LOS ANGELES -- After crossing containment lines overnight, the Bobcat fire moved within striking distance of the Mount Wilson Observatory, the U.S. Forest Service said Tuesday, with firefighters mounting an aggressive defense from the air and on the ground.

The fire continued to push in multiple directions Tuesday, with firefighters stationed at the observatory ready to do battle and multiple aircraft making water drops through the day. The observatory is one of the crown jewels of astronomy, and officials vowed to save the historic complex.

Officials at the 116-year-old observatory tweeted Monday night that the fire was "knocking on our door," noting that all observatory personnel had been evacuated.

But shortly after noon Tuesday, forest officials tweeted the blaze was less than 500 feet from the observatory. Fire crews "are in place ready to receive the fire," officials said, noting that strategic firing was taking place to the south, where air operations were strengthening bulldozer lines.

"They are in a firefight right now, because it is so close," Los Angeles County Fire Capt. David Dantic said of crews positioned at Mount Wilson.

In addition to the observatory's iconic structures, there are several communications towers atop Mount Wilson that are threatened by the blaze.

"There's major infrastructure for radio, television and cell towers up there," Dantic said, "so there might be possible disruptions."

Progress on the fire has been slow. After hovering at 6% containment for several days, the blaze, which started Sept. 6 and grew to more than 40,000 acres, is now only 3% contained.

"It's a bigger area now," Dantic said Tuesday morning. "Before, we had 6% containment when it was about 30,000 acres, but now the fire has gotten bigger. It's a bigger footprint. That's why the containment is down."

Mount Wilson is a major site in the development of astronomy. The observatory was founded by George Ellery Hale in 1904, and its first telescopes -- weighing hundreds of pounds each -- were transported in pieces up the nine-mile Mount Wilson toll road on the backs of burros. Now the observatory lets visitors view space through its 60-inch telescope, which has been in place since 1908.

When the 100-inch Hooker telescope gathered its first light on Nov. 1, 1917, it overtook its 60-inch neighbor and became the largest telescope in the world -- a position it held for more than three decades.

Other observatories -- in the high plains of Chile, in orbit around the Earth -- have rendered Mount Wilson obsolete, but the campus is still a destination for astronomers and stargazers, who for decades have been drawn by its famed "seeing," a shared term of appreciation for the quality of light that defines Los Angeles.

The Bobcat fire is approaching the Mount Wilson Observatory from its most vulnerable front: the steep eastern slopes that rise precipitously from canyons cut by creeks flowing into Big Santa Anita Canyon beside Chantry Flat. The terrain cannot be defended by cutting a fire break, leaving airdrops and backfires as the only strategies.

"If the fire does arrive, I suspect it will come first hit the ground near Echo Point," said Robert Anderson, a docent with the Mount Wilson Institute, a nonprofit that operates and maintains the historic campus. "The terrain allows no stopping that without the air support."

Anderson is most concerned about the nearly 100-year-old dormitory, known as the Monastery, where astronomers would sleep during the day before working the night shift at the observatories.

"The Monastery is perched on a ridge, a finger of granite, that is exposed on three sides with steep walls," he said. "That whole ridge down to the Monastery could easily to be lost without fire suppressant on it. The home of the 20th century's most celebrated astronomers is on the line."

Additional personnel have been added to the fire crew, which has been stymied by the aggressive, fuel-driven blaze for days, but the fire crossed the contingency line and was burning along the east side of Little Santa Anita Canyon -- the closest it had been to the observatory and threatened communities, Dantic said.

"We were in the process of making a containment line around the whole fire, but unfortunately, with the steep terrain and the fuel there, the fire got over a contingency line that we were trying to make," he said.

Contingency and containment lines are barriers made by bulldozers and hand crews that help reduce fire fuel, but they're not foolproof.

"When flame lengths get so big," Dantic said, "there's always the possibility of an ember starting another fire outside of the containment line."

In the forest's Big Santa Anita Canyon, officials said they feared 80 historic cabins, along with the 1893-built Sturtevant Camp, might have been burned as flames swept through the area Sunday and Monday.

"These are historic buildings made of wood and rock and preserved as such under Forest Service rules," said Ben Fitzsimmons, president of Big Santa Anita Canyon Permittees Association. "It is unlikely that this 100-year-old community has survived unscathed."

Fitzsimmons said he didn't know the extent of damage in the area but mourned the possibility that many of the historic structures "may no longer exist."

The cabins, built between 1907 and 1936, are not typical real estate. Owners are issued a "special use permit" to lease the land from the U.S. Forest Service, according to the group's website. Cabins can't be purchased using a mortgage, and owners can't use them as their primary place of residence.

The result, Fitzsimmons said, is a small, close-knit community of people who often spend every weekend together -- people who "gather to work on each other's cabins and socialize together, bonded by our shared experience and love for this unique place that has been a community since the early 1900s."

Dantic, the L.A. County fire captain, couldn't confirm whether the fire had reached the Big Santa Anita cabins, but community members braced for bad news.

"We think we lost most of it," said Deb Burgess, a cabin owner.

Burgess was doubtful that the Forest Service would allow any burned cabins to be rebuilt, noting that she was previously told that structures more than 50% destroyed could not be rebuilt. Even small work on the cabins must follow strict guidelines about materials, size and other historic characteristics.