Page:Concepts for detection of extraterrestrial life.djvu/59

 CHAPTER XII

The first spacecraft destined for Mars will be Mariner C, a NASA planetary flyby scheduled for launching in 1964.

Mars and Earth both orbit the' Sun in the same direction, but not at the same speed or distance. The mean distance of Earth from the Sun is 93 million miles, while the mean distance of Mars from the Sun is 141 million miles. Earth makes one revolution around the Sun each 365¼ days, while Mars revolves around the Sun once every 687 earth days.

The maximum distance between Earth and Mars is about 247 million miles, which occurs at the aphelion conjunction. However, when Mars is at opposition, the two planets (Earth and Mars) can be as close together as 34.5 million miles or as far away from each other as 63 million miles. Oppositions are usually considered in planning missions to Mars. It is emphasized that neither spacecraft launch nor encounter would occur exactly at these opposition distances because the planets are in orbital motion at different velocities, as was already mentioned. From a search-for-life point of view, it is important to launch so that arrival coincides with those times when the wave of darkening on Mars is most pronounced.

Mariner C is not intended to land on Mars, but to fly past it at a distance of about 15,000 km. It will carry instrumentation to obtain data on interplanetary dust and plasma and to take television photographs of Mars. In addition, experiments are to be aboard to obtain data on the magnetic field and cosmic rays. The final dimensions of Mariner C will be similar to Mariner II which flew past Venus on December 14, 1962.

The hexagonal framework of Mariner II housed a liquid-fuel rocket motor for trajectory correction. It had six modules or compartments containing the attitude control system, electronic circuitry for the scientific experiments, power supply, battery and charger, data encoder, and command subsystem for receiving and obeying signals from Earth, digital computer and sequencer, and radio transmitter and receiver. 49