On a small moon with low gravity, a fastball pitcher could throw a ball into orbit. Gravity always pulls the ball towards the center of the moon. However, if the ball was moving fast enough, the moon's surface would curve away beneath the ball as quickly as gravity curved the ball downward. The ball would be in orbit!
Click on image for full size
Windows to the Universe original artwork.
The Heart of Satellite Orbits
Starting at the same altitude above the surface of a small moon, balls are thrown at different speeds. Regardless of the horizontal speed of the balls, they all fall at the same rate and hit the surface at the same time. The faster balls go further, in the same elapsed time as the slower balls, before hitting the ground. Finally, a ball travels so fast and thus so far before falling, that the ground doesn't appear flat but curves away beneath the ball. At just the correct speed, the ground curves away at the same rate that the ball drops. The ball has achieved orbit.
If the baseball player were to climb a high mountain and repeat the same exercise, the moon would appear smaller with the ground curving away more sharply. The speed that the ball needs to achieve orbit gets slower and slower as altitude increases and the curvature gets more pronounced. The combination of forward speed and falling determine the curvature of the ball's path. Matching this curvature to the curvature of the moon's surface at each altitude is at the heart of satellite orbits. The satellite drops forever towards the moon's surface but since the moon's surface curves away at the same rate, the satellite remains suspended forever at the same height above the surface. It is in orbit.
Why do satellites sometimes fall out of the sky? Frictional forces like the drag of the atmosphere on the satellite can cause the forward speed of the satellite to decrease. As a result, the curvature of the orbit changes, decreasing so that it no longer matches the curvature of the moon's surface at this altitude. It is now like one of the "too slow" balls in the cartoon. The satellite begins to drop. As it drops it increases in speed in the vertical direction due to gravity, but never again moves fast enough in the horizontal direction to maintain its orbit. The satellite will slowly descend to the surface unless it is reboosted to speeds sufficient to achieve orbit at its new altitude.
Shop Windows to the Universe Science Store!
Our online store
includes fun classroom activities
for you and your students. Issues of NESTA's quarterly journal, The Earth Scientist
are also full of classroom activities on different topics in Earth and space science!
You might also be interested in:
With Tycho Brahe's observations in hand, Kepler set out to determine if the paths of the planets against the background stars could be described with a curve. By trial and error, he discovered that an...more
The key to understanding satellite motions is in recognizing that gravity acts only in the downward direction - forward motion and falling are totally independent of each other. If there were no gravity...more
Satellites and robotic spacecraft are crucial to space exploration. They let us explore space without having to support life, and can travel longer, in more dangerous areas, and at lower risk than manned...more
Gravity is one of the universal forces of nature. It is an attractive force between all matter, and is very weak as compared to the other forces of nature. The gravitational force between two objects is...more
Starting at the same altitude above the surface of a small moon, balls are thrown at different speeds. Regardless of the horizontal speed of the balls, they all fall at the same rate and hit the surface...more
PAH is the short name for a "polycyclic aromatic hydrocarbon". A PAH is a stable structure made up of multiple rings that are fused together. The rings are made of carbon and hydrogen (though other elements...more
Acids are well known as substances capable of dissolving things. If you've ever gotten some battery acid on your clothes and had a hole develop in a couple weeks you'll know what we mean. In this regard,...more
Like an acid, a base is a substance capable of dissolving things. Unlike an acid in which the active agent is a substance which has a positive electric charge (H+), in a base the active agent has a negative...more