Asteroids and Comets and Meteors, Oh My!

By Ted Smolinski

From the beginning of time, man has been fascinated with celestial bodies. The sun, the moon, the stars -- all of these were constants in primitive life.

Every once in a while, though, a heavenly body would suddenly appear, "threatening" war, famine, pestilence and death in the world of primitive man. These harbingers of doom were asteroids and comets.

The superstitions surrounding these interplanetary projectiles have dissipated over the past few centuries. This is the result of the advent of modern astronomy and the realization that asteroids and comets have nothing to do with natural disasters on Earth; at least not the way ancient man perceived it. In this day and age, there are very few people who still believe that an asteroid or comet, in the simple act of passing by Earth, will have any effect on them.

However, there is actually a very real threat from incoming asteroids and comets, in the possibility they could crash into Earth and cause tremendous damage.

This is a threat that has been gaining more and more popularity and scientific interest by the year. Recently, there have been several media events that have focused on and amplified public interest on the dangers of asteroid and comet impacts. These include a National Geographic documentary, a Discovery Channel documentary, a feature article in the New Yorker, several newspaper articles and television stories, as well as the NBC original miniseries, "Asteroid."

Part of the reason why the subject is popular is it lends itself to so much speculation, and sometimes even borders on science fiction.

Imagine what would happen if a rock the size of a small town (a few kilometers in diameter) hit the surface of the planet traveling at 25 kilometers per second. There would be a massive explosion, several times greater than the effect of the world's most powerful nuclear weapon. This

An asteroid colliding with Earth
could be catastrophic.

would be followed by about a half hour of scorching heat in the immediate area, slowly toasting any nearby plant and animal life. Dust thrown up into the atmosphere from this massive impact would produce a pitch-black night which would last for several months. The average temperature of the planet would drop more than 10 degrees Celsius and nitric acid produced from the explosion would rain down on the planet, acidifying lakes, streams and the soil. The acid, the darkness and the cooling would all lead to a global failure of agriculture, resulting in the starvation and death of billions of people.

Finally, after the atmosphere cleared, there would be an enhanced greenhouse effect, which would bring global temperatures 10 degrees above normal for the next few decades. Months of cold and darkness and years of high temperatures would be extremely taxing on the environment. This would lead to massive human casualties and a decrease in the populations of every species on the planet.

The damage done by these projectiles all depends on their size, their speed, their chemical composition and their landing spot. According to Dr. Raymond J. Pfeiffer, professor of astronomy and astrophysics at The College of New Jersey, the composition of an asteroid would determine how it reacts to entry into our atmosphere. If an asteroid is comprised mostly of metal, the heat resulting from the friction would be conducted throughout the body and it would vaporize. However, if an asteroid is comprised mostly of rock, the inner layers would survive the outer layers because rock does not conduct heat nearly as well as metal.

"An asteroid made out of rock poses much more of a threat to humans, because it has a much greater potential for damage to the surface of Earth," Pfeiffer said.

On the other hand, comets, which are comprised largely of ice, are more likely to break up upon entry into Earth's atmosphere, but since they are so much larger and because they travel at twice the speed of a typical asteroid, they still pose a major threat. However, comets still only make up about 10 percent of celestial matter that is expected to hit Earth.


The damage done by these
projectiles all depends
on their size, their speed,
their chemical composition
and their landing spot.
There are many differences between asteroids and comets. Asteroids are rocky and/or metallic bodies, all under 1,000 kilometers in diameter, which orbit the sun like the planets. Most of the asteroids in our solar system orbit between Mars and Jupiter, with some asteroids crossing the orbit of Earth on regular intervals. These objects are often called Near Earth Objects or NEOs.

Comets, on the other hand, travel in orbits much different than those of the planets. Long-term comets take over 10,000 years to orbit the Earth and short term comets can take anywhere between 76 and 10,000 years to complete the same orbit. Their orbits can take them a tremendous distance from the solar system, which leads astronomers to discover new comets that will pass by Earth only a year or two after their discovery. Comets usually range in size from a few kilometers to 50 kilometers, with a few extremely rare cases where they can be as large as 950 kilometers in diameter. Comets are made up of clay, organic material and ice. The reason comets are so much more visible in the sky than asteroids is because light is reflected by the ice as they get closer to the sun and the materials inside become active.


