SPACE ROCKS!
ASTEROIDS IN THE SOLAR NEIGHBORHOOD
“All truths are easy to understand once they are discovered; the point is to discover them.” Galileo
In 1801, the Italian monk Giuseppe Piazzi learned that what he thought was at first a star, then a planet
and finally a comet, was, quite literally, none of the above. Without understanding fully what he saw,
he named this moving "star like" object Ceres.
A year later, the German physician Heinrich Olbers found
another star-like object and named it Pallas. Sir William
Herschel, who had previously discovered the planet Uranus
in his back yard with a home made telescope, was one of
several scientists who tried to come up with a name for what
Ceres and Pallas might be. While he never quite determined
what they were, his term, "asteroid," from the Arabic root
"aster," became the name that most astronomers used.
Like many scientific phenomena, asteroids were observed
and named before anyone understood what they were. We
now know of more than 150,000 of these objects within the Solar System. So many asteroids remain to
be discovered that the quest to find, identify, catalog and understand asteroids has become an
international pursuit that has transcends political and cultural distinctions. Thanks to an ever increasing
number of professional and amateur astronomers, asteroid research has become a science that unites
the world.
Image 1: Matilde (NASA/ JPL)
Our understanding of asteroids has also come a long way
in 200 years. We’ve learned that asteroids are leftover
debris, or planetesimals, from the formation of the Solar
System. Locked in orbit about the sun, these chunks of
rock never combined to form planets. Most of them they
tumble at great distances from each other in a belt
between Mars and Jupiter. Though asteroids can measure
a mile or more in length, if every one that has been
discovered since Piazza’s find were formed into a single
body, the result would be smaller than the Earth’s moon.
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Image 2: Ida (NASA/ JPL)
So Who Found Them?
Image 3:
Gaspra (NASA/ JPL)
Although Piazzi's discovery of the first asteroid was by accident,
most of what followed was intentional. Astronomers actively hunted
the night sky for “star-like” objects with telescopes. By the middle
of the 19th Century, they had found ten. Others predicted the orbit of
an asteroid mathematically using the Titus-Bode Law. When the
German astronomer Maximilian Wolf pioneered the field of
astrophotography in 1891, about 300 asteroids had been discovered
using observational and mathematical methods. Wolf took
photographic images of the sky and noticed streaks of light against a
field of motionless stars. His work brought the total of known
asteroids to 500.
During the last century, more asteroids were found in photographic surveys of the sky using wide-field
telescopes and stereoscopic techniques. Since 1998, the majority of asteroids have been identified from
automated sky surveys and CCD cameras. The LINEAR system has made the greatest strides, with 97,
470 discoveries as of 2008.
Sky surveys pick up many of the obvious bright objects. They tend to miss fainter objects. This leaves
room for more discoveries of what astronomers call the higher magnitudes, objects that appear to be
dimmer in the night sky. It is in these magnitudes that amateur astronomers, armed with moderately
capable telescopes and a passionate interest in the night sky, continue to make significant contributions
to asteroid research.
Secondary school programs such as Hands-On Universe (HOU), the
International Astronomical Search Collaboration (IASC), the Killer
Asteroid Project and ARI Observatory with Bob Holmes provide
opportunities for students to make original discoveries and
confirmations of outer space objects. On February 2, 2009 amateur
researchers from IASC and Holmes received national attention for
making a significant discovery of a virtual impactor Near-Earth Object
(NEO).
Beginning in 2010, the NASA WISE space telescope will survey the
sky in the mid-infrared region and reveal new objects, providing even
more opportunities for amateur scientists to contribute to the
international scientific community’s understanding of asteroids.
Image 4: 2009 BD 81 Discovery of
Potentially Hazardous Asteroid (PHA)
by ARI in Feb 2009.
Before anyone can make their contribution, it is useful to review what we’ve come to understand about
these elusive, enticing and potentially dangerous objects.
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What are “Asteroids?”
All objects that orbit about the Sun that aren’t planets or moons, regardless of their sizes, are
categorized as Minor Planets. If the object is composed mostly of rock, it's an asteroid. If it's mostly
ice, it's a comet. If the object is found in an interplanetary debris field, it is meteoroid. A meteoroid that
enters the earth's atmosphere is referred to as a meteor or "shooting star." A group of meteoroids
entering during the same time will become a meteor shower.
And if the meteor doesn't burn up from entry and part of it hits the ground, the remnant, which may or
may not have melted and fused with a planet's surface, is called a meteorite.
How Big Are They?
Asteroids large enough to cause significant damage to our
planet are actually rare. According to NASA, our solar
system contains only 200 asteroids greater than 100 km in
diameter. About 750, 000 fit within the range 99 to 1 km in
diameter. It is thought that millions more are smaller than
that.
