Sep 1 1979

From The Space Library

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Current revision

September 1-7 NASAs Pioneer 11 had encountered Saturn's bow shock (where the solar wind-a stream of electrified particles from the Sun-bent around Saturn's magnetic field), project manager Charles Hall reported from ARC, adding that solar storms during the past few days had played havoc with Pioneer's radio signals. Pioneer was not designed in 1967 to fly to Saturn; that mission arose only after a successful Jupiter flyby in 1974, calling at first for Pioneer to fly through Saturn's rings on what the Washington Post called a "kamikaze mission." The most dangerous moment of the 6.5-year 2-millionmile journey would be crossing September 1 through belts of unknown material beyond the ring systems around Saturn that had been visible from Earth, during which Pioneer would risk a potentially fatal collision.

Because of intense radiation (solar-particle interference was degrading the quality of Pioneer images), mission controllers at ARC had reduced the data transmission rate by half. Solar storms had occurred late in July and in mid August, and high-energy particles from the second storm had reached Saturn at the same time as slower particles from the first storm. Reducing the transmission rate would diminish effects of interference and also the quality of the images. Pioneer would enter the dangerous area outside the visible ring system at 10:44 a.m. EDT, but scientists would not know for 86 minutes whether it had survived (the time the signals needed to reach Earth). No one knew the composition of the 2 minute-wide third ring; if it consisted of boulders, Pioneer could probably get past it safely, but a ring of dust particles could riddle the fast-moving craft and end its career. Project scientist Dr. John Wolfe said chances for survival were half and half; pictures already transmitted by Pioneer were better than any yet taken from Earth telescopes.

The Washington Post reported September 1 that solar storms had increased the noise level in space to 10 times normal. In early afternoon, Pioneer had flown into a detectable magnetosphere, showing that Saturn was surrounded by a strong and sizable magnetic field estimated at 700 to 1,000 times the size of Earth's but roughly equivalent in strength. The New York Times reported September 2 that, as of 12:31 p.m. EDT September 1, Pioneer 11 had survived a hail of fine particles to get within 13,000 miles of Saturn for a 4-hour first encounter. During the approach, scientists were keeping watch on the plane of the visible rings through which Pioneer was descending: Dr. James Van Allen of the University of Iowa was first to note Pioneer's approach to "dangerous ring debris" in its path. However, the 10:30 a.m. crossing at a speed of 48,000 mph had taken only a second, though ARC had to wait until 2:02 p.m. to learn what had happened; when the radio signals continued past that moment without interruption, "a cheer went up." LaRC's Donald H. Humes said that Pioneer 11 might have had a close call, as the micrometeoroid detector had registered two impacts during the crossing: this was the limit of the instrument's counting; ability, and more impacts might actually have occurred. The impacts apparently caused no damage. The sharp rise in radiation encountered by Pioneer before it flew under the rings had fallen as abruptly as it rose, and by the time Pioneer was under the rings (closer to the planet) the radiation reading had disappeared completely. Dr. John Simpson of the University of Chicago said the rings were big enough to "scour out" nearly all the radiation trapped by the planet's magnetic field. (The New York Times quoted Dr. Van Allen as saying that the radiation count while Pioneer flew under the rings was even lower than while the spacecraft was sitting on its launching pad at Cape Canaveral, Fla., 6.5 years ago. Saturn's radiation, although it would be lethal to an unprotected human, posed no hazard to spacecraft, Van Allen added.)

After its close approach, Pioneer swung behind the planet, out of radio contact for 78 minutes, during which its on-board instruments recorded data for later transmission. Upon reemerging, the spacecraft had to cross the ring plane again on its outward journey; the outbound crossing came at 2:32 p.m., and controllers again had to wait for indication that the spacecraft was safe. At 3:55 p.m. the signals affirmed that Pioneer 11 had survived another crossing. This was the last predicted hazard for the spacecraft, which would continue past Saturn to the outer edge of Earth's solar system and enter interstellar space in 1993, by which time its radio would no longer be audible.

The safe passage meant that Pioneer could spend September 2 and 3 photographing Titan and measuring Saturn's rings from the dark side of the planet; also, that the two Voyager spacecraft following Pioneer could approach without fear of damage. In its two-hour passage, besides recording impacts, Pioneer had photographed a new ring previously undetected by Earth telescopes, called the F ring: scientists already knew of four rings called A through D (in order of discovery, not location), three of them bright enough to be seen from Earth telescopes, and a fifth (or E) ring had been suspected close to the A ring although Pioneer did not report it. Pioneer had also tracked what might be a new Saturn moon orbiting about 70,000 miles out; some thought it might be a known moon, Janus, but it was in a position indicating that it was an entirely new satellite that would bring Saturn's count to 11, one short of Jupiter's 12.