"An asteroid made out of rock
poses much more of a threat to
humans, because it has a much
greater potential for damage to
the surface of earth."
- Dr. Raymond J. Pfeiffer
Comets may have even played a crucial role in the formation of life on Earth. According to Gloria Rall, staff member at the New Jersey State Planetarium and former director of the Newark Planetarium, comets have dumped a lot of organic material onto Earth that could have been the precursor molecules for organic life. These materials form naturally out in space by non-living processes and eventually find their way to Earth. This, of course, was billions of years ago when asteroids and comets were much more likely to crash into a planet because they were more numerous. According to Rall, there is no doubt that a substantial amount of water has been added to the Earth by comets.

Humans have been studying these natural space travelers for centuries. But it seems the fear of cosmic impacts on Earth is mainly something that's restricted to the 1990s.

"I think that Hollywood is hyping up the threat of asteroids," said Scott Gershman, a junior communications major at The College of New Jersey in Trenton. "In the '80s there were a ton of movies that had to do with nuclear war, but that kind of thing isn't really believable anymore, so Hollywood has to come up with new ways to end the world. You've got all kinds of disaster movies about volcanoes, tornadoes and earthquakes, so asteroids fit right in there with the rest of them.

While this may be true, there is actually a more scientific explanation for this whole phenomenon.

"(The hype) all started with astronomers writing about the possibility of dinosaurs dying out from asteroid impact, which is now actually a very well accepted theory," Pfeiffer said.

Paleontologists have known for quite some time that dinosaurs, along with about 70 percent of the other species on Earth, faced extinction about 65 million years ago. Many different theories have been discussed by scientists trying to figure out what had caused the "Cretaceous-Tertiary Mass Extinction," also known as the "K-T event." In 1980, Luis Alvarez, his son and two other scientists came up with the theory that dinosaurs died as the result of a huge asteroid impact.

After people began to consider that an asteroid might have caused the extinction of the dinosaurs, they started contemplating that the same thing might eventually happen to mankind.

Rall can attest to this sudden rise in interest ever the last couple of years. The public program "Death of the Dinosaurs," is a two to three month program that ran three times in the past five years at the New Jersey State Planetarium.

"There's been so much curiosity on the whole subject," Rall said. "We run it as a public program and we get people of all ages. It basically describes the death of the dinosaurs, and then shows modern events that have been close calls and how we can prevent them from happening again."


The Shoemaker-Levy comet collided with Jupiter
in 1994.

The second, and even more recent, reason that the asteroid and comet threat has gained so much interest is the break-up and collision of the fragments of comet Shoemaker-Levy with the planet Jupiter in July 1994.

The Shoemaker-Levy comet broke apart and left several marks in Jupiter that were the size of our entire planet. This knowledge made the threat of cosmic impacts seem like a much more real and tangible occurance to people across the globe. NASA re-assessed its study on the threat of asteroids that year.

Another possible cause for the surge in interest over the last couple of years could be the upcoming turn of the millennium -- the dawn of the year 2000. There are some that think the year 2000 has some kind of mystical importance.

Of course, there are cults, like the Zeta Reticulans, who think that an alien invasion is on its way, or Heaven's Gate, whose members killed themselves in order to hitch a spiritual ride on the Hale-Bopp comet before the year 2000 hit. Heavenly bodies in general are going to gain a little extra attention in these superstitious times before the current millennium ends. Despite human reliance on superstition, the year 2000 doesn't have any actual scientific weight, experts say.

"People are fascinated with numbers and yet they have no bearing on physical reality," Pfeiffer said.

Cultures start counting at different times and this has no meaning in an absolute sense of nature.

"A calendar is really just an arbitrary thing," Pfeiffer said. "Nature doesn't know our time."

Many people wonder about the chances of a comet or asteroid hitting the earth. The smaller an impact is, the more likely it is to happen, and the larger it is, the less likely it is to happen. For example, a fist sized projectile might hit somewhere on Earth roughly every two hours, but an asteroid the size of the one that may have wiped out the dinosaurs will only hit once or twice in the next 100 million years.

