In both cases, the viewing target is measured and analyzed from a distance. From the moment Galileo peered into space through his early telescope, our knowledge of the Earth's surface features, its weather, and its place in the universe has been enriched through remote sensing. Hundreds of years before the dawn of planetary exploration, Galileo used this tool of science to discover the four largest moons of Jupiter. In fact, the science of remote sensing has already played an important role in each of the previous Passport to Knowledge projects. Active remote sensors transmit some form of energy and detect its reflection from the viewing target.
Radar is one example of active remote sensing.
In order to map the surface of Venus, NASA scientists used radar transmitted from the orbiting Magellan spacecraft to pass through its surrounding thick cloud layer and reflect back. In a similar manner, the laser altimeter on board the Mars Global Surveyor will be firing a beam of light down to the Martian surface. Scientists will use the timed reflection back to the spacecraft to create an accurate map of Martian topography and elevation. Passive remote sensors collect reflected energy from natural sources such as the sun. The camera you use to take your favorite pictures is an example of a passive remote sensor.