2573 - 1991 29c Space Exploration: Jupiter
US #2573 – from the 1991 Space Exploration Issue

On April 5, 1973 (some sources cite April 6 UTC (Coordinated Universal Time), the Pioneer 11 space probe launched from Cape Canaveral, Florida.

The Pioneer program was a series of unmanned missions that explored the planets before leaving the solar system. Pioneer 10 and 11 were both approved in February 1969 and were the first probes to be designed to explore the outer solar system.

The mission of Pioneer 11 was to collect data about the atmosphere of Jupiter and Saturn, as well as to further man’s understanding of solar winds and cosmic rays. The spacecraft was just over a foot in diameter weighed 571 pounds, and had six 30-inch panels projecting from it. It was powered by a radioisotope thermoelectric generator, which converted heat from the decay of radioactive material into electricity. Its instruments included analyzers for detecting solar wind particles and telescopes for collecting data about cosmic ray and atomic particles.

1556 - 1975 10c Pioneer 10 and Jupiter
US #1556 pictures Pioneer 10 (which was similar to Pioneer 11) passing in front of Jupiter.

Sensors detected meteoroids and hydrogen and helium levels. Another telescope relayed images of Jupiter and Saturn back to earth. The computer on board was primitive by modern standards. It could store about 6,000 bytes of information. Many of today’s smartphones hold billions of bytes of data. Like Pioneer 10 before it, Pioneer 11 carried a golden plaque picturing a man and a woman along with other information about our origins and the probe, in case it was ever found by extraterrestrials.

305035 - First Day Cover
US #1556 – Classic First Day Cover

Pioneer 11 was launched toward Jupiter on April 6, 1973, about 13 months after Pioneer 10 had launched. Initially, Pioneer 11 was built as a backup to Pioneer 10. Once Pioneer 10 successfully completed its primary mission of observing Jupiter, NASA came up with a new plan for Pioneer 11. They would adjust the probe’s trajectory so that it could use Jupiter’s gravity to slingshot around the planet and head toward Saturn.

The adjustment worked and in November 1974, Pioneer 11 encountered Jupiter and began sending back data about the planet. It was closest to Jupiter on December 2, when it passed over 26,000 miles above the clouds. A year earlier, Pioneer 10 had passed within 81,000 miles. The space probe recorded the first detailed images of the Great Red Spot and the planet’s polar regions. After two months, Pioneer 11 was on its way toward Saturn.

The probe became the first to encounter Saturn in September 1979. The controllers on earth decided to send the probe through the planet’s ring to test the route for the Voyager spacecraft, which were also approaching Saturn. Pioneer 11 made it through without damage, paving the way for future missions.

While studying Saturn, the space probe discovered an unknown moon and an additional ring. It barely missed colliding with another moon when it flew within 2,500 miles of it – a near miss in space terms.

5073 - 2016 First-Class Forever Stamp - Views of Our Planets: Jupiter
US #5073 shows a near-infrared image of Jupiter captured by the Hubble telescope in 2004.

On September 29, 1995, the research center connected with the mission released a statement that said in part, “After nearly 22 years of exploration out to the farthest reaches of the Solar System, one of the most durable and productive space missions in history will come to a close.”

5074 - 2016 First-Class Forever Stamp - Views of Our Planets: Saturn
US #5074 shows an image of Saturn captured by Hubble in 1998.

Pioneer 11 continued to travel into space. In July 2015, it was estimated to be over eight billion miles from earth. In the words of a NASA administrator, the space probe was “a venerable explorer that has taught us a great deal about the Solar System and… about our own innate drive to learn…”

Click here for more info and images from the Pioneer 11 mission from NASA.

Click here to see what else happened on This Day in History.

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    1. A very informative, historical account of mankind’s attempt at space exploration. The amount of knowledge culled from the information sent by the Pioneer 10 and 11 were truely valuable.
      Hats off to NASA!
      Darius D. Dubash

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