ON November 20, 2007, Andrea Boattini of the University of Arizona's Catalina Sky Survey spotted an extremely faint object moving across the stars. The story of that object—an Apollo asteroid named 2007WD5—had the makings of a science fiction thriller. Unfortunately for fans of conventional thriller structure (a form marked by mounting tension), most of the story was over by the time Boattini discovered WD5.

Every four years this asteroid—a rock measuring about fifty meters in diameter—drops in from the asteroid belt to cross the Earth's orbit. On November 1, 2007, WD5 had zipped by the Earth at a distance of 7.5 million kilometers, just thirty times farther away than the Moon—a near miss by astronomical standards.

In any thriller worth its salt, one hair's-breadth escape is quickly followed by another. Shortly after Boattini spotted WD5, astronomers at NASA's Near Earth Object program realized that this asteroid was going to pass very close to Mars on its return to the asteroid belt. In fact, there was a chance that the asteroid might collide with the planet—exciting news for anyone interested in cratering and planetary impacts. If WD5 hit Mars, scientists estimated it would make a crater about a kilometer in diameter and 200 meters deep. High-resolution orbiting cameras circling Mars were poised to provide a ringside seat to the aftermath of the impact.

When news of the possible impact went out, the Exploratorium in San Francisco sprang into action, preparing to create a webcast of the impact if it happened. Alas (for the Exploratorium, astronomers, and disaster enthusiasts everywhere), the asteroid passed close to—but missed colliding with—Mars on January 30, 2008. WD5 passed so close to Mars that the asteroid's orbit was disturbed by the gravitational interaction. Astronomers have lost track of WD5 for now, but they expect that it will return in four years.

But we're not willing to wait four years to share all the research Paul did about what happens when an asteroid bashes into a planet. Scientists and science fiction writers are both in the business of asking "What would happen if…?" In this case, we'll take a look at what would have happened if WD5 had struck the Earth or Mars.

BELLY FLOPS AND ASTEROID IMPACTS

If you've read Robert Heinlein's The Moon is a Harsh Mistress, you already know that an asteroid hitting the Earth will mess things up in the immediate vicinity of the impact. In that novel, moon colonists use a catapult to "throw rocks"—really big rocks—at the Earth.

But if you're like us, you're curious about exactly how destructive that impact will be. We can help you there. The level of destruction depends on three factors: what makes up the asteroid, how big it is, and how fast it's traveling.

Natural objects that are likely to strike a planet come in three major flavors: ice, iron, and stone. Each type of object behaves differently on hitting the planet's atmosphere, if it has one. We'll start close to home and consider what happens to an object entering Earth's atmosphere.

To appreciate what happens when an object hits the atmosphere, remember the last time you did a belly flop. If you fell flat into the water from the edge of the pool, the water welcomed you with a soft gentle splash. If, however, the flop was the result of a bad dive from a platform that's ten meters (thirty feet) above the water, you probably remember the pain to this day. (Paul certainly does!)

At slow impact speeds, it takes small forces to accelerate the water out of your way. At higher speeds, it takes extremely high forces. Diving from the higher platform, you hit the water a lot faster. Your body exerts a lot of force on the water. Newton's third law (For every action, there is an equal and opposite reaction) dictates that when your body slaps the water, the water also slaps your body—a nasty example of physics in action.

Remember the pain of your last belly flop as you consider a space object striking the Earth's atmosphere. That object is experiencing a similar situation. The molecules in the air have no warning. Suddenly—"Wham!"—they are hit by the object! In keeping with Newton's third law, the air molecules hit back, exerting large forces on the object. One of those forces, the friction of the object passing through the atmosphere, heats the object's surface to incandescent temperatures—so hot that the object emits light. That bright light is what we see from Earth as a falling star or meteor.

The forces on the object may also break it apart or rip off pieces as it travels. (Search YouTube for "meteor" to see some exciting footage of incandescent meteors breaking apart and shedding bits as they travel.)

