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Asteroid Discovery
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Historical
Discovery Methods
Asteroid discovery methods have drastically improved
over the past two centuries.
In the last years of the 18th century, Baron Franz
Xaver von Zach organized a group of 24 astronomers
to search the sky for the "missing planet"
predicted at about 2.8 AU from the Sun by the
Titius-Bode law, partly as a consequence of the
discovery, by Sir William Herschel in 1781, of
the planet Uranus at the distance "predicted"
by the law. This task required that hand-drawn
sky charts be prepared for all stars in the zodiacal
band down to an agreed-upon limit of faintness.
On subsequent nights, the sky would be charted
again and any moving object would, hopefully,
be spotted. The expected motion of the missing
planet was about 30 seconds of arc per hour, readily
discernable by observers.
Ironically,
the first asteroid,
1 Ceres, was not discovered by a member
of the group, but rather by accident in
1801 by Giuseppe Piazzi director, at the
time, of the observatory of Palermo, in
Sicily. He discovered a new star-like object
in Taurus and followed the displacement
of this object during several nights. His
colleague, Carl Friedrich Gauss, used these
observations to determine the exact distance
from this unknown object to the Earth. Gauss'
calculations placed the object between the
planets Mars and Jupiter. |
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Three other asteroids (2 Pallas,
3 Juno, and 4 Vesta) were discovered over the
next few years, with Vesta found in 1807. After
eight more years of fruitless searches, most astronomers
assumed that there were no more and abandoned
any further searches.
However, Karl Ludwig Hencke
persisted, and began searching for more asteroids
in 1830. Fifteen years later, he found 5 Astraea,
the first new asteroid in 38 years. He also found
6 Hebe less than two years later. After this,
other astronomers joined in the search and at
least one new asteroid was discovered every year
after that (except the wartime year 1945). Notable
asteroid hunters of this early era were J. R.
Hind, Annibale de Gasparis, Robert Luther, H.
M. S. Goldschmidt, Jean Chacornac, James Ferguson,
Norman Robert Pogson, E. W. Tempel, J. C. Watson,
C. H. F. Peters, A. Borrelly, J. Palisa, Paul
Henry and Prosper Henry and Auguste Charlois.
In 1891, however, Max Wolf pioneered the use of
astrophotography to detect asteroids, which appeared
as short streaks on long-exposure photographic
plates. This drastically increased the rate of
detection compared with previous visual methods:
Wolf alone discovered 248 asteroids, beginning
with 323 Brucia, whereas only slightly more than
300 had been discovered up to that point. Still,
a century later, only a few thousand asteroids
were identified, numbered and named. It was known
that there were many more, but most astronomers
did not bother with them, calling them "vermin
of the skies".
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Modern
Discovery Methods
Until 1998, asteroids were discovered by a four-step
process. First, a region of the sky was photographed
by a wide-field telescope. Pairs of photographs
were taken, typically one hour apart. Multiple
pairs could be taken over a series of days. Second,
the two films of the same region were viewed under
a stereoscope. Any body in orbit around the Sun
would move slightly between the pair of films.
Under the stereoscope, the image of the body would
appear to float slightly above the background
of stars. Third, once a moving body was identified,
its location would be measured precisely using
a digitizing microscope. The location would be
measured relative to known star locations .These
first three steps do not constitute asteroid discovery:
the observer has only found an apparition, which
gets a provisional designation, made up of the
year of discovery, a code of two letters representing
the week of discovery, and of a number so more
than the one discovered one took place in this
week (example: 1998 FJ74).
The final step of discovery is to send the locations
and time of observations to Brian Marsden of the
Minor Planet Center. Dr. Marsden has computer
programs that compute whether an apparition ties
together previous apparitions into a single orbit.
If so, the object gets a number. The observer
of the first apparition with a calculated orbit
is declared the discoverer, and he gets the honour
of naming the asteroid (subject to the approval
of the International Astronomical Union) once
it is numbered.
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Latest
Technology: Detecting Hazardous Asteroids
There is increasing interest in identifying asteroids
whose orbits cross Earth's orbit, and that could,
given enough time, collide with Earth (see Earth-crosser
asteroids). The three most important groups of
near-Earth asteroids are the Apollos, Amors, and
the Atens. Various asteroid deflection strategies
have been proposed. The near-Earth asteroid 433
Eros had been discovered as long ago as 1898,
and the 1930s brought a flurry of similar objects.
In order of discovery, these were: 1221 Amor,
1862 Apollo, 2101 Adonis, and finally 69230 Hermes,
which approached within 0.005 AU of the Earth
in 1937. Astronomers began to realize the possibilities
of Earth impact.
Two events in later decades increased the level
of alarm: the increasing acceptance of Walter
Alvarez' theory of dinosaur extinction being due
to an impact event, and the 1994 observation of
Comet Shoemaker-Levy 9 crashing into Jupiter.
The U.S. military also declassified the information
that its military satellites, built to detect
nuclear explosions, had detected hundreds of upper-atmosphere
impacts by objects ranging from one to 10 meters
across.
All of these considerations helped spur the launch
of highly efficient automated systems that consist
of Charge-Coupled Device (CCD) cameras and computers
directly connected to telescopes. Since 1998,
a large majority of the asteroids have been discovered
by such automated systems. A list of teams using
such automated systems includes: The Lincoln Near-Earth
Asteroid Research (LINEAR) team
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The Near-Earth Asteroid Tracking (NEAT) team
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Space watch
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The Lowell Observatory Near-Earth-Object Search
(LONEOS) team
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The Catalina Sky Survey (CSS)
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The Campo Imperatore Near-Earth Objects Survey
(CINEOS) team
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The Japanese Space guard Association
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The Asiago-DLR Asteroid Survey (ADAS)
The LINEAR system alone has discovered 50,484
asteroids as of May 24, 2005 Between all of the
automated systems, 3353 near-Earth asteroids have
been discovered including over 600 more than 1
km in diameter.
Also see
vAsteroids
In Solar System
vAsteroids
Exploration
vAsteroids
Classifications
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