerized version was mainly made by
Ghislain
Plesier (Belgium), and the work was continued by Rainer Arlt
(Germany),
who are very much indebted to Mr. Dale (Belgium), Mr. Vanderbecq
(Belgium), Jurgen Rendtel (Germany), Jeff Wodd (Australia), Dr.
Alexandra Terentjeva (Russia), Evelyne Blomme (France), Ludwig
Cluyse
(Belgium), Albert de Kersgieter (Belgium), Dirk Laurent
(Belgium), Jean
Meeus (Belgium), Ann Schroyens (Belgium), Christian Steyaert
(Belgium),
Sabine Bohmer (Germany), and Manuela Trenn (Germany) for their
help and
support.
I would be most grateful for any comments, suggestions or hints
on
errors.
--
Rainer Arlt, visual@imo.net,
1998 Dec 29.
=====================
(3) LATEST NEAR NEWS
NEAR Major Engine Burn Completed
January 3, 1999
http://near.jhuapl.edu/news/flash/99jan03_1.html
At noon, EST, January 3, the NEAR mission team conducted a
24-minute,
large bipropellant engine burn, to increase the spacecraft's
speed for
a rendezvous with Eros in February 2000. Preliminary indications
are
that the burn was successful. NASA's Deep Space Network, which is
tracking the NEAR spacecraft, is expected to confirm the accuracy
of
the burn early Monday morning, January 4.
The burn increased NEAR's speed by 2,100 mph (940 meters per
second) to
catch up to the faster-moving Eros asteroid, which overtook NEAR
during
the Dec. 23rd flyby. At the time of the burn, the spacecraft was
565,650 miles (910,100 kilometers) from Eros
Once accuracy figures from the burn are received, plans will be
finalized for a small hydrazine engine burn to correct any
deviation
from the spacecraft's intended location. This burn is expected to
take
place in one to two weeks. Periodic trajectory correction
maneuvers
will be executed by the Mission Operations Center as deemed
necessary
to keep the spacecraft on course during its yearlong journey to
the
asteroid.
For now, NEAR continues on its orbit around the sun, traveling at
about
43,000 mph (19 kilometers per second) as it gains on asteroid
Eros.
=====================
(4) QUADRANTIDS
From Mark Davis <MeteorObs@charleston.net>
Quadrantids...
The major shower of January is the Quadrantid meteor shower
(QUA),
named after the ancient constellation Quadrans Muralis. It is
expected
to reach a maximum on January 3rd at about 23h UT, Universal
Time. This
is very near the time of full moon. The radiant is at 230
degrees, ie.
RA 15h20m, Dec +49, which is just past the halfway mark on a line
from
the end star of the handle of the Big Dipper to the upraised arm
of
Hercules.
The ZHR, Zenithal Hourly Rate, is about 120 meteors per hour with
the
naked eye. These meteors are at an average speed of 41 km per
second.
The duration of the shower is from January 1st to 5th, but this
shower
is noted for an extremely narrow peak with some studies finding
rates
higher than about 60 meteors per hour for only about 16 hours.
Hence,
whether or not we are in darkness or daylight at the maximum time
becomes rather critical for seeing the best rates.
Recent studies have found a similarity between the orbit of this
shower
and that of Comet 96P/Machholz 1, and indeed, possible
relationships
with both the Delta Aquarid and daytime Arietid meteor showers.
Another
possibility may be a relationship with Comet 1491 I. Because
orbits
change over time (in this case, thousands of years) due to
outside
factors, such as perturbations by Jupiter, it is difficult to
always
match meteor showers to parent bodies.
Is this shower favorable this year? Unfortunately not. The moon
is bad,
so the shower will be adversely affected. However, due to the
high
rates that can be associated with this shower, it is still worth
observing!
Upcoming Meetings...
May 11-13, 1999:
The Leonid Meteoroid Storm & Satellite Threat Conference is
being held
in Manhattan Beach, California. For more information, please
contact
leonid-conf@aero.org or
check out the website at:
http://www.aero.org/conferences/leonid
Papers are solicited in many areas, including UV, optical, IR and
radar
observations of the 1998 Leonid storm; dynamics, composition,
occurrence of the Leonid meteoroids; and orbital and meteoroid
dynamics: 1997-2000.
July 26-30, 1999:
The Asteroids, Comets, Meteors 1999 Conference is being held at
Cornell
University, near Ithaca, in New York State. Details are available
at
their website: http://scorpio.tn.cornell.edu/ACM.
