A massive explosion with more energy than the atomic bombs dropped in World War II ripped through the sky as a huge chunk of rock traveling close to fifty-thousand miles per hour was overpowered by the sheer force of aerodynamics and exploded into oblivion (Tagliaferri, 1998). This explosion was an asteroid from space that was on a collision course with the earth; in February of 1994 the asteroid struck. “The asteroid actually broke into several pieces, with one large piece detonating about 34 km above the surface, and a second, much larger piece detonating at an altitude of approximately 21 km” (Tagliaferri, 1998). Scientists were lucky to record this data via satellite to study the force an asteroid can have when impacting firsthand. There is evidence of asteroid impacts covering the surface of the earth and other surrounding planets in space. There are also many theories involving asteroid impacts causing mass extinctions on Earth, the most common being the extinction of the dinosaurs 65 million years ago (deGrasse Tyson, 2005). Most of the impact evidence shows that the earth has been attacked by these asteroids since its formation billions of years ago. Evidence all around us tells us that the Earth will be struck, and another major asteroid impact will occur. The most common and highly argued impact event is the one that may have wiped out the dinosaurs. This event in itself has many theories, but the one common element in most sound dinosaur extinction theories is that a major impact event occurred around the same time the dinosaurs stopped roaming the Earth. Most of the scientific community agrees that an asteroid impact did destroy the dinosaurs. Some scientists; however, claim that the dinosaurs were dying off well before the asteroid strike, due to climate changes, and that the impact only “finished them off” (Archibald, 2005). This very well could be the case, because it is extremely difficult for paleontologists to create an accurate timeline of the cretaceous period through studying the sediment layers of the Earth. The reason for some discrepancy is that most of the differences in the sediment layers were created over thousands and even millions of years, so being precise is nearly impossible (Hildebrand, A., Boynton, W., 1991). Complete accuracy in the timescale of years does not disprove the fact that the dinosaurs were wiped out in the final stages of the cretaceous period, or the fact that one giant asteroid struck the Earth with enough force to help bring about the end of the dinosaurs. The proof of the asteroid impact is found deep in the Earth’s sediment layers and in the fossil record. The boundary between the cretaceous and tertiary (K-T) periods holds an element uncommon on the earth, iridium (Hildebrand et al., 1991). Although iridium is a rare element to find in any quantity on the earth’s crust, iridium is a common element found in space rock and other objects in outer-space. Some scientists discard the theory of a major impact extinguishing the dinosaurs because there is no distinguishing evidence of a crater large enough to have come from an asteroid in that period. For most scientists though, there is other evidence that is more convincing than simply not finding a crater. Roughly 70% of the earth’s surface is covered by water which could easily cover up a massive crater. Erosion of the Earth’s crust also helps to explain how a crater could disappear over long periods of time (Hildebrand et al., 1991). The amount of iridium deposits found at the K-T boundary in the sediment layers is more than enough proof for scientists who are constantly researching the sediment layers from that period around the world. At first “pencil-thick” sheets of clay were being found in the same boundary layer across the globe all which contained similar levels of iridium (Hildebrand et al., 1991). This layer of iridium that is found all over the world, has been traced back to its assumed source by measuring the amounts of iridium left behind, and the probable impact site, most of a crater named Chicxulub found near the Yucatan peninsula in Mexico’s northern coast. The Chicxulub crater measures more than 100 miles in diameter (Hildebrand et al., 1991). “The evidence suggests that the Chicxulub crater could be the sole K-T boundary crater.” (Hildebrand et al., 1991) Most people have common knowledge that the dinosaurs once roamed the Earth and eventually became extinct because of an asteroid impact. This; however, was not the first major extinction to occur on our planet. “Five major extinctions have occurred in the past 500 million years: the Ordovician, the Devonian, the Permian, the Triassic and the Cretaceous” (Merali, 2006). (Merali, 2006)
This chart signifies the time between the five major extinction events in the earth’s past.
Common acceptance of the impact theory for the Cretaceous extinction allowed scientists to accept the same theory for all other mass extinction events. There are now some differing theories that do not involve any such asteroid impacts for the earlier extinctions. Merali (2006) states, that “There is mounting evidence that the Permian extinction around 250 million years ago was caused by huge volcanic eruptions in Siberia, which led to catastrophic climate change.” Merali (2006) also adds a rebuttal that “large-scale volcanic eruptions happened throughout geological history, they didn’t always cause mass extinctions.” With these major mass extinction events going so far back in the Earth’s history we may never know what the true causes were, the thing to keep in mind is that there have been five major mass extinctions in the past and these will not be the last. Space and most of the larger bodies that occupy space show even more evidence of the impact an asteroid or comet can create. The closest object to the Earth is the most obvious place to start, the moon. When looking at the moon, even through a cheap telescope, one can easily see craters large and small, created by asteroid and comet impacts throughout the course of time. “Planetary scientists now realize that rocks of various sizes have been colliding with solar system members since the whole system formed some 4.65 billion years ago” (Reynolds, 2006). Space is still an active place for asteroids but many astronomers believe there is little more than five percent of the original number of asteroids left in the solar system (Reynolds, 2006). The number of asteroids is constantly depleting, every time there is an impact with a larger more stable object the smaller of the two impactors is destroyed. The fact that there are fewer asteroids does make space a safer place than it was in the beginning. Reynolds (2006) says “Objects with ancient surfaces, like the Moon and Mercury, show impacts used to happen much more frequently than they do now.” There is evidence of these impacts on every large body in space. Some planets like the Earth do not show as many signs of the damage they have sustained. Jupiter, for example, does not have any craters on its surface. Jupiter is a “gas-giant” planet the impacts do not leave craters because there is no solid surface. Only through observation do we know Jupiter has undergone heavy bombardment. Reynolds (1996) tells us how and when.
