Guys, we are talking about Libya here. Gaddafi has long ago gutted the military in Libya. After all, Gaddafi came to power in a coup, and he certainly didn’t want anyone to get any ideas.
The Libyan Air Force has one Mirage fighter and maybe a few dozen MiGS used mainly, and most of those MiGS have been grounded long ago. There are actually hundreds of planes in the Air Force, but most are slow and have little capacity for bombing or maneuverability. Most are based upon designs from the 1950s and 1960s.
There are some SAM missiles which could shoot down a U.S. plane. But, most of those SAM missiles are a bunch of very obsolete SA-2 systems. (The SA-2 gained fame by shooting down an American U-2 flying over the Soviet Union in 1960).
With half of the Libyan coast in rebel hands, almost all of the Libyan Air Force Gaddafi has available is located in and around Tripoli. It wouldn’t be hard to monitor the few air bases in Libyan hands and shoot down anything that launches from it.
The main question isn’t whether or not the U.S. is capable of enforcing a no-fly zone, but whether it will help or hinder the rebels who don’t want to look like tools of a foreign power. Until very recently, the rebels have asked the West not to interfere.