1 Russian cosmonaut Vladimir Titov hoisted a gleaming red satellite from Discovery's cargo bay early Saturday to help NASA pinpoint what causes a cosmic phenomenon called shuttle glow.
2 The experiment was one of several secondary tasks for Discovery's crew leading up to the mission's highlight -- a planned rendezvous Monday with the Russian space station Mir.
3 Titov, the second cosmonaut to fly aboard a shuttle, grappled with the boxy 2,800-pound (1,200-kilogram) satellite using Discovery's 50-foot (15-meter) robot arm.
4 He lifted the craft, called Spartan, high overhead and aimed its ultraviolet telescope toward the shuttle tail for a 4 1/2-hour study. The telescope was aimed later toward a shuttle jet to gather ultraviolet images of exhaust plumes, and then placed back in the cargo bay.
5 Scientists hope to confirm a theory that the faint glow surrounding some shuttle surfaces in flight is caused by molecules of atomic oxygen and nitrogen combining as the spaceship slams into them.
6 The glow could interfere with delicate science instruments, and the military is interested in the phenomenon as a way of tracking missiles.
7 Spartan is to be released next week for two days of free-flying astronomical research before crew members retrieve it for the trip home.
8 Mission Control continued monitoring a leaky shuttle steering jet as Discovery pursued Mir. The leak was detected after Friday's launch and was still spewing icy propellant into space early Saturday.
9 NASA called the seepage minor and said it would have no impact on Monday's rendezvous, assuming it doesn't worsen. If it does, Discovery will have to stay at least 1,000 feet (300 meters) from Mir instead of coming within 35 feet (10.5 kilometers).
10 Late Saturday morning, commander James Wetherbee said that most of the time the leaking propellant looks like 'a very slow snowstorm' with just a few particles. But every 30 seconds or so, 'it looks like a geyser.'
11 The 250-mile-high (400-kilometer-high) rendezvous with Mir is the top priority of the eight-day flight. It's intended as practice for the first shuttle-Mir docking in June. By early Saturday, Discovery had closed to within 6,000 miles (9,600 kilometers) of the Russian station and was catching up at a rate of about 160 miles (250 kilometers) every 1 1/2 hours.
12 Late Friday night, astronauts ejected three metal spheres and three thin wires from a canister in the cargo bay for an experiment designed to help NASA calibrate radars that track potentially hazardous space junk.
13 The spheres, one about the size of a golf ball and two about softball-sized, will circle the planet for up to 10 months, eventually burning up in the atmosphere. The pin-length to pencil-length wires should burn up within a month.
14 NASA will attempt to follow the objects through radar stations in Massachusetts, Florida and North Dakota and in Germany and the South Pacific.
15 The U.S. Space Command continually tracks about 7,000 large objects, mostly old rocket and satellite pieces. A collision with any one of them would destroy a shuttle.
16 But there are also an estimated 112,000 smaller man-made objects, so-called 'risk objects,' that can't yet be tracked by radar.
17 The study is focused on that category of space junk, which has caused minor damage to shuttles in the past. An orbiting paint fleck caused a dime-sized dent in a window of shuttle Endeavour last year.
18 Discovery and its crew of six are due home Feb. 11.

Scientists hope to confirm a theory that the faint glow surrounding some shuttle surfaces in flight is caused by molecules of atomic oxygen and nitrogen combining as the spaceship slams into them .
confirm a theory
What other theories, if any exist?