Let's get back to the three flavors of objects that might collide with Earth. Most icy bodies disintegrate in the atmosphere. They fall apart and melt long before they reach the Earth's surface.

Iron meteorites, on the other hand, are strong enough to hold together in large pieces despite the forces and the heat. Pieces of iron meteorites strike the Earth regularly. In fact, if you have access to the output of the downspout of a building, you can collect iron meteorite particles. Get a magnet (a refrigerator magnet will do fine) and paint it white. Place the magnet where water from the downspout will pour across it.

After a rainstorm, check the magnet for small black specks. Many of those specks are tiny iron meteorite particles that fell onto your roof and paused there until the rain flushed them onto your magnetic trap.

But don't let those cute little meteorites lull you into feeling all warm and cuddly about rocks that fall from the sky. Meteor Crater in Arizona was made by an iron meteorite about fifty meters in diameter—approximately the size of WD5. (Be happy that one didn't land on your roof!)

As a rule of thumb, an object impacting Earth will make a crater that's about twenty times the diameter of the impacting object. Meteor Crater follows this rule, measuring about 800 meters in diameter. (For a more precise estimate of the size of a crater produced by a meteorite impact, the Lunar and Planetary Laboratory has a website that allow you to calculate the size of the crater made by an object of a specific diameter and speed. See http://www.lpl.arizona.edu/tekton/crater.html.)

The third flavor—the rocky asteroid—is stronger than ice but weaker than iron. Like icy bodies, most rocky asteroids don't ever reach the ground. The forces resulting from impact with the atmosphere are large enough to break the rock into pieces. Each new piece is then exposed to collisions with more air, which breaks it up. This continues in a cascade so that the rocky object is pulverized high in the atmosphere.

How can we say this with such confidence? Well, we have a pretty good idea of what happens when a rocky asteroid of that size hits the Earth, since it happened near Tunguska, Siberia back in 1908. We'll get back to that in a minute.

HOW FAST? HOW BIG?

As we said before, an asteroid hitting the Earth will create a mess in the spot it hits. One way to measure the expected destruction is to estimate the energy of the object as it strikes the atmosphere and the surface of the Earth. To do that, you need to know the object's mass and speed. When an asteroid is in motion, its kinetic energy is proportional to the mass of the asteroid times its speed squared.

There are two parts to the velocity of an object that will hit the Earth: the velocity it gains by falling into Earth's gravity well (about eleven kilometers per second or seven miles per second) and its orbital velocity around the sun relative to the Earth. Suppose an object has the same orbit as the Earth and is orbiting at about the same speed as the Earth (about thirty kilometers per second or twenty miles per second). If it's traveling in the same direction as Earth, it could slowly overtake the Earth, in which case it will drop in from nearly rest and have the lowest possible speed of impact—about eleven kilometers per second.

At the other extreme, an asteroid might have the same orbit as Earth, traveling in the opposite direction. The speed that an orbiting object travels depends on the mass of the body it's orbiting (in this case, the sun) and the radius of its orbit (about 150,000,000 kilometers). So the asteroid that's in Earth orbit will be traveling at thirty kilometers per second, the same speed as the Earth. That asteroid could collide with Earth head-on, in which case it will start falling into the Earth with a velocity of sixty kilometers per second.

But an asteroid does not have to be in the same orbit as the Earth. It could drop in from the outer solar system, as WD5 did. Such an asteroid can be moving faster than an asteroid in Earth orbit—racing along at more than forty kilometers per second. If a such a rock hits the Earth head on, its forty plus kilometers per second velocity will add to the Earth's thirty kilometers per second orbital velocity producing the fastest speeds we see for meteors: seventy kilometers per second. Such an object enters the atmosphere at over Mach 200, more than 200 times the speed of sound.

