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Who am I?
Edward Tagliaferri has been a consultant to the aerospace industry since July
of 1992. He has a Master's in math and a PhD in physics from the University
of California, Los Angeles. Most of the work he does involves satellites, space
travel and asteroids.
He has been chairman of several committees including the American Institute
of Aeronautics and Astronautics (AIAA) and the Space Systems Technical Committee
(SSTC).
You may have seen Dr. Tagliaferri on TV news reports about asteroids or Discovery
Channel's "Asteroid."
(From the NEAR press kit)
Asteroids are metallic, rocky bodies without atmospheres that orbit the Sun
but are too small to be classified as planets. Known as "minor planets," tens
of thousands of asteroids congregate in the so-called main asteroid belt: a
vast, doughnut-shaped ring located between the orbits of Mars and Jupiter from
approximately 2 to 4 AU (186 million to 370 million miles/300 million to 600
million kilometers). Gaspra and Ida are main belt asteroids.
Asteroids are thought to be primordial material prevented by Jupiter's strong
gravity from accreting into a planet-sized body when the solar system was born
4.6 billion years ago. It is estimated that the total mass of all asteroids
would comprise a body approximately 930 miles (1,500 kilometers) in diameter
-- less than half the size of the Moon.
Known asteroids range in size from the largest -- Ceres, the first discovered
asteroid in 1801 -- at about 600 miles (1,000 kilometers) in diameter down to
the size of pebbles. Sixteen asteroids have diameters of 150 miles (240 kilometers)
or greater. The majority of main belt asteroids follow slightly elliptical,
stable orbits, revolving in the same direction as the Earth and taking from
three to six years to complete a full circuit of the Sun.
Our understanding of asteroids has been derived from three main sources: Earth-based
remote sensing, data from the Galileo flybys, and laboratory analysis of meteorites.
Asteroids are classified into different types according to their albedo, composition
derived from spectral features in their reflected sunlight, and inferred similarities
to known meteorite types. Albedo refers to an object's measure of reflectivity,
or intrinsic brightness. A white, perfectly reflecting surface has an albedo
of 1.0; a black, perfectly absorbing surface has an albedo of 0.0.
C-type (carbonaceous): Includes more than 75 percent of known asteroids. Very
dark with an albedo of 0.03-0.09. Composition is thought to be similar to the
Sun, depleted in hydrogen, helium, and other volatiles. C-type asteroids inhabit
the main belt's outer regions.
· S-type (silicaceous): Accounts for about 17 percent of known asteroids.
Relatively bright with an albedo of 0.10-0.22. Composition is metallic iron
mixed with iron- and magnesium-silicates. S-type asteroids dominate the inner
asteroid belt.
· M-type (metallic): Includes many of the rest of the known asteroids. Relatively
bright with an albedo of 0.10- 0.18. Composition is apparently dominated by
metallic iron. M-type asteroids inhabit the main belt's middle region.
The relationship between asteroids and meteorites remains a puzzle. The most
common meteorites, known as ordinary chondrites, are composed of small grains
of rock and appear to be relatively unchanged since the solar system formed.
Stony-iron meteorites, on the other hand, appear to be remnants of larger bodies
that were once melted so that the heavier metals and lighter rocks separated
into different layers.
A long-standing scientific debate exists over whether the most common asteroids
-- the S-types -- are the source of ordinary chondrites. Spectral evidence so
far suggests that the S-type asteroids may be geochemically processed bodies
akin to the stony-irons. If S-types are unrelated to ordinary chondrites, then
another parent source must be found. If the two are related, then scientists
need an explanation for why they aren't spectrally similar.
Asteroids with orbits that bring them within 1.3 AU (121 million miles/195
million kilometers) of the Sun are known as Earth-approaching or near-Earth
asteroids (NEAs). It is believed that most NEAs are fragments jarred from the
main belt by a combination of asteroid collisions and the gravitational influence
of Jupiter. Some NEAs may be the nuclei of dead, short-period comets. The NEA
population appears to be representative of most or all asteroid types found
in the main belt.
NEAs are grouped into three categories, named for famous members of each:
1221 Amor, 1862 Apollo, and 2062 Aten.
Amors: Asteroids which cross Mars' orbit but do not quite reach the orbit
of Earth. Eros -- target of the NEAR mission -- is a typical Amor.
Apollos: Asteroids which cross Earth's orbit with a period greater than 1
year. Geographos represents the Apollos.
