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Target Earth

Target Earth

The Lawrence Hall of Science

University of California, Berkeley

Written by Alan Gould, John Hewitt,

Janet Lopez, and Nathalie Martimbeau

Revised by Toshi Komatsu


  • Have calculator handy. This is for demonstration purposes only.

  • Get out Role Playing Cards. Sort and collate them, if necessary.


Daylight remains on while reading lights fade out.

Introduction—Origin of Life on Earth

Welcome to our planetarium and our program, Target Earth.

IMAGE: Earth

Our beautiful planet has been home to many forms of life through the ages. What makes Earth a good planet for life? [Accept any answers, e.g., water, hydrogen, oxygen, air, soil, rock, carbon, right temperature, right size.] These are some very important ingredients!

Earth is mostly water on the surface. But our planet didn’t always have such a good environment for life. Did you know most of the ingredients for life, and ingredients for building solid planets on which life may arise, come from inside stars? So, the story of life on Earth begins with the stars.

Pan away from Earth, centering on Orion.

Deep inside, stars are like giant pressure cookers—fiery, hot nuclear furnaces where the simplest ingredients of the Universe (mainly hydrogen) build up into more complicated ones. For millions of years, lightweight atoms combine to make heavier atoms (such as oxygen, nitrogen, carbon, and iron). Basic ingredients for building Earth-like planets where life might begin are cooked up inside stars—but they are trapped inside the stars. Can anyone think of a way for the insides of a star to get outside? What could happen? [Some stars explode.] The contents of a star might burst out in a supernova—an exploding star.

Fortunately, stars like our own Sun will not explode. Let’s find one that probably will go supernova. Let’s look for a type of star called a red super giant. It should look pretty red, and there may be more than one in our sky.

Allow audience to point to red stars. Finally, focus on Betelgeuse in Orion.

This [point at Betelgeuse] is a star called “Betelgeuse”. To find it, I usually look for these stars first [point to Belt stars]. Does anyone know a name for these three stars? [Orion’s Belt.]

Show Orion outline.

This whole group of stars is often called “Orion,” but we’re just interested in his upper left shoulder right now. Betelgeuse is one of those stars that we think will go supernova. However, like the next big earthquake, we can’t predict exactly when Betelgeuse will explode. It might be tomorrow or 100 years from now or…

Flash supernova.

…it might happen now! A supernova explosion blows apart a star, releasing all its elements in a second, sending them far out into space to mix in with the vast clouds where new stars and planets form. Now freed, some of the supernova’s ingredients may be planted on some future planet to help give it a good environment for life.

End supernova dot.

Just below Orion’s Belt, you can spot a fuzzy-looking “star”. [Point to middle “star” in Orion’s Sword.] This is not a place where stuff is coming apart—it’s a place where stuff is coming together.

IMAGE: Nebula Locator

IMAGE: Great Nebula in Orion

Telescopes reveal huge clouds of stardust and glowing gas, a great nebula where new stars are forming. Supernova explosions like the one we just witnessed have enriched these clouds with carbon grains and other heavy elements made inside stars.

PENDING: Solar System formation video. Need both inner planet formation & outer planet formation

Our Solar System started as a great swirling nebula, like the Orion Nebula. Slowly, the cloud pulled together ever more tightly by gravity. Spinning faster, it flattened. Most of it gathered at the center to form the new Sun. Gases and the lighter stuff got blown out and away. Heavier stuff like metals and rocks stayed close.

Like gathering dust bunnies under your bed, the leftover stuff started to come together, and started to build small chunks. Vast swarms of small bodies formed first, some crashing together and breaking up into tiny fragments. Many others stuck together, growing ever larger as more and more pieces fell onto them. This is how planets like Earth formed, built up from chunks of metal and rock crashing down for millions of years. Does our early Earth look like a good place for life? [No!]

Further out away from the Sun, gases came together to form the giant planets—Jupiter, Saturn, etc.—and lots of icy chunks were leftover. Those leftovers gathered far away in the “land of the comets.”

Observing and Describing Comets

People like to find comets. They are fun to watch, and if you’re the first to spot one, the comet is named for you.

IMAGE: Views of Comet McNaught

Here’s a beautiful comet that graced our skies some years ago (2007), Comet McNaught. It had a long, bright, colorful tail. Let’s search for a new comet up in our planetarium sky.

Turn on comet projection very faint.

