PASADENA, California--Venus, the cloud-shrouded mystery planet, may yield some of her secrets in mid-October to Mariner V experimenters at Caltech's Jet Propulsion Laboratory and several allied installations.
Scientists and engineers involved in the Mariner Venus Project of the National Aeronautics and Space Administration hope instruments aboard the spacecraft will record more exact data on the sister planet's atmosphere, ionosphere, temperatures and perhaps even surface qualities. Several of the project's seven planned experiments have been specially designed to probe Venus close-up.
To measure the density of the Venusian atmosphere an S-band radio occultation experiment will be performed by a group of scientists headed by Dr. Arvydas Kliore of JPL.
They will study changes in the Mariner's radio signal before and after the spacecraft passes behind the planet. Mariner V is scheduled to fly within 2,500 miles of Venus on October 19. The occultation experiment is similar to one performed by Mariner IV in its 1964-5 flight past Mars.
In the case of Mars, the occultation (radio-signal disruption) studies showed the Martian atmosphere to be only one-tenth as thick as previously assumed.
Since then, radar-beam experiments conducted at JPL's Goldstone, California, tracking station indicate that Venus' surface is not smooth and may have similar promontories and depressions.
With extreme luck, the close-in measurements of the Venus atmospheric density might be used to determine something about the planet's surface. However, the expected high density of the Venus atmosphere--probably 40 miles deep beneath the opaque cloud tops--may nullify efforts to make such deductions on this flight.
Dr. Kliore believes, though, that if the cloud mixture is uniform, it may be possible to learn the composition, pressure, density and lower elevation temperatures of the atmosphere. At least, his group hopes to obtain basic data for a density-altitude profile of the Venusian atmosphere and ionosphere.
The Venus atmosphere has been estimated at up to 8,000 miles high, with a pressure perhaps five to several hundred times heavier than Earth's atmosphere.
Temperature and components of the planet's upper atmosphere could be determined by another experiment using three ultraviolet photometers. These instruments are designed specifically to detect hydrogen and oxygen atoms.
A scientific team led by Dr. Charles A. Barth of the University of Colorado will conduct this test. By measuring the amount of ultraviolet light radiated by the upper atmosphere, these photometers should supply information on atmospheric components from several thousand miles altitude down to the top of the clouds.
Carbon dioxide, nitrogen, neon, helium and perhaps argon are believed present in the Venus atmosphere. The chances of finding hydrogen and oxygen are rated good.
From the 1962 flight of Mariner II, scientists calculated Venus cloud-top temperatures at from -30 to -70 degrees Fahrenheit. At the base of the cloud layer, heat may be higher than 200 degrees. Temperatures on the planet's surface were estimated as high as 800 degrees. However, these readings were obtained from a spacecraft which flew no closer than within 21,600 miles of Venus.
A dual-frequency radio occulations experiment to study the planet's ionosphere will be conducted by Prof. Von R. Eshleman at Stanford University. Signals of two different frequencies will be beamed from the 150-foot diameter antenna at Palo Alto, California. The effect of both the Venus ionosphere and atmosphere on the signals received by Mariner then will be studied. The changes observed could reveal the altitude, thickness and daynight variation of the ionosphere, the electrified layer high in the planet's atmosphere.
Dr. John D. Anderson of JPL will compare the NASA-JPL Deep Space Network tracking data with that of the 1962 Mariner mission to refine the estimated masses of Venus and the Moon and to determine more accurate orbits for Earth and Venus.
Throughout the flight, the network will receive data indicating very precisely the position of the spacecraft relative to the Earth, and in October it will observe the change of path caused by Venus' attraction.
Venus, roughly 95 percent the size of Earth, has a mass about 82 per cent of ours. Its distance at Mariner V encounter on October 19 will be 49.5 million miles.
In sum, the scientific findings of Mariner V could contribute some leading answers--not final, of course, but helpful--to the interesting questions:
How much is our sister planet really like Earth?
What chance is there of finding life on Venus via robot landers?
And, ultimately, would there be any point in sending a manned spacecraft to Venus in the distant future?
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