JOAN CARTAN-HANSEN, HOST: Asteroids and comets are more than just frozen rocks flying about in space.

They're key to understanding the early history of our solar system.

(MUSIC) (BELL RINGS) ANNOUNCER: Let's meet two small solar system bodies.

First, comets.

CARTAN-HANSEN: Comets formed far from the sun.

They're made of dust, ice, and gas left over after the sun and planets formed.

Most comets come from two places, the first, the Kuiper belt, a region of space beyond Neptune.

Comets from here are known as short-period comets because they take 200 years or less to make one orbit around the sun.

The other place is the Oort Cloud, a far distant cloud maybe a trillion comets that surround the solar system.

Comets from here are known as long-period comets because one trip around the sun can last as long as 30 million years.

Comets are sometimes called "dirty snowballs" because they contain mostly ice and frozen gases, like carbon dioxide, with just a little bit of dust.

Sometimes the gravitational pull of a passing star stirs up the comets in the Oort Cloud, sending them flying toward the sun.

And sometimes gravity from a planet can knock a comet in the Kuiper belt on its journey falling inward toward the sun.

The comet follows an elliptical path, moving faster and faster as it gets closer to the sun.

The heat of the sun starts to warm the surface of the comet's nucleus, a ball of ice and dust at the comet's center.

As the ice warms, it makes a cloud of gas and dust around the nucleus called a "coma" The gas and dust in the coma can escape into space forming two tails.

The dust tail is curvy because it falls behind the comet.

The gas tail is straight because it points outward from the sun.

Comet's nuclei have diameters of about 6 to 25 miles, but the tails can grow millions of miles long.

Some comets show up just once, but more than 200 comets come back past the Earth at predictable times.

Halley's comet is one of the most famous periodic comets, flying past the Earth about every 75 years.

Comets are often named after the person who discovered it.

Halley's comet is named after the English astronomer, Edmond Halley.

Although he officially discovered it, Chinese astronomers first noted the comet's visit in 239 B.C.E.

Halley's Comet was even included in an old tapestry showing the 1066 invasion of England by William the Conqueror.

It was said William felt Halley's comet heralded his success.

(BELL RINGS) ANNOUNCER: Let's meet our next small solar system body, asteroids.

CARTAN-HANSEN: Asteroids form closer to the sun than comets, so they're basically just small, rocky objects - no ice, it was just too hot.

Most asteroids orbit the sun in the asteroid belt between Mars and Jupiter.

Because of their small size, most don't have enough gravity to form into ball-like shapes, so they can be lumpy, and they could be covered with craters from impacts with other asteroids and comets over billions of years.

Asteroids range in size from a few yards to more than 335 miles in length.

The largest asteroid is Pallas.

Asteroids are usually the color of lead in a pencil.

They're made up of clay, metals, and silicate rock.

Silicates usually are made up of silicon and oxygen atoms and are the most common form of minerals on Earth.

Asteroids generally take three to six years to orbit the sun, and those orbits can come pretty close to Earth.

The gravity from planets, especially Jupiter or Mars, can also toss asteroids out into space.

And when asteroids collide, the leftover bits and pieces can make their way toward Earth, as they did over Russia in 2013.

(EXPLOSION) (BELL RINGS) ANNOUNCER: And what do these two small solar system bodies have in common?

CARTAN-HANSEN: Asteroids and comets both played an important role in the Earth's history.

Scientists think comets may have brought water and some of the chemical compounds needed to form life here on Earth.

Comets and asteroids have hit the Earth countless times, changing our planet.

For example, an asteroid or comet more than 6 miles wide smashed into the Yucatan Peninsula of Mexico more than 65 million years ago, wiping out most of the dinosaurs and many plants and animals.

NASA and other agencies now track asteroids and comets to keep our planet safe.

Asteroids and comets don't experience erosion like the Earth does, so their surfaces can be billions of years old.

And because they've changed so little over time, asteroids and comets can help scientists understand the conditions that existed when the solar system was formed.

And when a comet is far away from the sun and doesn't have a coma or a tail it can be pretty hard to tell the difference between it and an asteroid.

So they have lots of similarities.

Both are important small solar bodies that can tell us a lot about our world and beyond.

If you want to learn more about comets and asteroids, check out the Science Trek website.

You'll find it at Science Trek dot org.

(MUSIC) ANNOUNCER: Presentation of Science Trek on Idaho Public Television is made possible through the generous support of the Laura Moore Cunningham Foundation, committed to fulfilling the Moore and Bettis family legacy of building the great state of Idaho, by the Idaho National Laboratory, mentoring talent and finding solutions for energy and security challenges; By the Friends of Idaho Public Television and by the Corporation for Public Broadcasting.