File image of Titan 4 rocket at Complex 40. Photo: Lockheed Martin

Workers at Cape Canaveral's Complex 40 launch pad are replacing a faulty pump in ground support equipment that caused Tuesday's nitrogen tetroxide leak during fueling of Lockheed Martin's Titan 4B rocket.

Liftoff of the classified mission for the National Reconnaissance Office has been delayed at least one day as a result of the incident. Launch is now targeted sometime between 11:40 p.m. EDT Monday and 5 a.m. EDT Tuesday. The exact launch window within that period remains a secret.

"There was an accident on Tuesday afternoon at LC40 when the oxidizer pump in the ground equipment failed, briefly spilling nitrogen tetroxide on the ground," said Col. John Insprucker, the Titan program manager at the Air Force's Space and Missile Systems Center.

"The propellant team safed the ground system in seconds. There were no injuries and the Titan rocket was not involved."

Air Force safety personnel said the general public was never in danger.

"Our emergency response team was immediately dispatched to the scene to secure the site and assess the potential risk to those at Cape Canaveral and the surrounding area," said Col. Ev Thomas, 45th Space Wing vice commander. "All those involved were in full protective equipment and were able to control the leak within minutes."

"For every fuel loading operation, worst case scenarios are modeled by Range Safety personnel to ensure the overall safety and security of the operation," the Air Force said in a statement Tuesday evening. "Calculations include weather conditions, amount of propellant and potential credible failures. If at any time a condition exists that exceed established safety criteria, the operation is immediately halted."

Tech. Sgt. Shannon Green, 3rd Space Launch Squadron booster section maintenance controller, dons the Self-Contained Atmospheric Protective Ensemble, also known as a SCAPE suit prior to fuel operations Tuesday. The protective equipment is a mandatory safety measure all maintenance personnel must wear prior to any hazardous operation at the launch pads. Photo: U.S. Air Force/1st Lt. Warren Comer

The Air Force said the plume from the leak dissipated harmlessly before reaching populated areas of Kennedy Space Center. As a precautionary measure, KSC officials were notified about the incident.

Officials estimate about 50 gallons of the deadly nitrogen tetroxide was spilled. The hypergolic oxidizer is consumed by the Titan 4's first and second stage main engines along with Aerozine 50 fuel -- which is a mixture of hydrazine and unsymmetrical dimethyl-hydrazine.

"The launch team is spending the next days decontaminating the propellant loading unit and replacing the pump and associated filters and hoses and pipes before finishing the loading of oxidizer on the Titan 4," Insprucker explained.

This rocket is valued at $500 million, the Air Force program office said. It features an 86-foot long payload fairing nose cone and a Centaur upper stage to loft the hush-hush cargo into orbit.

The upcoming launch is one of four remaining Titan 4 missions scheduled. Two flights are expected from Cape Canaveral in 2004, followed by the finale in early 2005 from Vandenberg Air Force Base in California.

The next-generation Evolved Expendable Launch Vehicles -- Boeing's Delta 4 and Lockheed Martin's Atlas 5 -- are replacing the Titan 4 and the other heritage rockets.