Does anyone see a faint fuzzy streak in the sky? [Give pointer to whoever “discovers” the comet, ask them to point it out, and ask their name.] Great job! Comets are named for the person or people who discover them, so this is Comet _______. [Brighten comet.] Over time, comets will brighten, move across the sky, and then fade away.

A comet travels in from the outermost reaches of the Solar System, growing brighter with bits melting and streaming away in a tail as it nears the Sun. A comet’s tail may be millions of kilometers long, but it’s thinner than the thinnest air. You could fly a spaceship right though the tail and it wouldn’t make any difference to your spaceship or to the comet. Perhaps Comet _______ will return again many years from now!

Let’s see how comets get their wonderful tails.

IMAGE: Comet Halley (1910)

Here is a picture of one of the most famous comets of all. Do any of you know which one it is? [Comet Halley.] It was not really “discovered” by Edmund Halley—people had seen and recorded this comet for centuries. However, Halley was the first to predict the comet’s return every 76 years.

This was a picture that was taken in 1910. Notice the coma (or head) of the comet. [Point to coma.] And here is the tail of the comet, streaming away from the head, pointed away from the Sun.

IMAGE: Halley Nucleus (Giotto Spacecraft view)

We got our first look at the heart of a comet in 1986. This is the nucleus of Comet Halley, a dark, dense mass of solid ice and rock over 15 km long. Would we want our spaceship to run into this? [No!]

IMAGE: Oort Cloud

New comets come in from a realm called the Oort Cloud, which surrounds the Sun and planets as a vast halo far beyond the orbit of Pluto (about 1 light-year from the Sun).

IMAGE: Solar System view

Chunks of metal and rock also are left over from the beginnings of our Solar System too. Let’s take a closer look. Here are the outer worlds, Jupiter and Saturn. Just inside Jupiter’s orbit, before we get to Mars, there is a region with many thousands of solid bodies much smaller than planets called the asteroid belt.

IMAGE: Gaspra

Many asteroids are big chunks of rock and metal. This is Gaspra. Would you want a spaceship to run into Gaspra? [Probably not!] It looks like lots of things have run into Gaspra, though. Look at all the circular marks. Does anyone know what we call holes like these? [Craters.] Gaspra never comes near Earth, but other asteroids do.

PENDING: Early hot Earth video

Chunks of metal and rock like Gaspra built our solid planet. Heavy elements like iron formed Earth’s core. Asteroids and comets kept pounding the surface relentlessly for hundreds of millions of years. Volcanoes pouring out molten lava also make the surface hot. Earth’s interior is still very hot. Does Earth look like a good place for life here? [No!]

Finally (about 3.8 billion years ago), Earth cooled enough for the first rains to fall and oceans to form. Which do you think more likely brought in a lot of Earth’s water—asteroids or comets? [Most will answer comets.] You would think comets, but it turns out comets have the wrong kind of water—sometimes called heavy water, with an extra neutron. Water can also be locked in the rocky matter that makes up asteroids too, and recent finds1 suggest water might be more common on asteroids than previously thought. The water discovered so far in asteroids also more closely resembles seawater on Earth than water found in comets. So comets brought in some water, but asteroids brought in more!

Life arose just after Earth cooled enough to have oceans. Both comets and asteroids brought our oceans carbon-rich matter and other important ingredients for life. Without their contributions, there would be no Earth as we know it, no life—and no us!

Comet and Asteroid Impacts

There are still plenty of asteroids and comets flying around in our Solar System. We can easily see their effects if we look around.

IMAGE: The Moon

Look at the familiar face of Earth’s companion, the Moon. What do you see everywhere on the Moon’s surface? [Craters.] Photos of other planets and moons taken by spacecraft show more signs of cosmic bombardment.


Looking down at the Red Planet from space, we see its mountains and deserts. What else do you see? [Craters.]

IMAGE: Mimas

Although we have four gas giant planets which have no surfaces to form craters, those planets have moons—and those moons are covered with craters too!Wherever there is a solid surface to hit, we can see craters made by comets and asteroids.

Does Earth have craters? [Yes!] There are two kinds of craters—ones formed by volcanoes, and ones formed from impacts. We are interested in impact craters today.

IMAGE: Barringer Meteor Crater

Here’s one—Meteor Crater—near Winslow, Arizona. It is easy to see there, but that is because it is a young crater, less than 50,000 years old. It is over 1 km across, and 200 m deep. It was formed by the impact of an iron asteroid 30 m in diameter, weighing 100,000 tons, traveling at 20 km per second, slamming into the ground. Like a speeding bullet, an asteroid does enormous damage not so much because of its size, but because it is moving fast. When it hits, the moving energy of the asteroid is transformed in an instant into a tremendously hot, violent explosion. The power of this explosion was equivalent to 15 megatons of TNT (1000 times greater than the first atomic bombs). Part of the original object may lie buried deep underground, but much of it vaporized.

