If the Pioneer 11 spacecraft now speeding toward Saturn is targeted to fly inside that planet's rings, it could be heading toward disaster, warned Bradford A. Smith, a University of Arizona associate professor of planetary sciences, at a press briefing during the Jupiter Science Symposium at NASA Ames Research Center. Particles of ice and rock probably exist in a faint inner ring where Pioneer 11 may fly in 1979 and could deliver such a barrage of blows that the spacecraft would not survive.
One current Pioneer 11 targeting strategy calls for the spacecraft to pass between Saturn's three bright visible rings and the planet itself. Though this 12,872-km area appears to be empty, observations at the University of Arizona, Paris Observatory, and New Mexico State University indicate that a fourth ring exists. It is difficult to measure because of glare from the brighter rings and from Saturn itself. If Pioneer 11 flies between the rings, it may be aimed at a 1609-km gap, called the Guerin division after its discoverer Pierre Guerin, between this inner ring and the bright rings. If it is, according to Smith, it will enter a region ‘about which very little is known and which could prove to be extremely hazardous or even fatal. It is very unlikely that the gap is completely free of particles, and it is possible that it is merely a region of lesser ring density.’ If the particles are all large in comparison to the size of the spacecraft, then its chances for survival would be good. If they are small, which Smith says is more likely, then the spacecraft might be bombarded by as many as 500 impacts for each square meter of surface area. According to Smith, it is highly doubtful whether Pioneer 11 could ever survive such a barrage, in which case any planned postencounter science measurements might be lost. Although he agreed that there is important and unique science to be learned by approaching Saturn so closely, he said that the possible advantages of such a kamikaze mission should be carefully weighed.