You can also leave
your name and address, to be contacted with more information.
Although
this is a professional conference, a number of North American
amateurs,
including a number from NAMN, are planning to attend.
September 23-26, 1999:
The 1999 International Meteor Conference (IMC), the annual
conference
of the International Meteor Organization, is being held in Frasso
Sabino, Italy. The cost, including conference, lodging, and
meals, is
approximately $200 U.S. For more information, see the IMO website
at
http://www.imo.net
Mark Davis, MeteorObs@charleston.net
Mt. Pleasant, South Carolina, USA
Coordinator, North American Meteor Network
And check out:
NAMN home page:
http://medicine.wustl.edu/~kronkg/namn.html
=================
(5) LIFE IN EXTREME ENVIRONMENTS
From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
LIFE IN EXTREME ENVIRONMENTS
http://www.nsf.gov/cgi-bin/getpub?nsf9943
Announcement of Opportunity and Special Competition for FY 1999
Planetary Environments:
In order to provide insights into the possibility of life beyond
our
own planet, research is also needed to characterize the
environments of
planets in the solar system and beyond and to understand the
commonalities of their formation and evolution. Examples of
relevant
topics include:
* studies of the formation of Earth, other planets and their
satellites;
* remote sensing of planets and their atmospheres;
* studies of interstellar grains and meteorites to establish
criteria for
the presence of biogenic substances;
* studies of interstellar and cometary chemistry, particularly of
biologically relevant molecules;
* the relationship between interstellar organic molecules and the
origin
of life; and
* research on the biogeochemical effects of microbes on their
environments on Earth to better design tests for life on
other
planets.
* Methods and Capabilities for LExEn Research
====================
(6) LOUIS FRANK REFUSES TO BACK DOWN: 'SMALL COMETS ARE REAL'
From Andrew Yee <ayee@nova.astro.utoronto.ca>
University of Iowa
University News Services
100 Old Public Library
Iowa City IA 52242
(319) 384-0009; fax (319) 384-0024
CONTACT: GARY GALLUZZO
e-mail: gary-galluzzo@uiowa.edu
UI's Louis Frank uses mathematical data analysis to show small
comets
are real
IOWA CITY, Iowa -- Asserting that critics have been looking at
the
wrong data, University of Iowa space physicist Louis A. Frank has
published a new paper supporting his "small comet"
theory that about 20
snow comets weighing 20 to 40 tons each disintegrate in the
Earth's
atmosphere every minute.
The paper, which appears in the Jan. 1, 1999 issue of the
American
Geophysical Union's (AGU) Journal of Geophysical Research-Space
Physics, uses an automated mathematical formula to filter out
electronic instrument noise from data gathered by NASA's Polar
satellite. The result, says Frank and his UI colleague John B.
Sigwarth, is a "hands-off" analysis showing that
"instrumental effects
were not major contributors" to the images of atmospheric
holes. Using
the mathematical formula, the two researchers found that the
atmospheric holes photographed by the Polar satellite cameras:
* Increase in number when photographed from lower altitudes.
* Increase in number when photographed during local-morning time
periods.
* Appear larger in size in satellite images when photographed
from lower altitudes.
* Vary in number, depending upon the season.
"What critics of the small comet theory were analyzing was
instrument
noise," Frank says. "If you strip away the noise from
the data, as they
properly should have done, what remains clearly validates the
reality
of atmospheric holes. Our most recent paper is the only
comprehensive
paper on this topic and shows, without reasonable doubt, that the
atmospheric holes are indeed a real phenomenon."
Frank says that the mathematical formula applied to the data
screened
out possible causes of electronic noise such as longer wavelength
radiation, energetic electrons and uneven sensitivity -- or
"hot spots"
-- among camera instrument pixels. Significantly, he found
mid-January
1998 data containing no atmospheric holes and used it as a
baseline
measurement.
"The period in mid-January during which no atmospheric holes
were
detected provided an excellent opportunity to have a very
effective
calibration series of images which were equivalent to an
extensive
post-launch laboratory calibration. These in-flight calibration
images
were extremely important in establishing the instrument noise
performance without the presence of atmospheric holes and with
the
actual temperatures and operating voltages for the instrument.
These
images verify the accuracy of our computations of random hole
rates,"
he says.
