ASTEROID
Asteroids are rocky and metallic objects that orbit
the sun but are too small to be considered planets. They are known as Minor Planets. Asteroids range in size from Cares, which has
a diameter of about 1000 km, down to
the sizes of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found from inside Earth’s orbits
to beyond Saturn’s orbit. Most, however, are contained within a main belt that
exists between the orbits of Mars and Jupiter. Some have orbits that cross
Earth’s path and some have even hit the Earth in times past.
Asteroids are materials left over from the formation
of the solar system. One theory suggest that they are the remains of a planets
the was destroyed in a massive collision long ago. More likely, asteroids are
materials that never coalesced into a planet, in fact, if the estimated total
mass of all asteroids was gathered into a single object, the object would be
less than 1,500 kilometers (932 miles)
across – less than the diameter of our Moon.
Much of our understanding of asteroids comes from
examining pieces of space debris that fall to the surface of Earth. Asteroids
that are on a collision course with Earth are called “METEOROIDS”. When a meteoroid strikes our atmosphere at high
velocity, friction causes this chunk of space matter to incinerate in a streak
if light known as “METEOR”. If the
meteoroid does not burn up completely, what’s left strikes Earth’s surface and
is called a “METEORITE”. Of all the
meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7
percent are composed of iron and nickel; the rest are mixture of these three
materials. Stony meteorites are the hardest to identify since they look very
much like terrestrials rocks.
Because asteroids are materials from the very early
solar system, scientists are interested in their composition. Spacecraft that
have flown through the asteroid belt have found that the belt is really quite
empty and that asteroids are separated by very large distances. The Galileo
spacecraft recently made close encounters with asteroids 951 Gaspra and 243
Ida.
Asteroids are minor planets,
especially of the inner Solar System. Larger asteroids have also been called planetoids.
These terms have historically been applied to any astronomical object orbiting
the Sun that did not resemble a planet-like disc and was not observed to have
characteristics of an active comet such as a tail. As minor planets in the
outer Solar System were discovered they were typically found to have
volatilerich surfaces similar to comets. As a result, they were often
distinguished from objects found in the main asteroid belt. In this article,
the term "asteroid" refers to the minor planets of the inner Solar
System including those co-orbital with Jupiter.
There exist millions of asteroids, many thought to be the
shattered remnants of planetesimals, bodies within the young Sun's solar nebula
that never grew large enough to become planets.The vast majority of known
asteroids orbit within the main asteroid belt located between the orbits of
Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter trojans).
However, other orbital families exist with significant populations, including
the near-Earth objects. Individual asteroids are classified by their
characteristic spectra, with the majority falling into three main groups:
C-type, M-type, and S-type. These were named after and are generally identified
with carbon-rich, metallic, and silicate (stony) compositions, respectively. The
sizes of asteroids varies greatly; the lagrest, Ceres, is almost 1,000 km (625 ml) across.
Asteroids are differentiated from comets and meteoroids. In the
case of comets, the difference is one of composition: while asteroids are
mainly composed of mineral and rock, comets are primarily composed of dust and
ice. Furthermore, asteroids
formed closer to the sun, preventing the development of cometary
ice.The difference between asteroids and meteoroids is mainly one of size:
meteoroids have a diameter of less than one meter, whereas asteroids have a
diameter of greater than one meter. Finally, meteoroids can be composed of
either cometary or asteroidal material
Only one asteroid, 4 Vesta, which has a relatively reflective
surface, is normally visible to the naked eye, and this only in very dark skies
when it is favorably positioned. Rarely, small asteroids passing close to Earth
may be visible to the naked eye for a short time. As of October 2017, the Minor
Planet Center had data on almost 745,000 objects in the inner and outer Solar
System, of which almost 504,000 had enough information to be given numbered
designation.
The United Nations declared 30 June as
International Asteroid Day to educate the public about asteroids. The date of International
Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over
Siberia, Russian Federation, on 30 June 1908.
In April 2018, the B612 Foundation
reported "It's 100 per cent certain we'll be hit [by a devastating
asteroid], but we're not 100 per cent sure when. Also in 2018, physicist
Stephen Hawking, in his final book Brief Answers to the Big
Questions, considered an
asteroid collision to be the biggest threat to the planet. In June 2018, the US
National Science and Technology Council warned that America is unprepared for
an asteroid impact event, and has developed and released the "National Near-Earth ObjectPreparedness Strategy Action Plan" to better prepare. According
to expert testimony in the United States Congress in 2013, NASA would require
at least five years of preparation before a mission to intercept an asteroid
could be launched.
Historical methods
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 years 1944 and 1945). Notable
asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis,
Robert Luther, H. M. S.
In 1891, Max Wolf pioneered the use of
astrophotography to detect asteroids, which appeared as short streaks on
longexposure
photographic plates. This dramatically increased the rate of
detection compared with earlier visual methods: Wolf alone discovered 248
asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had
been discovered up to
that point. It was known that there were many more, but most
astronomers did not bother with them, calling them "vermin of the
skies", a phrase variously attributed to Eduard Suessand Edmund Weiss. Even
a century later, only a few thousand asteroids were identified, numbered and
named.
Until 1998, asteroids were discovered by a fourstep process.
First, a region of the sky wasp hotographed by a wide-field telescope, or
astrograph. Pairs of photographs were taken, typically one hour apart. Multiple
pairs could be taken over a series of days. Second, the two films oprla tes of
the same region were viewed under as stereoscope. Anybody in orbit around the
Sun would move slightly between the pair of films. Under the stereoscope, the
image of the body would seem 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 letter representing the half-month of discovery and
finally a letter and a number indicating the discovery's sequential number
(example1:9 98 FJ74).
The last step of discovery is to send the locations and time of
observations to the Minor Planet Center, where computer programs determine
whether an apparition ties together earlier apparitions into a single orbit. If
so, the object receives a catalogue number and the observer of the first
apparition with a calculated orbit is declared the discoverer, and granted the
honor of naming the object subject to the approval of the International
Astronomical Union.
There is increasing interest in
identifying asteroids whose orbits cross Earth's, and that could, given enough
time, collide with Earth (see also: why are Planet all the same). The three most important groups of
near-Earth asteroids are the
Apollos, Amors, and Atens. Various asteroid deflection strategies
have been proposed, as early as the 1960s.
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, andfinally 69230 Hermes, which approached within 0.005 AU of Earth in 1937.
Astronomers began to realize the possibilities of Earth impact.
Two events in later decades increased the alarm: the increasing
acceptance of the Alvarez hypothesis that an impact
event resulted in the Cretaceous–Paleogene extinction, 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 ten meters across.
All these considerations helped spur the
launch of highly efficient surveys that consist of charge-coupled device (CCD)
cameras and computers directly connected to telescopes. As of 2011, it was
estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in
diameter had been discovered. A list of teams using such systems Includes:
As of 20 September 2013, the LINEAR system alone has discovered
138,393 asteroids. Among all the surveys, 4711 near-Earth asteroids have been
discovered including over 600 more than 1 km (0.6 mi) in diameter
Traditionally, small bodies orbiting the Sun were classified as comets,
asteroids, or meteoroids, with anything smaller than onem eter across being called
a meteoroid. Beech and Steel's 1995 paper proposed a meteoroid definition
including size limits. The term "asteroid", from the Greek word for
"star-like", never had a formal definition, with the broader term
minor planet being preferred by the International Astronomical Union.
0 Comments