IMAGE: Henbury, Australia

Here’s one of Earth’s youngest craters at Henbury, Australia. It’s about 150 meters across and about 4,200 years old. But this one was formed when a meteor or comet broke up upon entry. In fact there are 13 craters in the area, formed probably by a small swarm of meteorites.

IMAGE: Gosses Bluff (Australia)

From the air and from space, though, we can still see some big impact craters. On the ground, Gosses Bluff just looks like a range of worn mountains. However, we can spot some really large circular features from space. This crater is about 20 km in diameter, and 140 million years old.

IMAGE: Manicouagan Crater (Quebec, Canada)

This great ring in Quebec is 100 km across—far bigger than the Arizona crater! It was formed when an asteroid struck over 210 million years ago. Now it is filled in with water. From the ground, it just looks like a lake.

The older a crater is on Earth’s surface, the more difficult it is to see. Why do you think craters on Earth get harder to see as time goes by? [Craters erode away and are buried; Earth’s surface is always changing.] Earth’s volcanoes make many fresh craters that are easy to see, but most impact craters have disappeared entirely.

Turn on meteor shower.

We know that Earth encounters small pieces of space debris all the time. At night, sometimes we see meteors appearing as fast streaks of light in the sky. Some people call them “shooting stars”. Are these really stars? [No.] Meteors are pieces of rock, metal, or ice (or a mixture). [NOTE: We call the pieces in space meteoroids. When we see them falling to Earth, burning in the air, we call them meteors. Any fragments that survive and hit the ground are called meteorites.] Meteors are mostly very small—like grains of sand—that do no harm.

How Often is Earth Hit?

We have seen large craters made by asteroids or comets. Small meteors slow down and usually burn up in Earth’s atmosphere. How often do you think big things hit Earth? [Take any answers.]

Maybe we can tell by looking at a near neighbor in space that gets hit by comets and asteroids just like Earth does. What is this near neighbor of Earth? [The Moon.]

Fade to Full Moon.

Let’s learn something about Earth by studying the Moon.

IMAGE: Earth and Moon (Galileo view)

Earth and the Moon have been targets for asteroids and comets since they formed over 4.5 billion years ago. Many important clues to Earth’s past are gone forever due to erosion. On the Moon, craters have not been erased by wind or water.

IMAGE: Full Moon

How many craters do you count here? [Too many!] Which parts have less craters—the light or the dark areas? [The dark areas.] Let’s focus on those dark areas.

IMAGE: Apollo Astronauts on the Moon

We know the ages of some parts of the Moon by studying soil and rocks collected by astronauts who landed there. They landed mostly on the smooth dark areas and brought many lunar samples back to Earth.

The dark areas are younger than the lighter areas. The lunar samples show they were formed by molten rock flows not more than four billion years ago. [Point out lunar maria.] If we looked at the Moon just after these dark areas formed, they would be blank, without impact craters. Every crater we see today on the dark plains was formed by an impact during the last 4 billion years. So, if we count the big craters in the dark areas only, and divide that number into four billion years, we can find out how many years go by between large impacts—on average.

IMAGE: Moon with outlined maria

Now let’s count some craters. This view of the Moon the darker regions outlined to help us find the large craters on them. Let’s look for craters bigger than 50 km—about the size of the San Francisco Bay (in fact, a little smaller). The dot is a “sample crater” to give you an idea how big a 50-km crater looks on the Moon. Here is crater #1. [Show appropriate crater] Who can point out a second crater? [Give pointer to volunteer.] How about a third crater? [Give pointer to another volunteer.]

Now, please count all the large craters that you can find in the dark areas of the Moon. How many do you see? [Take a few answers. For the purposes of this script, we will say there are five—the actual number astronomers count.]

IMAGE: Earth and Moon comparison

Which is the bigger target—Earth or Moon? [Earth.] Would you expect there to have been more big impacts on Earth, or fewer? [More.] Why? [Since Earth is bigger, it has a lot more surface area for asteroids or comets to hit.]