Compared to the speed of these falling rocks, the speed of sound in air is positively glacial. Sound pokes along at a mere 350 meters per second or just 0.35 kilometers per second (700 miles per hour depending on altitude). The speeds at which objects hit the Earth range from 30 to over 200 times the speed of sound.

Combine these tremendous speeds with a substantial mass and you've got a lot of destructive potential. WD5 massed about a billion kilograms and would have hit the Earth at about twenty kilometers per second or 50,000 miles per hour. (Just for reference, that's fifty times the speed of sound or mach 50.) Calculate the kinetic energy of WD5, and you'll get many megatons—the energy of a large hydrogen bomb!

GROUND ZERO—SIBERIA

That brings us to Tunguska, a region of Siberia known mainly for peat bogs and pine forests. According to eyewitness accounts in 1908, a bright, flaming object came down from the sky at an angle, followed by a giant bright blast. The heat wave and wind blast flattened huts and trees for 800 square miles. Forty miles from the impact site, windows shattered, ceilings collapsed, and people and livestock went flying. Hundreds of miles away, the Earth shook and people heard the "thunderclaps" of the impact.

Recent supercomputer simulations of the Tunguska collision made at Sandia National Laboratory (http://www.sandia.gov/news/resources/releases/2007/asteroid.html) produced movies of a simulated Tunguska impact that look like works of modern abstract art. These simulations have helped scientists understand what happens during such an impact.

Scientists figure that a rock measuring fifty meters in diameter was pulverized in the atmosphere high above the ground. This explains why no one finds large pieces of the Tunguska object scattered over the ground, although tiny pieces have been found embedded in trees.

The rock's energy was released as an explosion that was the equivalent of a five-megaton nuclear bomb high in the atmosphere. Originally, scientists had estimated that the explosion at Tunguska was closer to twenty megatons. They revised their estimates as they gained a better understanding of how the energy of the blast spread. A five-megaton explosion requires a smaller rock than the twenty-megaton explosion. This is an important distinction since small rocks in space are much more common than larger ones. So Tunguska-like collisions may be more common than we thought. That's one reason we are glad that NASA pays the folks at the Catalina Observatory to watch the skies for us. (Paul suspects that government agencies are keeping watch for nuclear-sized explosions around the Earth, and hears rumors that they may pick up one large nuclear bomb equivalent blast per year due to rocks from space striking the atmosphere.)

WHAT ABOUT MARS?

To understand what happens when an asteroid hits Mars, you need to remember that Mars has only 1/100 the atmosphere of Earth. You have to go up forty kilometers above the surface of the Earth to enter air with the same density as gas at the surface of Mars. Without the atmosphere to break them up, much smaller rocks can make it to the surface of Mars.

The spacecraft with high-resolution cameras that orbit Mars have documented more than twenty new impact craters that have formed in the last decade. These Martian craters tend to be twenty-five meters or so in diameter, which means that they were made by falling rocks as small as one meter in diameter.

The creation of twenty-five-meter-wide craters on Mars is common. Scientists say that if you lived on Mars for a decade you would hear or feel the shockwaves from an impact. Thank goodness our atmosphere protects us on Earth.

THROWING ROCKS, MAKING CRATERS

Before we describe the actual impact on Mars, let's pause to make some craters. (We know that all this talk about falling rocks has made you want to throw some.)

At NASA, scientists study cratering made by hypervelocity impactors by using an extremely high-speed gun. Paul develops activities for middle school science students and decided rather quickly that arming students with a hypervelocity gun might be unwise. So he did some experimentation and discovered a safer model of crater formation.

The key ingredient to Paul's model is table salt. Take a plastic tray at least five centimeters (two inches) deep and a foot in diameter. (Garden supply stores sell trays like this to place under potted plants.) Fill the tray full of table salt. (In an interesting tangent, Paul has discovered that salt costs almost the same regardless of the size of the package indicating that the price of the salt is almost entirely due to packaging and advertising.) Get a spherical ball bearing or fishing weight as big as a fingernail (at least one cm or 1/2 inch in diameter). Drop the weight into the salt from about a meter (a yard) above the tray. Watch what happens.

