COMETS, ASTEROIDS, AND METEORS

OverviewRecent advances in space technology have allowed scientists coming from

different background like physics, chemistry, biology, and geology to collaborate on studying Near-Earth Objects (NEO) like comets and asteroids . With more powerful telescopes and space probes, the study of comets and asteroids provides more clues about the origins of our solar system. Over the past three years, amateur and professional astronomers have discovered several NEO’s that came close to Earth, the most recent asteroid being Asteroid 2012 DA14. It made a very close approach to Earth as it orbited the Sun on February 16, 2012 (Philippine Time). On the morning of February 16, 2012, an asteroid entered Earth’s atmosphere and exploded over Lake Chebarkul in Russia hurting about 1,000 people in the process. These two events triggered superstitions, fears, and doomsday prophecies held by different cultures. But do these things have scientific basis?

HAS EARTH EVER BEEN HIT BY A COMET OR AN ASTEROID?

IF YES, HOW HAVE SOME IMPACTS AFFECTED EARTH?

HOW OFTEN DOES A COMET OR AN ASTEROID HIT EARTH?

comets

Comets and asteroids are referred to be astronomers as Near-Earth Objects (NEO).

Comets -are icy bodies or objects while asteroids are rocky fragments.

-they are remnants from the formation of our solar system 4.6 billion years ago.

The table summarizes the similarities and differences between a comet and an asteroid.

As you can see in the table, comets and asteroids have irregular shapes and varied sizes.

-They both reflect light from the Sun at varying amounts depending on the size and composition.

-The presence of more silicates allows a comet or asteroid to reflect light.

Silicates -are minerals that contain the elements silicon, oxygen, and at least one metal.

If an asteroid has smaller amounts of silicates relative to its other components, it would be more difficult to see it even with a telescope because only a small area of the asteroid can reflect light thus it may appear smaller than what it really is when viewed.

Characteristic Comet Asteroid

Origin Kuiper Belt and Oort Cloud Main Asteroid Belt

Shape Varied/Irregular Varied/Irregular

Size range of diameter(kilometer)

1 – 10 (nucleus only) 1 – 100 ++

Chemical composition Ice (frozen water); frozen gases (ammonia, methane, and carbon dioxide); other organic compounds (Carbon – containing compounds)

Orbit Highly elliptical More rounded

Orbital period (years) 75 to 100,000++ 1-100

Table 1. Comparison of some characteristics of comets and asteroids

Comets

A Comet is a small body of ice, rock and cosmic dust loosely packed together in a highly elliptical orbit

around the sun.

Structure of a Comet

To Sun

Ion Tail

Dust Tail

Coma

Comet Structure

• Nucleus• 10 km “Dirty Snowball”

• Coma• Cloud of evaporated ices and ions• may be 100,000 km in diameter

• Tail• Always points away from Sun• Solar Wind and Radiation Pressure

Tail always points away from Sun because of solar wind and radiation pressure

?

Nucleus10 km “Dirty Snowball”

Comet and asteroid both orbit the Sun and move relatively slow when viewed from Earth. This means, you can see a comet for up to a year in the night sky (or even during the morning if the comet is bright enough).

Comets usually come from the

Oort Cloud which is beyond our

Solar System, and a few from Kuiper Belt which

is just beyond Neptune’s orbit. Long-period comets come from the Oort Cloud,

while short-period comets come from Kuiper Belt.

The Oort Cloud

In 1950 Jan Oort -noticed that no comet has been observed

with an orbit that indicates that it came from interstellar space, there is a strong tendency for aphelia of long period comet orbits to lie at a distance of about 50,000 AU, and there is no preferential direction from which comets come.

Comet Halley, the most famous comet of the 20th century

is the only known short-period comet. It takes 75-79 years for Comet Halley to orbit the Sun. we see it in the sky every time it makes its

nearest approach to the Sun. All other comets that have been identified are classified as long-period comets and takes 200 to hundred millions of years to complete their orbit around the Sun.

Asteroids, on the other hand, originate from the Main Asteroid Belt between Mars and Jupiter. This belt is theorized by scientists to be remnants of a planet that did not completely form.

