Fifteen to twenty years ago the discovery of 1 or 2 Near Earth Asteroids (NEAs) per year was typical from one systematic search program, Palomar Planet Crossing Asteroid Survey (PCAS), and the incidental discovery from a variety of other astronomical program. Sky coverage and magnitude were both limited by slower emulsions, requiring longer exposures. The 1970's sky coverage of 15,000 to 25,000 sq. deg. per year led to about 1 NEA discovery every 13,000 sq. deg. Looking at the years from 1987 through 1990, it was found that by comparing 1987/1988 and 1989/1990, the world discovery rate of NEAs went from 20 to 43. More specifically, PCAS' results when grouped into the two year periods, show an increase from 5 discoveries in the 1st period to 20 in the 2nd period, a fourfold increase. Also, the discoveries went from representing about 25 pct. of the world total to about 50 pct. of discoveries worldwide. The surge of discoveries enjoyed by PCAS in particular is attributed to new fine grain sensitive emulsions, film hypering, more uniformity in the quality of the photograph, more equitable scheduling, better weather, and coordination of efforts. The maximum discoveries seem to have been attained at Palomar Schmidt

Helin, Eleanor F.

English

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"25 995
Near-Earth Asteroid Discovery Rate Review, Eleanor F. Helin, Jet Propulsion Laboratory
Fifteen to twenty years ago the discovery of 1-2 Near-Earth Asteroids (NEA's) per year
was typical from one systematic search program, Palomar Planet-Crossing Asteroid
Survey (PCAS), and the incidental discovery from a variety of other astronomical
programs. Sky coverage and magnitude were both limited by slower emulsions, requiring
longer exposures. The 1970"s sky coverage of 15,000 to 25,000 sq. deg. per year led to
about one NEA discovery every 13,000 sq. deg.
Looking at the years from 1987 through 1990, we find that by comparing 1987/1988 and
1989/1990 the worldwide discovery rate of NEA's went from 20 to 43, a little more than
a twofold increase from the first two year period to the latter two years. More specifically,
PCAS" results when grouped into the two year periods, show an increase from 5
discoveries in 1987/1988 to 20 in the 1989/1990 time period, a fourfold increase. Also,
that our discoveries went from representing about 25 percent of the worldwide total to
contributing roughly 50 percent of the discoveries worldwide in the 1989/1990 period.
This trend continues into 1991, whereby PCAS is discovering about a NEA per month.
As the discovery rate continues to spiral up, with significant contributions coming from
McNaught/Steel in Australia and Gehrels, Scotti and Rabinowitch in Arizona, I anticipate
a doubling again in discoveries in the 1991/1992 period. Of course, an important aspect
of these more recent discoveries is the inclusion of fainter than magnitude 20-21 objects
extending out as faint as H magnitude 28 (1991 BA). At the same time several very bright
asteroids have been discovered which indicates that in the NEA population not all of the
objects to magnitude 13-14 have been discovered. The PCAS discovery of the 1990 SQ
at H magnitude 12.5 surpasses (1627) Ivar as the brightest known asteroid in the NEA
population. It is rather remarkable that our most recent NEA discoveries in 1990/1991
include the brightest (1990 SQ) and the faintest (1991BA) on record, certainly suggesting
that a wide range of undetected objects still roam in earth-approaching and crossing
orbits. The surge of discoveries enjoyed by PCAS in particular is attributed to new fine
grain sensitive emulsions, film hypering, more uniformity in the quality of the photograph,
more equitable scheduling, better weather, excellent team members and coordination of
efforts. Although one hopes that more improvements and fine tuning may be possible,
it seems that we have just about attained our maximum output for the discovery of NEA's
using current techniques at the 0.46 M Palomar Schmidt. I anticipate working diligently
to maintain our current rate of discovery. Significant increase in discoveries in the next
two year period will most likely come from elsewhere (the southern hemisphere and
fainter, smaller objects found by CCD - electronic sensor detector-facilities). With the
stunning increase in the last 2 year period over the previous two years, an order of
magnitude, if not greater, increase in the discovery rate of NEA's can be anticipated by
the end of the decade with the advent of CCD Schmidt Telescopes distributed around the
world.
I wish to thank Palomar Observatory for observing time and necessary support. This
research was carried out in the Planetary/Astronomy programs at Jet Propulsion
Laboratory, California Institute of Technology, under contract with National Aeronautics
and Space Administration.
https://ntrs.nasa.gov/search.jsp?R=19910016681 2020-03-19T16:51:23+00:00Z
ACH '91
SPECTROSCOPIC OBSERVATIONS OF COMET AUSTIN (1989c)