The atmosphere protects the planet from most of the small projectiles that fall from the skies. In fact, every hour, a ton of weight is added to the total weight of the Earth in the form of cosmic dust. Even asteroids that do make it to the earth's surface are significantly worn down by their entry into the atmosphere.

But asteroids and meteors (which are basically asteroids too small to observe until they start to burn up in the atmosphere) do actually reach the surface on occasion. On Friday, October 9, 1992, a fireball was seen streaking across the sky all the way from Kentucky to Peekskill, N.Y., where a meteorite fell through the trunk of a parked car. There are several other accounts in the United States of small meteorites nearly hitting people.

A 10-meter diameter asteroid usually falls somewhere on Earth about once every 10 years, but usually goes unnoticed because it either falls into the sea or to some remote uninhabited part of the world, such as Antarctica. On August 10, 1972, an asteroid sailed past Grand Teton National Park and burned in the atmosphere for about a hundred seconds before bouncing back out into space. In June 1908, an asteroid exploded over the Tunguska region of the Siberian wilderness and leveled 2,000 square kilometers of forest. The asteroid exploded at an altitude of about eight kilometers and was about 60 meters in diameter. The blast was similar to that of a 20-megaton bomb. It lit up the sky for thousands of miles, even as far away as London, where it was so bright one night that people could read books by the light of the sky, according to eyewitnesses.

According to Pfeiffer, asteroids of these sizes can actually be more of a problem than the larger ones. We worry about the small ones because they're not detectable until they get close, he said. Impacts of this class usually happen a few times every millennium. One-kilometer asteroids hit about once every 5000 years, five-kilometer asteroids hit about once every million years and anything much greater than that only hits once every 10 to 30 million years.

Several events in Earth's history are now being re-evaluated in light of the new information on cosmic impacts. It is now being strongly theorized that most, if not all, of the major evolutionary shifts in our planet's history had to do with comet or asteroid impacts. Also speculated on is the theory that asteroids have been the occasional cause of typhoons, which could mean that asteroids actually have been responsible for the deaths of many people.

The first step in reducing the threat of asteroids lies in charting the locations and paths of as many asteroids as possible. This will enable astronomers to identify problematic asteroids much further in advance, so appropriate actions can be taken.

This is the general idea of the Spaceguard Survey, which has been proposed to detect all asteroids greater than one kilometer in diameter over the next 25 years. Only about 10 percent of the asteroids in our solar system have been located by astronomers. The survey would also deal with comets and would determine their potential threat as they enter the solar system. The survey has not been funded in the United States or through any other sources and NASA only spends about $1 million a year on comet and asteroid search programs.

Knowing that an asteroid or comet is coming still leaves one crucial problem: how to physically stop or divert one from striking Earth.

Smaller objects up to 100 meters in diameter could be stopped by smashing other large objects from Earth into them. Boeing's Exoatmospheric Projectile, a large impactor device, developed under the Strategic Defense

Boeing's Exoatmospheric
Kill Vehicle (EKV).

Initiative for the Cold War, would work well, according to researchers. However, larger objects would require much more drastic measures, and the only real way to deal with them would be with nuclear weapons. Unlike in the movie "Asteroid," there aren't any lasers in existence that would do anything significant to a large oncoming asteroid or comet. A U.S. Titan missile, or a Russian Proton, which are used to carry military payloads into space, could carry a nuclear explosive to the asteroid or comet, and change the angle of its descent so that it does not hit Earth. This would work well combined with a complete survey of the asteroids in the solar system, because the more warning time given, the less energy is needed to move an object out of a collision course with Earth.

Before developing a defense system centered around any of these scenarios, however, much more research must be conducted on this subject. The study of the different impact dangers has only increased in the last decade or so. The chances of an asteroid or comet larger than a kilometer wide striking anywhere on the planet in the next century are 1 in 10,000. It's probably safe to say that we'll have some time to figure this one out.


Images obtained from:
http://www.ssd.bna.com/ekv
http://www.exepc.com/~maas/extinction/image2
http://www.jpl.nasa.gov/s19