Video Transcript
In this video, we will learn how to
define asteroids, comets, meteors, and meteorites, what they are made of, and where
they can be found.
The largest object in the solar
system is the Sun. This is a hot, bright ball of
hydrogen and helium as well as some heavier elements. We can recall that there are eight
planets which orbit the Sun in approximately circular orbits. These planets are some of the
next-largest objects in the solar system after the Sun itself. In order from closest to furthest
from the Sun, these planets are Mercury, Venus, Earth — that’s our planet — Mars,
Jupiter, Saturn, Uranus, and Neptune. Then, smaller still are the moons
of some these planets. These moons are some of the
next-largest objects in the solar system after the planets.
Let’s remember that just as the
planets orbit the Sun, the moon or moons of a particular planet orbit that
planet. Then, in addition to the Sun,
planets, and moons, the solar system also contains many smaller objects. Among these are asteroids and
comets.
Let’s start by looking at
asteroids. Asteroids are objects with
diameters ranging from a few hundred meters to a few hundred kilometers across. That’s roughly anywhere from the
length of a football pitch to the length of the Grand Canyon. Asteroids are typically rocky and
can contain large quantities of metals, such as iron and nickel. This image is a photo of one of the
largest asteroids in our solar system. This particular asteroid, named
Vesta, is 573 kilometers wide. It orbits the Sun between Mars and
Jupiter.
Asteroids are, in general,
irregularly shaped rather than spherical. Here are some photos of a few
different smaller asteroids. We can notice the varied shapes of
the different asteroids. We should also take care to notice
that these photos are not to scale with each other. The actual sizes of the different
asteroids are written below each photo.
Asteroids are found orbiting the
Sun in roughly circular orbits throughout the solar system. Between Mars and Jupiter, though,
there’s a really large number of them, millions in fact. This collection of asteroids is
known as the asteroid belt. Now, we should point out that the
sketch we’ve drawn of the Sun, planets, and asteroid belt does not show the sizes of
the objects and the orbital radii to scale. The important thing here is the
relative positions. So that’s asteroids.
Now we’ve said that the solar
system also contains comets. Comets are similar to asteroids in
some ways, but there are some important differences. Comets are often slightly smaller
than asteroids. Typically, they are a few tens of
kilometers wide, while we’ve seen that asteroids can be up to hundreds of kilometers
across.
Perhaps a more significant
distinction is what the two objects are made up of. While asteroids are rocky objects
often containing metals, comets are typically made more of ice. Now we’ve written “ices” here,
which is the plural of the word “ice.” When we hear the word “ice,” we
might instinctively think of frozen water. However, in this case, the term
“ices” is more general than this and is referring to multiple kinds of ice. Comets do typically consist largely
of frozen water. But they also contain frozen
ammonia, methane, carbon dioxide, and carbon monoxide. In addition to these ices, comets
also contain dust and rock.
The other major difference between
comets and asteroids is the shape of the orbit. We’ve seen that asteroids orbit the
Sun in roughly circular orbits, similar to the planets. Comets, on the other hand,
typically have highly elliptical orbits, that is, orbits shaped something like
this. In a highly elliptical orbit like
this, the comet’s distance from the Sun changes considerably at different points in
the orbit. The comet starts out a long way
from the Sun. It is then pulled much closer to
the Sun by gravity, before being flung back out far away into the outer solar system
again.
A comet may cross the orbital paths
of many different planets during this process. For example, let’s look at the path
of one of the most well-known comets, called Halley’s Comet. The path of Halley’s Comet is shown
here in pink. This comet crosses the paths of
Neptune, Uranus, Saturn, Jupiter, Mars, Earth, and Venus. That is, it crosses the paths of
all the planets, except for Mercury.
When a comet comes close to the Sun
during its orbit, it can get close enough that the Sun heats the ices in the comet,
turning them into gas. These gases are ejected from the
comet and can be seen as a bright blue and white tail. This bright tail makes a comet
easier to see. And in fact, often a comet that is
in the inner solar system near to the Sun can be seen from Earth by eye without a
telescope. Here is a photo of Halley’s
Comet. This photo was taken from Easter
Island here on Earth on the 8th of March, 1986. We can clearly see here the bright
tail of gases. It’s worth mentioning, by the way,
that the tail of a comet always points away from the Sun.