The crater will be much larger than the impacting object—as much as ten times larger. The same thing happens in real cratering events. (Of course, the real cratering events happen in solid rock and your event occurred in loosely packed salt.)

Notice also the pattern of the ejecta, the material thrown out of your crater. Some salt grains were probably thrown entirely out of your plant tray.

Rocks are found surrounding craters in real life as well. To study ejecta patterns, cover the salt in the tray with black paper. Cut a fist-sized hole (ten cm or four inches in diameter) in the middle of the paper. Drop a weight into this hole and observe the ejecta pattern. There will be more ejecta near the crater than farther away, and the ejecta will spread to a distance of many crater diameters from the point of impact. In fact we know that impacts on Mars have given some Mars rocks escape velocity. We know this because scientists have found Mars rocks on Earth.

You can also experiment with throwing your weight into the salt at an angle. Even when an object impacts at an angle far from vertical the impact crater is still circular. It takes an impact at an angle that skims the surface (under twenty degrees from the horizontal) to create an elliptical crater. (Meteor Crater Arizona is more circular than elliptical even though the main body of the impactor is located underground, beyond the rim of the crater.)

When the odds were in favor of the asteroid hitting Mars, Paul decided to create a simulation of the impact in Second Life (www.secondlife.com, Exploratorium (132, 163, 251)). Alas, the full scale model was too large even for a virtual world. The resulting crater would have been one km in diameter and the virtual land for such a simulation would have cost over $16,000. So Paul made a smaller crater impact, one that created a crater that was only fifty meters across. The cratering event portrays the same details you will see in your hands-on model of cratering.

MEANWHILE, BACK ON MARS

With your new understanding of cratering, let's return to our consideration of WD5 on a collision course with Mars. It would have gone something like this: WD5 enters Mars atmosphere at hypersonic velocities, glowing incandescent and breaking up a little, shedding debris as it falls. It hits the surface at twenty kilometers per second, taking only three milliseconds from the time the bottom of the asteroid hits the surface until the top passes the level of the surface.

The energy of the impact spreads down and outward as a shock wave compresses rocks and actually changes the minerals in the rocks through shock metamorphism. (Finding shock-metamorphic rocks is one way to make sure you are dealing with a meteorite crater on Earth.) Because the crater is excavated by the shock waves and is larger than the impactor, the crater is hemispherical to start. The compressed rocks rebound, throwing the ejecta up and away from the crater at high speed. The ejected blocks fall back to the planet surface where they may create secondary craters. The ejecta that hits nearest the rim has been thrown out at the lowest speeds and so creates smaller secondary craters. If the surface is made of layered rocks, the layers of rock near the rim are flipped over creating a rare instance where older rock is on top of younger rock. The flipped-over rock and ejecta near the rim create a raised rim for the crater. In all but the smallest craters, the bottom of the crater rebounds since the rock above has been removed.

Ejecta falls back into the crater and avalanches roll down the sides of the crater immediately starting the process of filling in the crater. The result is a crater that is wider than it is deep. Seismic waves spread out from the crater and sound waves too. Even the most enthusiastic cratering expert would agree: A meteorite impact is definitely something you want to observe from the side or above at a distance of many miles.

WHAT NEXT?

Calculations made shortly after Boattini spotted WD5 gave the asteroid a 1/25 chance of actually striking Mars. As data accumulated, the odds dropped to 1/10,000.

Scientists are fairly confident that the asteroid missed Mars, although there is a chance that it struck the planet without being captured on camera. WD5 has not been seen since its close approach. If it did pass Mars without colliding, its close encounter with the planet threw it into a new orbit.

Fortunately for Paul and other cratering enthusiasts, WD5 will remain an Earth-orbit-crossing asteroid even in this new orbit. In four years, Paul will be watching the skies and dusting off his lecture notes.

--------