Comet Halley 1910•POPE CALLIXTUS III EXCOMMUNICATED HALLEY'S COMET IN 1456 •IN 1910, CHARLATANS SOLD "COMET PILLS"

Bayeaux TapestryNorman Invasion of 1066

Comet of 1577

Two Showers for Halley

Comet Hale-Bopp

Comet Hale-Bopp, which made its closest approach to the Sun in April 1997, was one of the largest and brightest comets of the second half of the 20th century, even though its closest approach to the Earth was still relatively distant. A very long-period comet, it had no previous known history in human observation, and will not be seen again by humans for thousands of years.

Bennett's Comet

In 1970 the South African amateur astronomer J. Bennett observed this brilliant comet. A computer technique called isopoda mapping has enhanced this image by assigning different colors to the various grey tones in the original black and white photograph. The comet's long gaseous tail is clear to see.

The return of Halley's CometHalley's Comet reappears approximately every 76 years. This photograph, taken in New Zealand in 1986, shows the comet during its most recent approach to the Sun.

Comet HyakutakeComet Hyakutake reached its closest point to Earth in its path through the inner Solar System on March 25, 1996. The comet was bright enough to be seen with the naked eye.

Comet West

A Tale of Two Tales

ION TAIL (top, blue): ionized gas pushed away from the Sun by the solar wind.

DUST TAIL (bottom, white): fine dust pushed away from the Sun by radiation pressure.

A Tale of Two Tales

Edmond Halley

The astronomer and mathematician Edmond Halley was the first person to calculate the orbit of a comet. He believed that comets travelled in elliptical orbits around the Sun. This was proved to be correct with the return, in 1758, of what is now known as Halley's Comet.

SkylabIn 1973 the United States

-launched the space station Skylab. This photograph of Skylab was taken over the Earth on February 8, 1974, by the fourth group of astronauts to occupy the space station. They took the photograph from their Apollo spacecraft as they prepared to return home. The sheet of gold-colored material stretched over the station is a makeshift Sun shield that the second group of astronauts to occupy Skylab set up. It replaced a Sun and micro-meteoroid shield that fell off during launch. Skylab operated for six years.

asteroids

asteroids• When the solar system came together about 4.6 billion years ago, many lumps

of rock did not become part of any of the planets. These bits and pieces have continued to orbit round the Sun. Sometimes they smash into each other and break into smaller pieces. Sometimes they crash into planets as meteorites. A few have been captured as moons, mainly by Jupiter. But thousands still wander round the Sun. They are called minor planets, planetesimals or asteroids.

• Most of the asteroids we know about are in a wide belt between the orbits of Mars and Jupiter. Another belt of small, icy objects lies at the outer reaches of the solar system, far beyond Neptune, called the Kuiper Belt. Other asteroids are sprinkled thinly between the orbits of the planets.

Asteroids

Asteroids are made of rock, metal, or a combination of

both.

They range in size from one metre ( or perhaps smaller ) up to 1000km.

Asteroids have different shapes. Big ones are usually round, but small ones are all sorts of knobby shapes, and sometimes look like potatoes.

They have craters where smaller rocks have crashed into them. Some asteroids form pairs going round each other as they orbit the Sun, in the same way as the Earth and Moon.

Some have been given names, such as Eros, Hermes, Vesta, Chiron and Ida, but most are listed by numbers. Most are dark in color because they have a lot of carbon.

These have not changed much since they were formed.

Some contain iron and other minerals and seem to have been melted in the past.

• A number of asteroids cross the path of the Earth. That means we might be hit by one of them. If a big asteroid collided with the Earth, the results would be catastrophic. Big meteorites have crashed into the Earth several times in the past. One may have caused the death of the dinosaurs, 65 million years ago. Such collisions are very rare, but they do happen.

• Eros is one of the asteroids near Earth. The Near Earth Asteroid Rendezvous (NEAR) spacecraft, sent by NASA (the National Aeronautics and Space Administration) went into orbit around Eros in 2000, and became the first spacecraft to land on an asteroid. It sent back pictures of Eros’s pitted and grooved surface, and information about what it is made of. Finding out more about asteroids will help us to predict what might happen if one did hit the Earth. Many scientists think that governments should set up a shared system that would knock an asteroid off course if it came towards us.