Earth’s whole surface is 80 times larger than the area of the dark lunar maria where we have been counting craters. So, we can expect about 80 times more impacts on Earth. If there were five large impacts on the lunar maria over the last 4 billion years, how many impacts have occurred on Earth during the same period? [Make a show of using a calculator, but also enlist audience help: 5 x 80 = 400 craters] If there have been 400 large impacts in the past 4 billion years, that means, on the average, there has been [Use the calculator again. Tip: 4 billion = 4000 million, so 4000 can be input to the calculator to yield the number of millions of years] one large impact every 10 million years!

Will there be an impact tomorrow? Next week? Next year? [Probably not.] But, over a long period of time—like a few million years—we do expect one to happen. Remember that this is just an average rate—impacts do not happen at regular intervals or on a schedule we can predict. Smaller impacts such as Meteor Crater in Arizona happen much more often.

Impact Theory and the Extinction of Dinosaurs

Could impacts affect the history of Earth?

IMAGE: Grand Canyon

Here is the Grand Canyon. Can you see the different layers? Earth’s crust is made up of layers of rock that have build up over time. What do you call someone who studies rocks and how rock layers form? [A geologist.] If you are a geologist who wants to study the most ancient rocks, do you look in the layers at the top of the cliffs or at the bottom? [The bottom.] The layers deepest down are the oldest, and each layer above formed more recently. Each rock layer represents an immensely long period of geologic time—some took millions of years to form. In these layers, we find lots of these:

IMAGE: Pterosaur fossil

What is this? [A fossil.] What are fossils? [Fossils are the remains and imprints of animals and plants that lived and died long ago, preserved in the rocks.] Many forms of life have come and gone since life began. Some scientists study the record of life preserved as fossils in the rock layers. What do we call a person who studies fossils? [A paleontologist.] The fossils we find are the best clues we have about past life on Earth. Fossils show how life changed over time, evolving into the plants and animals we see today.

IMAGE: Paleontologist with fossils

This paleontologist is uncovering a duck-billed dinosaur’s skeleton. How can she tell how long ago it lived? [By the depth of the rock layer it is in.] By studying the fossils and the ages of the rock layers, we’ve learned that dinosaur species lived all over Earth for more than 165 million years, during a time called the Mesozoic Era. At the end of this era—about 65 million years ago—they seemingly disappeared from Earth.

What happened to the dinosaurs? Let’s see how paleontologists and geologists work together to learn more about past life on Earth and try to solve the mystery of the dinosaurs!

IMAGE: Walter & Luis Alvarez and the K-Pg Boundary

One special layer of clay-like rock caught the attention of UC Berkeley geologist Walter Alvarez (shown here in the blue shirt, next to his father, Luis Alvarez—a Nobel Prize-winning Berkeley physicist). Alvarez and a team of chemists have found some strange differences between this thin shiny layer and the layers above and below it. [Point out layers.] The lower rock layers formed during a time geologists call the Cretaceous period, when dinosaurs still lived. The younger rocks above formed in the Paleogene period2. We call this very unusual borderline between the Cretaceous and Paleogene periods the K-Pg boundary layer.

IMAGE: K-Pg Boundary layer close-up

Keep this image on through the following activity to help the audience read their clue cards and to clarify their discussion.

The clay layer marks the K-Pg boundary at many places in the world. From its position in the rock layers, we know it formed about 65 million years ago. It is fairly thin compared to the layers above and below. Look at the size of the pocketknife in this close-up for reference. What caused the K-Pg layer worldwide? Did it form slowly or quickly? What clues about the history of life are hidden in this clay? These are questions the Alvarez team asked themselves.

What on Earth Happened 65 Million Years Ago?

What might have happened on Earth when the clay layer formed? We need clues from different fields of science to help us find answers. Let’s all play roles of different kinds of scientists. Some of you will be geologists, and some of you will be paleontologists. We’ll also need some astronomers. What do astronomers study? [Things in space.] Finally, we’ll need some chemists—people who study what things are made of. You will get a card telling what kind of scientist you’ll play and what clues you have found. You’ll be one of a team of experts. Please share your clue with the other people on your team. You must listen carefully to what each expert on your team has to say. At the top of each card is the question we are all trying to answer: “What was happening on Earth (65 million years ago) when the clay later was formed?” One important clue is the shocked quartz shown in this photo.

IMAGE: Shocked quartz

Keep image on during activity.