Image 5: Ceres.
Put them all together and you don’t get much. The
combined mass of every asteroid would probably be no
more than 0.0005 times the mass of Earth. If all the
asteroids were combined into one big minor planet, its
diameter would be less than 2000 km, about half the
diameter of our moon.
It is believed that when the Solar System formed, the original asteroid
belt was anywhere from 100 to 1000 times more massive. Where did
all that mass go? One theory involves the planet Jupiter. Jupiter's
strong gravitational force will perturb the orbit of any close object.
Some of these asteroids could have fallen into highly eccentric orbits,
either leaving Solar System or headed inwards toward Sun. Some
might have suffered the fate seen in the Shoemaker-Levy "String of
Pearls" incident when, in July 2009, something large hit Jupiter. The
impact scar could have been caused by a collision with a comet or an
asteroid.
Image 6: Jupiter's scar, July 2009
The largest asteroid is Ceres. At just under 1000 km in diameter, it
comprises a third of the total mass of all asteroids within our Solar System. In 2006 Ceres became the
only asteroid to be reclassified as a “dwarf planet” by the IAU. Ceres is so massive that its gravity has
shaped it into sphere.
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The other 150,000 known asteroids are highly irregular in shape.
Consider the elongated potato shape of the asteroid Ida to the
left.
Image 7: Ida (left) and moon Dactyl.
Turn, turn, turn.
Most asteroids rotate rapidly. What causes this spin? Light from the sun creates uneven areas of heat
on asteroids less than 10 km in length. This
contributes to a rotational torque as the hotter
areas are displaced by cooler ones. To an
observer with an optical telescope, these
spinning asteroids flash and flicker like
beacons.
Astronomers measure the change in brightness
for the rotation period over a few nights, and
use these measurements to construct a graph
known as a light curve.
The curves are created by measuring the
brightness of an object as viewed through a
telescope with an imager and a software
program with a photometry tool. The curve
demonstrates the period of rotation,
temperature and brightness.
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Can asteroids have moons?
Many asteroids, such as Ida and its moon Dactyl, are binary systems that rotate about a common center
of mass. The Clearwater Lakes in Canada are evidence of an impact from a binary system about 300
million years ago.
Another moon system is 22 Kalliope and Linus. Kalliope is a
large main belt asteroid and has a stony surface composition,
although it is technically classified as a metallic asteroid. It's
precise composition has not been determined.
Image 8: The Clearwater Lakes, Canada.
Image 9: 22 Kaliope (right) and Linus
Can asteroids collide with other asteroids or objects in space?
They can, and do. Asteroids that have experienced major collisions
can be observed with a "family" of small fragments that share the
same orbit and inclination. Some families have hundreds of
members while others contain a just few.
Image 10: 951 Gaspra
One of the largest in the main belt is the Flora family with almost
590 members. The star of the family is the asteroid Gaspra, which
was imaged by the Galileo satellite on its way to Jupiter in 1991.
The Flora family orbit is close to an unstable zone in the asteroid
belt. Occasionally a member will be flung out and away. It's likely
that 38% of all the meteorites that have hit Earth are from this
family.
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Some asteroids are categorized as "rubble piles." These
are asteroids that were smashed to bits by a collision, and
over hours and weeks, fell back together to form a loose
collection with very low densities, cracks and cavities. 87
Sylvia and her two moons Romulus and Remus are
example of rubble piles.
When images of Gaspra are compared with Itokawa, a
known rubble pile asteroid, it becomes obvious that
Gaspra has impact cratering while Itokawa does not.
Itokawa's odd appearance may be the result of the final
collision of a binary rubble pile system.
Image 11: Illustration 87 Sylvia and Moons Romulus
and Remus
The Japanese spacecraft Hayabusa
landed on Itokawa for a short time in
2005 to collect samples and then blasted
off. It is expected to return to earth in
2010, landing in Australia. If samples
survive the trip, we will know a great
deal about this elusive asteroid's
composition.
Image 12: 25143 Itokawa
Where are asteroids located in our solar system?
Most inhabit the “asteroid belt,” a region of space
between the orbits of Mars and Jupiter. The
average distance between each asteriod is several
million kilometers so that if you were standing on
any asteriod within the Belt, you would barely be
able to see your closest neighbor. .
The Belt is the only place in the Solar System
where rocky planetesimals have remained for
billions of years. The other rocky planetesimals
condensed to form Mercury, Venus, Earth and
Mars. It is believed that the perturbing gravitational
influence of Jupiter prevented the asteroids from
forming into an additional planet.
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Image 13: Asteroid Belt and Jupiter's Trojans and Greeks.
