THE PATH OF THE PHOTOGRAPHERS

Meanwhile we had also discussed among ourselves and with trajectory experts at the Jet Propulsion Laboratory the question of what regions of Mars to photograph.] We knew that if everything functioned perfectly we would at best be able to photograph about 1 percent of the planet's entire surface. We concluded that the best scan paths were those that crossed the largest number of light and dark regions. Naturally we were also anxious to photograph regions in which canal-like markings had most consistently been reported. [Finally, we wanted the camera to view the side, of the planet including Syrtis Major in at list some of its pictures.

The needs of the television experiment were not the only ones, however, that had to be considered in selecting Mariner IV’s flight path. For example the spacecraft could not be allowed to pass in the shadow of Mars or it would lose its fix on the sun; it could not pass above Mars or it would lose its fix on Canopus, the star that would be used to control the orientation of Mariner IV around its axis. [In addition, the flight path had to carry the spacecraft behind Mars so that its radio would be blacked out in the occultation experiment that would provide information about the density of the Martian atmosphere.

One final requirement, however, placed such a restriction on the flight path that in the end it proved impossible to have the camera pointing anywhere near Syrtis Major. This was the requirement that California be facing Mars at the time of encounter, so that the powerful transmitter at the Goldstone tracking station near Barstow, Calif., would be in a position to send last-minute commands to the spacecraft if that proved necessary. The desired time of encounter was to be achieved by making an appropriate adjustment in the spacecraft's trajectory at the time of the mid-course maneuver, which actually took place on the eighth day of flight. The maneuver could adjust the encounter time to any desired value over a period of several days, but because the earth and Mars rotate on their axes at nearly the same rate, it was impossible to delay or accelerate the encounter sufficiently during the favorable 1965 launch period for Syrtis Major to be facing Mariner IV’s camera at the same time that Goldstone was facing Mars.

Date: 2015-04-20; view: 1046