PLEASE NOTE:
*
CCNet DIGEST, 24 April 1998
---------------------------
(1) IAUC 6879 AND AN EXPLANATION
Brian G. Marsden <bmarsden@cfa.harvard.edu>
(2) ANOTHER FIREBALL OBSERVATION ON 22 APRIL OVER WASHINGTON
STATE &
BRITISH COLUMBIA
Edward Majden <epmajden@mars.ark.com>
(3) SPECIAL PUBLICATION ON THE 1999 NEAR SPACECRAFT ON ITS
RENDEZVOUS
WITH ASTEROID 433 EROS
A.F. Cheng et al., Johns Hopkins APL
(4) THE NEAR MULTISPECTRAL IMAGER
S.E. Hawkins, Johns Hopkins APL
(5) THE DESIGN AND TESTING OF THE NEAR SPACECRAFT
T.J. Hartka & D.F. Persons, Johns Hopkins
APL
(6) THE NEAR SOLID-STATE DATA RECORDERS
R.K. Burek, Johns Hopkins APL
(7) COOPERATIVE FABRICATION OF THE NEAR SPACECRAFT
J.R. Dettmer, Johns Hopkins APL
(8) THE NEAR SCIENCE DATA CENTER
K.J. Heeres et al., Johns Hopkins APL
========================
(1) IAUC 6879 AND AN EXPLANATION
From Brian G. Marsden <bmarsden@cfa.harvard.edu>
Own up to one inconsequential mistake,
and all's right with the world!
This tactic learned in one's youth seems to work in the NEO arena
too.
At Benny's request, I give below the item concerning 1997 XF11
that I
published on IAUC 6879 last Saturday evening. The item was
prepared after
more than a month of discussions among some of those involved
with the
assessment of the uncertainties in the computation of the
object's orbit.
That it should take so long to resolve the issues does not augur
well for
those who think lips should be sealed until there is consensus
that the NEO
thought to be bearing down on us really is...
Two quite different procedures are used
at the Minor Planet Center for
error estimation. One of these procedures is quite simple
to use and
allows for some consideration of non-Gaussian error distribution,
but it
tends to underestimate the error; this is the procedure that was
used for the
statement on IAUC 6837. The other procedure is both more
powerful and more
time-consuming (although we have been working on ways to speed it
up), and it
tends to overestimate the error; this is the procedure that was
used to give
on IAUC 6879 a maximum miss distance that was fully an order of
magnitude
larger than was indicated as "virtually certain" on
IAUC 6837.
So what difference did this make?
In the context of the original IAU
Circular, the purpose of which was to encourage further
observations and on
which no other statement is in dispute, the answer is:
"nothing of
significance". IAUC 6879 also gives Muinonen's
statement that there was
(only) a 10-percent chance that the miss would be by more than my
original
value of 0.002 AU. Muinonen was unhappy that this
statement, which he made on
March 12, was not included on IAUC 6839 (because of space
limitations), so it
is given here. However, it is worth noting that this
statement seems
incompatible with the Yeomans-Chodas upper limit of 0.009 AU,
given already
on March 11 and also acknowledged here. In a remark to me
last week,
Chodas notes that this limit should actually be considered more
of a
a 4-5-sigma value; he and I therefore agree that 0.002 AU is
indeed a 1-sigma
value, and he finds the chance of a larger miss distance to be 60
percent.
I never understood why my wording on
IAUC 6837 led some to believe
that the impact probability in 2028 was as high as 0.1
percent. With
a change from "virtually certain within 0.002 AU" to
"absolutely certain
within 0.03 AU", those who went through this naive exercise
would presumably
have revised the probability to 0.01 percent. This
linearization was the
same trap I fell into in 1992 when, at the urging of a member of
the press
but against my better judgment, I came up, on the fly, with much
the
same probability for a 2126 impact by comet
109P/Swift-Tuttle. I carefully
resisted giving impact probabilities for 1997 XF11, but I get
blamed either
way. Damned if I do, damned if I don't...
As I have said before, since I gave no
impact probability in the first
place, I refused to respond positively to the "order"
that I should give on
IAUC 6839 the Yeomans-Chodas zero impact probability. For
one thing, it was
rather flippantly given as "that's zero, folks" at a
time when there was in
the minds of many people some doubt that it was exactly zero, and
for another,
this statement was clearly inconsistent with the miss distance of
0.00058 +/- 0.00892 AU provided with it! In an e-mail
message by Bowell
that was widely distributed on March 13, he remarked that he
"could not
get the miss distance down below 0.00020 AU". In a
response a few hours
later, I remarked that this was "a very useful point",
because we were
essentially dealing with what is usually termed a
line-of-variation problem,
and I suggested that we could get an excellent handle on the
accuracy of our
respective calculations if we all went through this same
exercise. Despite the
extremely vitriolic remarks with which some of my critics greeted
my message,
we did in fact eventually do this (and one of the critics
carefully described
the line-of-variation principle in the CC DEBATES several weeks
later).