Asteroid BeltAsteroids are mostly in the asteroid belt between Mars and Jupiter.

Asteroid Orbits

Asteroids are strongly influenced by the gravity of Jupiter

Asteroids Elsewhere

The orbit of an asteroid is more rounded and less el l iptical than the orbit of a comet.

In February 2013, Asteroid 2012 DA14 -made a very close approach to Earth as it orbited the Sun.

Distance in spaced is measured in l ight years and this Asteroid was just 0.4 l ight year away from Earth; the closest that any asteroid has ever been close to Earth.

In December 2012-during the midst of the dooms day prophecies, Asteroid Toutatis

also made a near approach to Earth but not as close as Asteroid 2012 DA14

Three asteroids

• This composite image shows the asteroids Mathilde, Gaspra and Ida. Mathilde, left, was photographed by the Near Earth Asteroid Rendezvous (NEAR) space probe in 1997. Gaspra and Ida, centre and right, were photographed by the Galileo space probe in 1991 and 1993, respectively, and are smaller and more oblong-shaped than Mathilde. The three asteroids are partially obscured by shadows.

At least 1.1 million asteroids larger than 1 km in the main Asteroid belt around the Sun .

Taking a Close Look

TAKING A CLOSE LOOK

The discovery of high contents of iridium in oceanic sedimentary layers in different parts of the world such as Italy, Denmark, and New Zealand during the late 1970’s led geologists.

Luis and Walter Alvarez- propose the Alvarez Hypothesis in 1980.

Iridium-is a metal belonging to the Platinum family. It is a very rarely found in Earth’s crust,

but more abundant in the mantle and core. It is also abundant in our solar system.They proposed that an asteroid with approximately 10 kilometers in diameter made

impact with Earth 65 million years ago. They thought that the impact caused material to be thrown up in the air, thus blocking sunlight, and brought about a period of winter long enough to cause a mass extinction of plants and animals, including the dinosaurs. Further, this event ended the Cretaceous Period and ushered in the Tertiary Period.

METEORS

Have you ever seen a shooting star in the night sky?

It appears as an object with a tail just like a comet.

It travels quickly and appears to fall on the ground. A shooting star is another name for meteor.

But the truth is: a meteor is not a star at all. Meteor is a light phenomenon or a steak of light that occurs when a meteoroid burns up as it enters Earth’s atmosphere.

Meteoroid is a broken up rock and dust from either a comet, asteroid, the Moon, or from Mars.

Meteoroid

- can be as small as grain of sand or as big as a boulder. When it enters Earth’s atmosphere, the air in front of the meteoroid

heats up, causing materials to burn up. From Earth, these glowing materials appears as a streak of light or a fast-moving bright object that appears to have a tail just like a comet.

-what differentiates the two when we see them in the sky is that a comet moves slowly and appears in the sky for a longer time.

- usually all burns up when it enters Earth’s atmosphere. But when a fragment is now called a meteorite.

- meteorite exploding over Russia in February 2013 , their use of the word meteorite inaccurate. Instead, a meteoroid exploded over Russia.

-the space fragments they collected on the ground is the meteorite.

Meteor

- moves swiftly and seems to fall on the ground.

-it “shoots” from a point in the sky, making people think that it is a shooting or falling star.

Also, a comet is difficult to see with the unaided eye because it is farther from Earth compared to a meteoroid entering Earth’s atmosphere.

Sometimes, a comet can be bright enough to seen by the unaided eye, but this is rare, just like in the case of Comet Hale-Bopp.

When Earth orbits the Sun and passes through the orbit os a comet where these comet fragments are found, we observe many streaks of light from Earth which is called a meteor shower.

During a meteor shower, meteors seem to originate from only one point in the sky because the meteoroids are travelling in parallel paths with the same velocity.

Meteor Shower

- is named after the constellation where they seem to originate from, but this does not mean that the meteoroids come from the associated constellation.

REMEMBER:A meteor and a meteor shower are light phenomena; they are not

stars.

The Cause of Meteor Showers

Why after midnight is best?

