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Black Holes, Comets, Asteroids and Meteors
What Is a Black Hole?
A black hole is a place in space where gravity pulls so much that even light cannot get out. The gravity is
so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.
Because no light can get out, people can't see black holes. They are invisible. Space telescopes with
special tools can help find black holes. The special tools can see how stars that are very close to black
holes act differently than other stars.
How Big Are Black Holes?
Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom.
These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or
"stuff," in an object.
Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the
sun. There may be many, many stellar mass black holes in Earth's galaxy. Earth's galaxy is called the
Milky Way.
The largest black holes are called "supermassive." These black holes have masses that are more than 1
million suns together. Scientists have found proof that every large galaxy contains a supermassive black
hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius
A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few
million Earths.
How Do Black Holes Form?
Scientists think the smallest black holes formed when the universe began.
Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When
this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into
space.
Scientists think supermassive black holes were made at the same time as the galaxy they are in.
If Black Holes Are "Black," How Do Scientists Know They Are There?
A black hole cannot be seen because strong gravity pulls all of the light into the middle of the black hole.
But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists
can study stars to find out if they are flying around, or orbiting, a black hole.
When a black hole and a star are close together, high-energy light is made. This kind of light cannot be
seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.
Could a Black Hole Destroy Earth?
Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black
hole because no black hole is close enough to the solar system for Earth to do that.
Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall
in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the
black hole as they orbit the sun now.
The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.
How Is NASA Studying Black Holes?
NASA is using satellites and telescopes that are traveling in space to learn more about black holes. These
spacecraft help scientists answer questions about the universe.
Comets
A comet is a small world which scientists sometimes call a planetesimal. They are made out of dust and
ice, kind of like a dirty snow ball.
Where do they come from?
Comets come from two places: The Kuiper Belt and Oort Cloud.
Imagine a place far, far away at the very edge of the solar system. A place where millions of comets can
be seen swishing around in every direction. These icy comets are orbiting the Sun in two different
places, both of which are very distant. One place is called the Oort Cloud, and the other is called the
Kuiper Belt.
Why do comets leave the Oort Cloud or Kuiper Belt?
A comet will spend billions of years in the Kuiper Belt or Oort Cloud. Sometimes two comets will come
very close to each other, or even crash into one another. When this happens the comets change
directions. Sometimes their new path will bring them into the inner solar system. This is when a comet
begins to shine. Up until now the comet has been among millions of others exactly the same, but as they
approach the warmer inner solar system they begin to melt leaving behind magnificent tails.
Unfortunately, comets don’t live very long once they enter the warmer part of the solar system. Just like
a snowman melts in the summer, comets melt in the inner solar system. Although it is the most glorious
part of their lives, travelling through the inner solar system eventually kills them. After several thousand
years they melt down to a little bit of ice and dust, not nearly enough to leave a tail. Some even melt
away completely.
Many people think that a comet’s tail is always following behind it, but actually the coma, or tail, can
either be behind the comet or in front of it. Which way the tail is pointing depends on where the Sun is.
That’s right, the Sun’s heat and radiation produce a wind called Solar Wind, as a comet gets close to the
Sun it begins to melt. The gas and dust that melt off are blown away from the Sun by the solar wins. So if
a comet is travelling towards the Sun then the tail will follow behind, but if the comet is travelling away
from the Sun the tail will be in front of the comet.
Asteroids
What are asteroids?
An asteroid is a large rock in outer space. Some, like Ceres, can be very large, while others are as small
as a grain of sand. Due to their smaller size, asteroids do not have enough gravity to pull themselves into
the shape of a ball. Astronomers group asteroids into different categories based on the way they reflect
sunlight.
How many asteroids are there?
26 very large asteroids have been discovered, which is probably most of the big ones; but, there are still
millions of smaller ones that we have yet to see because they are too tiny, only a mile or so across.
If we stuck all the asteroids together, how big would the new planet be?
If all the materials of all the asteroids were squashed up into one planet it would be smaller than our
moon.
Quaoar is the largest known non-planet object in the solar system. It is one-tenth the size of the Earth,
and is larger than all other asteroids put together. For many years, Ceres was thought to be the biggest
asteroid in the solar system. It is about 600 miles wide and contains about 25% of the mass of all the
asteroids combined. However, in the summer of 2001, scientists discovered an even bigger asteroid
orbiting the sun near Pluto.
Asteroid Belt
The asteroid belt is divided into an inner belt and an outer belt. The inner belt, which is made up of
asteroids that are within 250 million miles (402 million km) of the sun, contains asteroids that are made
of metals.
The outer belt, which includes asteroids 250 million miles (402 million km) beyond the sun, consists of
rocky asteroids. These asteroids appear darker than the asteroids of the inner belt, and are rich in
carbon.
Where did the Asteroid Belt come from?
Asteroids are left over materials from the formation of the solar system. These materials were never
incorporated into the planet because of the proximity to Jupiter’s strong gravity.
Meteors
Meteors are asteroids that get stuck in orbit around a planet because of the planet’s gravitational pull.
A meteorite is the parts of a meteor that has fallen through the atmosphere and struck earth. There are
three main types of meteorites: stony, iron, and stony-iron. A lot of them have been smashed off from
very large chunks of rock, called asteroids, in collisions before eventually finding their way to our planet.
Iron meteorites, for example, are bits of metal iron cores of large asteroids that were once hot enough
to have melted, causing all of their iron to stick to the center. Stony meteorites look most like the stones
that you find on Earth and come from the outer layer of asteroids, whereas stony-iron meteorites are a
mixture of the two.