In a 1998 study, Frank and Sigwarth analyzed 1981 data collected
by the
Dynamics Explorer 1 satellite and compared it to data gathered by
Polar
in 1997, finding a mid-January lull in both sets of data. Despite
the
fact that observations of seasonal variations in atmospheric
holes were
made 16 years apart by different spacecraft carrying different
cameras,
criticism remained. Several papers refuting the theory were
presented
at the spring 1998 AGU meeting, one of them suggesting that
measurements made by another satellite show that the atmosphere
some 15
to 35 miles above the Earth is much drier than the small comet
theory
would suggest.
In December 1997 Frank presented a study at the AGU fall meeting
showing that dark spots (called "atmospheric holes"
because of their
appearance on film) captured in June 1997 on Polar photographs
decrease
in size and number as the satellite's altitude and distance from
the
holes increases. Earlier, Frank had created a stir at the May
1997 AGU
meeting when he revealed a series of Polar satellite photographs,
ranging from a picture of a small comet the size of a two-bedroom
house
disintegrating thousands of miles above the Atlantic Ocean to an
image
of light emitted by the breakup of water molecules from a small
comet
less than 2,000 miles above the Earth. Frank and Sigwarth, who
co-discovered the small comets and designed and built the three
Visible
Imaging System (VIS) cameras aboard Polar, offered the pictures
as
proof of their theory.
Frank first announced the small comet theory in 1986 after
examining
images recorded in photographs taken by Dynamics Explorer 1.
Frank and
his colleagues had designed and built a special camera to take
pictures
of the northern lights, including the first images of the
complete ring
of the northern lights from above the North Pole. But some of the
images contained unexplained dark spots, or atmospheric holes.
After
eliminating the possibility of equipment malfunction and numerous
other
explanations, Frank and Sigwarth concluded that the atmospheric
holes
represented clouds of water vapor being released high above
Earth's
atmosphere by the disintegration of small comets composed mostly
of
snow.
They calculated that more than 25,000 comets enter the atmosphere
each
day. At that rate, the steady stream of comets would have added
about
one inch of water to the Earth's oceans every 20,000 years --
enough to
fill the oceans over billions of years. The theory was
immediately
controversial, with people asking why such objects hadn't been
observed
previously. Frank countered that not only their small size --
20-to-30-feet in diameter -- makes observation difficult, but
also that
water striking the upper atmosphere glows very faintly as
compared to
the bright glow of metal and rock in solid meteors. The
controversy
re-ignited after the 1996 launch of Polar, carrying two sensitive
visible light cameras and one far-ultraviolet light camera, made
it
possible to photograph the small comets with greater resolution.
For further information, see:
* Small comet web site:
http://smallcomets.physics.uiowa.edu
* Small comet website press release:
http://smallcomets.physics.uiowa.edu/automated1.html
* Full text of JGR paper:
http://smallcomets.physics.uiowa.edu/pdf/jan99_jgr.html
* New small comet photos:
http://smallcomets.physics.uiowa.edu/newphoto1.html
==========================
(7) JANUARY'S CHILLY METEORS
From NASA Science News < expressnews@sslab.msfc.nasa.gov
>
28 Dec. 1998 January's Chilly Meteors - The 1999 Quadrantids
One of the year's most intense meteor showers, the Quadrantids,
begins
tonight. The shower stretches from Dec. 28 through Jan. 7
with a sharp
maximum on Jan. 3, 1999. The Quadrantids are the only major
annual
meteor shower whose source, presumably a comet or an asteroid,
remains
unknown. Readers are invited to observe the upcoming shower
and to
submit their data for analysis by scientists studying the
structure and
origin of the Quadrantid meteoroid stream.
FULL STORY at
http://science.nasa.gov/newhome/headlines/ast28dec98_1.htm
==============
(8) ROSETTA PASSES IMPORTANT DESIGN REVIEW
From Andrew Yee < ayee@nova.astro.utoronto.ca
>
ESA Science News
http://sci.esa.int
14 Dec 1998
Rosetta Passes Important Design Review
The Rosetta comet rendezvous mission has passed another
significant
milestone. According to ESA's usual practice for major projects,
a
Rosetta Mission System Design Review took place at ESTEC in The
Netherlands on 10 December 1998. During the review an independent
team
of engineers and ESA officials closely scrutinised all the
elements of
the mission, including the ground stations, the spacecraft, the
payload
of scientific instruments and the launcher.
The review came at the end of several weeks of very intensive
discussion focused around a number of severe constraints which
the
mission team will have to overcome. They include:
* the spacecraft's thermal design -- how Rosetta will cope with
high
temperatures close to the Sun and much lower temperatures beyond
the
asteroid belt
* the available mass for the spacecraft and its scientific
payload,
based on the lifting capacity of the Ariane-5 rocket
* providing sufficient electrical power supply from the
spacecraft's
solar panels in the dark depths of the Solar System
* the very challenging construction, assembly and test schedule,
for
the January 2003 launch date.