Collectively, when the experiment was completed, we obtained
gratifyingly
similar results over the narrow range 0.00019-0.00021 AU (as
stated on IAUC
6879), and this allowed Chodas to give a correct lower limit for
the
Yeomans-Chodas miss distance. Since there was at no time
any discussion of
what actually went into our computations (in some cases
non-Gaussian error
distributions, perturbations by Pallas, etc.), I think this
comparison was
important. For one thing, it provided more confidence in
similar computations
made from observations of 1997 XF11 covering different
arcs. Nevertheless,
as recently as last week, Muinonen was claiming that some of his
orbit
computations involved earth impact in 2028. Despite a
mounting suspicion that
there was no way these orbits could satisfy the 1997-1998
observations, he
did not reveal how bad the residuals really were until last
Friday, and it is
this that accounts for the proviso "in the absence of
effects that would be
highly unusual".
The last part of IAUC 6879 acknowledges
the near-simultaneous
observational and computational activity of a number of people on
March 12.
The Bowell 1990 measures arrived 50 minutes and the Chodas
1990-1998 orbit
computation arrived 100 minutes after IAUC 6839 was completed.
The clearest message of the past few
weeks is that the computation of
an impact probability for a one-opposition asteroid on some
identified future
passage near the earth is a meaningless exercise, given that it
will
invariably be less than the background impact probabilities of
unknown
objects, even if the earth happens to be inside the error
ellipse. (Did the
dinosaurs engage in sophistry? One wonders...)
Whether one is taken in by
this or not, it supports my often-stated view that more of an
effort should be
taken to ensure that PHAs are observed at a second
opposition. If the earth
is then unfortunate enough to remain in the error ellipse, there
will at
least then be an impact probability of some significance.
Although new
PHAs were being added at a rate of one per month during the two
years
beginning in March 1996, no fewer than nine such objects have
been added
during the past six weeks, bringing the total to 117.
We are making a point of (discreetly)
drawing attention to these new
PHAs on the MPECs containing the initial observations and orbit
determinations. Furthermore, in addition to our regular PHA
page, there is a
new Web page (http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html)that
shows
calculated passages within 0.05 AU of the earth between now and
the year 2100.
Newly computed orbits of minor planets
are contained on nightly MPECs.
For NEOs the latest astrometric data are also included. The
files of PHAs,
including that listing approaches to the earth until 2100, are
also updated
nightly. It was perhaps not widely noticed that, for a
recent 48-hour
interval, the PHA 1998 DV9 headed the list with a particularly
small miss
distance in 2095, prior to moving to a more distant spot.
This
instability is a characteristic of the nominal orbits of new
single-opposition
PHAs (as the 1997 XF11 case illustrates), and all such objects
should be
considered among the leading targets for identification on past
films.
Even with the initial 15-day arc on MPEC 1997-Y11, the 1990
position of
1997 XF11 could be predicted within 0.7 degree. And rather
than fret
that the March 3-4 positions were not published until March 13,
those
interested in past searches could have produced a better 1990
prediction
without them.
Anyway, more important than IAUC 6879
and this explanation is the
fact that the changes instituted by the Minor Planet Center
should mean
that cases like 1997 XF11 will not occur in the future.
Much more
likely is that the next "scare" will involve a
(hopefully) tiny object
discovered just a couple of days before predicted but essentially
harmless
entry into the earth's atmosphere. So, would the public be
informed
beforehand or not?
**************
[From IAUC 6879, 1998 Apr. 18]
1997 XF11
The estimate by the undersigned on IAUC
6837 that passage within
0.002 AU of the earth on 2028 Oct. 26 was "virtually
certain" was
incorrect; this was a 1-sigma miss distance, and detailed
computations
allowed miss distances of up to 0.02-0.03 AU. The nominal
miss distance
and error by D. Yeomans and P. Chodas (see also IAUC 6839) should
have
been given as 0.00058 (+ 0.00892 / - 0.00039) AU (3 sigma on the
plus side),
and K. Muinonen early remarked on a 10-percent chance for a miss
by more
than the lunar distance. All concerned, including E.
