Man's first chance to obtain information from another planet will come on December 14 when the Mariner II spacecraft passes approximately 21,000 miles from Venus.

Mariner's radiometers will pierce the cloud cover to determine surface temperature and temperatures in the atmosphere. Instruments will determine the strength of the magnetic field and nature of the radiation belts. The entire spacecraft will measure the strength of the gravitational field as it speeds and slows on its curving path near Venus.

The 447-pound spacecraft was launched by the National Aeronautics and Space Administration on August 27, 1962, at 1:53 a.m. from the Atlantic Missile Range, Cape Canaveral, Florida. It was built, and is now being tracked, by the California Institute of Technology Jet Propulsion Laboratory. The launch vehicle was the Atlas-Agena B.

When the Mariner arrives at Venus it will have traveled 182,000,000 miles during its 109-day journey through space. During its long cruise, that extended almost halfway around the Sun, the spacecraft set a long distance communication record of 36,000,000 miles, and performed the first successful guidance maneuver in space.

The Mariner II carries six scientific experiments. Four of these, turned on by ground command two days after launch, are: a magnetometer, an ion chamber and particle flux detector, a solar plasma detector, and a cosmic dust counter. They have been making invaluable measurements during Mariner's curving trip towards Venus and when the spacecraft arrives will measure magnetic fields, radiations and dust particles around the planet.

Two other experiments--a microwave radiometer and an infrared radiometer--will scan the surface and the atmosphere of Venus for 42 minutes as Mariner rushes by.

All telemetry information gathered during Mariner's voyage is transmitted by Mariner to a network of ground receiving stations called the Deep Space Instrumentation Facility (DSIF) which are located in Goldstone, California, Woomera, Australia, and Johannesburg, South Africa.

Mariner was launched in a way that would cause it to fall inward toward the Sun. This was accomplished by timing the injection so that the spacecraft would leave earth in a direction opposite from that of the Earth in its orbit around the Sun. Since Mariner's speed around the Sun was less than that of the Earth, it could not maintain a circular orbit like Earth and the Sun's gravity caused it to be drawn inward so that it would eventually intercept the trajectory of Venus.

By nine days after launch, DSIF tracking data processed at JPL's Space Flight Operations Center in Pasadena, showed that Mariner would arrive at a rendezvous ahead of Venus, missing the planet by 233,000 miles. This launch dispersion was well within the correction capability of the 50-pound-thrust rocket motor aboard the spacecraft. During the midcourse maneuver, the motor was fired for 27 seconds affecting a slight decrease in velocity. This guidance correction will bring Mariner to a point 21,000 miles from the planet. This point is well within the original target area, a pie-shaped region extending between 8000 to 40,000 miles from Venus.

Before Mariner passes Venus a sequence of events will begin.

The first of these will be the activation of a stored command in the spacecraft's central computing and sequencing system that turns on the radiometer scan device. If for some reason this command is not initiated by the spacecraft, it will be sent by the DSIF Goldstone station.

The radiometers, located on the hexagonal deck of the spacecraft, are 20 inches in diameter and five inches deep. They are mounted on a swivel and are driven by an electric motor in a 120 degree scanning motion. During the pass by Venus, the microwave and infrared energy will be collected and transmitted to Earth.

Prior to activation of the radiometers, the Mariner was in a "cruise mode." In this mode, it was continuously telemetering the first 20 seconds of information provided by its four interplanetary scientific instruments, and then 16 seconds of engineering data. Engineering data is concerned with the conditions aboard the spacecraft and include temperatures, pressures, voltages and angular positions.

During the fly-by, the data format changes from "cruise mode" to "encounter mode" and Mariner devotes itself exclusively to gathering and sending scientific data.

At the time that the radiometer's scan mechanism is turned on, Mariner will be approaching the planet from the dark side and moving in a downward direction. As seen from Venus, the spacecraft will be moving in a direction to the right and below the Sun.

As Mariner cruises past Venus its solar panels will remain locked on the Sun to obtain electrical power, as they did throughout the long mission. The radiometers point in a direction perpendicular to the roll axis of the spacecraft and move in a nodding motion across the surface of Venus at a rate of one-tenth of a degree per minute. As Mariner passes Venus, the radiometers will first scan the dark side and then the sunlit side.

This planetary scanning period will last for 42 minutes. During this time, the findings of all sox scientific experiments will be transmitted to the Woomera and Goldstone DSIF stations.

At 66 minutes before the point of closest approach, or 10:55 a.m., December 14, Mariner will be 25,262 miles from Venus. At that time its velocity will have increased to approximately 87,000 mph due to the gravitational pull of the planet. At this time the radiometers should detect the planet's surface for the first time.

At 44 minutes before the point of closest approach, or 11:17 a.m., Mariner will pass the planet's terminator, or dividing line between light and darkness. It will still be moving downward and picking up speed.

Drawn by the gravitational field of Venus the spacecraft continues to accelerate. By 11:37 a.m. the scanning period ends as Venus moves out of sight of the radiometers. At that point in time, Mariner will be going approximately 87,000 mph. Venus will be approximately 21,700 miles away while the Earth is about 36,000,000 miles away.

Twenty-three minutes later, at 12:01 p.m., Mariner will reach the position of closest approach, approximately 21,000 miles from Venus. It will be traveling approximately 88,400 mph.

The gravitational attraction of Venus will have increased Mariner's velocity by 1400 mph in one hour. As the spacecraft starts moving away from Venus, gravity reverses its effect and starts slowing the spacecraft down. In addition to changing the speed of the spacecraft, the gravitational field also will bend Mariner's trajectory by about 25 degrees during encounter.

After closest approach Mariner will be instructed to turn off its radiometers and return to the cruise mode. When the command is obeyed the spacecraft will resume the sending of engineering data and will continue to take measurements with its interplanetary instruments.

It will continue in this mode until the mission is completed.

On December 27, it will reach its closest point to the Sun, 65,539,000 miles. At this time, its velocity will be approximately 85,300 mph. It will be 2,700,000 miles from Venus and Mariner then will be 44,213,000 miles from Earth in a heliocentric orbit around the Sun.

Uncertainties in Mariner's trajectory resulted from: the effect of solar pressure, the mass and gravitational fields of the Earth and Venus, the exact location of ground tracking stations and the astronomical unit.

Refinements in these uncertainties will be achieved by analysis of the tracking and doppler data collected during Mariner's trip and during the encounter phase when Mariner's trajectory is perturbed by Venus gravity.

The doppler effect is a principle of physics in which the frequency of radio waves appear to increase when a transmitter and receiver are approaching each other, and to decrease when they are moving apart. The speed of Mariner is determined by analysis of the frequency of its signals.

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