Rod tteyd_ Susan Wyckott and Peter Wehinger, Arizona State University
Terry l:tettig, University of Notre Dame
Peter Mack, Michigan-Dartmouth-MIT Observatory
Longs[it CCD spectra(_ - 5100 - 6400 _, A_ ,,, 3 ,_) were obtained with the Michigan-
Dartmouth-MIT 1.3 meter telescope in May 1990(r -- 0.74 AU, A - 0.50 AU). The spectra
have been reduced with IRAF. Spectral extractions offset sunward and tailward from the
nucleus were analyzed. Species observed in the spectra include: C=, NB=, H20 +, CO +,
and several unidentified features. Spatial extractions of rotational lines in the NH2(10-0)
band extend _ 104.5 km from the nucleus. A fit of the vectorial model to the NtI=(10-0)
spatial profile is consistent with an NHs parent molecule. The NH2 production rate and
an ammonia to water abundance ratio have been derived.
91
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Comet Austin
EDM 1.3-m Telescope
Coma
7 May 1990
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5200 5400 5600 5800 6000 6200
Wavelength (._)
92 ACM '91
IR Spectroscopy of Comets: Methanol at 3.52 gm
S. Hoban, M. Mumma, D. Reuter, M. DiSanti (NASA/GSCF),
R. Joyce (NOAO) and A. Storrs(U. Texas)
As part of an ongoing program to use infrared spectroscopy to search for molecular
species which are directly sublimated from cometary nuclei, we present medium reso-
lution (X/Ak = 103) long-slit spectra of several recent comets in the 3-1am region. In
the 3.44-3.64 Ixm spectral region, we detect the red wing of the broad "organic" feature
which is centered at 3.36 _tm, and a narrower feature is apparent which peaks at 3.52
_m, which we tentatively identify with methanol. We find that the spatial profile of
the 3.52-1sm emission is consistent with that of a directly sublimated species. We will
compare our results with the detections of methanol and formaldehyde in comets at
millimeter wavelengths. We will report upper limits for formaldehyde retrieved from
our spectra, and we will discuss the implications of a possible extended source of
H2CO on our results.
LIGHT CURVE DERIVATIVES AS A TOOL FOR THE ANALYSIS OF
ASTEROID REFLECTION PROPERTIES, M. Hoffmann, Observatorium
I_her List, Universit_ts-Sternwarte Bonn, Daun, F. R. Germany
Signatures of the colllsional origin, subsequent cratering,
and of albedo variegation show up in the light curves of
_teroids, but it is impossible to separate these effects by a
_ght curve inversion uniquely. For smooth ellipsoidal and
multifractal surface models as well as for several aspects of
the asteroid 7 Iris some characteristic properties of their
rotation light curves are shown by a spectral analysis of their
derivatives. Contrary to light curve analysis techniques
involving spherical harmonics the high frequencies of their
brightness changes are utilized by this method.
ACM '91 93
U.S. GEOLOGICAL SURVEY-LOWELL OBSERVATORY ASTEROID SURVEY: FIRST
RESULTS; H.E. Holt, USGS, Flagstaff, Ariz., E. Bowell, Lowell
Observatory, Flagstaff, Ariz., C.S. Shoemaker and E.M. Shoemaker,
USGS, Flagstaff, Ariz.
Each year during the course of the asteroid and comet survey
conducted by us at Palomar, about 1,000 fields, 8.75 degrees in
diameter, are photographed with multiple exposures on hypered
Kodak 4415 film. Under favorable conditions of observing, the
detection threshold for main belt asteroids is about B = 18.5.
Single fields near opposition may contain more than I00
asteroids, most of which are new. Altogether, several thousand
asteroids are found and marked on the films each year. Until
recently, because of limited manpower, astrometry was carried out
only for a modest number of objects, chiefly planet-crossing
asteroids, Trojans, high inclination asteroids, and comets.
A systematic effort was begun in the fall of 1990 to
complete astrometry for the majority of asteroids observed in
selected months. The immediate objectives of this pilot effort
are to update the orbits of numbered asteroids, to extend the
orbital arcs of previously designated objects and search for
linkages, and to determine orbital elements for numerous new
asteroids. To date, asteroids observed in 50 fields have been
measured on 2 to 3 pairs of films separated by 2 to 6 days during
a single dark run. More than 2,000 asteroids have been measured;
among those, 820 objects were initially regarded as new
asteroids. We are attempting to obtain at least one-month arc
orbits for as many of the new objects as possible. So far,
updated orbits have resulted for about 200 numbered asteroids;
improved orbits have been obtained for about i00 unnumbered
asteroids, of which about 20 have now been numbered. New short
arc orbits were derived for about 300 asteroids from our
observations; in many cases it has permitted the linkage of
previously designated asteroids, resulting in multiple opposition
orbits. A few of the new objects have already been numbered.
Our principal objective in this investigation is to
substantially increase the rate of discovery and of numbering of
main belt asteroids. Our long range goal is to double or triple