Bowell, agree that in
the absence of effects that would be highly unusual there was no
possibility
of an earth encounter within 0.00019-0.00021 AU; Chodas remarks
(and the
undersigned agrees) that this was already evident with the
issuance of
MPEC 1997-Y11. Nevertheless, the likelihood of an unusually
close approach
in 2028 was also clear already in Dec. 1997, and it is
unfortunate that
there was not then greater awareness of this, for the recognition
of 1990
observations (also found by Bowell on films taken by C. S. and E.
M.
Shoemaker) would have been possible at an earlier date and
physical
observations may have been attempted. Given the
Helin-Lawrence 1990
measurements, simultaneous computations by G. V. Williams and by
Chodas
immediately made it clear to all concerned that the 2028 miss
distance
would be in the range 0.006-0.007 AU.
Brian G. Marsden
=========================
(2) ANOTHER FIREBALL OBSERVATION ON 22 APRIL OVER WASHINGTON
STATE &
BRITISH COLUMBIA
From Edward Majden <epmajden@mars.ark.com>
Another fireball was observed over Vancouver Island, Washington
State
and mainland British Columbia at 21:22 PDT on April 22. We would
be
interested in receiving any reports on this fireball. Information
required is as follows:
Your Longitude and Latitude from a topo survey map or GPS
reading.
Compass Direction of first sighting, corrected to true north
Elevation
above the horizon measured with a clinometer or similar device
Compass
Direction of last sighting, corrected to true north Elevation
above
horizon, as noted above.
Any photographs or video tapes would be most helpful. We will not
pay
for these however as is requested by one individual who claims he
has a
tape. Anyone who cooperates will be given credit in a scientific
report.
Forward reports to : epmajden@mars.ark.com
Thanks for your help: Ed
------------------------------------------------------------------------------
Edward
Majden
epmajden@mars.ark.com
1491 Burgess
Road
Meteor Spectroscopy
Courtenay,
B.C.
AMS Affiliate
CANADA
V9N-5R8
MIAC Associate
======================================
(3) SPECIAL PUBLICATION ON THE 1999 NEAR SPACECRAFT ON ITS
RENDEZVOUS
WITH ASTEROID 433 EROS
A.F. Cheng, R.W. Farquhar & A.G. Santo: NEAR overview. JOHNS
HOPKINS
APL TECHNICAL DIGEST, 1998, Vol.19, No.2, pp. 95-106
APL, PLANETARY SCIENCE SECTION, LAUREL, MD, 20723
The Near Earth Asteroid Rendezvous (NEAR) mission inaugurates
NASA's
Discovery Program. It will be the first to orbit an asteroid and
will
make the first comprehensive scientific measurements of an
asteroid's
surface composition, geology, physical properties, and internal
structure. NEAR was launched successfully on 17 February 1996
aboard a
Delta II-7925. It made the first reconnaissance of a C-type
asteroid
during its flyby of the main-belt asteroid 253 Mathilde in June
1997
and will orbit the unusually large near-Earth asteroid 433 Eros
for
about a year at a minimum orbit radius of about 35 km from the
center
of the asteroid. NEAR will obtain new information on the nature
and
evolution of asteroids, improve our understanding of planetary
formation processes in the early solar system, and clarify the
relationship between asteroids and meteorites. The NEAR Mission
Operations Center and Science Data Center are both located at
APL. The
latter will maintain the entire NEAR data set on-line and will
make
data from all instruments accessible over the Internet to every
member
of the NEAR science team. Copyright 1998, Institute for
Scientific
Information Inc.
===========================
(4) THE NEAR MULTISPECTRAL IMAGER
S.E. Hawkins: The NEAR Multispectral Imager. JOHNS HOPKINS APL
TECHNICAL DIGEST, 1998, Vol.19, No.2, pp.107-114
The Multispectral Imager, one of the primary instruments on the
Near
Earth Asteroid Rendezvous (NEAR) spacecraft, uses a five-element
refractive optics telescope, an eight-position filter wheel, and
a
charge-coupled device detector to acquire images over its
sensitive
wavelength range of approximate to 400-1100 nm. The camera
operates at
a frame rate of 1 Hz, and the detector is passively cooled. The
primary
science objectives of the Multispectral Imager are to determine
the
morphology and composition of the surface of asteroid 433 Eros.
The
camera will have a critical role in navigating to the asteroid.
