Many animals including dinosaurs were exterminated 65 million years ago (Diagram 6). Roughly speaking, the geological time shifted from the Mesozoic Era to the Cenozoic Era, or more precisely, from the Cretaceous period to the Tertiary period. Large-sized animals or dinosaurs, who had thrived till then, suddenly disappeared, and mammals became the dominant creatures on Earth.
Why did this huge change in the animal world occur?
There are various theories, and one of them was due to a collision with a small planet such as an asteroid. According to this theory, the Earth's environment drastically changed with the collision, and many kinds of animals, including dinosaurs, could not live with these changes. On the other hand, mammals and other animals survived and have since thrived.
In the Yucatan Peninsular of Mexico, a crater has been discovered that is believed to have been made by a collision with an asteroid about 65 million years ago. Although it is called a "crater", it doesn't look like one on the surface, as it is covered by soil. This crater is named "Chicxulub (Chik-sah-loob) Crater" (meaning "devil's bottom" in Maya) and is a whopping 180 km in diameter. The asteroid, estimated to be 10 km in diameter, collided with the Earth, measuring some 13,000 km. The collision was more like a mosquito hitting a large truck, and our planet itself was barely physically affected. However, it seems that the surface environment suffered drastic changes.
In 1994, the Comet Shoemaker-Levy 9 collided with Jupiter, and about 20 pieces of cometary nuclei hit Jupiter one after another (Diagram 7). Each cometary nucleus was estimated to be 5 km in size, still they left a huge impact that is equivalent to the size of the Earth on Jupiter. This shows us how powerful the energy of the collision packed. We, of course, cannot simply compare Jupiter and the Earth, but suppose an asteroid collides with the Earth, we can easily imagine major problems. Therefore, as a prevention measure, the activity "Spaceguard" was started.
Spaceguard aims to detect, at an early stage, asteroids and comets that may collide with the Earth. In the case of an asteroid, we can precisely determine its orbit by continuously observing it and accumulating data on its location. By determining the orbit accurately, we can define if and when it will possibly collide with the Earth. Fortunately, none of the asteroids that have been monitored so far seem set to collide with the Earth in the near future. However, there are many other asteroids that have not been discovered yet, so it is essential for us to find them as soon as possible.
A collision with an asteroid is certainly something to fear, but let's consider the issue from a different angle. Suppose that the theory is true that the extinction of the dinosaurs was caused by a collision with an asteroid, dinosaurs would still be on the Earth if such a collision had not taken place. In that case, the existence of mammals like us would still be in the minority. In other words, we are indebted to the asteroid that collided with the Earth for the birth of mankind. Today, another asteroid collision with Earth is a definite threat. Therefore, we need to better understand asteroids for the future security of mankind.
