PLEASE NOTE:


*

CCNet SPECIAL, 13 July 1999: ASTEROID 1999 AN10 PROBLEM SOLVED
--------------------------------------------------------------

(1) ASTRONOMERS SOLVE ASTEROID 1999 AN10 IMPACT PROBLEM
   - AT LEAST FOR SOME CONSIDERABLE TIME
   Benny J Peiser <b.j.peiser@livjm.ac.uk>

(2) "IT PAYS TO SAVE OLD PHOTOGRAPHS"
    Brian G. Marsden <bmarsden@cfa.harvard.edu> and
    Gareth V. Williams <gwilliams@cfa.harvard.edu>

(3) 1999 AN10
    MINOR PLANET CENTER, 12 July 1999

===============
(1) ASTRONOMERS SOLVE ASTEROID 1999 AN10 IMPACT PROBLEM
   - AT LEAST FOR SOME CONSIDERABLE TIME

From Benny J Peiser <b.j.peiser@livjm.ac.uk>

It is exactly half a year ago that asteroid 1999 AN10 was discovered - 
and three month to the day that the its potential threat became
widely known. The story about 1999 AN10 made headlines around the
world after I drew attention to the web-paper by Andrea Milani,
Steven Chesley and Giovanni Valsecchi on the potential risk of
asteroid 1999 AN10 hitting the Earth in 2039 (CCNet Special, 13 April
1999). Two weeks ago, the team based at Pisa University even claimed
that the impact risk had significantly increased to a probability of
1:100,000.

Now, two amateur and two professional astronomers have dispelled the
concerns about potentially hazardous asteroid 1999 AN10 colliding
with planet Earth in 40 years time. The discovery of 44-year-old
photographic images of the asteroid by Arno Gnadig and Andreas Doppler,
two German amateur astronomers, and orbital computations by Brian
Marsden and Gareth Williams of the Minor Planet Center in Cambridge
(Mass.), show that there is no longer any risk of asteroid 1999 AN10 
colliding with Earth at least for most of the twenty-first century.
Marsden and Williams cannot say what will happen after 2076, but they
don't expect anything immediately precipitous. The important result of
Marsden's and William's calculations is that the impact probability in
the 2040s is now essentially zero. 

Interestingly, Arno Gnadig and Andreas Doppler found the 1955 image of
the asteroid in the Digital Sky Survey (DSS) which is generally
available to the scientific community. It is quite astonishing that the
teams involved in calculating impact probabilities for 1999 AN10
apparently failed to check this data before going public. After all,
they could have avoided announcing a short-term "problem" right from
the start.

The main lesson to come out of this latest asteroid scare underscores
what Brian Marsden, the Director of the Minor Planet Center, has
stressed ever since the similar 1997 XF11 affair errupted: More
important (and much more reliable) than inherently erratic probability 
calculations are hard facts and real observational data. Unless we can
improve this astronomical data base substantially, we will have to rely
on short-lived and highly speculative probability statistics which
begin to look like a game of pure gamble.

Benny J Peiser

=======================
(2) "IT PAYS TO SAVE OLD PHOTOGRAPHS"

From Brian G. Marsden <bmarsden@cfa.harvard.edu> and
Gareth V. Williams <gwilliams@cfa.harvard.edu>


So wrote Malcolm Browne in The New York Times on March 14 last year
with reference to some eight-year old photographs of 1997 XF11.  It was
of course the identification and measurement by Eleanor Helin and Ken
Lawrence of images of 1997 XF11 on films obtained with the 18-inch
Schmidt telescope at Palomar on two consecutive nights in March 1990,
and the resulting orbital computations by us and others, that removed
any possibility that the infamous object could impact the earth during
the foreseeable future. Now it seems that the recognition of
44-year-old Palomar images and the resulting orbital computations show
that the earth is also quite safe from impact by the almost equally
infamous 1999 AN10, at least through most of the twenty-first century.

