The Voyager 2 spacecraft that left Earth for the outer planets on Aug. 20, 1977, has begun to photograph Uranus, and scientists are getting ready for Voyager's historic encounter on Jan. 24 with the seventh planet from the Sun.
Now 50 million miles from Uranus, Voyager will come within 64,000 miles of Uranus on that day and within 18,000 miles of Miranda, the smallest and outermost of the planet's five known moons.
So far, the pictures show a featureless planet colored blue-green, reflecting the methane haze that scientists long have known covers almost the entire disc of the eccentric planet.
The pictures reveal sketchy details of the planet's ninth and outermost ring, encouraging scientists who had worried that Voyager might not see the thin dark circles until the spacecraft was almost on top of the planet. The key questions about the rings:
*Are there more than nine?
*Are they made mostly of methane ice?
*How big are the particles that make up the rings?
Uranus, one of the solar system's strangest planets, is hidden in such a veil of mystery that scientists don't even know how long its day is. They think that it is 16 hours, but Voyager Project Scientist Edward C. Stone says that this is only a "derived" number.
"That's what it has to be, if we think we know what the interior is like," he explained. "To be honest, it could be as long as 24 hours, just like our own day here on Earth."
Uranus is known to be cold, because it is almost 1.9 billion miles from the sun. But nobody knows what its weather is like, because of the planet's crazy tilt. Uranus' south pole now faces the Sun; in 42 years, its north pole will face the Sun. This means that one pole is in daylight and the other in darkness for 42 years. How cold does it get on the dark side? How much heat is transported from the sunlit side to the dark side? These are some of the questions that may be answered in January.
COMET-TRACKING . . . The ease with which NASA's International Cometary Explorer flew through the dust tail of Comet Giacobini-Zinner almost two months ago has prompted the European Space Agency to consider a plan to reroute its Giotto spacecraft to another comet after it flies by Halley's Comet next March.
ESA has asked California's Jet Propulsion Laboratory to plot new flight plans that would change Giotto's course and bring it on a path toward any of several comets on their way to or from the Sun. Giotto carries more than enough fuel for several new course corrections after it makes its midcourse maneuver in March to bring it within 310 miles of Halley's Comet.
The Soviet Union has not disclosed any post-mission plans for its two Vega spacecraft, now on their way to Halley's Comet. One NASA source said he understood that when the Soviets turn Vega's cameras on to photograph the comet, they will be unable to send the spacecraft any new commands to change course. So the Vega missions may end at Halley's Comet.
SHUTTLE LANDINGS . . . The successful landing test earlier this month of a new nose wheel steering system on the space shuttle Challenger means that its sister ship, Columbia, will resume shuttle landings at the concrete runway at Florida's Kennedy Space Center when it returns to Earth Dec. 23 at the end of a five-day mission.
All shuttle landings since April have been at California's Edwards Air Force Base, where the endless desert runways allow shuttle pilots to roll to a stop without leaning on their brakes.
At Kennedy, however, crosswinds tend to push the shuttle toward the edges of the narrow runway after the spacecraft touches down and coasts to a halt. In past landings, pilots have countered these crosswinds by making "preferential" use of brake assemblies on the landing gear under the shuttle wings. By braking on one side or the other, the shuttle was kept on the runway. But a landing in April shredded three of the shuttle's tires and burned out a brake assembly.
The new nose wheel steering mechanism was built into the shuttle so the pilot could steer the spacecraft without using his brakes. Challenger Commander Henry Hartsfield said he was "very pleased" with the assembly, but felt that one more test on the concrete runway at Edwards wouldn't hurt. Shuttle managers have decided against another test, however, so they can get back on a tighter launch-and-land schedule at Kennedy.