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In fact, some astronomers have long suspected that it was a catastrophic event, perhaps a collision with an earth-size object, that toppled Uranus on its side (see chart); it spins with its rotational axis practically perpendicular to those of most of the other planets. The spacecraft raised even more questions about Uranus when it discovered that the planet has a magnetic field about as strong as earth's but topsy-turvy by terrestrial standards, with the north magnetic pole displaced by 55 degrees from the south geographic pole. The odd arrangement led scientists to speculate that Voyager had caught the magnetic field in the process of reversing its polarity, a phenomenon that has occurred often on earth, most recently about 700,000 years ago.
The magnetic field also helped scientists calculate the length of a Uranian day. By detecting the changing radio emissions caused by the interaction of the field with the solar wind as the planet turns on its axis, the spacecraft established that Uranus rotates once approximately every 17 hours. The technique, explained Physicist James Warwick, can be likened to standing on a lawn and "feeling the water drops every time a sprinkler goes around." By tracking clouds in the atmosphere, Voyager discovered high-altitude winds moving around the planet at 220 m.p.h., more than twice as fast as they travel above the earth.
Other conclusions from Voyager's findings, according to Project Scientist Edward Stone: Uranus has a core consisting of rock and liquid, is covered by a deep ocean of water laced with dissolved ammonia, and is wrapped in a 5,000- mile-thick atmosphere consisting largely of hydrogen, with 10% to 16% helium and a scattering of methane and other gases.
At week's end, as it looked back and saw Uranus receding in the distance, Voyager seemed to be in perfect health. Scientists at J.P.L. are confident that it will stay that way not only through its encounter with Neptune in August 1989 but perhaps until 2010, when it will be far out of the solar system. Says Richard Laeser, the Voyager project manager: "I have no desire to do much else except to ride this thing all the way out into interstellar space."