Beginning this past April, attention has often been drawn to the
possibility that 1999 AN10, a presumed half-mile object discovered by
the LINEAR project in January, could hit the earth at some time during
the next six centuries or so, conceivably even only 40-50 years from
now.  Any specific impact probability was initially very small indeed,
largely because it required the object first to make a very close
approach to the earth in 2027, and it was initially not at all clear
that this would happen. Interest in 1999 AN10 was therefore
considerably augmented, in the scientific community, at any rate, when
the post-conjunction recovery observations in May showed the
inevitability of the 2027 close approach, after which the impact
probabilities for the 2040s were substantially increased, perhaps even
just to the point where one might want to take notice. This situation
was very comparable to that of the possibility of impact in the 2040s
by 1997 XF11 on the basis of the observations available before the 1990
observations were discovered, the day after 1997 XF11 made the
headlines.

The similarity between the two situations is now even more complete. 
Despite a widespread feeling that pre-1999 images of 1999 AN10 did not
exist, the German amateur astronomers Arno Gnadig and Andreas Doppler
have now found one, from 1955 Jan. 26, when, as at its 1999 discovery,
the object was located above north declination 70 degrees. 
Furthermore, Gnadig and Doppler found the image in a resource that is
readily available, namely, the Digital Sky Survey. The long trail on
the digitization of the 45-minute red-sensitive exposure from the
Palomar Sky Survey is very weak, but it was located well within a
moon's diameter of the expected position. Furthermore, the length of
the trail matches what would have been expected of 1999 AN10, then some
20 million miles away. But in a case like this it is better to utilize
hard copies of the Survey exposures, which are also available at most
professional observatories. For one thing, there were two separate
images, a 12-minute blue-sensitive exposure having been begun shortly
after the red-sensitive one was completed. Not only does the paper copy
of the red plate show the image somewhat more clearly than does the
digitized image, but that of the blue plate shows a very much clearer
image.  Measurement of the ends of the 1955 trails is not easy, the
result in any case being compromised by the fact that the beginnings
and ends of the exposures are timed only to the nearest minute.

Nevertheless, despite these shortcomings, the 130-fold increase in the 
arclength permits a very dramatic improvement in the orbit
determination of 1999 AN10. The miss distance on 2027 Aug. 7 will be
0.0026 AU, just the distance of the moon, and therefore now the closest
confirmed approach of a sizeable minor planet. For a few hours it
should become as bright as eighth magnitude. Its subsequent revolution
period of slightly more than 1.74 years completely eliminates the
possibility of close approaches in 2044, 2046--and even 2034, when the
minimum distance will be comparable to that at the time of the 1955
observations.  At the time of the purported 2044 encounter the object
will be more than 200 million miles away, on the far side of the sun! 
The next significant approach after 2027 will not be until Feb. 2076,
when the distance is likely to become less than 0.05 AU, although a
miss distance as small as 0.008 AU would not be possible. Computations
into the past show miss distances of 0.0062 AU on 1946 Aug. 7 and 0.025
AU on 1990 Aug. 8. 

===================== 
(3) 1999 AN10

From the MINOR PLANET CENTER, 12 July 1999
http://cfa-www.harvard.edu/mpec/J99/J99N21.html

M.P.E.C. 1999-N21                                Issued 1999 July 12, 21:23 UT

     The Minor Planet Electronic Circulars contain information on unusual
         minor planets and routine data on comets.  They are published
   on behalf of Commission 20 of the International Astronomical Union by the
          Minor Planet Center, Smithsonian Astrophysical Observatory,
                          Cambridge, MA 02138, U.S.A.

             BMARSDEN@CFA.HARVARD.EDU or GWILLIAMS@CFA.HARVARD.EDU
          URL http://cfa-www.harvard.edu/iau/mpc.html  ISSN 1523-6714


                                   1999 AN10

                           Revision to MPEC 1999-K07

Observations:
     J99A10N 5 1955 01 26.40208 08 38 25.08 +72 39 45.7                      675
     J99A10N 5 1955 01 26.43333 08 37 35.21 +72 40 05.4                      675
     J99A10N 5 1955 01 26.43750 08 37 26.05 +72 40 07.3                      675
     J99A10N 5 1955 01 26.44583 08 37 13.24 +72 40 11.7                      675

Observer details:
675 Palomar Mountain.  Earlier trail located and measured by A. Gnadig and
    A. Doppler.  Later trail measured (and earlier trail remeasured) by
    G. V. Williams.  1.2-m Oschin Schmidt.