The Review Board concluded that there is a high confidence on the
success of the mission and its objectives. Work will now continue
on
detailed design activities, with the aim of starting hardware
manufacturing and engineering model testing next year.
----------------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK (CCNet)
----------------------------------------
The CCNet is a scholarly electronic network. To subscribe, please
contact the moderator Benny J Peiser at < b.j.peiser@livjm.ac.uk
>.
Information circulated on this network is for scholarly and
educational
use only. The attached information may not be copied or
reproduced for
any other purposes without prior permission of the copyright
holders.
The electronic archive of the CCNet can be found at
http://abob.libs.uga.edu/bobk/cccmenu.html
*
REACTIONS TO THE OPEN LETTER TO THE PLANETARY SOCIETY
From Louis Friedman <tps.ldf@mars.planetary.org>
Mr. Peiser:
Thank you for your "open" letter to me about the
predictions of the orbit
for 1997XF11. There are two points the first is
technical accuracy;
which is discussed in the scientific literature and which The
Planetary
Society plays no role in evaluating. We are satisfied that
the process
of evaluating the technical and scientific work is proceeding
properly.
I certainly do not know of any evidence of cover up or data being
suppressed.
Contrary to what you have stated, there are not two groups of
scientific conclusions about the orbit prediction. No
analysis has
suggested that there is anything more than an infinitesimal
(essentially zero) probability of an impact in the next century
from
1997XF11.
The second point is Dr. Chapman's position as a columnist with
The
Planetary Report. We deeply appreciate his long time voluntary
role as
our News & Review columnist. We respect his right to opinions
and
reporting based on his deep scientific involvement and
professional
standing in the field of planetary science, even, sometimes, when
we
don't agree with some of those opinions or
characterization. We review
his column (as we do all material in The Planetary Report) with
the
high standards of our editorial staff, in consultation with our
Editorial Advisory Board and professional colleagues. On this
matter,
Dr. Chapman's analysis is in the scientific mainstream, and, as
such,
it really isn't very controversial. His opinions about the
procedures
involved in reporting and publicizing the prediction, may not be
exactly our own, but we accepted his right, and scientific
credibility,
to express them in his column. My own opinion is that there has
been
too much personalizing of this whole subject about the 1997XF11
prediction, and rather than augment it, will do my best to
minimize it.
It is a tempest in a teapot.
We appreciate your interest and welcome suggestions from our
members.
Sincerely,
Louis Friedman
========================
From Paolo Farinella <paolof@keplero.dm.unipi.it>
wrote:
Dear Benny,
I think that in your letter to the Planetary Society you
misreported
the point of view of those scientists (I am among them) who think
that
1997 XF11 shouldn't have been announced as a short-term potential
hazard in March this year.
You say:
> While it was apparent at the time of the initial
announcement that XF11
> could come exceptionally close to Earth, little attention
was paid to
> the object until the report on 11 March 1998 that it would
in fact do
> so in 2028. Since then, the experts have been deeply split
about
> this asteroid, and no unanimity was reached with regards to
the future
> risk this object might pose to Earth. After three months of
> observations, astronomers involved in orbit calculations
were divided
> into two groups: those who believed that any risk of impact
in the next
> century could be ruled out altogether, and those who
maintained that
> the limited data available at that time did not allow for
such
> unequivocal conclusions.
Actually I belong to a third `group' (the vast majority of the
experts, I
think) who claimed to different things: (1) that an impact in
2028
could be safely ruled out from the first three months of
observation;
(2) that as a consequence the short-term (~1 century)
hazard posed by
this object is much lower than the collective hazard level posed
by the
undiscovered NEAs of similar size (the so-called `background
level'). I
think that, after many bitter discussions, Brian Marsden now
agrees
with (1); and I have never seen a calculation by him or anybody
else
showing that (2) is false. Nor I think that there are other
issues
worth being presented to the public in this context.
Best regards and season greetings,
Paolo Farinella
*
THREE FAVORITE NEOs AND THOSE DREADED IMPACT-PROBABILITY
CALCULATIONS
From Brian G. Marsden (bmarsden@cfa.harvard.edu)
"NASA should fund experts to make valid impact-probability
calculations
[rather than] conclude that all it needs to do is fund more
observations ... when in fact we really need better early
calculations."