The Trojan asteroids are located in Jupiter’s orbit
at the stable Lagrangian points L4 and L5 some 60o ahead and behind Jupiter
Gravitational encounters with Jupiter, and other objects, have given a few asteroids eccentric orbits
that take them from the asteroid belt through the inner Solar System. Those that cross Mars’ orbit are
called Amor asteroids. Those that pass through Earth’s orbit are called Apollo or Aten asteroids.
What are asteroids composed of?
Most asteroids have been discovered by observing them
visually using land-based or space-based telescopes. So
the brighter they are, the easier it is to detect them and
collect and analyze their data based on their light
curves. Asteroids spin and their elongated shape
produces a particular pattern. However, the brightest
asteroids compose only 8% of the population, while the
darkest, which are best observed in the infrared range
compose 75%. So they are classified by their
appearance in context of how they are imaged for
research:
C-type: Dark carbonaceous objects - 75%
S-type: Brighter silicaceous (stony) objects – 17%
X or M-type: Primarily high composition of metals
(iron or nickel) with varying albedos – 8%
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As research targets, their spectral brightness and composition provide valuable clues to the origin of
the universe.
Seeing Asteroids In a Special Light
Almost all asteroids discovered so far have been observed because they reflect enough visible sunlight
to be resolved in a telescope or on a digital frame. As well as being reflected, the sunlight also warms
the surface of the rocks and they emit infrared. Depending on the composition, some emit more
infrared radiation than they reflect visible, so they are "brighter" in the infrared spectrum. The table
below reveals that most asteroids, because of their composition, are better detected in the infrared
range than the visible.
% of
Population
Brightness
Hot
8
brightest
.10 - .18
Warm
17
Medium
bright
.10 - .22
Cold
75
dark
Albedo
.
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Composition
Class
Class Name
Fe-Ni
Fe-Ni &
magnesium
silcates
early solar
system materials
& presolar
grains
M
Metallic
Detection
Methods
Visual, IR
S
Silicaceous
Visual, IR
C
Carbonaceous
IR
Observing infrared light emitted by objects in space on earth is challenging because water in our
atmosphere "mops up" and absorbs much of what enters. So high altitude observatories, or even better,
those that are orbiting in space, are the best ways to catalog infrared objects like asteroids.
The WISE satellite will conduct a whole sky survey over less than a year and deliver over a million
images, in four mid-infrared wavelengths, of the sky. The catalog of objects that will be discovered
will include dark (cold) asteroids that reflect little visible light and so were invisible to the view.
Besides discovering asteroids, the WISE data will help in refining the orbits of known asteroids and
determining their sizes. Both pieces of information are critical in assessing risks from Near Earth
Objects.
WISE data will also help to study how uneven heating of an asteroid by the Sun can alter its orbit. This
is also important to understand in assessing the risk of a NEO. How likely will an orbit change once
figured and how will it be changed?
Sorting by Threat Level: PHA's
According to the JPL website in July 2008, 964 known potentially hazardous asteroids, or PHAs, have
a probability of approaching earth's orbit within 0.05 AU, where they could collide with the Earth. The
Earth's orbit has a gravitational "keyhole" no more than 400 m across. When an asteroid passes
through the keyhole, it perturbs the orbit. The asteroid may move closer, or even collide, on the next
orbital pass.
The PHAs are further classified into three varieties:
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1. Earth-Crossing- When the orbit of an asteroid has been so perturbed by Earth's gravity that it may
potentially pass close to or collide with the planet.
2. Earth-Crossers- A special group of Apollo asteroids with orbits that cross Earth's. A significant
number of these will hit earth.
3. Near Earth Object- an asteroid that approaches but does not cross the section of Earth's orbit that
would perturb it and bring it closer to collide with the planet.
Asteroid 99942 Apophis will be doing a close flyby in Earth in 2029.
There is a very small possibility that it may orbit through Earth's
gravitational keyhole. Upon returning in 2036, Apophis just might
collide with Earth.
The asteroid is 320 km in diameter and too small to be a planet killer,
but the damage it might do could be severe. Astronomers will study
Apophis during the next observing opportunity in 2013, and learn more
about what the future may hold.
Image 14: Illustration of Asteroid 99942
Apophis
Very few asteroids that are discovered or known will ever collide with
our planet. Those that will collide probably won't result in a global catastrophic consequence as
predicted by the Torino scale. So how important is discovering asteroids, shaping their orbits,
collecting data and scrutinizing their qualities to find out what makes them spin the way they do?
Well, as an author of thriller novels explains: Use the tiny to evoke the tremendous, the ignorable to
depict the crucial, a blur to focus what is in front of our eyes, if we only took a second look.
Perhaps there are mysteries about the origins of our solar system that we will never know, until we
unlock the secrets kept in these few and very cold space rocks.
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