"This has been a very difficult time for all of us. We have witnessed a senseless tragedy of monumental proportions," the NASA chief said in an address broadcast at the agencys headquarters and its 10 national field centers.

But "through these terrible days, NASA work has continued. The International Space Station is still in orbit, research continues. Telescopes continue to send us remarkable data."

And then he commended all NASA and contractor employees for "their strength, their courage and their commitment to our nation during this difficult time, and for moving forward with our important work. It is imperative that we do so."

NASA headquarters and agency space centers around the nation reopened to properly credentialed workers Thursday, and Goldin asked employees to "be patient" amid heightened security levels.

"Work with our security people. They are here to assure your safety and the safety of your co-workers," he said, adding that the threat of additional terrorist acts still exists.

"We cannot forget that there is still potential for further violence," Goldin said. "It may not be over yet."

Tight security was event at NASA facilities Thursday. Here at Kennedy Space Center, some 12,000 workers faced hour-long traffic jams at perimeter gates to the coastal spaceport, a restricted federal enclave that covers 140,000 acres (56,000 hectares).

Cars, trucks and vans were backed up for 1.6 miles (2.5 kilometers) outside the main gate to the space center, which is the homeport to NASAs $8 billion space shuttle fleet.

Random checks were being made inside vans and other large vehicles, and only employees with permanent NASA, Air Force and contractor identification badges were allowed to enter the center.

Those with temporary credentials had to arrange escorts with employees holding permanent identification badges, and Goldin indicated that tight security would remain in place indefinitely.

"The public is genuinely concerned about keeping our launch vehicles and facilities safe. We will continue to refine and strengthen security measures at all NASA installations, and we will continue our important work," he said.

Some of that work faces a bit of a delay due to the terrorist attacks, and more specifically, their affect on air travel.

NASA had planned to launch an Athena rocket from the Kodiak Launch Complex in Alaska next Monday, but the mission has been postponed until Sept. 21 at the earliest.

NASA and contractor personnel on their way to Alaska were stranded en route as the nations commercial airlines were grounded in the immediate wake of the attacks.

"We have folks who are stuck in Tulsa, Denver and even here at Kennedy Space Center," said KSC spokesman Bill Johnson. "Obviously, we cant launch until they are able to make it to Kodiak."

Built by Lockheed Martin, the Athena rocket is slated to carry a NASA educational payload and three Air Force technology experiments on what will be the first orbital launch from the island spaceport, which is located about 250 miles (400 kilometers) south of Anchorage.

Goldin, meanwhile, said getting back to the business of space exploration is essential not only for NASA but also to the psyche of a nation still reeling from the horrific attacks.

"We can help by getting about our business, by doing our jobs, by living our lives," the NASA chief said. "One of the goals of these despicable terrorists is to disrupt our way of life and our society. If we let that happen, we will hand them a victory. Lets not do that. Lets hand them a resounding defeat."