ASTEROIDS AND COMETS

 

 

 

Unit Overview

This unit will investigate the composition and orbits of asteroids and comets. A study of the effects that asteroids and comets have on other planetary bodies will also be included.

 

Unit Directions

Read the unit below, complete all activities, and complete the test at the end of the unit. It is helpful to review the key terms before you begin the unit.

 

 

GLOSSARY OF KEY TERMS

Asteroid - A medium-sized rocky object orbiting the Sun; smaller than a planet, larger than a meteoroid.

Asteroid belt - Region of the solar system, between the orbits of Mars and Jupiter, in which most asteroids are found. It is the dividing line in our solar system between the inner and outer planets.

Ceres - The largest asteroid in our solar system.

Coma - A diffuse, luminous cloud of dust and gas that develops around a comet's nucleus as it nears the Sun.

Comet - A relatively small extraterrestrial body consisting of a frozen mass that travels around the Sun in a highly elliptical orbit .

Comet Nucleus - Solid part of the comet containing ices and dust close to the Sun, the source of a coma, dust, and plasma tails.

Dust Tail - One of the two tails of a comet made of dust grains that curves away from the Sun from the action of the photons in the Sunlight pushing the dust grains away from the Sun. It has a yellow-white color from reflected Sunlight.

Ejecta - Material such as glass (melted silicon) and fragmented rock is thrown out of an impact crater during its formation.

Impact Crater - Impact craters are the remains of collisions between an asteroid, comet, or meteorite and the Earth.

Ion Tail - One of the two tails of a comet made of ionized particles that point directly away from the Sun from the action of the solar wind. It is bluish from the emission lines, mostly of ionized carbon monoxide.

Kuiper Belt - A large ring of icy, primitive objects beyond the orbit of Neptune. Kuiper Belt objects are believed to be remnants of the original material that formed the solar system. Some astronomers believe Pluto and Charon are Kuiper Belt objects.

Meteor - A streak of light in the sky at night that results when a meteoroid hits the earth's atmosphere and air friction causes the meteoroid to melt or vaporize or explode.

Meteorite - Matter that has fallen to the earth's surface from outer space.

Oort Cloud - A large spherical cloud of billions to trillions of comets surrounding the Sun at distances between roughly 50,000 to 100,000 AU from the Sun. It has not been directly observed; its presence is inferred from the behavior and orbits of the long-period comets.

Orbit - The (usually elliptical) path described by one celestial body in its revolution about another.

Shooting Star – A term used for meteor.

 

Print out the Key Term worksheet for extra practice. Key Term Worksheet

 

Space Junk?

A great deal of matter is flying around in outer space! You already know about planets, the Sun and stars, and the forces that affect them. This unit will look at some of the smaller pieces of matter that seem to be no more than space junk, and that term may be more accurate than you think.

 

Asteroids

Asteroids are formed from leftover material remaining after the formation of our solar system. They are composed of rock and metals that did not form on the main planets. They orbit our Sun just as the planets do. Many of the asteroids are significant and have even been given names by scientists! Ceres is the largest asteroid in our solar system and is around 1000 km in diameter. Many asteroids are the size of pebbles. All the asteroids in our solar system have orbits, most of which are located in the asteroid belt. This belt is located between Mars and Jupiter.

 

   

 

 

 

 

As you can see in the picture above, asteroids come in different sizes and many different shapes. Ceres is a very spherical, giant asteroid that looks like a small planet. Scientists sometimes refer to the asteroids as the minor planets.

 

Below is a chart showing some physical data and the history of the discovery of several asteroids.

 

 

ASTEROID SUMMARY

Num

Name

Radius
(km)

Distance*
(10^6km)

Albedo

Discoverer

Date

1 

 Ceres

466

413.9

0.10

G. Piazzi

1801

511 

 Davida

168

475.4

0.05

R. Dugan

1903

433 

 Eros

17.5 x 6.5

218

?

G. Witt, A. Charlois

1893

15 

 Eunomia

136

395.5

0.19

De Gasparis

1851

52 

 Europa

156

463.3

0.06

Goldschmidt

1858

951 

 Gaspra

17x10

330.0

0.20

Neujmin

1916

10 

 Hygiea

215

470.3

0.08

De Gasparis

1849

243 

 Ida

58x23

428

?