Now, we’ve looked in some detail at
the defining features of both asteroids and comets. As well as asteroids, comets,
moons, planets, and the Sun, the solar system is also filled with millions of
smaller fragments of rock that are just a few meters wide. These fragments can hit Earth as
Earth moves along its orbit.
When a fragment of rock enters the
Earth’s atmosphere, the fragment experiences resistance from the air. This heats it to extremely high
temperatures. This in turn causes the rock
fragment to burn. When this happens, the fragment is
known as a meteor. A meteor is visible as a bright
streak across the sky. This photo shows a meteor as seen
from Earth. The reason that the meteor is
visible in this way as a bright streak is that the fragment burns really brightly
and that it is also moving very fast.
Often, the fragment will burn up
completely before it reaches the ground. However, this is not always the
case. Sometimes, a fragment will reach
the surface of the Earth. A meteor that reaches the ground is
known as a meteorite. The impact of a meteorite with the
surface of the Earth will often leave behind a crater, or large hole, in the
ground.
Now that we’ve learnt a bit about
asteroids, comets, meteors, and meteorites, let’s have a look at an example
question.
Complete the following
sentence. Blank is a large object made of ice
and dust that orbits the Sun along a highly elliptical orbit. (A) An asteroid, (B) a comet, (C) a
meteorite, (D) a meteor.
In this question, we’re being asked
to identify which of these four objects fits this description of being a large
object made of ice and dust with a highly elliptical orbit around the Sun. Let’s have a look at each of the
options we have been given to fill the blank in the sentence.
Beginning with option (D), let’s
recall that a meteor is the name given to a fragment of rock that enters the Earth’s
atmosphere. These fragments are relatively
small, typically a few meters across. Now, the air resistance the meteor
experiences from the Earth’s atmosphere will often cause it to burn up entirely
before it reaches the ground. However, this isn’t always the
case. And a meteor that does reach the
ground is known as a meteorite. We can see then that neither a
meteor nor a meteorite is defined as a large object in a highly elliptical orbit
about the Sun that’s made of ice and dust. We can therefore eliminate answer
choices (C) and (D).
Next, let’s consider answer option
(A). Now, in comparison to meteors and
meteorites, asteroids are much larger objects. They can be anywhere from hundreds
of meters to hundreds of kilometers in width. So an asteroid does fit the
description of being a large object. However, asteroids orbit the Sun in
roughly circular orbits. And they are rocky objects that
often contain metals. So asteroids are not a good match
for the rest of the sentence. That means we can rule out answer
choice (A).
This just leaves option (B), which
claims that the object in question is a comet. Let’s recall that a comet does have
a highly elliptical orbit about the Sun. Comets are made of dust and ices
that melt when the comet gets to the part of its orbit that is close to the Sun. We might remember that the melted
ice is ejected as gas, which creates a visible, bright tail. Comets are also pretty big,
typically tens of kilometers across.
We have then that a comet is indeed
a large object, that it is made of ice and dust, and that it does have a highly
elliptical orbit about the Sun. Our answer then is option (B), a
comet.
Okay, now let’s finish up by
summarizing what we have learnt in this video.
We have seen that asteroids are
large, irregularly shaped rocky objects in space. Asteroids have widths that range
from hundreds of meters up to hundreds of kilometers. We then saw that though asteroids
are found throughout the solar system, there is a large number of them between the
orbits of Mars and Jupiter. This collection of millions of
asteroids is known as the asteroid belt.
Next, we saw that comets are made
of ices and dust and that they have highly elliptical orbits about the Sun. Comets have a bright blue and white
tail when they’re in the part of their orbit that is near the Sun, as a result of
heat from the Sun melting the ices in the comet. Because of this, a comet that is
near the Sun may be visible by eye from Earth.
We then went on to see that meteors
are fragments of rocks, typically much smaller than asteroids or comets, that enter
Earth’s atmosphere. Air resistance causes the fragments
to heat up and burn so that a meteor may be visible as a bright streak in the
sky. While most meteors burn up
entirely, some reach the ground. A meteor that hits the ground is
known as a meteorite.
