Now that there are two launch pads at Florida's Kennedy Space Center, two space shuttles sit on the pads awaiting launch. The rebuilt spaceliner Columbia is scheduled to lift off on Jan. 6, the workhorse spaceliner Challenger on Jan. 23.

The two January launches will begin what is billed as the National Aeronautics and Space Administration's busiest year. It includes 15 scheduled missions involving both launch pads and all four space shuttles: Discovery, Atlantis and the two now on the pads.

Two launches are scheduled for March, two for May and three for September. Four classified missions will be flown for the Pentagon, two from the new West Coast spaceport at California's Vandenberg Air Force Base.

The two May shuttle missions will leave Earth five days apart. The first will send the Ulysses spacecraft into orbit on a roundabout path over the sun's north and south poles; the second will put the Galileo spacecraft on a path to Jupiter.

The first January launch is Columbia's rescheduled flight of Dec. 19, scrubbed when a high-speed turbine that helps steer the shuttle during ascent went out of control 15 seconds before liftoff. The turbine was removed and replaced with one tested just before Christmas.

The second January launch is to carry into orbit the second $75 million Tracking Data and Relay Satellite and the first Amer- ican teacher to fly into space, Christa McAuliffe of New Hampshire.

VOYAGER 2 IN THE STRETCH . . . On its way to a Jan. 24 encounter with the planet Uranus, the Voyager 2 spacecraft underwent a successful midcourse maneuver last Monday that aims it at a point 64,000 miles from the seventh planet from the sun.

Now moving at more than 88,000 mph, Voyager 2 was 22.3 million miles from Uranus late last week and 1.843 billion miles from Earth. Voyager 2 has been in space since Aug. 20, 1977, flying past Jupiter July 9, 1979 and Saturn Aug. 25, 1981. Its last planetary encoun- ter will be Aug. 25, 1989, when it flies past Neptune, the eighth and next-to-last planet from the sun.

Voyager 2's cameras already have photographed three of Uranus' nine known rings, made of some of the darkest material in the solar system. The planet's widest and outermost ring is charcoal black, reflecting no more than 5 percent of the sunlight that reaches it.

"The suspicion is that all of the rings are black," Voyager Project Scientist Edward C. Stone said, "which makes us suspect that they may be made of water ice and frozen methane which has been discolored over time by the ultraviolet radiation from the sun."

SOARING ART . . . The first work of art to be flown aboard the space shuttle will be a six-pound crystal cube created by Artist Lowry Burgess aboard a still-unidentified shuttle flight in next spring.

Titled "The Boundless Aperture," the crystal cube contains water collected from 18 of the world's best-known rivers, including the Mississippi, the Amazon, the Yangtze and the Nile as well as 18 other water sources such as glaciers, geysers, wells and ponds. Dissolved and suspended in the water are traces of all the elements in the periodic table or an appropriate substitute for those too radioactive and those with too short a half life to be included. Floating in the water is a cube, half the size of the larger crystal cube, in which a vacuum has been created.

On its return to Earth, the cube will be placed inside a petrified sycamore tree taken from the Grand Canyon and replanted inside a rock formation on the grounds of the De Cordova Museum near Sandy Pond, Mass. The museum is paying NASA for space on the shuttle that will carry the cube.