The space shuttle Columbia returned to Earth December 5, wrapping up a 16-day mission that included a second, unscheduled spacewalk to complete tasks not finished on the first spacewalk.
     Astronauts Winston Scott and Takao Doi spent five hours in the shuttle's cargo bay early on December 3, conducting a series of tests on equipment that may be used during the assembly of the International Space Station.
     The spacewalkers tested a crane that would be used to move equipment outside the space station. They also worked with AERCam Sprint, a camera with its own thruster system that allows it to fly under remote control. NASA hopes the camera will cut down on the number of spacewalks needed to inspect the space station.
     The tests conducted December 3 had been scheduled for a November 24 spacewalk, but were not conducted when the astronauts spent the first few hours of the spacewalk retrieving the errant Spartan-201 satellite, which had failed to deploy properly from the shuttle three days earlier.
     The crew, which included Japanese astronaut Doi and Ukrainian astronaut Leonid Kadenyuk, completed a set of microgravity experiments on board the shuttle during its 16+ day mission.
     Columbia landed at the Kennedy Space Center at 7:20am EST (1220 UT) December 5.

A request by the Russian Space Agency, coupled with a minor mishap in a shuttle hangar, will delay the launch of the shuttle Endeavour on the next-to-last Mir docking mission next month.
     NASA announced December 9 it was delaying the launch of Endeavour by five days to January 20, following a request from the Russian Space Agency. Russian officials cited the heavy work load of American astronaut David Wolf and his Russian crewmates, who are trying to complete a set of experiments before the shuttle's arrival.
     "It's a very busy time up there on Mir," NASA spokesman Rob Navias said. "There are three spacewalks, a Progress resupply ship docking and we do not want Wolf's science program to be impacted."
     The delay also makes it more likely that Wolf will be able to perform a spacewalk outside Mir with Anatoly Solovyov. The spacewalk, which had been delayed a month, is now scheduled for January 9.
     The launch was expected to be delayed regardless of the Russian request after a minor accident took place the preceding weekend in the shuttle Endeavour's hangar. A support structure on the left cargo bay door of the orbiter broke, denting the door and poking a hole in a thermal blanket.
     The accident delayed the mating of Endeavour to its external fuel tank and solid rocket boosters by several days. The delay would have likely have carried over to the launch date regardless of the Russian request.
     The Endeavour mishap is the latest in a string of minor problems with three shuttle orbiters. Crews inspecting Columbia after it landed December 5 noticed an unusually high number of damaged thermal tiles. Three hundred tiles were found to be damaged, twice the normal number, and over 125 had holes more than 2.5 cm (1 in.) in diameter.
     Last month the shuttle Atlantis was flown from the Kennedy Space Center to California with two key washers missing from a bolt that attached the shuttle to its 747 carrier aircraft. The shuttle was not damaged but the bolt was bent during the cross-country flight.

The Federal Aviation Administration pulled Orbital Sciences Corporation's license to launch Pegasus launch vehicles Thursday, then restored it after the company agreed to make modifications to the fourth stage of the rocket.
     The license suspension, the first since the FAA started issuing commercial launch licenses in the 1980s, came after the agency found that a piece of equipment designed to vent fuel from the Pegasus XL's last stage was missing in the final design of the booster.
     The suspension, announced December 11, came just before the scheduled launch of the Pegasus XL carrying 8 ORBCOMM communications satellites. OSC officials worked with the FAA during the day, and got the agency to restore the company's license after OSC agreed to install a venting system on the booster, Spacecast reported.
     The venting system is required by the FAA to release any remaining hydrazine fuel in the tank of the fourth stage of the booster after engine shutdown. Officials fear that otherwise pressure in the tank could build up and cause an explosion, adding to the orbital debris problem.
     The venting equipment had been in the design of the booster when the current license had been granted, but was missing when the rocket was assembled.
     The effort to add the venting equipment will delay the launch, originally scheduled for the 11th, by several days. No new launch date had been announced by press time.

A theory that the Earth is pelted with thousands of house-sized comets each day was the subject of a number of rebuttals presented at a conference last week, while the leading proponent of the theory provided new evidence which he says further supports his explanation.
     A series of independent papers by researchers at the University of Arizona, University of Washington, and NASA Marshall Space Flight Center presented at the American Geophysical Union (AGU) meeting in San Francisco all found evidence for small comets lacking, and believed that the original evidence was instrumental error.
     At an AGU meeting in May Louis Frank of the University of Iowa presented images of the Earth from the Polar spacecraft which showed black specks at ultraviolet wavelengths. Frank explained the specks as small comets, weighing 20 to 40 tons, breaking up in the Earth's upper atmosphere.
     Research by George Parks, a geophysicist at the University of Washington, points to a different explanation. Parks said other images taken from the camera, including some taken in a lab on Earth before launch, show the same black specks, leading him to believe that the spots are just instrument noise.
     The same conclusions were found by James Spann of the Marshall Space Flight Center. Spann compared the black dots to the random noise seen in other instruments that use image intensifiers, including night-vision goggles.
     Three different studies by University of Arizona scientists searched for additional evidence for small comets, but failed to find any. Bashar Rizk and Alex J. Dessler found that if small comets hit the Earth's atmosphere at the rate predicted by Frank, they would create clouds of ice and dust that would be easily visible as one of the brightest objects in the sky.
     "The two-hour periods after sunset and before sunrise ought to produce the most spectacular sightings -- intermittent punctuations of bright, rapidly moving points of light," they said, but noted, "Where are they? We should see them."
     Timothy D. Swindle and David A. Kring noted that the elemental composition of small comets, had they struck the Earth at the same rate throughout its history, would have given the planet 500 times as much krypton and 30,000 times as much argon as found today.
     Jennifer A. Grier and Alfred S. McEwen estimated that small comets should strike the Moon every nine minutes, leaving behind a crater at least 50 meters (165 ft.) in diameter. Images of the Moon from the Apollo missions and the 1994 Clementine spacecraft do not show the hundreds of thousands of craters that should form every year.
     Despite the mounting evidence against the small comet theory, Frank held firm to the theory and presented new evidence to support it at the conference. In what Frank dubbed the "ultimate" test of his explanation, he and John Sigwarth compared data taken by Polar at low altitudes -- 3 to 5 Earth radii (19,100-31,850 km) above the surface, to data taken the same day at high altitudes, 5 to 8 radii (31,580-50,960 km) high. The high altitude data showed an 80 percent drop in dark spots, or "holes", seen in the data.
     "This result is a marvelous confirmation of the reality of atmospheric holes," Frank said, claiming that the spots would be more visible at lower altitudes, but should not vary in frequency if they were just instrument noise. Frank called the instrument noise explanations "nonsense" at a press conference.
     Frank, who originally proposed the small comet theory in 1986 based on data from the Dynamics Explorer 1 satellite, said resistance to his theory has been strong. "Despite all of the evidence that the atmospheric holes were a geophysical phenomenon and not an artifact of the camera, many members of the scientific community refused to accept the reality of the atmospheric holes because of the immense implications of the large fluxes of small comets in the vicinity of our planet."