Orbital elements:
1999 AN10                                                            PHA 0.015A
Epoch 1999 Aug. 10.0 TT = JDT 2451400.5                 Williams
M  70.04961              (2000.0)            P               Q
n   0.55941450     Peri.  268.25229     -0.56757144     +0.68457751
a   1.4587479      Node   314.55304     -0.21820270     -0.66076120
e   0.5621418      Incl.   39.93185     -0.79388302     -0.30781206
P   1.76           H   18.0           G   0.15           U   2
Residuals in seconds of arc
550126 675  3.1-  0.3+    990123 071  0.1+  0.0     990214 587  0.9-  0.2-
550126 675  6.5+  2.0+    990124 046  0.4+  0.0     990214 587  0.3+  0.2+
550126 675  3.4-  1.7+    990124 046  0.4+  0.1-    990215 046  0.5-  0.3-
550126 675  1.4+  1.7+    990124 046  0.0   0.3-    990215 046  0.3+  0.6+
990113 704  1.2+  0.8+    990124 046  0.8+  0.3-    990215 046  0.1+  0.6+
990113 704 (0.1+  2.5-)   990124 658  0.3+  0.6-    990215 046  0.3+  0.5+
990113 704  0.6-  0.5-    990124 658  0.1-  0.3-    990218 104  0.1+  1.0-
990113 704  0.4+  0.4+    990124 658  0.3+  0.6-    990218 587  0.0   1.0+
990113 704  1.9-  0.1-    990126 071  0.4+  0.4+    990218 587  0.8-  0.2-
990114 557  0.2-  0.5-    990126 071 (0.2-  2.8+)   990218 587  0.6+  0.7+
990114 557  0.0   0.0     990126 402  0.5+  0.7+    990218 587  1.0+  0.7-
990115 046  0.1+  0.0     990126 402  0.2-  0.3+    990220 428  0.8+  0.7-
990115 046  0.0   0.2+    990126 402  0.5+  0.7+    990220 428  1.0+  1.4-
990115 046  0.3-  0.5-    990126 360  0.4+  0.3+    990515 426  0.7-  0.2+
990115 046  0.1-  0.2-    990126 360  0.1-  0.6+    990515 426  0.2-  0.4-
990115 046  0.3-  0.1+    990126 360  0.4+  0.4+    990515 426  0.5-  0.2+
990115 587  0.8+  0.7+    990127 587  0.4-  0.0     990516 426  0.8-  0.2-
990115 587  0.0   1.0+    990127 587  0.2-  0.4-    990516 426  1.3-  0.1-
990116 704  0.6+  0.4-    990130 402  0.8+  0.6+    990516 426  0.8-  0.7+
990116 704  0.2+  0.0     990130 402  0.1+  0.2-    990523 426  1.7-  0.2-
990116 704  0.7-  0.3+    990130 402  0.2+  0.3-    990523 426  1.6-  0.5+
990116 704  0.3+  0.6+    990204 402  0.3+  0.1-    990526 950  0.3-  0.6+
990117 046  0.0   0.0     990204 402  0.1+  0.1-    990526 950  0.5+  0.1+
990117 046  0.2+  0.3-    990207 587  0.5+  0.1-    990526 950  0.8-  0.2-
990117 046  0.0   0.1+    990207 587  0.8+  0.3+    990527 950  0.3+  1.3+
990117 046  0.1+  0.0     990207 587  0.6+  0.5-    990527 950  0.0   1.2+
990117 402  0.4-  0.1+    990208 704 (0.9+  2.8+)   990609 675  0.3+  0.6+
990117 402  0.1-  0.1-    990208 704  0.6+  0.3+    990609 675  0.1+  0.6+
990117 402  0.4-  0.1+    990208 704  0.4-  0.8+    990609 675  0.1+  0.7+
990117 540  0.6-  0.3+    990208 704  0.2-  0.0     990609 675  0.1+  0.6+
990117 540  0.5-  0.1+    990208 704  0.6-  0.7+    990609 675  0.1+  0.6+
990117 540  2.0-  0.1+    990208 900  0.2-  0.0     990609 675  0.1+  0.6+
990119 151  0.8-  0.3-    990208 900  0.4+  0.4-    990620 046  0.9-  1.9-
990119 151  0.6+  0.1+    990208 402  0.0   0.4-    990702 413  0.4+  1.1+
990119 151  3.3-  1.4+    990208 402  0.1+  0.8-    990708 587  0.4-  1.4-
990119 587  0.1+  0.1-    990208 402  0.4+  0.8-    990708 658  0.1-  0.8-
990119 587  0.2-  0.1+    990209 360  0.5+  0.0     990708 658  0.2+  0.1-
990121 046  0.0   0.3-    990209 360  0.7+  0.3+    990708 658  0.7+  0.2+
990121 046  0.5+  0.2-    990209 360  0.4+  0.2+    990709 658  0.1-  0.5+
990121 046  0.1+  0.1-    990212 587  0.0   0.8+    990709 658  0.7-  0.7+
990121 587  0.2-  0.5-    990212 587  0.1+  0.8-    990709 658  0.6+  0.4+
990121 587  0.1-  0.3-    990212 587  0.1+  0.2-
990123 071  0.0   0.1+    990214 587  0.4-  0.1+

