Question

identifying asteroids, comets, and craters
but what are asteroids? (from nasa)
asteroids, sometimes called minor planets, are rocky, airless remnants left over
from the early formation of our solar system about 4.6 billion years ago. most
asteroids can be found orbiting the sun between mars and jupiter within the main
asteroid belt. asteroids range in size from vesta - the largest at about 329 miles
(530 kilometers) in diameter - to bodies that are less than 33 feet (10 meters)
across. the total mass of all the asteroids combined is less than that of earths
moon.
sometimes, asteroids and comets are nudged into earths neighborhood by the
gravity of nearby planets. these objects are called near - earth objects, or
neos. there are more than 35,000 neos, according to the center for near
earth object studies (cneos) at nasas jet propulsion laboratory (jpl).
about 99% of neos are asteroids. their closest approach to the sun is less
than 1.3 times earths distance from the sun.
the three broad composition classes of asteroids are c -, s -, and m - types.

• the c - type (chondrite) asteroids are most common. they probably consist of clay

and silicate rocks, and are dark in appearance. they are among the most ancient
objects in the solar system.

• the s - types (\stony\) are made up of silicate materials and nickel - iron.
• the m - types are metallic (nickel - iron). the asteroids compositional differences

are related to how far from the sun they formed. some experienced high
temperatures after they formed and partly melted, with iron sinking to the center
and forcing basaltic (volcanic) lava to the surface.
the orbits of asteroids can be changed by jupiters massive gravity - and by
occasional close encounters with mars or other objects. these encounters can
knock asteroids out of the main belt, and hurl them into space in all directions
across the orbits of the other planets. stray asteroids and asteroid fragments
have slammed into earth and the other planets in the past, playing a major
role in altering the geological history of the planets and in the evolution of
life on earth.
scientists continuously monitor earth - crossing asteroids, whose paths
intersect earths orbit, and near - earth asteroids that approach earths orbital
distance to within about 28 million miles (45
million kilometers) and may pose an impact
danger. radar is a valuable tool in detecting and
monitoring potential impact hazards. by
reflecting transmitted signals off objects, images
and other information can be derived from the
echoes. scientists can learn a great deal about an
asteroids orbit, rotation, size, shape, and metal
concentration.
figure 1. an asteroid - note that even
asteroids have impact craters from
impacts with smaller space debris!

Explanation:

• For the first question, the text clearly states that asteroids flying near Earth are called Near - Earth Objects (NEOs).
• For the second question, the three types of asteroids (C - type, S - type, M - type) are directly mentioned in the text.
• For the third question, the text says that radar is used by scientists to find out more about asteroids as it can detect and monitor them, and derive information like orbit, rotation, size, shape, and metal concentration from the echoes of transmitted signals.

Answer:

• What do we call asteroids that fly nearby planet Earth? Near - Earth Objects (NEOs).
• What are the three types of asteroids? C - type (chondrite), S - type ("stony"), M - type (metallic).
• How do scientists find out more about asteroids? By using radar which reflects transmitted signals off objects and derives information from the echoes.