Fade on the reading lights, and pass out the clue cards, dividing the audience into teams of 4. It may sometimes be necessary to have two people share a clue or for one person to play two roles. Give the groups just a few minutes. Eavesdrop, and help explain difficult words as necessary. When the time seems right, ask for someone to make a report to the audience. Try to get reports from each team. Make sure people identify which clues led to their conclusions. Try to separate ideas that are firm conclusions from ideas that are hypotheses. The following is a summary of important points:

  • There was a great explosion that shattered the rocks, changed the structure of quartz grains, and scattered them far and wide.

  • Clouds of dust and debris thrown up by the explosion settled down worldwide to form the clay layer.

  • The explosion was caused by the impact of a comet or an asteroid. Iridium from the impactor is spread throughout the clay layer. Make sure people realize iridium is not a special element unique to extraterrestrial bodies—it is just rare in Earth’s crust compared to comets and asteroids.

  • Dinosaurs became extinct about the same time the clay layer formed.

So, the explosion from the impact led to the extinction of the dinosaurs. But wait! Does any of the evidence prove this assertion? [No!]

Dinosaurs disappeared from the fossil record 65 million years ago, about the time the clay layer formed. A major asteroid or comet impact probably caused the worldwide clay layer. Clues in the clay strongly suggest a connection between the explosion and the extinction. This idea of an object from space crashing on Earth and bringing the reign of dinosaurs to a sudden and catastrophic end is called the Impact Theory. The Alvarez team announced this startling theory in 1980.

Evidence of an impact does not prove that is what caused the dinosaurs to die out, however. They may be been extinct many thousands of years before the impact. Can you think of anything that might disprove the impact theory? [Finding dinosaur fossils above the K-Pg boundary layer.] Paleontologists are seeking dinosaur fossils above the clay layer, but so far have found none.

At the end of the Cretaceous era, many kinds of animals and plants became extinct—over 75% of all Earth’s species died out! We call such an event a mass extinction. Just how could a great explosion at one place on the planet affect life everywhere? [Take any answers.]

Suppose our fascinating impact story is true. What might have it been like to witness the impact and to live through the environmental catastrophe it caused?

IMAGE: Dinosaurs

Imagine we are back in the time of the dinosaurs. Here they are, munching happily away on plants (and on each other) as they had done for so many millions of years. Let’s see what might have happened.

IMAGE: 1 minute before impact

Imagine up in space a large comet or asteroid (10 km across) traveling at very high speeds finds Earth in its path.

All images and reading lights off. Audience should only see a dark sky with stars.

Look at the sky. Can you see the “big one” coming in?

Show bolide and flash effect.

There it goes! Oh, but we must be okay since it crashed so far away…

AUDIO: Explosion!

Play after a short pause following the flash.

Oh, maybe not. When it hits, the meteor explodes in a tremendous blast of energy. A gigantic fireball shoots up, hurling debris into the atmosphere. It hits our planet hard enough to penetrate kilometers below the surface. The blast sends an earthquake of magnitude 11.9 rumbling though the ground, and a powerful shockwave through the air. Even more earthquakes and increased volcano activity follow.

IMAGE: What dinosaurs might have seen

Everything near the fireball would be vaporized or burned instantly. Huge masses of molten rock would splash upward. Even hundreds of kilometers away, the explosion would have looked like this!

AUDIO: Wind, and music

Slowly dim stars to darkness.

What do you see happening to the stars? [They’re fading out.]

Turn on Meteor Shower

IMAGE: Tektite

These are not harmless meteors! For hours and days afterward, red-hot molten fragments called “tektites” thrown into space by the blast reenter the atmosphere around the world. These tektites fall back, heating up and setting ablaze everything they touch.

Slowly brighten red lighting to show a “burning” effect.

Earth’s forests are all burning. In the iridium clay layer, we find a fine layer of soot from those fires. Massive wildfires send smoke pouring into the atmosphere to join trillions of tons of dust, making thick black clouds that swarm over Earth. What do you think would happen on Earth if thick dark clouds blocked out the sunlight for a year? [It would keep light and heat from reaching the surface, making Earth a cold place to live.] Life would feel the effects of a dark year-long winter for at least ten years.

What would happen to plants and animals that were alive during this time? [Without sunlight, plants would die. The entire food chain would begin to break down.] There would be a global ecological crisis. Without plants for food, herbivores would die. Carnivorous dinosaurs would begin to starve and they, too, would die. Their habitats destroyed, all species unable to adapt to cold temperatures would perish. Small animals like lizards, furry animals, and birds would have an easier time surviving than larger animals like the dinosaurs. It’s amazing how many species made it though such worldwide havoc!

