1. MGS reached Mars in September 1997. Its primary mission lasting one Mars year began in March 1999. The first extended mission began in February 2001 and covered an additional Mars year. Since that time the mission has been extended to cover a third and currently a fourth martian year. Because the MOC narrow-angle camera has a field of view of only 3 km and the combination of onboard computer space and downlink capabilities limit the number of images acquired during a given Mars year the camera has only imaged about 5.2% of the martian surface through October 2006.
2. Mars Observer camera
3. Mars Orbiter Camera geodesy campaign
4. A simple estimate of the impact parameters can be made by calculating the energy of the impact. Such calculations are often heavy on assumptions. Assuming most of the energy goes into excavating the observed pits the energy is proportional to the work done against martian gravity lifting the volume of rock up out of the hole and piling it up adjacent to the hole. For our calculations we assumed that the craters are roughly hemispherical that the impact velocity was equivalent to the escape velocity (∼5 km/s) and that the target density was 3 g/cm 3 . Forsuch assumptions about 260 m 3 with a mass of 785 000 kg would be excavated to form a crater with a 10-m diameter. It would take about 13 MJ to lift this mass out of the crater and for a density of 3.5 g/cm 3 (typical of chondrite meteorites) the object hitting the ground was probably about 10 cm across with a mass around 1 kg. The largest crater seen (148-m diameter) was probably made by an object a couple of meters across with a mass close to 50 000 kg. The smallest craters seen about 2 m across are probably close to the smallest that can be formed given that the object would be so small that it would lose an appreciable portion of its total mass owing to ablation during passage through the atmosphere and it would also experience significant deceleration.
5. For convenience and brevity we use the term “blast zone” to cover all of the atmospheric effects associated with the impact including the interaction with the surface of the shock wave accompanying the penetration of the atmosphere by the meteoroid the centrifugal shock wave generated by the actual impact the centrifugal overpressure wave trailing the shock wave the centripetal back-flow of atmosphere after the initial outflow turbulence created by these flows and airflow and turbulence induced by ejecta moving through the atmosphere.