This statement, which appeared in the latest issue of The
Planetary
Report, prompts me to make a few remarks about (4179) Toutatis,
1998 XB
and 1997 XF11, three of everybody's favorite NEOs of 1998.
In the CCNet Digest for Oct. 15, Duncan Steel alluded to Grzegorz
Sitarski's recent Acta Astronomica paper on Toutatis. I fully
concur
with Duncan's statement about the epic orbital studies Grzegorz
has
done in his characteristically quiet but competent way over
several
decades. Actually, I don't agree that it is necessary to
postulate the
action of nongravitational forces on Toutatis in order to link
the
observations since its 1989 discovery with the two 1934 Uccle
prediscovery observations that were identified by former MPC
Associate
Director Conrad Bardwell soon after Toutatis was found. As with
Rob
McNaught's 1993 Apr. 27 precovery measurement of comet C/1995 O1
(Hale-Bopp) and the wealth of 1995-1996 postdiscovery data, the
problem
is that the enormous weight of the postdiscovery observations
(with
their associated errors, mainly the systematic errors arising
from an
imperfect reference-star system) can cause the isolated earlier
observations to show departures from the solution that are
significantly larger than would be expected. By increasing the
relative
weight of the prediscovery/precovery data (either by giving them
a
weight of ten, say, rather than one, or by literally decimating
the
recent data), those earlier data can be made to fit without doing
any
injustice to the representation of the later data.
Be that as it may, the whole point of Grzegorz's analysis is
that,
despite the well-known fact that Toutatis will come to a distance
of
little more than 0.01 AU from the earth in 2004, there is no
possibility that this 5-km object will strike the earth in the
near
future. The reason an impact is impossible is that the object's
nodes
are currently more than 0.4 AU from the earth's orbit. This is
true
even though the 0.5-deg orbital inclination to the ecliptic means
that
the orbits of Toutatis and the earth are only 0.006 AU apart. By
the
end of the 22nd century, however, the descending node will be
substantially closer than it is now--though still more than 0.1
AU from
the earth's orbit. Coupled with the approaching node is the fact
that
the motion of Toutatis is extremely chaotic, mainly because the
object
is only 1 AU from Jupiter at aphelion and in 3:1 resonance with
that
perturber. The chaos is such that the position of the
object in its
orbit starts to become quite uncertain, perhaps already by the
24th
century, after which the nodal distances become uncertain
too. The
outcome of this is, not only that one cannot currently say
whether or
not an earth impact will occur in, say, the 26th century, but
that one
cannot say whether or not the circumstances will make it
meaningful to
consider whether an impact is then even possible.
Understandably, the phrase "impact-probability
calculations" appears
nowhere in Grzegorz's paper. Because of the extreme chaos,
it is
obviously impossible to make such calculations for the 26th
century and
beyond. This would be the case whether or not one considers
nongravitational forces, whether or not one remeasures the 1934
plates,
whether or not one includes Steve Ostro's radar data in the orbit
solution. Of course, one could compute, perhaps, that the impact
probability through the 22nd century is 10^{-9772} (to pick a
number at
random). But in such a case one could worry that the impact
probability might be as large as 10^{-97}, say, because a close
encounter with an unconsidered asteroid just might change
Toutatis'
orbital inclination by 0.5 deg and reduce that 0.006 AU (and
decreasing) distance between the orbits all the way down to
0.00004 AU
and less... Whatever Clark Chapman may think, I suspect our
successors
will deem it prudent to continue to observe Toutatis from time to
time
over the next several centuries. A project they might in fact
undertake
would be to place a transponder on Toutatis--or maybe even
colonize
it--the better to monitor the rake's progress.
In another masterly study of real celestial mechanics, Andrea
Milani
(in CCNet Digest for Dec. 21) has examined the prospects for
close
passages of 1998 XB by the earth (and Venus) over an interval of
25
millennia. One almost wonders why this object should even come up
with
regard to consideration of the dreaded impact-probability
calculations.
As Andrea says, neither node is currently much within 0.2 AU of
the
earth's orbit. Furthermore, because of the 14-deg inclination,
the
minimum distance between the orbits of 1998 XB and the earth is
more
than 0.1 AU--and it would take a near collision with a
Jupiter-sized
asteroid (yet the aphelion distance is only 1.2 AU!) to make this
drop
precipitously. But Andrea shows that the slow combination of
perturbations by Venus and the earth can raise the specter of a
possible earth impact some eight millennia from now. As with
Grezgorz
and Toutatis, Andrea is of course well aware of the impossibility
of
estimating an actual probability of an earth impact with 1998 XB
at
that time.