the number of main belt asteroids with high precision orbits in
order to resolve questions concerning the dynamical structure of
the belt. In particular, we are interested in the identification
of collisional families. The present statistics of proper
elements are sufficient to recognize only large families with
high confidence. Greatly expanded knowledge of the asteroid belt
is required to demonstrate the reality of small families and
thereby document many specific events in the collisional history
of the belt.
94 ACM '91
ORIGIN OF ASTEROIDS AND COMETS
Zhon9-wei Hu, Er-kan9 Wang, Nanjin9 University, China
The physico-chemical conditions of the nebular disk around the sun,especially
the temperature profile, caused the asteroid region to be a transition region
between non-condensation and condensation of ice-matter, resultin9 in a
discontinuity in the projected surface density of solid acrretable matter. Because
the ice-matter can condense in the Jupiter region and there was more acrretable
matter, lar9e planetesinals formed very early and 9few rapidly. The mutual
perturbation and encounders of these planetesimals chan9ed their orbits,makin9 some
of them pass through the asteroid region. At that time, the planetesinals in the
asteroid region were still small, so the interlopers accreted and took away with
them the local accretable matter, causin9 the local planetesimals to stop at the
"half-finished product" stage, formin9 only asteroids and not major planets.
Perturbation by interlopers also increased the random velocity of the local remant
planetesimals and their break-up through mutual collision 9ave rise to the 9reat
orbital veriety of asteroids and their mass spectrum, this basic view is proved by
quantitative analysis, includin9 accretion of Jet streams.
The structure in the outer region of the nebular disk is analysed and no
formation zone of comets can exist there. Our view is that the comets evolved from
the residual ice-planetesimals in the zone between Jupiter and Nepture.
ANALYSIS OF MEI VILLAGE FALLEN ICE
Zhon9-wei Hu,Zhao-lin Li,Ru-lian9 Li,Hao-shen Jian9,Er-kan9 Wan9
Nanjin9 University,China
Xi-yuan Xu,Science and Technology Commitee of _u-Xi county,China
A piece of ice fell down from the sky into a field near cottaHe Chen Shah Fan9
of Mei village,Wu-Xi county in China,on 17 november 1984. By investisatin9 the
scene of the fallen ice and witnesses and departments of meteoroloHy and aviation,
possibilities that the ice was of artificial or meteorological or airplane origins
was excluded. A few samples of the ice were submitted to serious experimental
analysis, and preliminary results show that there are some differences between the
ice and terrestrial matter. It is in favour of the viewpoint that the ice
originated from extraterrestrail space, maybe from a small comet. Further
researchin9 fallen ice miHht be of 9reat sisnificance for understandin9 nature of
comets and water resource on the earth as well as relevant problems of meteorolo9y,
9eoloHy and astronomy.
AC'H ' 91 95
METALLIC ATOMS AND IONS IN COMETS
S. Ibadov
Institute of Astrophysics, Dushanbe 734670, USSR
Investigation of metallic component in comets is impor-
tant in different aspects. Emissions of refractory metallic
atoms (Fe, Ni, Si, etc.) have usually been detected in Sun-
grazing comets at heliocentric distances R _ 0.01AU. Mean-
while, in situ measurements carried out withing the coma of
comet Halley 1986 III by VEGA and GIOTTO spacecraft near
R=I AU led to the discovery of metal ions of the Fe + type.
Anomalous distribution of the Na D-line emissions in
the head of the bright comet Mrkos was detected by 200-inch
Palomar telescope in 1957, the origin of which is the puzzle
in the physics of comets.
Metallic atoms have the low ionization potentials and
in th_s connection the short photoionization time scale
(_ 10 _ sec at R=I AU_ so that generation mechanisms of metal
atoms may also by important in the problem of the ionization
of the cometary matter_ particularly of the inner coma.
Theoretical considerations of the different mechanisms
of generation of metal atoms and ions in the cometary comae
led to the following conclusions. Depression of the tempe-
rature of the cometary dust - of the potential source of
cometary coma metallic component - by the outflowing cryo-
genic gas from the cometary nucleus may occur in the near-
nuclear region of bright comets and therefore the gas pro-
duction rate of such comets may be determined on the basis
of anomalous distribution of the sodium D-line emissions.
Ions of refractory elements (Fe +, Si +, etc.) in the
cometary coma at large heliocentric distances will be effec-
tively produced by high velocity ¢_10 km/s_ collisions of
cometary and zodiacal dust particles and therefore such me-
tallic ions may serve as indicators of the passage of dusty
comets of the Halley type through interplanetary dust
clouds.


Near-Earth asteroid discovery rate review

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