Pioneer 11 photos of Saturn showed dark blue and green stripes across it just below the north pole; the colors, not so vivid from Earth, probably came from sunlight scattering in upper-atmosphere clouds. Jet streams like those seen on Jupiter appeared to whirl around Saturn at speeds of 300 mph. Pioneer images could give a better idea of the dimensions and composition of the rings when compared with Earth-based photographs.

In the six years it had taken to reach Saturn, Pioneer 11 had reported a space-particle strike only once every two months or so, LaRC's Humes recalled. Once past Saturn, Pioneer was to photograph Titan from about 221,000 miles, record its temperature and atmospheric methane, and try to detect "life chemicals"; as no life was evident on Venus or Mars, this might be the last chance to find extraterrestrial biology in the Earth's solar system. First images of Titan had shown a fuzzy ball of light indicating a stratospheric layer of orange smog with blue streaks that might denote an atmosphere beneath the smog and clouds. The closest approach at 2:04 p.m. September 2 was 220,000 miles, and Pioneer had taken the first pictures at 260,000 miles. The pictures would be enhanced by computer before scientists could make detailed analysis. On September 3, Pioneer was already more than a million miles from Saturn, relaying no ill effects from the planet's rings or radiation belts.

Early on the morning of September 2, NASA reported loss of about two hours of Titan data from Pioneer transmissions to the Madrid station; ARC controllers did not know whether the loss occurred between spacecraft and station or between Spain and the control center about 40 miles south of San Francisco. A loss between spacecraft and station would be irretrievable: Pioneer had only the one opportunity for Titan observations after its close approach. It was not sending images at the time, but about 15 minutes of a CalTech infrared sensor's readings on Titan temperature were lost. The cause might have been electrical storms in space or on Earth, or passage of the signals close to the surface of the Sun on their way to the Earth, any of which would make it difficult or impossible for the Madrid station to lock on to the signals. A dramatic difference in day-night Titan temperatures would suggest a thin atmosphere; a thicker, warmer atmosphere might be more hospitable to life forms.

Project manager Charles Hall reported September 4 that the Titan data loss previously attributed to radiation disturbances was now thought to result from radio frequency interference by a Soviet satellite. Both Pioneer 11 and three USSR satellites had been operating on a frequency reserved for scientific use. The Soviet Union had twice agreed to shut down during Saturn encounter, and Hall said NASA had "no doubt the Soviets would have acted" if they had been asked; "we did not request them to turn off their satellite on Monday," U.S. scientists did not realize at first that signals from a recently launched Soviet satellite might interfere with reception of Pioneer data, being "between 100 and 1,000 times stronger than the signals we were receiving from our spacecraft." Aerospace Daily said September 6 that the three Soviet satellites that had stopped transmitting September 1-2 for the benefit of Pioneer's Saturn flyby were early-warning satellites designed to detect U.S. ballistic missile attacks on the Soviet Union. Cosmos 1024, 1109, and 1124 had suspended transmissions September 1 and 2 at NASA's request; however, no request was made regarding September 3, and Cosmos 1124 had operated as usual. Western analysts questioned, Aerospace Daily noted, "whether the U.S. would suspend communications with any of is early-warning satellites if the Soviets made such a request" during a planetary mission.

Pioneer project officials told a September 6 news briefing that the loss of Titan data was not the fault of the Soviet satellite, which was broadcasting before but not during the lost transmission. Poor data had resulted from the combination of solar storm and poor transmission from Madrid. Some data had in fact reached ARC, but was so "noisy" as to be useless. Scientists said the moon discovered by Pioneer was inside the orbit of Janus, previously the innermost known satellite of Saturn; the new ring (labeled G) was between the orbits of Rhea and Titan, beyond the rings visible to Earth telescopes. (NY Times, Sept 2179, 1; Sept 3/79, A-1; Sept 4/79, C-1; Sept 7/79, A-1; W Post, Sept 1/79, A-3; Sept 2/79, A-1; Sept 5/79, A-10; W Star, Sept 1/79, A-4; Sept 2179, A-1; Sept 7179, A-1; AID, Sept 6/79, 17; Today, Sept 1/79, 4A; Sept 2/79, IA)

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