I do have one small criticism of Andrea's account, and this is
that I
wish to point out that the fact that the semimajor axis of 1998
XB is
as small as 0.906-0.908 AU was already quite clear on Dec. 14,
when MPC
Associate Director Gareth Williams published such a value on MPEC
1998-X37, using the observations made during Nov. 25-Dec.
11.
Furthermore, with regard to Al Harris' remark about the
"revised
orbit", it seems not to be widely understood that the
problem with the
early orbit determination is that there were in fact two discrete
solutions. Double solutions are quite the rule for objects like
1998 XB
that are discovered at elongations of only 90 degrees from the
sun.
Usually, of course, it is rather obvious which of the solutions
should
be accepted. What was remarkable in the case of 1998 XB is
that the
two solutions should be so similar. Gareth's first published
orbital
solution for 1998 XB, which appeared on MPEC 1998-X20 on Dec. 5
using
observations extending to earlier that day, gave a semimajor axis
of
1.021 AU. This orbit was the result of a least-squares fit
to the 41
available observations with a mean residual of 0.45 arcsec.
At the
same time, Gareth had also computed the other solution, with
semimajor
axis 0.903 AU, finding that this least-squares result represented
the
same observations with a mean residual of 0.59 arcsec. Although
the
fits were essentially indistinguishable, Gareth rather
understandably
adopted the solution showing the smaller mean residual. The whole
point, surely, was to provide an ephemeris that would yield
further
observations that would enable the physically correct solution to
be
isolated. By the time the observations extended to Dec. 9 it was
starting to become apparent that the wrong initial choice had
been
made. The switch to the correct solution then followed
automatically.
But while Andrea finds the long-term orbital evolution
interesting, and
I find interesting the fact that the two early orbit solutions
were so
similar, others were drawn to 1998 XB by its presumed great size,
or
more specifically, its low absolute magnitude. And Al
correctly points
out that the earlier enthusiasm on this issue (CCNet Digest, Dec.
15)
was premature. The use of the physically correct initial orbit
solution
changes the initial calculation of the absolute magnitude H from
14.2
to 14.7. Furthermore, in calculating the absolute magnitude, the
MPC
has a practice of essentially giving each observed magnitude unit
weight. But one should understand that these magnitudes are
provided,
almost as an afterthought, by astrometrists who, for the most
part,
have no particular interest in photometry. Except for the few
astrometrists who also specialize in photometry, the magnitudes
they
give are just numbers that come out of the computer reduction,
with no
consideration of the color band to which the CCD is sensitive,
the
reliability and appropriateness of the magnitudes catalogued for
the
reference stars, etc. Only four of the six early observers
of 1998 XB
chose to provide any magnitude observations at all. As later
observations came along, extending through Dec. 26, the
unit-weight
process changed H to 15.4. If zero weight is given to the data by
the
two most prolific observers, however, H changes all the way to
16.0.
As it happens, those same two observers were already the most
prolific
through Dec. 5, and if zero weight had been given to their data
in the
initial calculation (and the correct orbit used), the value of H
would
already then have been 15.9.
It is good that Al drew attention to the problem (which is much
more
general than in the case of 1998 XB, of course), but I'm sorry he
did
not mention the role of the Czech astronomer Petr Pravec in
coming up
with the solution. Petr has been concerned with this problem for
some
years now, and he is working with the MPC on finding a
satisfactory
general solution. This solution will require the preparation of a
table
giving, for each observatory, both a weight and a standard
adjustment,
possibly also with a dependence on the time (which could be a
real
nightmare, as observers change their habits). The incorporation
of such
a table will still allow the MPC to calculate the H values in a
very
automatic manner, something that is essential, given that several
hundred orbits (sometimes more than a thousand) are typically
being
computed (or recomputed) every day.
I hope the above remarks make it clear that it is fully
appropriate for
NASA and other grant-giving groups to fund ever more
observations, both
astrometric and photometric (as well as the processing of those
observations). On the other hand, the calculation of impact
probabilities is surely of quite limited value.
If the reader is still unconvinced, let him or her read on.
I almost hesitate to bring up again the matter of 1997 XF11, but
it
seems that my earlier messages concerning this object (notably in
the
June 8 and July 27 CCNet issues) still have not got through to
everyone. On the morning of March 11 Gareth and I noticed that
the
particular orbit I had published five days earlier on MPEC
1997-E13
gave an unprecedentedly close approach to the earth on 2028 Oct.
