For billions of years before Hollywood made the blockbuster movie Deep Impact, Near-Earth Objects (NEOs) have bombarded the Earth. According to the popular “giant impact theory,” our young Earth (around 50 million years old) was hit by another planet estimated to be about the size of Mars, creating our Moon. Since then, Earth has witnessed the impact of millions of NEOs of varying sizes and frequency. While astronomers and astrobiologists are divided on the details, it is generally believed that water and hydrocarbons brought to the Earth by NEOs played a significant role in the origins of life.
Our planet orbits the Sun among a swarm of asteroids whose orbits cross (or nearly cross) Earth’s orbit. These are not the asteroids that make up the asteroid belt between Mars and Jupiter, but rather the near-Earth asteroids whose orbits take them much closer to the Sun, and who regularly approach the orbit of Earth. These asteroids are remnants of the formation of our solar system, and range in size from pebbles to many miles across.
NEOs can also be Comets. Composed mostly of water ice with embedded dust particles, comets originally formed in the cold outer planetary system while most of the rocky asteroids formed in the warmer inner solar system between the orbits of Mars and Jupiter.
The estimated statistical frequency of impacts can provide a false sense of security: on the basis of statistical studies a “Tunguska-like event” is predicted to occur once every 1000 years; the actual Tunguska event, however, occurred only 100 years ago (and the size of the object is still being debated). Statistical analyses indicate a 300 m diameter object should pass by the earth at a distance of 30,000 km once every 70,000 years. However, the next occurrence of such an event, namely the close approach of the potentially hazardous asteroid Apophis, will be in 2029!