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As a helicopter buzzed over Yellowstone National Park last week, avalanche-triggering explosives were dropped out of an open door and onto snow-covered Sylvan Pass.

The drops hit their targets, the packages exploded, and snow slipped off the hillside as planned, a pre-emptive strike against a potentially more dangerous avalanche that nature could dish up.

Park officials expect 10 or more similar missions over Sylvan Pass this winter.

Dangerous efforts

What they've been doing for years - using a 105 mm howitzer to shoot 55-pound shells into the hillside - isn't safe, park officials say, especially when crews have to go through avalanche-prone areas to reach the gun.

The howitzer shells are also notoriously unpredictable. Park officials estimate there may be as many as 300 unexploded shells in the hills around Sylvan Pass, which is west of the East Entrance. Four more were added to that list last winter.

Using choppers for avalanche control has its limitations, too, but it doesn't pose the same kind of hazards to employees and the public.

The Park Service is starting the second year of a two-year pilot program testing the use of the helicopter. No decision has been made about whether helicopters will become a permanent fixture in avalanche control.

"There's not consensus here in Yellowstone whether the helicopter is the way to go or the howitzer is the way to go," said Steve Swanke, the park's east district ranger.

What park officials do know is that the howitzer program is about to get more expensive. The military said the price per round is going to increase from $17 to about $200, meaning a typical 20-shot mission could cost about $4,000 just in ammunition.

Gun on loan

There's also the chance that Yellowstone could lose its howitzer, which is on loan from the military.

Last year, a 20-pound shell used for avalanche control in Utah overshot its mark by three miles and exploded in the backyard of a home. No one was hurt, but military officials said they would take back all of the howitzers if a similar incident happened again, Swanke said.

"They have loaned them to us, and if they say they want them back, we say where and when," Swanke said.

With mounting costs and ongoing concerns about safety, park officials began looking into using helicopters to prevent avalanches, a practice that's used by highway departments and ski areas elsewhere in the West.

Last winter, the Park Service signed a contract with Carisch Helicopters Inc. of Bozeman to do similar work in Yellowstone.

The crew made 10 passes last year, hitting all of its targets and leaving no unexploded packages on the ground.

The howitzer worked on five avalanche-triggering missions, which were successful except for the duds left on the hill.

This winter, park officials are hoping to use the helicopter as the primary tool for avalanche control, firing the howitzer only when bad weather or other problems keep the helicopter grounded.

The crew will use "cast boosters" this year, which include packaged explosives, blasting caps and a fuse with a two-minute delay. Drops are typically made from about 50 feet above the ground.

The helicopter also could be used elsewhere in the park, including to clear Dunraven Pass before roads open in the spring, Swanke said.

Park officials are refining the costs of the chopper program.

Initially, they estimated it would cost about $285,000 annually. But with one year of the pilot program completed, Swanke said the costs could be closer to $125,000 a year.

By comparison, using the howitzer costs about $105,000 each year.

Some have questioned why the park would spend money on avalanche control at Sylvan Pass when the East Entrance brings in a small percentage of visitors each winter.

But Park Service officials say the road is also used by park employees traveling between the East Entrance and the rest of the park. Besides, they said, avalanche control would still be needed in the spring, when roads open.