26.
This miss distance of 0.00031 AU was a FACT (thus worthy of an
exclamation point), later fully confirmed by at least five other
groups, including Grzegorz and Andrea, who all obtained (using
the same
data) values between 0.00023 and 0.00090 AU, significantly less
than
half the distance of the moon. I immediately made some tests on
the uncertainty of the miss distance and found that the 88-day
arc of
observations was also fully consistent with the object's passage
at the
moon's distance, although these quick tests made it seem unlikely
that
the object would pass at a distance much greater than that of the
moon.
While we found this intriguing, it also immediately told us that
we
probably did not have much to worry about. A careful and detailed
statistical analysis carried out by Karri Muinonen over the
course of
several months (i.e., much longer than the few minutes I spent on
the
topic) concluded that the data then available indicated a
72-percent
chance that 1997 XF11 would come within the moon's
distance. This is
the most interesting result in the papers Clark mentions as
having been
presented at the meeting in Madison in October. The real 1997
XF11 was,
curiously, right in the tail of the distribution.
At this point on March 11, Gareth and I considered what we should
do
with the information. Since we appreciated that (a) anybody else
could
come to the same conclusion from published data (the March 3-4
McDonald
Observatory observations were actually irrelevant from this point
of
view, but we were going to publish these on March 13 anyway), and
(b)
further observations, both astrometric and photometric, were very
desirable, we decided to issue a request for further data on an
IAU
Circular, backed up with an information page in the WWW that
included
ephemerides showing where 1997 XF11 would have been in 1990 and
in
earlier years. We also considered saying nothing at all
about the
object until the presentation I was scheduled to make at the NASA
meeting to be held in Houston on March 17. We rejected this
second
course because (a) we were almost sure that someone else would
remark
on the 2028 encounter in the mean time anyway (for, after all,
the
March 3-4 observations themselves would be available on March
13), and
(b) the object was progressively getting more difficult to
observe, and
a delay of a week could make the difference between success and
failure
(as it happened, the very last observations made of the object
were
only on March 23).
But let us suppose that we had taken the second course, and I had
indeed dramatically dropped the bombshell at the end of my March
17
presentation in Houston. Bear in mind that six or seven of
my principal
antagonists were to be in the audience, which altogether totaled
about 20.
What would have happened? Certainly, the dreaded i.p.
phrase would
have been uttered, perhaps with the assertion by one of my
antagonists
that this was surely as high as 0.1 percent... But bear in
mind, too,
that rather than spend March 12-16 trying to stave off both the
press
and my antagonists, I should have had ample time to perfect my
presentation. Remember that all I needed was a few hours of spare
time
in order to come up with the "2037" calculation (June 8
CCNet), and a
few hours more to come up with the even better "2040"
calculation (July
27 CCNet). No new computer programs had to be written to do
this.
Given a weekend that included both a Friday the Thirteenth (with
a full
moon!) and the Ides of March, please do not doubt that I could
have
come up with this post-2028 twist, so that I could then spring
the
whole scenario on the assembled group on St. Patrick's Day! Of
course,
we all know what the response would be: "Give us
i.p.'s!" Well, as far
as I am aware, we still don't have i.p.'s! But it is hard
to argue
with a single trajectory that could have been taken by 2-km-wide
1997
XF11 to bring it--if unimpeded--only 3000 km from the center of
the
earth on 2040 Oct. 26! Remember, too, that we should not then
have had
salvation in the 1990 observations. Indeed, I should not even
have
known myself that those crucial 1990 images had impressed
themselves on
those Palomar films...
Maybe there would be no precovery films. The assembled
throng could
then argue about i.p.'s until it was blue in the face, but the
savior
would not arrive until the first observer pointed his or her
telescope
in the direction of 1997 XF11 in January 2000... Would
everybody in
the room have to take an oath not to speak to the press assembled
in
Houston for the annual Lunar and Planetary conference?
