2.4 Comets

Natural sciences

Grade 9

The earth, galaxies and space programmes

Module 16

Comets and other travellers in space

Comets are pieces of ice and rock that travel in orbits around the sun. According to scientists they are remains of matter from the creation of our solar system. However, most of the remains nearest to the sun have been destroyed. Only those in the cold parts behind Neptune and Pluto still exist. They come from an area called Kuiper's Belt.

a comet consists of three parts:

• A nucleus (mostly rock, ice and carbon)
• A head, or coma
• A tail (caused by heating and the reflection of the rays of the sun)

Tail of gasTail of dustNucleusComa

Astronomers are honoured by having a comet named after them if they record such a space traveller for the first time. The comet Hale-Bopp, for example, got its name when Alan Hale and Thomas Bopp spotted and registered it at the same time. In 1997 this comet passed close to the earth and caused great excitement, especially since its nucleus was 40 km in diameter.

Comets travel in oval orbits around the sun. Sometimes when orbits cross, major collisions can occur.

Thus the planet Jupiter was bombarded by fragments from the comet Shoemaker-Levy 9 in July 1994. This was televised and millions of people watched the phenomenon. ( www.dustbunny.com/afk/planets/jupiter/jupimpact.htm )

Halley’s comet was observed in South Africa in 1986. It takes 76 years to orbit the sun.

Comets probably contributed to the formation of our planet and probably also brought carbon and water to our planet.

Meteorites

• Particles that travel through space are called meteors. When large meteors hit planets, huge craters are caused. The craters on the moon were formed in this way. Some of the largest craters in South Africa are the one at Pretoria and the Tswaing crater, as well as the Vredefort crater in Northwest.
• Thousands of tons of small meteors usually hit the earth every day, but most are as small as a grain of sand. When they enter the earth’s atmosphere, we call them METEORS, once they have hit the earth’s surface they are called meteorites. Some are large enough to be seen with the naked eye and people call them shooting stars.
• Meteors as bright as Venus are called fireballs.
• The largest meteor that has ever hit the earth is known as Hoba. It hit Namibia and weighed about 60 tons. You can still see it near Grootfontein.

Asteroids

• Asteroids are giant meteors with a diameter of normally up to one kilometre. Approximately 40 000 asteroids revolve around the sun in orbits between the planets Mars and Jupiter, and for this reason this area is called the asteroid belt.
• Ceres is the largest asteroid and has a diameter of about 1 000 km.
• Many people believe that an enormous asteroid such as this hit the earth, creating dust clouds that blocked out all sunlight and causing dinosaurs to become extinct 65 million years ago.
• The last large asteroid hit the earth in Siberia in 1908.
• Scientists believe that the earth could be hit by an asteroid every few hundred years and so they keep a careful watch on the skies above us.
• Thousands of amateur astronomers around the world watch faithfully every night and report all “new” objects. These objects are referred to as “near earth objects” or NEOs.
• It is estimated that there are about a thousand NEO’s in space relatively close to us and that only half of them have been discovered to date.
• Comets become visible only a few months before they present a danger to us.
• Marvin Kilgore, an American, currently has the largest collection of asteroid fragments and explains that asteroids have incredible mineral compositions that, if man had the technology, could be mined!

On the web:

Do visit the following websites:

neo.jpl.nasa.gov/neo.html

planet.dm.unipi.it/neodys/

impact.arc.nasa.gov

See how the skies above us are monitored by scientists!!

Activity: questions on comets and other travellers in space

1. What is:

a) a comet?

b) an asteroid?

c) a shooting star?

d) a crater?

2. Which comet:

a) collided with Jupiter?

b) was in our skies in 1986?

3. Where are examples of craters found in Southern Africa?

4. Where will you find:

a) Kuiper's Belt?

b) the Asteroid belt?

5. What is the name of the largest asteroid in the asteroid belt?

Assessment of QUESTIONS ON COMETS:

Did you answer the questions correctly?

[LO 2.4]

Assessment

LO 2: Constructing Science Knowledge:

The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.

This is evident when the learner:

2.4 applies knowledge.

Memorandum

Comets and other travellers through space

ACTIVITY: QUESTIONS

1. a) Blocks of ice and rocks that are in orbit around the sun

b) giant meteorites

c) meteors

d) cavities formed by impact

2. a) Shoemaker- Levy 9

b) Halley

3. Tswaing in the vicinity of Pretoria, Vredefort Dome, Hoba in Namibia

4. a) Kuiper 's girdle – cold regions beyond Neptune and Pluto

b) between Mars and Jupiter

5. Ceres