The boost portion of the Mariner mission consists of three phases: ascent into a circular parking orbit of approximately 115 miles, coast in the parking orbit to a pre-determined point in space, and burning out of the parking orbit to greater than escape speed.

The Atlas D/Agena B space booster will rise vertically and pitch over in the required direction determined by the exact time of launch. The vehicle will gain speed and altitude until a signal from the ground guidance system commands shutdown of the Atlas engines and separation of the Agena/Mariner from the Atlas. The Agena engine ignites after a short coast period and accelerates itself and the spacecraft into the parking orbit at a speed of 17,450 mph.

The Agena/Mariner will be traveling in a southeasterly direction over the Atlantic Ocean towards the coast of South Africa. Just before reaching Africa, at a point in space determined by the launch date, time of launch, and desired flight time to Venus, the Agena engine will re-ignite and accelerate the spacecraft to a speed of about 25,820 mph.

Shortly after the Agena engine shuts down, the Mariner spacecraft is separated from the Agena. This is "injection." The speed of the spacecraft exceeds the escape velocity at this altitude by 1215 mph and the spacecraft moves off in the hyperbolic orbit relative to earth. Because of the rapid change of altitude, the rate at which it moves around the earth decreases until it is traveling essentially in a straight line outward from earth. During the time from injection to escape, the radius vector from the earth's center to the spacecraft moves through an angle of about 145?o\.

At the same time it is moving out, the spacecraft is slowing down relative to earth because of earth's gravity. When it reaches a distance of about 600,000 miles, after about three days, and has essentially "escaped earth", the velocity will have decreased from the original 25,820 mph to 6550 mph. The time of the second Agena burn will have been chosen so that this velocity relative to earth is in a direction opposite to that of the earth in its orbit about the sun. Thus, the spacecraft will be moving about the sun 6550 mph slower than the earth's approximate 66,000 mph; that is, about 59,100 mph.

Because of the lower orbital velocity about the sun, the spacecraft will be moving too slowly to maintain a circular orbit against the sun's gravity. It will, therefore, start falling inward toward the orbital Venus. The combination of the inward motion and the circular motion around the sun produces an ecliptic orbit that will intersect the orbit of Venus some 100 days later.

About eight days after launch the accumulated tracking data will be used to compare the trajectories of the spacecraft with the trajectory necessary to provide the planned Venus encounter. The midcourse maneuver will depend on the different between these two trajectories.

Now Mariner will begin to curve in towards the sun and gradually increase its speed. Eventually, due to the inward curving path, Mariner's speed will exceed that of the earth and it will catch up and pass earth. Later, it will catch up with rapidly moving Venus, approaching the planet on its dark side at a speed of over 83,000 mph relative to the sun.

Entering the sphere of gravitational influence of the planet, Mariner's path will begin to be deflected due to its pull. Its speed will be increased even greater, reaching over 90,000 mph relative to the sun, as it passes Venus on its sunny side at a distance of about 10,000 miles from the surface. In addition, Mariner's path will be bent about 36 degrees in traveling past the planet.

At about 65 minutes before closest approach, or at a distance of 18,600 miles from the planet's surface, the planetary experiments will begin to scan Venus. They will operate for 30 minutes, after which the mission is officially over.

The path of the spacecraft in the vicinity of Venus has been designed so that Venus will not block the spacecrafts' view of either the sun or earth. This is necessary to insure continuous communication with earth and proper functioning of the sun and earth sensors. The latter provide reference directions for attitude control of the spacecraft. The communication distance at the time of arrival is about 36 million miles.

After leaving the sphere of influence of Venus, the spacecraft will have even greater speed than when it entered. In essence, it will experience an increase in energy and speed due to the bending of its course by Venus. This phenomena is similar to that sometimes experienced by comets which travel too close to the planet Jupiter. The energy increase is sometimes sufficient to cause the comet to escape the solar system. Such will not be the case for Mariner, however,

Designing an interplanetary trajectory is a complex task that taxes the capabilities of high-speed computers. The trajectory engineer faces a task complicated by the interactions of the motion of the earth about the sun, the motion of Venus, the spin of the earth, and the effect of gravitational fields of the earth, sun, moon, Venus, Jupiter and even the pressure of the sun's radiation, on the path of the spacecraft.

The trajectory designer, therefore, must calculate a trajectory from minute to minute for that portion of each day during the launch opportunity that launch could occur. He must keep his trajectory with range safety limits (the early portions of the launch must be over water, not land masses) and he must keep the trajectory within range of the tracking stations.

Meshing all these factors into a successful trajectory, spanning millions of miles and nearly four months in time, is a formidable task.

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