Observations of a planet from an orbiting spacecraft have two main advantages. First, with the spacecraft in a polar orbit, the entire surface of a planet may be observed within a few weeks or months. Second, an orbiting spacecraft may make repeated observations of the same area, thereby recording any changes that may have taken place on the planet.
Mariner 9 was launched May 30, 1971. On November 13 it went into orbit around Mars. At that time, there was a great global dust storm on Mars that obscured Mariner's view of the surface. The storm eventually subsided and in the year following its arrival at Mars, Mariner transmitted over 7,000 pictures of the surface and measured the composition and structure of the Martian atmosphere.
The Viking 1 and 2 spacecraft each consisted of an orbiter and a lander. The orbiters surveyed Mars for four and two years respectively, and returned thousands of photographs of the Martian surface.
Launched November 7, 1996, Mars Global Surveyor's primary mission was to perform a comprehensive study of the Red Planet over the course of one Martian year (about 687 Earth days). Tracking a full cycle of Martian seasons, the orbiter acquired more than 80,000 images by the end of the primary mission in January 2001. These images revealed gullies that appear to have formed in relatively recent times and vast layered outcrops of sedimentary rock that could very well indicate places where ancient Martian lakes once existed. Entering an extended mission phase from February 2001 to at least April 2002, Mars Global Surveyor will focus on imaging specific places on Mars.
Orbiting Jupiter since 1995, Galileo has studied the gas giant and its moons in more detail than any previous spacecraft. What started out as a two-year study of the Jovian system was extended twice with the Galileo Europa Mission and the Galileo Millennium Mission. Returning an enormous wealth of scientific information, the spacecraft has discovered evidence of a sub-surface ocean on Europa, collaborated with the passing Cassini spacecraft in the study of a new volcanic plume on Io, and provided the only direct observation of Comet Shoemaker-Levy's impact into Jupiter. Its final mission extension had Galileo perform five more flybys of the gas giant's moons before taking a final plunge into the crushing pressure of Jupiter's atmosphere in August 2003.
Spacecraft in orbit around a planet have the ability to observe the same area again and again over a period of time. (We of course utilize this advantage every day for weather predictions made with data from Earth-orbiting weather satellites.)
Environmental Change on Earth
In these satellite photos of the Aral Sea, you can see the change in sea level over a period of 25 years. This type of monitoring helps to assess water resources on Earth.
The Magellan spacecraft was designed to map the surface of Venus. Shrouded by thick clouds, the surface of Venus was hidden from the view of earlier spacecraft in the visible wavelengths of light. Magellan used radar to see through the clouds and reveal surface details never seen before. Magellan orbited Venus from 1990 to 1994 and successfully mapped 98% of the planet's surface. Magellan was also the first planetary spacecraft launched from the space shuttle.