Nov 2 1974
From The Space Library
2 November-3 January 1975: NASA'S Pioneer 11 interplanetary probe, launched 5 April 1973, flew past Jupiter three times closer than Pioneer 10 had a year earlier and sent hack the first polar images of the planet. The spacecraft then headed on toward a September 1979 encounter with Saturn.
Pioneer 10, launched 2 March 1972, had been the first spacecraft to reach Jupiter-3 Nov. 1973-9 Jan. 1974. The success of that mission permitted NASA scientists to attempt a somewhat riskier approach with the backup Pioneer 11, a clockwise trajectory by the south polar region and then straight up through the intense inner radiation belt by the equator and back out over the north pole. Although the spacecraft went within 42 000 km of the planet at 12:22 am EST 3 Dec., its peak speed of 171 000 km per hr and the trajectory's high latitude prevented serious radiation damage.
The Jupiter mission of Pioneer 11 was to map the magnetic field; measure distributions of high-energy electrons and protons in the radiation belts and look for auroras; find a basis for interpreting radio emissions from Jupiter; detect and measure the bow shock and magnetospheric boundary and their interactions with the solar wind; verify the thermal balance and determine temperature distribution of the outer atmosphere; measure the hydrogen-helium ratio in the atmosphere; measure the structure of the ionosphere and atmosphere; measure the brightness, color, and polarization of Jupiter's reflected light; perform two-color visible-light imaging; and increase the accuracy of orbit predictions and masses of Jupiter and its moons.
The interplanetary mission was to map the interplanetary magnetic field; study the radial gradient of the solar wind and its fluctuations and structure; study the radial and transverse gradients and arrival directions of high-energy charged particles (solar and galactic cosmic rays) ; investigate relationships between the solar wind, magnetic field, and cosmic rays; search for the boundary and extent of the heliosphere (solar atmosphere) ; determine the density of neutral hydrogen, and determine the properties of interplanetary dust.
Hurtling toward and then away from Jupiter, Pioneer 11 sent data back to the earth at 2048 bits per sec throughout most of the month before and after encounter. Picture-taking of Jupiter began 18 Nov., at 14 800 000 km out from the planet. From 25 Nov. through 8 Dec., imaging and polarimetry observations were made 23 hrs a day.
On 26 Nov. Pioneer 11 crossed the detached bow shock wave (7 772 000 km out) at the juncture of the solar wind and Jupiter's magnetic field and entered the magnetosphere (6 923 000 km out) . In five hours the spacecraft passed undamaged through this first field of intense radiation. On 1 Dec. it crossed the orbit of Callisto-outermost of the five inner moons-and began returning its best pictures of Jupiter. On 2 Dec. Pioneer 11 crossed the orbits of Jupiter's four closest moons and entered the inner radiation belt. During this last leg of the approach to the south polar region, the spacecraft continued the imaging and sensing of the planet and moons and made a closeup, two-hour scan that included the red spot portion of the surface.
Less than 12 hrs before the spacecraft passed behind Jupiter, an in-tense peak of electron radiation built up a 4000-v electric charge within the spacecraft. The charge created an electric arc that signaled heater lines to turn on-drawing spacecraft power and weakening the signal to earth. In about three hours the problem was identified and override commands from earth switched the heaters back off.
At midnight on 3 Dec. Pioneer 11 entered Jupiter radio occultation-a blackout that lasted 42 min while the spacecraft swung behind the 142 744-km-dia planet-moving from the south pole straight up over the north pole. Since Pioneer 11 radio signals required 46 min to cross the 737 million km separating the earth and Jupiter at the time of encounter, the spacecraft had already circumscribed the planet before the earth lost contact with its signal. During occultation the spacecraft passed through the most intense radiation (up to 40 000 times as much in the earth's radiation belts) , reached its greatest speed (171 000 km per hr) and came closest to the planet (within 42 000 km of the cloudtops) . At 1:24 am EST-62 min after periapsis-the signal reached earth, indicating that the spacecraft had emerged from behind the planet and that all systems had survived the most critical phase of the mission. Temporary failures had lost some data, but no permanent damage was sustained.
Following the Jupiter encounter, Dr. James C. Fletcher, NASA Administrator, renamed the spacecraft Pioneer-Saturn as it headed toward the planet Saturn. The exact target point had not been selected but scientists considered an interesting possibility was to target the spacecraft to pass between Saturn and its rings and then close by Titan, one of Saturn's moons and a possible location of life. The five-year, 2.4-billion-km trip to Saturn would carry Pioneer-Saturn about 160 000 000 km above the orbit planes of most planets.
Pioneer 11 was the first spacecraft to use the gravity of an outer planet to throw it to another planet farther out, came the closest to Jupiter, was the fastest spacecraft yet launched, and was the first spacecraft to photo-graph Jupiter's polar regions. Pioneer-Saturn was expected to become the first spacecraft to pass far from the ecliptic plane and the first to reach Saturn.
Pioneer 11 confirmed, but somewhat modified, the findings of Pioneer 10. The model of a relatively flat, disclike magnetosphere was challenged by new evidence suggesting a large, blunt magnetosphere of irregular and fluctuating shape. Temperature measurements of about 125 K at cloud-tops supported Pioneer 10 readings, which were lower than those indicated by earth-based observations. Imaging revealed a decrease in the banding structure of the planet with increased latitude and a more mottled structure across the poles. (NASA Releases 74-292, 74-300; Hall, Opp, et al., Science, 2 May 75, 445-77; NASA Pioneer Mgr, interview, 12 Nov 75; NASA press conference transcript, 19 Nov 74; Perlman, SF Chron, 3 Dec 74; O'Toole, W Post, 3 Dec 74, Al; Sullivan, NYT, 4 Dec 74, 27; Mead, San Jose Mercury, 5 Dec 74; NYT, 9 Dec 74)
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