Rotational Velocity

Orbital Velocity

Midnight

The Leonid meteor storm of 1833

On the night of November 12-13, 1833, the annual Leonid meteor shower unexpectedly turned into a raging storm of up to 150,000 meteors per hour—some 40 per second—all appearing to radiate from a single point in the constellation Leo (hence the shower's name). It was the greatest meteor storm in recorded history. It was terrifying in its ferocity, and caused widespread panic where it was visible in the eastern United States.

The 1966 storm

1997 Leonids from Orbit

Meteor CraterMeteor Crater in Arizona in the United States was created about 50,000 years ago by a meteorite up to 50 meters in diameter. The crater is some 1,200 meters across and about 180 meters deep. For many years it had been thought that the crater had been caused by a volcano. It was finally identified as having been caused by a meteoric impact by the geologist and astronomer Eugene Shoemaker in the 1950s. It was the first large crater on Earth to be identified as an impact feature.

An iron meteorite 100 feet across and 70,000 tons slammed into the Earth at about 43,000mph in the Arizona desert near Flagstaff 40,000 years ago. Bar ringer Crater is 4,100 feet wide and 571 feet deep.

Large Impact Craters Crater

Other Impact Craters

Tunguska, 1908

Sporadic Meteors

Irons Stony-Irons

Chondrites Carbonaceous Chondrite

Achondrite

Comets Hitting Earth

The number of meteors that can be seen during a meteor shower vary. It starts with the appearance of a few meteors per hour, increasing in frequency until it reaches its peak of 1-2 meteors per minute, and then declines. The table shows some of the more famous annual meteor showers and the month when they reach their peak. The dates in the peak month vary and astronomers make forecasts of the peak days (usually lasting for three days.

Month Source of meteoroid

Name of the Meteor Shower

Constellation(where the meteor shower

seem to come from)

August Comet Swift-Tuttle Perseid Perseus

October Comet 21P/Giacobini-Zinner

Draconid Draco

October Comet Halley Orionid Orion

November Comet Tempel-Tuttle Leonid Leo

November Comet Encke Taurid Taurus

December Asteroid 3200 Phaethon

Geminid Gemini

Table 2. Some examples of Famous Annual Meteor Showers

Usually, the meteoroids that cause meteor showers come from comets, but they may also come from an asteroid like in the case of the Geminids. Earth passes through Asteroid3200 Phaethon’s orbit where some fragments from the asteroid are found.

Once these fragments enter Earth’s atmosphere, they burn up as well.

Meteoroids from comets appear fuzzy because of the ice particles while those from asteroids are clearer and distinct because they do not have these ice particles.

Meteorites are of importance to scientists in studying the occurrence of different elements and compounds on Earth. This information is in turn important in studying our mineral resources which is an important industry in any country.

Generally, there are three types: stony, stony-iron, and iron meteorites. In the Philippines, there are only five meteorites that have been accepted internationally.

The table enumerates this meteorites.

Meteorite Year of Discovery

Place Discovered

Type Chemical Composition

Pampanga 1859 Pampanga Stony Iron-Nickel (7-11%); Ferrous sulfide (FeS); Magnesium Iron silicate (a.k.a. olivine or (Mg, Fe)₂SiO₄); Calcium-Aluminum intrusion (Ca-AI); pyroxene or XY (Si, AI)₂O₆(X can be calcium, sodium, iron⁺2 and magnesium and more rarely zinc, manganese and lithium; Y represents smaller-sized ions like chromium, aluminum, iron⁺3, magnesium, manganese, scandium, titanium, vanadium and iron⁺2

Paitan 1910 Paitan, Ilocos Stony Iron; Magnesium iron silicate (a.k.a. olivine or (Mg, Fe)₂SiO₄); pyroxene

Calivo 1916 Western Visayas Stony Not yet determined

Pantar 1938 Central Mindanao

Stony meteorite

Iron; Magnesium iron silicate (a.k.a. olivine or (Mg, Fe)₂SiO₄); pyroxene

Bondoc 1956 Southern Tagalog Stony-iron Metallic iron-nickel; silicates (olivine and pyroxene)

Table 3. Five Meteorites Found in the Philippines (Internationally Validated)

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Comets, asteroids, and meteors