Fade stars on again.

The tektites, iridium, shocked quartz, and soot found in the K-Pg clay later all point to a tremendous impact by an asteroid or comet. Still, many scientists were not convinced. For years, the best piece of direct evidence was missing. Who can guess what the Alvarez team needed to look for? [A big crater.] The Alvarezes and others began hunting for a very large impact crater of the right age.

IMAGE: K-Pg site map of North and Central America

Tektites found along the Gulf of Mexico gave a hint about where to look for the missing crater. Shocked quartz is concentrated around the Gulf of Mexico and Caribbean too—more evidence for a massive explosion.

In the late 1970s, a geophysicist named Glen Penfield was hired by the Mexican government to make a special map of part of Mexico, called the Yucatán Peninsula, to look for oil. He scanned the region from an airplane with special instruments sensitive to underground terrain.

IMAGE: Chicxulub Crater Map & 3D gravity map

The map here shows where a huge circular ring was found, centered near the little town of Chicxulub (pronunciation: CHEEK-sheh-loob). Most of the crater is buried under jungle and ocean, and is highly eroded, making it difficult to make out. The above image shows a 3D gravity map of the crater, with a mound in the middle. [Point out crater.] The entire Chicxulub crater is 180 km across and is dated at 64.98 million years old! Walter Alvarez discovered more tektites the same age. The Chicxulub Crater is solid evidence that an asteroid or comet struck the Yucatán coast about the time the last dinosaurs disappeared. The impactor would have been at least 10 km across—about the size of Manhattan!

IMAGE: 1 minute after impact

If the asteroid or comet struck the ocean, vast amounts of water and sulfates would be thrown into the atmosphere (causing global climate change and cooling) and would have caused acid rains to fall. Huge waves would wash over nearby lands. We call this giant impact wave a mega tsunami or mega wave!

IMAGE: Mega tsunami map

An explosion this big could send waves up hundreds of meters high, rushing hundreds of kilometers away from the impact point, even on land. If such an object hit beyond the Golden Gate Bridge today, waves might splash all the way to Denver, Colorado. From the K-Pg impact site on the Yucatán coast, waves would have rushed across the Gulf of Mexico to flood modern Texas, and might have reached Kansas. We do find evidence of such a wave 65 million years ago all around the Gulf Coast area. Widespread extinctions of ocean life also happened at this time.

Despite all the evidence of an impact, many paleontologists still dispute the impact theory of mass extinction. It doesn’t exactly explain why certain animals and plants died out, while many vulnerable species that should have been wiped out were not. Also, other changes were happening to the environment. Many diverse phenomena could cause mass extinctions and dinosaurs may already have been on their way out. There could also have been a series of smaller impacts happening around the same time.

The truth is that scientists have not solved the mystery of the dinosaurs. We do not yet know all the factors that may have been at work—and we may never know! The rock layers give us clues, but not enough to fit together all the pieces of the puzzle. Many scientists who agree that a comet or an asteroid struck Earth 65 million years ago do not agree that the impact was the main reason for the mass extinction, but perhaps the final straw.

The K-Pg event is not unique. Scientists have found evidence that huge impacts may have caused mass extinctions even before the dinosaurs ruled Earth. For example, in February 2001, scientists announced evidence that an impact by a comet or asteroid 250 million years ago wiped out 90% of all life on Earth.

Could It Happen Again?

Could it happen again? Could it happen to us?

IMAGE: Doomed city of the future

We may never know for sure if an asteroid or comet long ago killed off the dinosaurs, but we can be sure that if one crashes into us today, it would be the worst disaster in human history—especially if it lands near one of our cities. In an instant, even a small asteroid impact would wipe out the entire San Francisco Bay Area.

IMAGE: Asteroid 1993KA

Let’s look again to the skies. Here’s a recent visitor to our neighborhood. In 1993, a tiny asteroid (1993KA) passed close by the Moon, zooming toward Earth. On May 20, 1993, it came to within nearly an hour of striking, but no one knew it. The asteroid was first noticed a day after its close approach! [Closest approach was 140,000 km—only about 11 Earth diameters.]

In January 2002, another asteroid passed Earth at a mere 830,000 km. Near-Earth asteroid 2001YB5 is about 300 meters wide. Had it impacted, it could have destroyed a medium-sized country, such as France. Although scientists never believed the asteroid would ever hit Earth, they discovered it a mere few weeks before its closest approach. That is just not enough time to do anything about it.

