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December 25, 2012 > Spacecraft's Signal a Multi-purpose Container

Spacecraft's Signal a Multi-purpose Container

One of these inventions is the way spacecraft that explore the solar system communicate with Earth. They use radio signals. The signal is a stream of radio waves. Radio waves are a form of light we can't see. However, NASA's big, sensitive dish antennas on the ground can "see" them.

But to send information like images and temperature readings, the spacecraft's transmitter changes, or "modulates", the radio signal. The signal is thus made to carry all kinds of information gathered by the spacecraft's cameras and other instruments.

But besides being a "container" for all the important information from the spacecraft's science instruments and internal workings, the signal itself can be used directly to do science experiments. For example, say the spacecraft is near Mars, as Mariner 4 was in 1965 when this type of radio science experiment was first tried. When Mariner 4 went behind Mars (as seen from Earth), there was a moment when the signal just grazed Mars' surface. For that short time, Mars' thin atmosphere changed the signal a little bit as it passed through. Before that, no one had accurately measured the properties of Mars' atmosphere!

Since then, scientists have set up many clever experiments on many different space missions using only the spacecraft signal itself, plus the ability of the big Deep Space Network antennas to "see" it. This radio science technique is used to learn about the atmospheres of planets, moons, and even the Sun. Spacecraft signals are also used to measure the mass of solar system objects, and how gravity varies on different parts of the object, and even to test Einstein's Theory of General Relativity.

Learn how NASA's Deep Space Network can communicate with faraway spacecraft 24 hours a day by playing the "Uplink-Downlink" game at

This article was written by Diane K. Fisher and provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

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