Probe Designs and Functions
On a cosmic scale, the solar system is not vast: The closest star to our sun, Alpha Centauri, is more than 1,000 times more distant from us than is the farthest planet of our solar system, Pluto. But distances within the solar system, though insignificant on the scale of the universe, are enormous on a human scale. Thus, to study the solar system in any detail from Earth is a difficult task. Recent advances in telescope technology have allowed some information about the composition of the atmospheres and surfaces of planets to be discerned with Earth-based telescopes. As advanced as these data and images are, however, the information they reveal is still limited. Sending probes to the various planets not only enables scientists to obtain much better images, it also permits them to directly study physical samples at the same time.
Probes carry many types of instruments. Cameras and other devices that are sensitive to different wavelengths of light, including infrared and ultraviolet, reveal surface features and atmospheric conditions of planets and their moons. Spectrometers divide sunlight that passes through a planet's atmosphere or is reflected from its surface into its component colors, producing patterns that reveal the makeup of soils, clouds, and atmospheric gases. Radiometers measure the intensity of infrared waves, from which scientists can calculate among other things the temperature of an object. Magnetometers measure the streams of electrically charged particles and magnetic fields emanating from the planets. All these data are radioed in digital form (as a series of 1's and 0's) back to Earth, where they are picked up by sensitive antennas.
Even a probe's radio system can be used to make observations. By bouncing radio signals off of a planet or moon's surface and measuring the time it takes for the signal to be reflected back up to the spacecraft, detailed computer-generated images can be made of a planet's surface features. This technique is known as radar mapping. A planet's atmosphere can be studied using a technique known as radio occultation, whereby a radio signal, aimed at the Earth, is beamed through the atmosphere of a planet. Scientists on Earth compare the signal with one sent by the probe through space. By analyzing the ways in which the signal has been changed by the atmosphere, they can infer what conditions (such as temperature, clouds, or charged particles) might have caused the changes.