So what WAS the impact probability for 2-km-wide 1997 XF11
during, say,
2033-2045? Sure, as with the possible eventual impacts of
Toutatis and
1998 XB, we are dealing with chaotic dynamics, and chaotic
dynamics are
not conducive to probabilistic arguments. Since we would be
dealing
with something only 40 years hence, rather than several hundreds
or
thousands of years, there may be some hope for a meaningful
result, but
I have not seen it yet. Yes, we know that 1997 XF11's descending
node
has to cross the earth's orbit during that time. We also know
that,
from the 1997-1998 data alone, 1997 XF11's revolution period,
currently
1.73 years, would end up somewhere between 1.53 and 1.99 years
after
the 2028 close approach. In his paper, Karri actually showed the
spreading out of the possible trajectories after 2028, but he
stopped
his computations already at the end of 2034. The limits on the
revolution period mean that the only possible encounter of
interest
during the time interval considered by Karri is in October 2033
(i.e.,
three times a period of 1.667 years), when I don't think 1997
XF11
could come within, say, 8000 km of the earth's surface: surely a
miss
like that would not startle anyone... This 2033 approach
does show in
Karri's plot, but his sampling of data was not sufficient to show
that
this date was more significant than other Octobers (and some
Mays).
One way of arguing is to say that the minimum distance between
the
orbits of 1997 XF11 and the earth, currently 150,000 km,
diminishes to
30,000 km in 2028, a reduction of 4000 km per year. This
reduction
continues beyond 2028, modified somewhat by how close 1997 XF11
approaches the earth in 2028 (which is unknown, without the 1990
data).
The minimum miss distance therefore takes about three years to
cross
the diameter of the earth. Thus it is reasonable that I should
find one
deep impact (2040), a graze (2037) and another half-dozen
possible
passages within a couple of earth radii. During the middle of the
three-year period, any impact would be quite central. One can
therefore
attempt a statement like: during at least one year during the
decade in
question, the impact probability may have been as high as the
ratio of
the diameter of the earth to the circumference of its orbit,
divided by
the object's revolution period in years. That number is close to
10^{-5}, which is, I think, larger than the annual impact
probability
by an unknown object 2-km across or larger. However, since I
really
have little use for impact probabilities, please don't argue with
me
about these rough estimates.
In conclusion, let me offer a cautionary story for those who are
inclined to solve all problems by statistics. Consider the
calculation
of the quantity N = n^2 + n + 41 for the integers n = 0, 1, 2,
and up.
The result is N = 41, 43, 47, etc. So what is the
probability that N
is not a prime number? Well, these three values of N are
prime, and
the reader can easily verify that the trend persists.
Statistically, I
suppose one can argue that, in this range of numbers, one integer
out
of four or five is prime. So one could argue that, with each
passing n,
the probability that N is not prime diminishes as 0.2, 0..04,
0.008,
etc. So almost before one realizes it, the probability
becomes
10^{-6}, 10^{-12}, 10^{-18}, etc. If you wish, you can
liken the
passage of n to the passage of time covered by the observations
of an
NEO, with then the probability of nonprime N equivalent to
earth-impact
probability for the NEO at some set time in the future. The
statistical
plodder will probably notice that primes become less frequent as
n
increases, but the formula still somehow manages to find them.
The
thoughtful geometer, however, suddenly appreciates the seemingly
unrelated geometrical fact that, given n^2, (n+1)^2 follows from
the
formula n^2 + n + (n+1). Bingo! If n+1 = 41, then N is 41^2 =
1681,
decidedly not a prime. So, whereas the plodder was thinking of
probabilities like 10^{-30} or so--maybe even "essentially
zero"--all
of a sudden the probability of impact becomes 2.4 percent!
By the time
n = 100 rolls around, the probability is up to 14 percent,
increasing
to 42 percent at n = 1000 and a whopping 59 percent at n =
10,000. Far
from being a remote possibility, the nonprimeness of N, and the
impact
of the NEO, is rapidly becoming a dead certainty!
Don't get me wrong. Statistics has its uses in attempting to
correlate
quantities having no obvious causal relationship. But the
computation
of orbits and the deduction of consequences therefrom also
involve
hearty doses of geometry and dynamics. And except in the
most simple
circumstances, dynamics means chaos. Most of the impact danger is
from
asteroids that orbit the sun in a quasi-stable state. An
eventual
impact is the product of countless previous perturbations,
generally
involving many earlier approaches to the earth.
A year ago, 1997 XF11 was catalogued as the 104th potentially
hazardous
asteroid in a series that began in 1932. Now there are 159 PHAs,
the
tremendous increase in 1998 being principally due to the great
success
of the LINEAR program. But it is important to examine
whether PHAs
really can pose a threat to the earth in the near future. To my
knowledge, 1997 XF11 is unique in that it is the only object
under
observation for more than a day or so that actually was a
demonstrated
threat only decades hence. My study is not completely exhaustive,
but
I have found only one other specific case--of a rather smaller
object--that may conceivably become a problem around 2090.
Brian G. Marsden