Science Guidelines

The most extensive data that the Team used to choose a landing site are images of the south polar region from the Viking and Mariner 9 spacecraft. Because they are low-magnification images taken from orbit, they were used mostly to rule out unsuitable regions. For example, areas with large obstacles (cliffs, craters, etc.) or areas with unexplained colorations were ruled out. Very recent data from the Mars Orbiter Laser Altimeter (MOLA) and the Mars Orbiter Camera (MOC) were very valuable in understanding the slopes and textures of different areas. These instruments are aboard the Mars Global Surveyor spacecraft, currently at Mars.

A major science goal is that the lander land on the Polar Layered Deposits, a geologic unit that contains fine-scale layering that may be a record of climate change. Accordingly, places where the layered material is absent, such as surfaces consisting of older, underlying terrain or slope debris, were ruled out.

Engineering Guidelines

Trajectory.
Where is the spacecraft capable of going?
One limitation is that the landing site location must allow the spacecraft to enter the atmosphere at a suitable angle. If the spacecraft enters the atmosphere too quickly, to much frictional heat will be generated and it will burn up. If the trajectory is too shallow, the spacecraft will skip out of the atmopsphere and miss the surface. Accordingly, the landing site was limited to regions equatorward of 78 degrees south. Because Mars rotates on its axis relatively fast (24.6 hours), any longitude on Mars can be reached by timing the spacecraft's arrival.

Safety.
Where is the spacecraft capable of landing and surviving?
The Mars Surveyor Operations Project (MSOP) at the Jet Propulsion Laboratory identified the following safety constraints that guided the selection of the landing site:

• Elevation - must be < 6000 m above "sea level" on Mars (6 mb atmospheric pressure)
• Slopes - must be < 10 degrees
• Rocks - rock coverage < 15% of surface; spacecraft clearance is 35 cm
• Topography - away from hills, craters, cliffs
• Temperature - above the frost point of water and carbon dioxide

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Targeting.
Where will the spacecraft actually end up?
The spacecraft's trajectory and time of arrival will be changed by firing on-board thrusters during the journey to Mars. The primary maneuver was on September 1, with further adjustments being made as the spacecraft reaches Mars. The accuracy in these maneuvers results in a spread of possible landing sites around the aim point, called an error ellipse (about 10 km by 200 km, see right). In practical terms, this means that the landing site can be chosen within a particular valley or between two craters (above), but not next to a particular rock.