J. Palisa

29 Sep 1884

704 

 Interamnia

167

458.1

0.06

V. Cerulli

1910

253 

 Mathilde

28.5 x 25

396

0.03

J. Palisa

1885

2 

 Pallas

261

414.5

0.14

H. Olbers

1802

16 

 Psyche

132

437.1

0.10

De Gasparis

1852

87 

 Sylvia

136

521.5

0.04

N. Pogson

1866

4 

 Vesta

262.5

353.4

0.38

H. Olbers

1807

*Mean distance from the Sun.

 

 

 

 

The Asteroid Belt


The asteroid belt is the dividing line between the inner and outer planets in our solar system. Many asteroids remain in this belt, but some wander away from this region. Some of these asteroids orbit in a plane far above the planetary orbits. But others pass very close to the Sun and cross Earth's orbit. Fortunately, the Earth is in a different position when these asteroids pass. Asteroids can hit the Earth, but the chances are minimal. Let's look at what happens to a planet when it collides with an asteroid.

 

We are most concerned with asteroid collisions with Earth. The Earth is hit quite often by minor asteroids – daily. These small asteroids are called meteors. Meteors are small pieces of asteroids that enter into Earth's atmosphere and look like burning fireballs in the night sky. Perhaps you have heard them called shooting stars. They are not stars; they are pieces of the asteroid as it breaks up in the atmosphere. These pieces of space debris burn up before they hit the Earth's surface. Pieces of meteors that do not completely burn up and eventually hit the surface are called meteorites.

 

Meteors

 

Meteorites

 

Meteorites have hit the Earth's surface, and scientists have recorded the damages done by their impact. In 1908, a meteorite hit a region of Siberia and flattened and burned a forest. This type of meteorite is referred to as a small impactor. There is also evidence of significant impacts by meteorites in the Earth's past. Scientists believe there is evidence of a large meteorite (12 miles in diameter) that impacted the Earth 3.4 billion years ago. This would be very early in the Earth's history, even before the formation of the continents. Deposits of the meteorite were discovered in South Africa and Australia. The scientists believed that the meteorite would have taken only 2 seconds to travel to the bottom of the ocean before impacting the floor. This collision would have caused huge tidal waves that eroded any newly formed land masses. Scientists have not been able to determine the exact impact area since they have not located an impact crater on the sea floor. Another cataclysmic meteorite collision is believed to be the cause of the extinction of the dinosaurs.

 

Meteor Crater in Arizona is evidence of an impacting meteorite from 50,000 years ago. The rock was composed of nickel and iron. The crater left an impact site 215 meters deep and 1.2 kilometers wide. Every time scientists study material left by a meteorite, and they learn valuable information about the formation of our solar system. wo new minerals were also discovered, coesite and stishovite. Both minerals are composed of silicon dioxide found on Earth, but they have undergone extreme heat and pressure that could only occur as the meteorite passes through the Earth's atmosphere.

 

Meteor Crater, Arizona

 

Meteors are composed of a variety of metal and rock-metal combinations. The chart below shows the main meteorite types.

 

 

 

Meteorite Types

Iron

primarily iron and nickel; similar to type M asteroids

Stony Iron

mixtures of iron and stony material like type S asteroids

Chondrite

by far, the most significant number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets

Carbonaceous Chondrite

very similar in composition to the Sun, but with lesser amounts of volatiles; similar to type C asteroids

Achondrite

similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites

 

   

 

 

Meteors have impacted other planets and moons. Looking at our moon, you can see that the evidence is clear. The Earth's moon must have had a violent past, as seen by all the impact craters on its surface. Other planets' moons have also had close encounters with meteors.

 

 

 

Near Side of Our Moon

 

 

Far Side of Our Moon



Surface of Calisto – Moon of Jupiter

 

Looking at the pictures above, you can see that a thick atmosphere can protect a planet or moon. Both moons above have thin or no atmospheres, so the meteors do not burn up before they get to the surface. For this reason, they have hundreds of impact craters on their surfaces. There is also evidence of collisions between asteroids and planets. Look at the pictures below.