VIDEO: Earth-skimming asteroid, Grand Teton National Park

Now here’s a really close call. This is a home movie made in 1972 by a family on their summer vacation camping trip in the Rockies. What do you see streaking across the sky? [An asteroid!] This small asteroid skimmed through the upper atmosphere, disappearing back into space. Such objects sometimes explode kilometers above the ground.

Blink Comparator Demonstration

So, do you think we should be keeping a close watch for things in space that might cross our path? [Yes!] Is another asteroid heading our way? Would we spot it in time?

IMAGE: Carolyn Shoemaker

Carolyn Shoemaker is an astronomer who searches for Earth-approaching asteroids and comets. She is shown here with a telescope, but mostly astronomers like her do not look through a telescope so much as take pictures. Astronomers photograph the same region of the nighttime sky at two different times and compare the photos. How does this help find asteroids?

IMAGE: Blink comparator image 1

Here is one photograph of a certain section of the sky. Somewhere in there is an asteroid. How could we tell which one it is? [We can’t, unless we can somehow see it move.] Okay, so let us take another photograph of the same section of sky, but a few days after the first one. Perhaps the asteroid will appear in a different place relative to the stars.

IMAGE: Blink comparator image 2

Now, can you tell which object is the asteroid? [Probably not.] Look really hard.

It is very unlikely the audience will be able to spot the asteroid. After a few moments of frustration, reveal the “trick.”

IMAGE: Blink comparison

The trick in a blink comparator is to switch back and forth rapidly between the two photos. Now, can you tell which object is the asteroid? [Yes—the spot that’s moving.]

IMAGE: WISE spacecraft

In addition to the blink comparator method, there is a new NASA mission that can help. The Wide-field Infrared Survey Explorer (WISE) mission is scanning the entire sky in infrared light, picking up the glow of hundreds of millions of objects and producing millions of images. The mission will uncover objects never seen before, including the coolest stars, the universe’s most luminous galaxies and some of the darkest near-Earth asteroids and comets.

IMAGE: WISE first asteroid discovery

Astronomers believe there are over 1000 Near-Earth Objects (NEOs) larger than 1 km—with about 80% identified (as of May 2011). Many smaller NEOs exist, however. Of the over 8000 NEOs discovered so far, over 1200 are considered Potentially Hazardous Asteroids—ones that could (but not necessarily will) impact Earth.

So, what about our future? There is no doubt that things have hit Earth. Sooner or later, an asteroid or comet will surely hit us again. It is not a question of if, but a question of when.


If an asteroid or comet were heading toward Earth on a collision course, is there anything we could do to protect ourselves? [Right now, no.] There are only theories and tentative plans on the drawing board, but nothing definite. Despite all of our technology and weapons, blowing up an asteroid at the last minute with a nuclear weapon like you may have seen in the movies is not feasible. You would get millions of radioactive pieces that would rain down on Earth, potentially causing just as much damage or more to people.

So, what can we do?

IMAGE: Rocket deflecting an asteroid

When an asteroid or comet really threatens Earth, perhaps we will be prepared to send rockets to nudge aside the object.

IMAGE Gravity tug spaceship

Another idea is to park a massive spaceship next to an asteroid, and let it’s gravity act as a tug to affect the asteroid’s trajectory.

However, we would need years or even decades to do anything. The closer the object is, the more we would have to push it away. A few hours or even a few weeks (as suggested in the movies) is not practical. This would be a major undertaking with our very existence at stake. To build such ships, long-term peaceful cooperation among many nations may be necessary. This would also give a wonderful opportunity for people on Earth to explore space together, as with the International Space Station. Space missions to asteroids and comets would give us clues to life’s ultimate origins.

Space debris is all over our Solar System. Small pieces give us beautiful meteor showers every year. Big pieces sometimes crash and create large craters. But remember, we are the only species with the ability to protect the life on this planet. And, this is the only natural disaster that can actually be prevented.

And remember, if it were not for asteroids and comets, our Earth would not be as we see it today. Life might never have arisen. We would never have developed to contemplate the wonders of the Universe. Let’s work together to keep Earth a good planet for life, so when the next big one comes...

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...let’s hope our luck is better than the dinosaurs, and we all stay safe!

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Background Information

The Oort Cloud: The Oort Cloud is part of our Solar System, and the objects in it are the oldest, most primitive relics of the Solar System’s formation. The Oort Cloud may extend out as far as 50,000 Astronomical Units (1 A.U. = the average distance between Earth and the Sun, about 150 million km) from the Sun—about 1000 times farther than Pluto.