Seven
narrowband spectral filters have been selected to provide
multicolor
imaging for comparative studies with previous observations of
asteroids
in the same class as Eros. The eighth filter is broadband and
will be
used for optical navigation. The Multispectral Imager has a focal
length of 168 mm and a 2.93 x 2.25 degrees field of view. The
spatial
resolution of the instrument is 16.1 x 9.5 m at a range of 100
km. An
overview of the instrument is presented, and design parameters
and
tradeoffs are discussed in the context of the fast-paced,
low-cost
Discovery Program. Copyright 1998, Institute for Scientific
Information
Inc.
==============================
(5) THE DESIGN AND TESTING OF THE NEAR SPACECRAFT
T.J. Hartka & D.F. Persons: The design and testing of the
NEAR
spacecraft structure and mechanisms. JOHNS HOPKINS APL TECHNICAL
DIGEST, 1998, Vol.19, No.2, pp.163-173
This article describes the primary structure and mechanisms of
the Near
Earth Asteroid Rendezvous (NEAR) spacecraft. Presented are design
requirements as well as a description of the primary structure
and
mechanisms to meet those requirements. The test philosophy for
this
cost-and schedule-driven program is outlined along with a summary
of
the test flow and results. The structure and mechanisms were
designed,
assembled, and tested at APL, with most of the structure
manufacturing
subcontracted. Testing continued at Goddard Space Flight Center,
and
the final spin balance test was performed at Kennedy Space
Center.
Copyright 1998, Institute for Scientific Information Inc.
===========================
(6) THE NEAR SOLID-STATE DATA RECORDERS
R.K. Burek: The NEAR solid-state data recorders. JOHNS HOPKINS
APL
TECHNICAL DIGEST, 1998, Vol.19, No.2, pp.235-240
APL, DEPARTMENT OF SPACE, SIGNAL PROC SECTION, LAUREL, MD, 20723
Data recorders make it possible for the Near Earth Asteroid
Rendezvous
(NEAR) spacecraft to delay and slow the transmission of
information to
Earth, thereby accommodating the temporal and bandwidth
limitations of
the communications link. NEAR is the first spacecraft developed
by the
Applied Physics Laboratory to employ solid-state recorders,
supplanting
magnetic tape recorders used previously. Also, the 132 dynamic
random-access memory devices, which are at the heart of the NEAR
recorders, constitute the first large-scale use of plastic
encapsulated
microcircuits on a Laboratory spacecraft. Earlier spacecraft
relied
almost exclusively on hermetically packaged microcircuits.
Several
measures, including two layers of error detection and correction,
were
taken to mitigate the effects of single-event upsets that may be
induced by charged particles in space. Copyright 1998, Institute
for
Scientific Information Inc.
======================
(7) COOPERATIVE FABRICATION OF THE NEAR SPACECRAFT
J.R. Dettmer: Cooperative fabrication of the NEAR spacecraft.
JOHNS
HOPKINS APL TECHNICAL DIGEST, 1998, Vol.19, No.2, pp.241-246
APL, ELECTRONIC SERVICE GROUP, LAUREL, MD, 20723
Cooperative fabrication was a key factor in building the Near
Earth
Asteroid Rendezvous (NEAR) spacecraft within the cost and
schedule
constraints dictated by the NASA Discovery Program. Because many
of the
traditional barriers between the engineering and the fabrication
teams
were avoided on NEAR, APL reaped the benefits of cooperative
planning,
design for ease of fabrication and assembly, and team problem
solving.
The result was a unified and high-spirited team focused on
accomplishing the task. That teamwork, in combination with many
of the
enabling technologies within the fabrication organization,
allowed APL
to meet NEAR's cost, schedule, reliability, and performance
goals.
Copyright 1998, Institute for Scientific Information Inc.
========================
(8) THE NEAR SCIENCE DATA CENTER
K.J. Heeres, D.B. Holland & A.F. Cheng: The NEAR Science Data
Center.
JOHNS HOPKINS APL TECHNICAL DIGEST, 1998, Vol.19, No.2,
pp.257-266
APL, DEPARTMENT OF SPACE, TIMED MISSION DATA CTR, LAUREL, MD,
20723
The Near Earth Asteroid Rendezvous (NEAR) Science Data Center
serves as
the central site for common data processing activities needed by
the
NEAR science teams and the scientific community. The Center
provides
instrument and spacecraft data to the science teams from around.
the
world and redistributes science products produced by those teams,
allowing the teams to focus on analysis. These data and the
accompanying documentation are available at
http://sd-www.jhuapl.edu/NEAR/.
In addition, the Science Data Center is
responsible for archiving spacecraft, instrument, and science
data to
the Planetary Data System. Copyright 1998, Institute for
Scientific
Information Inc.
----------------------------------------
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.