Britannica Student Encyclopedia - 2010

orbit, or travel around, the sun. They

are also called minor planets or

planetoids. In general, they are

materials left over from when the

planets formed. They are also created

The Assiniboin

were friendly

with the Cree.

Their enemies

were the

Blackfoot and

the Dakota

Sioux.

212 Astana BRITANNICA STUDENT ENCYCLOPEDIA

 

when other bodies in space collide and

break apart. Most asteroids travel

around the sun in a loose path that falls

between the orbits of the planets Mars

and Jupiter. Astronomers identified the

first asteroids in 1801.

Asteroids are much smaller than planets.

The largest asteroids are between 62

miles (100 kilometers) and 620 miles

(1,000 kilometers) across. Millions of

asteroids are the size of large boulders.

Asteroids can be almost any shape. Some

are nearly round while others are irregularly

shaped chunks. Asteroids are made

up of stone and metal.

Many small asteroids enter Earth’s

atmosphere each day. Almost all burn up

before they hit the ground. As they burn

they create a streak of light called a

meteor. An asteroid that survives the fall

through the atmosphere and reaches

Earth’s surface is called a meteorite.

#More to explore

Planets

Asthma

Asthma is a long-term medical condition

that affects a person’s lungs. From

time to time people with asthma suffer

attacks, or episodes of sickness, in which

they find it difficult to breathe. People

can get asthma at any age. It is one of

the most common long-term health

problems in children. Asthma in children

usually begins by the age of 5.

Symptoms

The symptoms, or signs, of an asthma

attack include coughing and the feeling

of not getting enough air. Wheezing, or

breathing that makes a whistling sound,

is also common. Asthma attacks usually

last from about 30 minutes to several

hours. The symptoms can range from

mild to serious. In rare cases an asthma

attack can lead to death.

A child uses an inhaler. An

inhaler is a device that quickly

delivers medicine to the lungs of

a person with asthma.

The asteroid called Gaspra is about 12

miles (20 kilometers) long.

BRITANNICA STUDENT ENCYCLOPEDIA Asthma 213

 

Causes

An asthma attack results from changes

in the small tubes in the lungs that carry

air. During an attack the muscles around

these airways tighten. This makes it

harder for air to pass through. The tubes

also swell up. This leaves less space

inside for the airflow. In addition, the

tubes fill up with extra amounts of a

thick fluid called mucus.

Scientists do not know exactly why

people get asthma. But once people

have the condition, their bodies tend to

overreact to certain things called

triggers. The triggers vary from person

to person. For some it may be

exercising. For others, breathing

polluted air or getting a cold may bring

on an asthma attack. Sudden weather

changes can cause attacks, too.

Asthma attacks are also often associated

with allergies. Allergies are a body’s

negative reactions to certain substances.

Things that cause allergic reactions may

trigger asthma attacks, too. These things

include plant pollen, mold, and tiny

insects that live in house dust.

Treatment

There is no cure for asthma. However,

several medications can help prevent

asthma attacks. Other medicines can

quickly relieve the symptoms of an

attack. People with asthma also try to

avoid the things that trigger their

attacks.

#More to explore

Allergy • Lung • Medicine

Astronomy

Astronomy is one of the oldest sciences

in the world. The study of astronomy

involves all of the objects outside Earth’s

atmosphere. These include the sun,

Moon, planets, stars, galaxies, and all

other matter in the universe. People

known as astronomers have studied

these objects for thousands of years.

Study of Astronomy

Astronomers study the universe in different

ways. Some are professionals,

meaning that they make their living at

the work. Others study objects in space

as a hobby. They are known as amateur

astronomers.

Some astronomers work in laboratories.

There they study actual samples of

objects that have been collected by

spacecraft. These include meteorites,

An astronomer points a telescope toward

the evening sky.

214 Astronomy BRITANNICA STUDENT ENCYCLOPEDIA

 

rock samples from the Moon, and dust

particles.

Many astronomers observe distant

objects with the help of telescopes. Telescopes

range in size. Some are small

enough to be carried by hand. Large

telescopes are often housed in buildings

known as observatories. The Hubble

Space Telescope is in orbit around Earth

as a floating observatory. The Hubble

magnifies and records images of distant

space objects. It then sends the information

to astronomers on Earth.

Astronomers also use cameras and other

instruments to record information about

distant objects. Photographs provide

astronomers with a visual record of their

observations. Other instruments can

help reveal faint, hard-to-detect objects

in outer space. These instruments measure

the light and other radiation coming

from the objects.

Some Early Astronomers

Thousands of years ago there were no

calendars or clocks. People kept track of

time by watching the sun and the stars.

It was important for them to know the