“We had front-row seats in July 1994, when fragments of Comet Shoemaker-Levy 9 smashed into mighty Jupiter. The Dark splotches that resulted from these impacts lasted for months, dramatic proof of what can happen when you release an energy equivalent to all the nuclear arsenal on Earth.” The other planets and their moons are not the only place we find evidence of these massive impacts. “Our planet’s surface is riddled with craters formed by high-speed cosmic impacts” (Reynolds, 2006). Our planet does not show as many signs of past impacts as the moon and other surrounding planets do. Reynolds (2006) explains “Plate tectonics has resurfaced Earth in the past 2 to 3 billion years, leaving no evidence of the early, heavy bombardment.” Even though the older craters are gone forever, there are still many signs of asteroid impacts on the Earth’s surface. The most popularly known crater is Meteor Crater in Arizona. Meteor Crater measures about 4,000 feet across and is nearly 570 feet deep; this massive crater is estimated to have been created some 50,000 years ago by an iron based meteor close to 150 feet in diameter (Reynolds, 2006). Based on the size of the impact crater and the force required to create such a massive hole in the Earth’s crust scientists have estimated the impactors speed to be close to 45,000 miles per hour when it hit. A good amount of early crater research was done at the Meteor Crater site, but despite its popularity this crater was not discovered until the late 1800’s, leaving only the past hundred or so years for scientists to search for other craters on Earth. “So far, scientists have identified about 150 impact craters on Earth, with several new ones joining the list each year” (Reynolds, 2006).
With the increases in technology over the last few decades, scientists have been given more tools to use in understanding the force and affect of asteroid impacts. One major advance came by accident when satellites orbiting the Earth picked up on explosions of energy in the atmosphere. The event in 1994 was detected by space based sensors, and the impact information was relayed to operators on the ground. This event left no crater, both pieces of the asteroid exploded in mid-air; because of the satellites detection; however, scientists were able to analyze the data and determine what truly happened (Tagliaferri, 1998). These space based sensors were not originally created for detecting asteroid impact events, some of their purposes are classified for national security reasons, but the best thing about them is that we can record impacts that occur even if the impactor is not strong enough to strike the Earth’s surface. The first satellite recorded event happened in August of 1972, when an “Earth grazer ‘impacted.’” The object entered the Earth’s atmosphere traveling close to 33,000 miles per hour and “grazed” past the Earth, then climbed back out of the atmosphere and back out into space (Tagliaferri, 1998). Tagliaferri (1998) tells us “This object is still in an Earth-crossing orbit around the sun and passed close to the Earth again in August 1997.” There are many objects out in space that could eventually end up on a collision course with Earth. Since the discovery of our satellites ability to track these impacts and collect information, and have helped scientists determine the size and force of the impacting asteroids steps have been made to search for, and track all possible near earth asteroids. When an asteroid is found in space that could strike it is given a name, one such asteroid is named 1950 DA, This asteroid is being tracked currently by NASA and is estimated to be around 3,600 feet in diameter that is more than 20 times the size of the meteor that formed Meteor Crater. NASA scientists have given this asteroid a 0.3 percent chance that it will collide with the Earth in the year 2880 (Ward, S., Asphaug, E. 2003). There is another asteroid currently being watched for possible collision, “It’s called Apophis, it’s 390 meters wide and it could hit Earth in 2036” (Jha, 2005). This asteroid has an outside chance of striking the Earth because we do not yet know how its orbit will be affected by coming close to the Earth in 2029. This asteroid is one of the top threats among near earth asteroids, and it will be coming soon. Strong evidence links the extinction of the dinosaurs to a major impact event. Earlier mass extinctions show some signs of impacts on Earth but nothing conclusive enough to have been the only cause of the extinction. There is; however, evidence on Earth, and all around it that proves with no doubt that asteroid impacts have been occurring since the formation of the galaxy. People on Earth have had many chances to witness impact events, and may not have realized what they are seeing, shooting, or falling stars are space objects that were to small to survive and burnt up in the atmosphere, creating something that is truly amazing to watch. There will come a time when one of the larger asteroids, that scientists are searching for right now, will strike.

References
Archibald, J. (2005, May). Were dinosaurs the victims of a single catastrophe? No, it only finished them off. Natural History, 114(4), 52-53. Retrieved June 1, 2007, from Academic Search Premier database.
DeGrasse Tyson, N. (2005). Knock ‘Em Dead. Natural History, 114(4), 25-70. Retrieved Wednesday, May 16, 2007 from the Academic Search Premier database.
Hildebrand, A., & Boynton, W. (1991). Cretaceous Ground Zero. Natural History, 100(6), 46-53. Retrieved Saturday, May 19, 2007 from the Academic Search Premier database.
Jha, A. (2005). It's called Apophis. It's 390m wide. And it could hit Earth in 31 years time. Guardian Unlimited Special Reports, Retrieved May 19, 2007, from http://www.guardian.co.uk/space/article/0,14493,1660485,00.html#article_continue
Merali, Z. (2006). Climate blamed for mass extinctions. New Scientist, 190(2545), 18-18. Retrieved Wednesday, May 16, 2007 from the Academic Search Premier database.
Reynolds, M. (2006). Earth Under Fire. Astronomy, 34(8), 40-45. Retrieved Wednesday, May 16, 2007 from the Academic Search Premier database.
Tagliaferri, E. (1998, November). Observation of meteoroid impacts by space-based sensors. Mercury, 27(6), 18. Retrieved June 1, 2007, from Academic Search Premier database.
Ward, S., Asphaug, E. (2003, June). Asteroid impact tsunami of 2880 March 16. Geophysical Journal International, 153(3), f6. Retrieved June 1, 2007, from Academic Search Premier database.