Asteroid -
Cont'd
Asteroid discovery
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
organised 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. Piazzi named it after Ceres, the Greek goddess of
agriculture.
Three other asteroids (2 Pallas, 3 Juno, 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".
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 (link).
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
tied together previous apparitions into a single orbit. If so, the
object gets a number. The observer of the final apparition is
declared a discoverer, and he gets the honour of naming the
asteroid (subject to the approval of the International Astronomical
Union) once it is numbered.
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 metres
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 include (link):
-- The Lincoln Near-Earth Asteroid Research (LINEAR) team
-- The Near-Earth Asteroid Tracking (NEAT) team
-- Spacewatch
-- The Lowell Observatory Near-Earth-Object Search team
-- The Catalina Sky Survey
-- The Campo Imperatore Near-Earth Objects Survey (CINEOS)
team
-- The Japanese Spaceguard Association
-- The Asiago-DLR Asteroid Survey
The LINEAR system alone has discovered over 40,500 asteroids as of
May, 2004 (link).
Between all of the automated systems, 2701 near-Earth asteroids
have been discovered (link)
including over 600 more than 1 km in diameter.
Issue #36