Far from the Sun where it is very cold, comets are big frozen chunks of ices and dirt, like “dirty snowballs”. They are also rich in ingredients for life, containing complex carbon compounds that make them look very dark. As comets travel in, sunlight heats the frozen gases (water, carbon dioxide, methane, ammonia, etc.). They begin to melt, vaporize, and glow, often growing long bright tails. A tail’s shape and brightness can change from day to day. Most comets have tails only while nearing and leaving the vicinity of the Sun. When comets move away from the Sun, the freeze again and fade from view. They become dirty snowballs again, losing their brightness and making it harder for us to watch them.

Comets coming in fresh from the Oort Cloud have very long orbits. New comets have orbital periods of thousands, even millions of years. Some comets’ paths are shortened by gravitational encounters with Jupiter and other planets. Comet Halley is an example of such a comet whose orbital path lies entirely within the planetary system. Some comets travel in orbits that cross the path of Earth. After many passes around the Sun, comets lose most of their ices and other volatile compounds. Eventually, we may no longer distinguish them from certain types of asteroids.

Meteor Crater, Arizona: The energy released in the Meteor Crater explosion was equivalent to 15 megatons of TNT. Because of the huge energy release, craters are much larger (up to 20 times) that the objects which cause them. For example, a 5-km-wide asteroid could create a crater 100 km wide! Most of the 140 craters known are larger than Barringer, ranging in size to about 200 km in diameter. The explosions that made them would have been much greater than the Arizona event. Most of Earth’s impact craters are less than several hundred million years old, but a few range up to 2 billion years. More craters have been identified in Australia, North America, and Eastern Europe party because those areas have been relatively stable for very long time periods. Only in the last few decades has the importance of impact cratering on Earth been recognized.

Late Heavy Bombardment: The lighter regions of the Moon’s surface are relics of an early era called the “late heavy bombardment”. During the first half-billion years following the formation of the Sun and planets, the impact rate was over 1000 times what it is today. Many more “leftover” objects were flying around the Solar System then, crashing into the Moon and Earth. Collisions happened far more often in the earliest part of the Moon’s history that during the period since the dark maria formed. After a final intense burst of heavy cratering, the impact rate decreased rapidly and leveled off to the current average rate nearly 4 billion years ago.

Dinosaurs: Dinosaurs dominated Earth for 165 million years. Dinosaur fossils are found in rocks of many different ages. Fossil life forms found in different rock layers help, but the dinosaurs’ fossil record is very incomplete and geologic time boundaries are fuzzy. In the late Cretaceous Period, however, the variety of dinosaur types seems to have dropped off drastically—by 70%—during the final 8 million years before the K-Pg extinction event. The fossil record shows dinosaur populations also decreased sharply. Species with small numbers have a higher risk of extinction than species with large numbers of individuals. Paleontologists are seeking non-avian dinosaur fossils above the K-Pg clay layer—so far without success. It is generally accepted today that some dinosaurs survived, and evolved into birds.

Tektites: Large impacts also create tektites—strange glassy rocks shaped like eggs or raindrops. These are pieces of hot molten rock ejected in tremendous explosions. The air slows and smoothly shapes them as they fall back to Earth. As tektites cool, their insides crystallize. The iridium clay layer contains tektites from a huge blast 65 millions years ago.

Explosions in the Atmosphere: Small asteroids and comets sometimes explode in the atmosphere. In 1908, an explosion above the Tunguska River in Siberia blasted 2000 square kilometers of forest flat. Pressure waves from the blast were recorded thousands of kilometers away. Powerful high-altitude explosions happen much more often than we used to believe, according to recently declassified data from spy satellites. Once a year, on average, there is an explosion in the upper atmosphere equal to about 15 kilotons of TNT—the energy released by the first atomic bombs.

Earth-Crossing Asteroids: Astronomers have identified 163 asteroids which travel in orbits that cross Earth’s orbit. As of the beginning of 1994, Ivar, the largest of these, is about 5 km wide. The impact of an object this size would cause worldwide environmental catastrophe, killing over 1 billion people.

1Studies of asteroid 24 Themis and asteroid 65 Cybele announced in 2010 found surface water ice on asteroids for the first time. See: (case-sensitive link)

2Formerly known as the Tertiary period. “Tertiary” is now being discouraged as a formal time or rock unit by the International Commission on Stratigraphy.

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