The seasonal changes take place because the tilt of the earth remains the same while the earth circles around the sun. To understand this clearly think of the plane that contains the path of the earth around the sun. The axis of the earth around which it rotates is not at right angles to this plane. It that was so the equator of the earth will always be the part of the earth nearest to the earth irrespective of the position of the earth in relation to the sun in the course of it's yearly path round the sun. However the axis of the earth forms an angle of 23.5 degrees with the perpendicular to the plane of its path round the sun. And the direction of its tilt remains same irrespective of its movement around the sun. Because of this parts of the earth closest to the sun keeps shifting as the earth circles round the earth, making one cycle of these shifts. As a result the amount of sun's heat received at any place on earth changes from time to time making on complete cycle every year, causing yearly cycle of changing seasons.
Asteroids are numerous small bodies that circle round the sun. Asteroids are also called planetoids or minor planets. Though asteroids circle around the sun, there is a theoretical possibility that if an asteroid passes too close to the earth or some other planet it can deviate from its normal path round the sun, and start circling in a different path. With these kind of changes there is certain amount of uncertainty associated with the possibility that an asteroid may pass so close to the earth that it will be pulled by the earth's gravity and crash on to earth.
Currently there is only one asteroid identifies that has any identifiable possibility of hitting the earth. This asteroid named Apophis will come very close to the Earth in 1929 If at that time it passes through a very narrowly defined specific path, the earth's gravitational pull can cause it to return to earth and strike it in 2036. The chance that this asteroid will actually strike the earth is estimated to be less than one in 233,000.