CAPE CANAVERAL, Fla. -- It is one of the most amazing sights associated with the U.S. space program.

The vision: a space shuttle orbiter, bolted to the back of a modified 747 jumbo jet, flying low over the beaches of Cape Canaveral -- or landing to refuel at an Air Force base somewhere between California and Florida during a cross-country ferry flight.

When it happens, thousands gather to see first-hand the most recognized symbol of America's space program riding piggyback on one of the most recognized airliners in the world -- itself a marvel to watch fly as you wonder how something so big can stay up in the air.

"It's complete excitement," said Gordon Fullerton, a veteran NASA astronaut who commanded shuttle missions in 1982 and 1985, but who is more well known around NASA's Dryden Flight Research Center at Edwards Air Force Base, California as one of the agency's most respected and experienced research pilots.

Fullerton is entrusted with flying NASA's 747 Shuttle Carrier Aircraft. He also trains new ferry pilots, including Bill Brockett, another NASA research pilot, who in early November made his first landing with the 747 while it carried shuttle Discovery and touched down on the concrete runway at the Kennedy Space Center (KSC) in Florida.

"The pressure was on," a smiling and relieved Brockett reported an hour after making that perfect, career-milestone landing at KSC.

"Bill did a great job," added Fullerton.

This particular job was required when shuttle Discovery was forced to land at Edwards on October 24, concluding an assembly mission to the International Space Station in California after bad weather on three consecutive days prevented the usual Florida homecoming.

It was the first time in more than four years that a space shuttle mission had not ended at KSC as planned.

When that happens, NASA's only way to return the 100-ton glider to its launch site is to bolt the spaceplane to the back of a 747 and ferry it cross-country, an exercise that costs about $1 million and a week's worth of time from the frequently tight launch schedule.

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And there's also some anxiety in a $2 billion orbiter defying gravity more often than needed.

"There's a certain degree of risk in transporting across the country like this," said Jim Halsell, a NASA astronaut who currently is serving as a shuttle program manager at KSC. "I'm nervous only in the sense that that represents a national resource and something that we want to take really good care of."

Every time the shuttle takes a ride it's a first class seat all the way.

In the first place, about 200 people are employed to make sure the orbiter has everything it needs before, during and after the flight.

Most of the workers are based at KSC and have to travel out to the high desert of California on short notice when it becomes evident that a shuttle mission isn't likely to end in Florida. The travel expenses for this group represents one of the larger shares of the $1 million cost.

Discovery lifts off again with the help of a 747 Shuttle Carrier Aircraft at Edwards Air Force Base in California, beginning a cross-country journey home to the Kennedy Space Center in November 2000.

But the trip apparently is no picnic for the technicians, engineers and managers.

"I would not for a second portray this trip as a vacation or as even a good deal," Halsell said of the effort in general.

And in the case of the team that supported Discovery's most recent ferry flight, Halsell said, "Most of the people worked 12-hour days everyday that they were out there [at Edwards]. Both days and nights were extremely cold, but especially the nights. When you combine the cold weather and the wind with a little bit of wet weather, those folks were earning whatever we were paying them, I'm sure of that."

Another perk the shuttle enjoys during the ride the escort service. Another jet is chartered to fly about 20 to 30 minutes ahead of the 747 to find where the ideal weather is so the shuttle doesn't have to risk flying through clouds or rain that might damage its fragile skin of heat protection tiles.

Usually that pathfinder jet is an Air Force cargo jet, often a C-141, which pulls double duty as the airliner which carries the 200 or so KSC workers and their equipment back home.

Why don't they fly aboard the 747? It's one of the most frequently asked questions and one of the easiest to answer, according to Brockett.

There are no seats inside.

"It's completely empty except for a couple of racks for monitoring equipment," Brockett said.

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All of the seats, galleys, overhead bins and other features of a commercial airliner have been stripped out of the 747 to save on weight, leaving the interior an open cavern with hundreds of square feet of unused floor space.

"You could host a pretty good party in the cabin of that airplane," Brockett said.

But not while a shuttle is attached; safety is the other reason why the number of people allowed on the 747 is limited. A shuttle that has just returned from orbit still has a lot of toxic rocket propellant and other hazardous fluids on board, so it's best to keep a safe distance away.

During a ferry flight the shuttle is completely powered down and essentially inert. The spacecraft is empty and there is no way to separate the shuttle should something go wrong with the 747. This was not the case during the late 1970s when the prototype shuttle Enterprise was dropped several times with pilots aboard -- including Fullerton -- to test the handling characteristics of the shuttle during landing.

And in all cases -- whether a shuttle is being ferried to Florida after landing or back to California for routine maintenance at the factory -- it's impossible to start up the shuttle's main engines while attached to the 747. This urban myth was fostered in part, by the opening sequence of the 1979 James Bond movie Moonraker, in which the bad guys did just that to steal the shuttle from NASA.

The aerodynamic streamlining tailcone on the shuttle and the two added vertical fins on the 747 are clearly seen in this dramatic air-to-air photo taken by a NASA chase plane as Discovery begins a cross-country journey home to the Kennedy Space Center in November 2000.

NASA actually owns two 747 carrier jets. The first was purchased from American Airlines in 1974 and has the tail number 905. It is the more historic of the two in that it was used for all the Enterprise approach and landing tests and has traveled the world carrying Enterprise on goodwill tours.

A second 747 was procured from Japan Air Lines via a contract with Boeing and was introduced in 1991 when it delivered Endeavour from its factory in California to KSC in Florida.

In order to handle a shuttle, both 747s feature several modifications, the most visible being the addition of two vertical fins to the tail of the airplane. This helps keep the plane stable despite the turbulence that comes from adding a shuttle to its back -- turbulence that is somewhat diminished thanks to a streamlining tailcone that covers the shuttle's three main engines during a ferry flight.

"There's a continuous kind of a rumbling throughout the airframe from the turbulent flow that comes off the back of the shuttle and hits the tail feathers," Brockett said.

The drag from the shuttle costs a lot in terms of fuel, too. According to Brockett, the combination 747-shuttle travels about one and a half times the length of the airplane per gallon of fuel.