LANDER -- Yellowstone National Park will be open to visitors this winter, but it's not clear what the rules regarding snowmobiles will be, a park official said Monday.

All bets are off for snowmobilers and guides aspiring to use the park this winter, after a federal judge in Washington, D.C., threw out the National Park Service's Yellowstone winter plan on Monday.

U.S. District Judge Emmet Sullivan sided with the conservation organizations that brought the suit, saying the potential air and sound pollution from more than 500 snowmobiles would be bad for Yellowstone and those visitors seeking to enjoy its winter atmosphere.

The judge's decision drew immediate praise from conservation and wildlife advocacy groups, but it was also slammed straight away by Wyoming's congressional delegation, among others.

In a strange twist, on the same day, a different federal judge in a U.S. District Court in Wyoming heard arguments in a separate lawsuit against the very same snowmobile plan.

In the Wyoming case, however, the Cowboy State is arguing the plan unduly limits snowmobile use, and should, instead, allow up to 720 snowmobiles per day, and should let some riders enjoy the park without licensed guides.

Wyoming Deputy Attorney General Jay Jerde was in court Monday in front of U.S. District Judge Clarence A. Brimmer for arguments in that case. Brimmer did not make a decision Monday, Jerde said.

It's unclear how the U.S. District Court decision in D.C. might affect the Wyoming decision, if at all. But the ruling is certainly not the last chapter in this legal battle, Jerde said.

"Not having had a chance to read Judge Sullivan's opinions, I can't say right now how it might affect (the Wyoming case)," Jerde said.

If two different federal district courts reach two very different decisions on the same topic, both decisions stand, Jerde said, no matter which one is rendered first, and each decision can be appealed.

Brimmer told The Associated Press he had not had a chance to review Sullivan's ruling in its entirety. But in light of Sullivan's order, Brimmer asked lawyers in the Wyoming case to offer stop-gap options that his court could give the Park Service for the coming winter.

Brimmer said the Park Service should have some kind of direction by Nov. 1.

The rules struck down Monday would have allowed up to 540 snowmobiles and 83 snowcoaches into Yellowstone every day, with all of the snowmobilers required to be accompanied by licensed guides.

For the last three winters, although more than 700 snowmobiles would have been permitted, only an average of 260 actually used the park's groomed trails daily.

Al Nash, spokesman for Yellowstone National Park, said following Monday's decision that officials with the National Park Service must now determine what rules will be implemented in lieu of the quashed plan.

"We certainly recognize that Judge Sullivan has taken issue with some of our analyses and decision-making," Nash said. "But we are still planning for a winter season that opens Dec. 15, and we're not speculating on the potential impacts of this ruling, and we'd encourage visitors not to (speculate) either. We're going to be open for the winter. The question we don't know the answer to is how this ruling might impact our winter operations."

The issue of whether Sylvan Pass should be open to snowmobilers did not appear to be addressed, specifically, in the ruling, Nash said.

The rules for snowmobiling in Yellowstone have been tied up in court for more than a decade. But Nash said the temporary rules by which the park had been operating in the meantime -- which had allowed for more than 700 guided snowmobiles per day -- expired in the winter of 2006-07.

"We don't know what rules we'll be operating under right now. The park will be open, but we don't know the details yet," Nash said.

Many conservation organizations immediately applauded Sullivan's decision, including the Wilderness Society, the Greater Yellowstone Coalition, the Sierra Club and the Natural Resources Defense Council. They argued, jointly, that winter visitors to the park should use more of the higher-occupancy, quieter snowcoaches, and fewer snowmobiles.

"This ruling will ensure that visitors are not disappointed by air and noise pollution when they make the one winter trip to Yellowstone of their lives," said Amy McNamara, National Parks Program director for the Greater Yellowstone Coalition. "We take our hats off to the tour businesses that didn't wait for this ruling. Their increasing investments in modern snowcoaches are already making it possible for winter visitors to access and enjoy Yellowstone while protecting it."

But Brian Hawthorne, public lands policy director for the Blue Ribbon Coalition, an advocacy group for motorized recreation, said snowmobilers continue to be unfairly singled out by environmentalists.

The 20,000 to 60,000 total snowmobiles that might visit the park each winter are required to be the lowest-emission models on the market, and they cause only a "tiny fraction" of the total human impact to the park annually, Hawthorne said -- especially when compared to the more than 2 million cars and trucks that can enter the park in a given year.

"We're frankly frustrated with these judges trying to appease radical environmental groups," Hawthorne said. "We believe that snowmobile use is appropriate in the park with reasonable restrictions, and our community is willing to live with those restrictions. We're committed to responsible use, and we continue to be committed to snowmobiling in Yellowstone."

Gov. Dave Freudenthal was aware of the decision Monday, his press secretary said, but he declined to comment for this story.

Litigation on this issue could continue for years, Nash said, but the National Park Service will nonetheless establish interim rules regarding snowmobiles prior to the start of this winter season.

Contact energy reporter Chris Merrill at 307-267-6722 or chris.merrill@trib.com