Atens: Asteroids which cross Earth's orbit with a period less than 1 year.
Ra-Shalom is a typical Aten.
NEAs are a dynamically young population whose orbits evolve on 100-million-year
time scales because of collisions and gravitational interactions with the Sun
and the terrestrial planets. Approximately 250 NEAs have been found to date,
probably only a few percent of their total population. The largest presently
known is 1036 Ganymed, with an approximate diameter of 25.5 miles (41 kilometers).
Estimates suggest at least a thousand NEAs may be large enough -- 0.6 mile (1
kilometer) or more in diameter -- to threaten Earth.
Many bodies have struck Earth and the Moon in the past, and one widely accepted
theory blames the impact 65 million years ago of an asteroid or comet at least
6 miles (10 kilometers) in diameter for mass extinctions among many lifeforms,
including the dinosaurs. Other theories suggest that the chemical building blocks
of life and much of Earth's water arrived on asteroids or comets that bombarded
the planet in its youth.
On June 30, 1908, a small asteroid 330 feet (100 meters) in diameter exploded
over the remote region of Tunguska in Siberia, devastating more than half a
million acres of forest. One of the most recent close calls occurred on March
23, 1989, when an asteroid 0.25-mile (0.4- kilometer) wide came within 400,000
miles (640,000 kilometers) of Earth. Surprised scientists estimated that Earth
and the asteroid -- weighing 50 million tons and traveling at 46,000 miles/hour
(74,000 kilometers/hour) -- had passed the same point in space just six hours
apart.
Since the Earth was formed more than four billion years ago, asteroids and
comets have routinely slammed into the planet. The most dangerous asteroids
are extremely rare, according to NASA.
An asteroid capable of global disaster would have to be more than a quarter-mile
wide. Researchers have estimated that such an impact would raise enough dust
into the atmosphere to effectively create a "nuclear winter," severely disrupting
agriculture around the world. Asteroids that large strike Earth only once every
1,000 centuries on average, NASA officials say.
Smaller asteroids that are believed to strike Earth every 1,000 to 10,000
years could destroy a city or cause devastating tsunamis. See:
http://www1.tpgi.com.au/users/tps-seti/spacegd7.html#tsunamiimpact
(From the NEAR press kit)
More than 160 asteroids have been classified as "potentially hazardous" by
the scientists who track them. Some of these, whose orbits come close enough
to Earth, could potentially be perturbed in the distant future and sent on a
collision course with our planet.
Scientists point out that if an asteroid is found to be on a collision course
with Earth 30 or 40 years down the road, there is time to react. Though the
technology would have to be developed, possibilities include exploding the object
or diverting it.
For every known asteroid, however, there are many that have not been spotted,
and shorter reaction times could prove more threatening. NASA puts the odds
at 1 in 10,000 of discovering an asteroid that is within 10 years of impact.
Two programs have been set up to actively search for Near-Earth Objects (NEO's):
NASA's Near Earth Asteroid Tracking (NEAT) program, and Spacewatch at the University
of Arizona.
Also, the Spaceguard Foundation was established in 1996 in Rome. The international
organization's goal is to protect Earth from the impacts by promoting and coordinating
discovery programs and studies of NEOs. A January report shows that NEOs 1 kilometer
or larger are being discovered at the rate of about five a month. The combined
goal of these agencies is to find 90 percent of all NEOs 1 kilometer or larger
within the next decade.
Papers
The paper that started it all! AIAA Position Paper (1995): "Responding to the Potential Threat of a Near-Earth-Object
Impact" AIAA Paper: "The History of AIAA's Interest in Planetary Defense" Historical Evidence of Recent Impacts on the Earth Other Links
What are asteroids?
Are we in danger?
What's being done?
Papers
Photographs
Other Links
Who am I?
What are asteroids?
The majority of asteroids fall into the following three categories:
Are we in danger?
What's being done?
AIAA Position Paper (1990): "Dealing With the Threat of an Asteroid Striking
the Earth"
View Now (PDF 17KB)
http://impact.arc.nasa.gov/reports/aiaa/index.html
View Now (PDF 685kb)
Ivan V. Nemtchinov, I.B. Kosarev, O.P. Popova, V.V. Shuvalov, V.V. Svettsov,
R.E. Spalding, C. Jacobs, J. Shavez, E. Tagliaferri
View Now (PDF 2.3M)
Last updated: February 2004
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