Traveling to Mars

Traveling to Mars

The first expedition to Mars did not wish to land in too rough a place. The spacecraft might have tipped over and crashed, or at the least its mechanical arm, intended to acquire Martian soil samples, might have become wedged or been left waving helplessly a meter too high above the surface. Likewise, we did not want to land in places that were too soft. If the spacecraft’s three landing pods had sunk deeply into a loosely packed soil, various undesirable consequences would have followed, including immobilization of the sample arm. But the humans did not want to land in a place that was too hard either – had they landed in a vitreous lave field, for example, with no powdery surface material, the mechanical arm would have been unable to acquire the samples vital to the projected chemistry and biology experiments.

The best photographs then available of Mars – from the Mariner 9 orbiter – showed features no smaller than 90 meters (100 yards) across. The Viking orbiter pictures improved this figure only slightly. Boulders one meter (three feet) in size were entirely invisible in such photographs, and could have had disastrous consequences for the Viking lander. Likewise, a deep, soft powder might have been undetectable photographically. Fortunately, there was a technique that enabled the scientists to determine the roughness or softness of a candidate landing site: radar. A very rough place would scatter radar from Earth off to the sides of the beam and therefore appear poorly reflective because of the many interstices between individual sand grains.

Safe harbors are, by any large, dull. Image : Brown Mosaic by Elena

While the scientists were unable to distinguish between rough places and soft places, they did not need to make such distinctions for landing site selection. Both, they knew, were dangerous. Preliminary radar surveys suggested that as much as a quarter to a third of the surface area of Mars might be radar-dark, and therefore dangerous for Viking. But not all of Mars can be viewed by Earth-based radar, only a swath between 25 degrees North and about 25 degrees South. The Viking orbiter carried no radar system of its own to map the surface.

There were many constraints – perhaps, the scientists feared, too many. The landing sites had to be not too high, too windy, too hard, too soft, too rough or too close to the pole. It was remarkable that there were any places at all on Mars that simultaneously satisfied all safety criteria. But it was also clear that the search for safe harbors had led the scientists to landing sites that were, by any large, dull.