Meteors and Comets
Pieces of comets and asteroids fall from space and cause the flashes of light in the sky that we see as meteors.
The space between the planets is not empty. It is filled with dust shed from comets and fragments of broken asteroids. As Earth’s orbit takes us through this debris, pieces of it enter our atmosphere. The smaller ones burn up because of friction with air molecules. The resulting flashes of light in the sky are meteors. Anything that survives the trip and lands on the surface is then called a meteorite.
The smallest and most plentiful particles come from comets, which contain ice and dust. As they orbit the Sun, Comets scatter dust along their paths. When Earth’s orbit takes us through those dust trails, the material enters our atmosphere and vaporizes. The smallest particles generate very little friction with our atmosphere and drift gently down to the surface. Meteors in showers come from comets.
Larger pieces of the sky generally come from asteroids, or minor planets. When asteroids collide, they scatter rock fragments throughout space. Eventually, some land on Earth. A few rare meteorites come from the Moon and Mars.
Meteor Showers Come from Dust in Comets
A comet nucleus is made of ice and dust, and formed in the original nebula where the Sun and planets were born. It is mainly water ice, with some methane and ammonia in the mix. Its dust particles are rocky grains. As a comet nears the Sun, the surface ices evaporate and form a cloud. This material streams out behind the comet, and the dust tail scatters particles along the comet’s path. When Earth moves through the dust trail, we get meteor showers. This model shows a typical comet nucleus with a dark, crusty surface coating that forms after the comet has rounded the Sun a few times.
Compared with a planet like Earth or Jupiter, the nucleus of a comet is very, very small. Most comets are somewhere between the size of a boulder and a city. Compared with the nucleus, a comet’s dust particles are extremely small. They range in size from a grain of sand to a tiny speck no larger than the dust we find on our furniture.
This is a model of a comet dust grain magnified 20,000 times. It simulates the look of dust from a comet that formed near the orbit of Pluto 4.5 billion years ago. The dust particle after which it is modeled was gathered by special collectors mounted on an aircraft.
The 1833 Leonid Meteor Shower Makes History
From the diary of Mary Hansard, Age 8
“In the year 1833 a most remarkable phenomenon occurred. It was called the falling of the meteors. It happened in the night, and as I was only a small child, I heard my parents describe it the next morning as being the most awful sight that was ever looked upon with mortal eye. They said that the firmament on high was one solid glare of fire and light, and looked as though every star in the sky was falling to the ground, and that they were certain that the Day of Judgment was at hand. There were many wicked men on their knees that night, praying to the Lord, and calling on other to pray for them, that had never been known to bow in prayer before.”
What Happens to a Meteor on Its Way Down to Earth
Every day, countless objects that came from comets and asteroids fall to Earth. What happens when they hit the atmosphere depends on their size and composition. Most of these particles are the size of dust grains. They simply float down to the surface. Slightly larger objects – the size of a grain of sand or a pea – are slowed by the atmosphere. They glow from the heat of friction and eventually burn up. The resulting flash of light is a meteor. Under a dark sky, you can see several meteors an hour on a typical night.
Incoming objects enter the atmosphere at average speeds of 12 miles (20km) per second. Friction heats them to 8,500 degrees F (4,700 degrees C) and slows them down. Large objects and those made mostly of iron often survive their trip through the atmosphere and fall to the surface as meteorites. The very largest ones continue glowing all the way down, making spectacular fireballs.
When Rocks Fall to Earth
Small pieces of space debris burn up as they pass through the atmosphere. We see their glow as small meteors. Only the largest, most solid pieces of rock and iron survive the fiery descent. We see them as fireballs. These larger pieces are heated and softened by atmospheric friction. The air pressure they encounter may cause them to explode into fragments just before they hit the ground. Those pieces land on the surface or bury themselves in the dirt.
The force of a large impact blasts out a crater like those on the Moon and sends material flying out from the impact site. Some of the Earth’s craters are more than a hundred miles across, though most have been covered or eroded away.
What Do You See?
Dust From Space
Look through the microscope to ﬁnd tiny pieces of actual asteroid or comet dust. They are smaller than grains of sand. Some spacedust particles began their journey to Earth when they were blasted from an asteroid during a collision or shed from comets passing near the Sun. The dust drifts through interplanetary space until it encounters Earth’s atmosphere.
The largest space-dust particles fall through our atmosphere and ﬂash as meteors we see in the sky, especially during meteor showers. Most space dust is vaporized before it reaches Earth’s surface. The particles shown here survived the journey, melting into spherical shapes on the way down. They were collected from a water well at the South Pole in Antarctica.