Pilots took off into clear skies Friday for a fifth day of searching Nevada's rugged high desert for any sign of missing adventurer Steve Fossett.

The search area was expanded to 10,000 square miles, roughly the size of Massachusetts, and there were still few clues to the 63-year-old aviator's whereabouts and no evidence that his small plane had crashed.

"Each day when I walk into the operational briefing, everybody's hopeful that this will be the day that we find Mr. Fossett," said Chuck Allen, a Nevada Highway Patrol trooper helping lead the effort.

He said the search conditions were ideal, with light wind and no clouds.

Click here for photos of Fossett.

"The weather's holding out," Allen said as search planes roared down the runway at the airport in Minden, which is serving as the search-and-rescue base.

Fossett took off Monday from a private airstrip about 80 miles southeast of Reno to scout possible sites for his planned attempt to break the land speed record. He didn't file a flight plan for his Bellanca Citabria Super Decathlon, and no signals had been picked up from the plane's locator beacon.

The terrain of western Nevada's high desert makes the search for him especially tough.

"It is not the flat desert that you think of when you think of the Mojave or the Sahara," Nevada National Guard member April Conway told NBC's "Today" show Friday. "The mountains are very, very tall, very rugged. Lot of deep ravines, lots of deep crevices, lots of places for people to hide.

"It is not uncommon for us to be on Day 4, Day 5 and not have any clues. We have an awful lot of people out there searching, and we will turn up something."

Although the small plane was thought to be carrying enough water for Fossett to survive about two weeks, the knowledge that he may be in trouble in a harsh landscape weighs on the searchers.

"You wonder what they're doing down there at this time, you know, whether they're really still alive or not," said Robert Todd, one of the Nevada Civil Air Patrol pilots involved in the search. "You always hold out hope that they are alive."

A squadron of 10 airplanes and two helicopters was deployed Friday to search the same area as the day before, Allen said. The crews are making repeated passes because the view of the region from the air — marked by 10,000-foot peaks and steep canyons — changes throughout the day, as deep shadows creep across the landscape.

Rescuers also turned their attention to Walker Lake, about 15 miles northeast of the private ranch where Fossett had been staying. They hoped to rule the lake out as a possible crash site and scanned its perimeter for debris.

That search was fruitless, and on Friday a sheriff's boat was scheduled to use sonar to peek beneath its surface.

The search has generated worldwide attention because of the adventurer's past exploits and his connection to British billionaire Richard Branson, who has bankrolled many of Fossett's record attempts.

Fossett became a multimillionaire operating a series of Chicago-based investment firms before turning his attention to long-distance and high-speed adventurers. He has set 116 land and air records, including becoming the first person to circle the globe in a balloon solo and the first to do so alone in a plane without refueling.

Many of his pursuits also have ended in failure, requiring costly and daring rescues. In 1998, one of his attempts to circumnavigate the globe ended when his balloon crashed into the Coral Sea about 500 miles off Australia's coast.

The search in Nevada, where Fossett had been scouting sites to attempt a new land speed record, was continued into the night Thursday with Nevada National Guard planes and helicopters equipped with thermal imaging systems. The intensive effort could continue for two weeks or longer, said Nevada Civil Air Patrol Maj. Cynthia Ryan.

"We can't always guarantee the right result that everyone would like," she said. "But I do guarantee results."