 

 

(above) The picture on the left is an impact crater on the face of Mars. The image on the right is the planet Uranus. The planet is tipped on its side, and scientists believe it may have collided with a planet-sized asteroid early in our solar system's history.

 

What exactly happens when a meteor hits the surface of a planet? The size, density, and force at which the meteor hits the surface determine the size and depth of the crater that forms. The first thing that happens is a fracturing of the surface rock, and that rock is forced out of its original position as the meteor pushes into the planet. The impact force sprays the surface rock out of the impact area. This debris is called ejecta. Some of the planet's rock will melt due to the high temperature of the meteor, and some of the meteor's pieces will melt or vaporize upon impact. Depending on the force and angle of impact, different types of crater formations will occur. Look at the following chart that describes crater formations:

 

 

 

 

Floor - Bottom of a crater, either bowl-shaped or flat, usually below the level of the surrounding ground.

 


Central peaks - Peaks formed in the main area of the floor of a large crater. For larger craters (typically a few tens of kilometers in diameter), the excavated crater becomes so great that it collapses on itself. The collapse of the material back into the crater pushes up the mound that forms the central peak. At the same time, the rock beneath the crater rebounds or bounces back up to add to the peak.

 

Walls - Interior sides of a crater, usually steep. They may have giant stair-like terraces created by slumping the walls due to gravity.

 

 

Rim - Edge of the crater. It is elevated above the surrounding terrain because it is composed of material pushed up at the edge during excavation.

 

 

Ejecta - Rock material is thrown out of the crater area during an impact event. It is distributed outward from the crater's rim onto the planet's surface as debris. It can be loose materials or a blanket of debris surrounding the crater, thinning at the outermost regions.

 

 

Rays - Bright streaks extending away from the crater, sometimes for great distances, composed of ejecta material.

 

 

 

 

The following activity may require teacher assistance.

Activity: Making Impact Craters

PURPOSE: To observe and record basic concepts about impact craters.

OBJECTIVES: Draw and describe the shapes made by the various objects dropped. Observe that a crater’s size and features depend on the mass and velocity of the impactor.

MATERIALS:

o   Pictures of craters from the Moon and Mars (Printed from the internet is best.)

o   Safety glasses

o   Large tub or aluminum pan

o   Fine white powder (sand, flour, sugar, etc.)

o   Fine colored powder (cocoa, powdered drink mix, dry powdered tempera paint, pudding, etc.)

o   Sieve, sifter, large spoon, or cheesecloth to sprinkle the dark powder

o   Two same-sized balls of different weights (e.g., marbles, ball bearings, gum balls, grapes, etc.)

o   Two same-weight balls of different sizes (e.g., rubber balls, golf balls, water-filled ping pong balls). Try to have one small and one large ball.

o   Small irregularly shaped rocks.

o   Tape measure

o   Toothpicks

o   3x5 index card (to smooth the surface of the powder)

o   Newspaper or drop cloths

o   Paper and pencils for sketches of craters

o   Paper to design a data chart

DIRECTIONS: Look at your pictures of the Moon and Mars. Find the parts of a crater and label them on the picture. From what you have learned in this unit, what factors influence a crater’s appearance?

1.Complete the following steps:

·       Fill a pan with white powder (sand, flour, etc.) to a depth of about 2.5 centimeters (1 inch).

·       Tap the pan on the table to settle the material and smooth the surface.

·       Sprinkle a fine layer of colored powder (see materials list) evenly and entirely over the white layer.

·       Sprinkle another layer of white powder over the top of the colored layer.

2. Use safety glasses to protect your eyes from flying powder.

3. Design your own data chart/sheet with 12 boxes so that you can fill in measurements for the drops.

4. Drop the different mass (weight) balls from three different heights. (Balls are similar in size.) Draw the crater and then measure the diameter of each crater and the distance the ejecta traveled after each impact. Use the toothpick to help you measure the depth of the crater. Record your results.

5. Drop the different-sized balls from three different heights. (Balls are similar in weight.) Draw the crater and then measure the diameter of each crater and the distance the ejecta traveled after each impact. Use the toothpick to help you measure the depth of the crater. Record your results.

