There are deep canyons, volcanic mountains and craters of all sizes formed by meteorite impact. To safely reach the surface of Mars, a spacecraft must decelerate from 13,000 mph in a matter of minutes and avoid these hazards or be able to protect its payload from the hazards as it lands. The Mars Exploration Rovers in 2003 will use the same proven airbag landing system as Pathfinder . Exploring Mars at the Suface Touching down accurately and safely isn't enough. After all, we wouldn't want to be stuck in just one place all the time since we never know what exciting discovery might be just around the next bend.
Rovers travel on the surface and can conduct close up studies of rocks and soils.
However, because the terrain is very rugged and there are no highways on Mars, roving range is limited. Aerial vehicles like airplanes and balloons are not impeded by the surface and can view much larger areas than rovers. Aerial vehicles also take images at higher resolutions than orbiters. For example, balloons or aerobots can circumnavigate Mars many times in a mission a few months long, producing more images that are of greater detail. But for many scientific questions, there is no substitute for bringing rocks back to Earth where they can be studied with an almost infinite variety of scientific techniques and complex, sophisticated instruments.