The Mars Reconnaissance Orbiter is a robotic spacecraft currently orbiting Mars at an altitude roughly 300 kilometers above the surface of Mars. The initial orbit around Mars was highly elliptical, with a closest approach of 426 kilometers and a furthest distance of about 48000 kilometers. The essentially circular final orbit was attained through a long series of some 445 aerobraking passes.
The Mars Reconnaissance Orbiter holds numerous scientific instruments to analyze the atmosphere, climate, weather, and geology of Mars, and to search for water in the polar caps and underground. One of the principal purposes of the Mars Reconnaissance Orbiter is to prepare the path for future spacecraft by studying potential landing sites and Martian weather patterns.
The HiRISE camera is still relatively functional, although the spacecraft has been shut down for several months to try to diagnose and fix a control system problem. This camera is an 20 inch diameter reflecting telescope, which is the largest ever sent on a deep space mission. It can resolve objects about a foot across from the Mars Reconnaissance Orbiter altitude – about 3 times higher resolution than available for Earth on Google Maps. HiRISE was able to photograph the Phoenix lander as it parachuted to the Martian surface. HiRISE can produce stereo images from which can be extracted topographical information to an accuracy of 0.25 m.
The topic question involves the speed of the Mars Reconnaissance Orbiter. As always, it is necessary to ask to what the speed is relative. If one wants to know the speed relative to the Martian surface, the answer is fairly easy to obtain. The orbital velocity of the Mars Reconnaissance Orbiter is about 3.4 kilometers per second, or a bit over 2 miles per second. From published reports it is unclear if the Mars Reconnaissance Orbiter is orbiting Mars in the same direction as Mars rotates or not. This factor would change the velocity relative to the Martian surface by about 0.2 km/sec.
To determine the speed of the Mars Reconnaissance Orbiter with respect to any other celestial body is a bit more difficult. Velocities add as vectors, so that it is incorrect, for example, to simply add the Mars Reconnaissance Orbiter orbital velocity of 3,4 km/sec to Mars’ orbital velocity around the Sun of 27.1 km/sec to obtain a net velocity relative to the Sun, or even a net velocity relative to the ‘distant stars’, which is the criterion for calculating inertial effects.