6.    Answer the following questions:

a.     How did crater size change when balls of different mass (i.e., weight) were dropped from the same height?

b.    How would you state the general relationship between a ball's mass and the crater size?

c.     How did the size of the balls affect the crater sizes?

d.    How would you state the general relationship between a ball's size and the crater size?

e.     How did the different speeds of the balls affect the crater sizes?

f.      How would you state the general relationship between a ball's speed and the crater size?

Re-examine your pictures of the moon and Mars. Choose two craters on each picture with different formations and write a hypothesis on how they formed. Use your data from the experiment to support your hypothesis.

 

Now that we have an in-depth look at asteroids let's investigate some other space debris in our solar system – the comets.

 

Comets

Comets are lumps of ice and rock that travel the expanse of our solar system. Like the planets and asteroids, they travel in predictable orbits. Scientists have observed comets for centuries and have recorded data on their comings and goings. Some mathematical theories state that comets come from a large area outside our solar system, forming a cloud around our system. The area is called the Oort Cloud. This theory was not developed until around 1950. In ancient times, the appearance of a comet could be interpreted as something magical and considered lucky or unlucky. This is understandable since some orbits take hundreds or thousands of years to return to Earth’s vision. There are recordings of Chinese sightings of the Halley Comet as long ago as 240 BC.

 

 

Some comets are believed to be coming from closer in than the Oort Cloud, from a place within our solar system. Scientists call this area the Kuiper Belt, located past the orbit of Pluto.

 

 

 

It is believed that as the comets travel in the Oort Cloud or the Kuiper Belt, they are loosely bound by the Sun's gravitational pull. But when they travel closer to a planetary orbit, they can be drawn off course and enter a new orbit traveling toward the Sun and passing through our solar system.

 

Let's take a closer look at the composition of a comet. As stated earlier, comets are made up of ice (both water and frozen gases) and particles of dust that did not go into the formation of the planets. As they travel near the Sun, they begin to evaporate, and scientists have given names to the distinct parts of the comet.

 

 

·        nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids;

·        coma: a dense cloud of water, carbon dioxide, and other neutral gases sublimed from the nucleus;

·        hydrogen cloud: huge (millions of km in diameter) but a very sparse envelope of neutral hydrogen;

·        dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye;

·        ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.

 

 

 

 

One of the most famous comets is Halley's Comet. Sir Edmond Halley was the first to predict the return of Halley's Comet, using Newton's Laws of Motion. He stated that the sightings of a comet that appeared in 1531, 1607, and 1682 were the same. He also predicted the comet's return in 1758. Unfortunately, Sir Halley was not alive to see the comet's return, but as indicated, the comet was spotted in 1758 and later named in his honor. Halley's Comet has an average orbital period of 76 years. While the year that the comet returns is predictable, the exact date of return can not be accurately calculated because as the comet passes different planets, the gravity of the planets affects the orbit of the comet. The last appearance of the comet was in 1986, and the subsequent return will be in the year 2062.

 

 

 

The picture above is the nucleus of Halley's Comet as taken by the spacecraft Giotto, which visited the comet as it passed in 1986. Scientists learn a great deal about the make-up of comets from these space crafts, but even more information can be known about the formation of our solar system since the comets contain matter that is left over from that formation.

 

Other famous comets have been studied in recent years. The Hyakutake and Hale-Bopp comets were observed in 1996 and 1997. Hale-Bopp was one of the brightest comets ever seen from Earth. The Shoemaker-Levy comet was seen from Earth in 1994 and, as it traveled near Jupiter, was caught in Jupiter's gravitational pull and crashed into the planet. Scientists have space crafts studying many comets, but an exciting comet mission will include a spacecraft landing on the comet Churyumov-Gerasimenko!

 

 

 

 

 

Unit Summary

Much has been learned about our planet and our solar system by studying asteroids and comets. Galileo and Newton watched all types of heavenly bodies to formulate their theories on the solar system's motion, structure, and history. Scientists today recognize that they can build on the knowledge of past scientists by using the latest technologies like spacecraft to gather data and samples of current meteorites and comets.

Not all of the information gained from asteroids and comets is used to study the history of our solar system. Our Earth has been hit in the past by asteroids and comets with very devastating results. Many scientists are watching the sky for potential danger from the impact of asteroids in the future. They are formulating theories, calculating risks, and eventually building technologies that will help protect our Earth from impacts that would harm our lives.