Answer to Question 1
The distribution of asteroids in the belt is strongly affected by Jupiter's gravitational force. Certain orbits in the belt almost free of asteroids are called Kirkwood gaps after their discoverer, Daniel Kirkwood. These empty orbits are at certain distances from the Sun where an asteroid would find itself in a resonance with Jupiter. For example, an asteroid with an average distance from the Sun of 3.28 AU will travel exactly twice around the Sun in the time it takes Jupiter to travel once. Such an asteroid would pass Jupiter at the same place in space every second orbit and be tugged outward. The cumulative perturbations would rapidly change the asteroid's orbit until it was no longer in resonance with Jupiter. Thus, Jupiter effectively eliminates objects from the orbit resonance. The example given represents a 2:1 resonance, but gaps occur in the asteroid belt at many other resonances, including 3:1, 5:2, and 7:3.
Answer to Question 2
Sixty-five million years ago, at the end of the Cretaceous period, more than 75 percent of the species on Earth, including the dinosaurs, became extinct due to a large impact that altered Earth's climate. If an object larger than 1 km was to hit the Earth, organisms near the impact would die instantly. An impact at sea would generate tsunamis hundreds of meters high and would travel around the world and flood the continents. On land, ejecta from the impact would fall back to Earth and produce a rain of hot meteorites, triggering forest fires and grass fires. Ash and dust in the atmosphere could block out the Sun for years, killing off plants because they require sunlight for photosynthesis.
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