Emerald Robinson: Hi! I'm Emerald Robinson. In this What is? video, we're going to take a closer look at asteroids.
Asteroids are small planetary objects orbiting the sun, their hard, rocky bodies, differentiate them from comets, which are made of small rocks, dust and ice. Asteroids range in size from a few meters across to objects large enough to trap smaller asteroids in their gravitational pull, like moons orbiting a planet.
Asteroids lack the gravitational force needed to round out like planets, so most are irregularly shaped. They have no atmospheres, and are not geologically active. Hundreds of thousands of asteroids exist in the solar system, most occupying an area between Mars and Jupiter called the Asteroid belt. Despite their numbers, the combined mass of all solar system asteroids only equals the mass of the moon.
Asteroids that cross the Earth's orbit are called near-earth asteroids. Astronomers have discovered approximately 4,500 near-earth asteroids, including up to 1,000 with 1 kilometer diameters; at least one asteroid with a 4 to 10 meter diameter hits the earth every year.
Impacts with large asteroids have triggered planet-wide extinction events several times in the Earth's past. The last mass extinction event wiped out the dinosaurs. A similar cataclysm could seriously test humanity's ability to survive. So astronomers track large asteroids capable of causing such impacts.
Going back to the beginning of the solar system, asteroid impacts helped grow planets, adding to a forming planet's mass while clearing it's planetary orbit. An impact with an enormous asteroid as large as Mars ripped a chunk off the early Earth, creating the moon.
Asteroids have changed little since the beginning of the solar system, over 4.
6 billion years ago. Their composition could teach us much about those early years. As for the future, rare minerals found on asteroids could provide raw materials for space exploration. And that's the rock-hard truth about asteroids.