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.

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