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NASA's large Deep Space Network radio receiving dish at Goldstone. California riASA/Jet Propulsion Laboratory

NASA's large Deep Space Network radio receiving dish at Goldstone. California riASA/Jet Propulsion Laboratory

The timing of the encounter placed the greatest burden on the Canlw and Goldstone sites. There, two 112-foot antennae were arrayed^ the 210-foot dish, and receptivity was increased by about 56 At Canberra, the receiving site for Voyager's data from Uranus at clo encounter, a further enhancement was made. The Australian govern^ linked its 210-foot |64-meter) Parkes radio telescope with the three^u array of NASA's Deep Space Network at Canberra to form a combine antenna with the reception power of a 400-foot (100-meter) dish.

77 ~ In addition to the chronic ailments of its remaining radio receiver its

DU11 S eye scan piatform an(j its computer memory, and in addition to the grea┬╗ distance that affected the strength of the radio signal from Vbyager2aus the time for instructions to reach the spacecraft, Voyager 2 faced a specif challenge at Uranus because of the planet's orientation.

The two Voyager probes had previously encountered Jupiter and Saturn planets that, like Earth, travel around the Sun with their rotational axa nearly perpendicular to their orbital planes. Because they travel arounc the Sun "standing up," their equators lie nearly in their orbital planes All the rings and a great majority of the moons in the solar system revoke around their planets' equators. So the rings and most of the moons oi Jupiter and Saturn lie nearly in the plane of revolution for that planet which is extremely close to the orbital plane of the Earth and hence essentially the plane formed by flight of Voyager 2. Thus as Voyager 1 and 2 approached Jupiter and Saturn, it was a little like approaching a city or, a flat map. Voyager first found itself in the suburbs, able to shoot a snapshot of one moon and then prepare for the next. In sequence, the spacecraft passed one moon after another, then closest to the planet, and then outward past additional moons one at a time. The Jupiter and Saturr. close encounters were each spread out over several days.

Uranus was not so accommodating. Voyager 2's closest encounters with Uranus, its rings, and all of its moons were crammed into a period oi less than six hours.

The rings and moons of Uranus revolve almost precisely around its equator, just as most of the moons and all of the rings of the other planets do. The problem is that Uranus lies on its side as it revolves around the Sun. So the orbits of its moons and rings are tilted, like the planet, almost perpendicularly to the orbital plane of Uranus. Even so, if the equatot of Uranus had been more or less edge on to Voyager as it approached (which is approximately the case for about half of the 84-year period oi Uranus), Voyager could have passed across that orbital plane and the moons could have been studied sequentially on approach and departure But the planetary alignment that allowed Voyager 2 to bounce from

hours

Oberon hours

Voyager 2's flight past Uranus. Voyager 2 passed nearly perpendicularly through the Uranian system because Uranus lies on its side as it orbits the Sun and its moons orbit above its equator. At the time of Voyager 2's arrival, Uranus had its south pole pointed toward the spacecraft and the Sun and thus presented a bull's-eye target to its visitor. Because of this geometry. Voyager 2's closest approaches to Uranus, its rings, and each of Its moons (its opportunities for best pictures and measurements) were squeezed into a period of less than six hours.

Oberon

Jupiter to Saturn to Uranus to Neptune just happened to catch Uranus with its south pole pointed toward the Sun, the Earth, and the approaching robot ambassador from Earth. Voyager 2 saw Uranus, its rings, and its moons laid out before it like a target with a bull's-eye and concentric rings. The spacecraft plunged perpendicularly through the Uranian system like a dart thrown at a paper target. The time when Voyager 2 was closest to Uranus was also very nearly the time when Voyager 2 had its closest view of each moon. The picture taking and scientific measurements that could be spread over several days at Jupiter and Saturn were compressed into a little over five hours at Uranus. While each image was made, another important target was near its best viewing.

It had taken four and a half years for Voyager 2 to fly from Saturn to j Uranus. Working against formidable obstacles of distance, darkness, and planetary orientation at Uranus; working against a crippled radio receiver, a sticky scan platform, and a degraded computer onboard the spacecraft, scientists meticulously planned every aspect of the Uranus enco so that Voyager 2 could take full advantage of the first practical oS?? tunity since the dawn of the space age to examine in detail the i of Uranus.

At Jupiter and Saturn, Voyager 2 had been the backup craft, follow up on Voyager 1's discoveries and duplicating measurements froniarfi ferent angle. Now, at Uranus, Voyager 2 was the only spacecraft, trav^ ing where no spacecraft had ever gone before. With each passing^ ment, Voyager 2 was expanding the absolute frontier of planed journeys.

On January 24, 1986, eight and a half years outbound from Earth NASA's Voyager 2 spacecraft passed 50,700 miles (81,600 kilometers, above the cloud tops of Uranus. Uranus was 1.75 billion miles (2.8 billio6 kilometers) from Earth, and the data radioed from Voyager took two and three-quarters hours to reach home traveling at the speed of light

Astronomy had come a long way in the 205 years since Herschel had caught sight of a tiny disk in his homemade telescope.

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