Ephemeris:
1999 AN10                a,e,i = 1.46, 0.56, 40                  q = 0.6387
Date    TT    R. A. (2000) Decl.     Delta      r     Elong.  Phase     V
1999 07 11    01 00.58   +31 25.4    1.096    1.393    82.4    46.4    20.7
1999 07 21    00 57.23   +37 29.3    1.082    1.474    89.2    43.6    20.7
1999 07 31    00 48.15   +43 13.5    1.070    1.551    96.1    40.6    20.7
1999 08 10    00 31.51   +48 22.8    1.064    1.624   102.8    37.5    20.7
1999 08 20    00 05.90   +52 32.2    1.066    1.692   109.0    34.4    20.7
1999 08 30    23 32.06   +55 13.2    1.080    1.756   114.3    31.6    20.7
1999 09 09    22 54.25   +56 05.3    1.107    1.816   118.3    29.2    20.8
1999 09 19    22 19.32   +55 11.0    1.150    1.872   120.5    27.6    20.9
1999 09 29    21 52.54   +52 58.1    1.208    1.924   120.8    26.6    21.0
1999 10 09    21 35.38   +50 02.4    1.280    1.972   119.3    26.2    21.2
1999 10 19    21 26.86   +46 54.5    1.366    2.016   116.3    26.3    21.4
1999 10 29    21 25.15   +43 55.2    1.462    2.057   112.4    26.5    21.6
1999 11 08    21 28.54   +41 15.8    1.567    2.094   107.7    26.8    21.8
1999 11 18    21 35.77   +39 01.9    1.679    2.127   102.7    27.0    22.0

     A trail of 1999 AN10 was located and measured by A. Gnadig and A. Doppler
on the Digital Sky Survey (DSS) copy of a 45-min red-exposure Palomar Sky
Survey I (PSS) plate.  Gnadig and Doppler remarked on the difficulty of their
trail measurement.  Examination of the DSS shows an extremely weak image.  In
view of this, it was decided to remeasure the trail from the PSS prints, as
well as measuring the corresponding trail from the 12-min blue-exposure plate.
Extracts from the two prints were scanned and the resulting images were
measured.  The r.m.s. of the fit of 25-30 USNO-A2.0 comparison stars was 0".22.
As examination of the residual block above shows, the trail-end measures are
clearly not consistent at the subarcsecond level, partly because of the poor
definition of the trail ends and partly because the times of exposure of the
PSS plates were recorded only to the nearest minute.  The apparent better
agreement of the start of the first trail with the second-trail measures
is presumably fortuitous.

     Computations by B. G. Marsden and G. V. Williams show that the miss
distance on 2027 Aug. 7.3 UT will be 0.0026 AU.  The subsequent revolution
period of a little over 1.74 years precludes the possibility of any further
significant approach to the earth until Feb. 2076, when the miss distance
can be expected to be less than 0.05 AU (although it cannot be as small
as 0.008 AU).

Gareth V. Williams           (C) Copyright 1999 MPC           M.P.E.C. 1999-N21

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