Think of them as America's security blanket in the sky.
Six months after Sept. 11, 100 fighter jets stationed at 30 air bases around the US and Canada still scramble heaven-ward with clock-work regularity to investigate suspicious activity in the skies.
In fact, these missile-laden planes have been sent up 292 times - more than once a day on average - to chase errant aircraft, escort planes with suspicious passengers, or circle over security-breached airports.
The missions not only tax America's military air teams, they also reflect the persistent national anxiety about aircraft again being turned into terrorist weapons. And some observers think this operation isn't worth the cost. Furthermore, the risk of accident, they say, - including mistaken shoot-downs - outweighs any real benefit these fighters provide.
The program is "not so much about an upgrading of safety as it is an example of the psychological and political pressure on America's leaders to show that they have a better handle on security," says Richard Bloom, a professor at Embry-Riddle Aeronautical University in Prescott, Ariz.
Yet if the planes are mostly a comforting presence, at least they're a formidable one.
Overseen by the North American Aerospace Defense Command (NORAD) in Colorado, the jets are ready to scramble in 15 minutes or less. And these F-15s, F-16s, and Canadian CF-18s are fast. The F-15s can fly 100 miles in just over three minutes.
"We can cover the bulk of the country fairly rapidly," says Major Barry Venable, a NORAD spokesman.
Once they intercept an airliner, the jets perform a series of tasks, depending on the situation.
If a plane's crew isn't responding to radio, pilots communicate by hand signs, tipping their wings, or even buzzing in front of the other plane. In extreme cases, White House officials have said NORAD commanders can order shoot-downs of suspicious planes.
So far, there aren't any reports of planes ignoring instructions. "These fighters tend to look rather convincing," says Major Venable.
Last week, planes shadowed an Air India flight into New York because a London screener thought one passenger resembled someone on the US "most wanted" list. The passenger was released after brief questioning.
In December, other fighter jets escorted the airliner carrying shoe-bomb suspect Richard Reid to Boston. And when Tampa, Fla., teenager Charles Bishop crashed his Cessna into an office building on Jan. 5, fighters were scrambled - though they didn't reach him in time.
If they had reached him, however, they would have faced an exceedingly thorny dilemma: whether to destroy a civilian plane to try to save more lives on the ground. On Sept. 11, for instance, the plane that crashed in Pennsylvania probably would have been taken down anyway if it had continued toward Washington - an arguably justified act.
Observers caution that there's a history of mistakes in this area. In 1988, the USS Vincennes accidentally shot down an Iranian jet with 290 passengers over the Persian Gulf. And last year, a US radar plane watched as a Peruvian military fighter shot down a private plane suspected of carrying drugs - but that turned out to be ferrying missionaries.
These were dramatic cases. But Dr. Bloom cautions, "you always have a human element - and these things are magnified when you have incomplete information and limited time."
Officials insist that detailed procedures will prevent mistakes. If time allowed, President Bush would make a shoot-down decision.
Meanwhile, there are more-mundane questions of sustainability. The 100 planes, for instance, represent more than one-fifth of the Air National Guard's 450 fighters.
And they're not all just sitting on the ground until needed. Many are involved in continuous combat air patrols over New York, Washington, and other cities. As early as January, Air Force Secretary James Roche discussed downsizing the 100-plane operation, which involves some 12,000 personnel and costs $100 million per month.
But so far, the level of air-alert activity has stayed consistent since Sept. 11. That's evidence that, for now, authorities think it's worthwhile - either as a truly effective tool, as mere comfort, or both.