Engineer Mark Branch flicked the switch on the blender and a monitor near him went alive with jagged lines.

"This demonstrates how we use equipment to measure noise," he said to the group of parents and children gathered before him inside a laboratory at the Environmental Testing Branch of NASA's Goddard Space Flight Center in Greenbelt. "The movement here shows the problem that noise could cause on [space] equipment."

Branch, 35, was among hundreds of Goddard employees who came out yesterday to demonstrate everything from the effect of vibration on space equipment to how computers can be used to create music as part of the center's Community Day 2004.

The annual open house, which gives people the chance to peek at the work that takes place inside the science facility, resumed yesterday for the first time since the Sept. 11, 2001, terrorist attacks.

NASA officials had stopped the event out of concern for the security of the work done at the center. Organizers estimate about 10,000 people showed up to walk through and observe places at the center normally restricted to employees with special government clearances.

Before Sept. 11, 2001, visitors could also tour the facility, but those tours are suspended indefinitely.

Yesterday's event offered activities including ham radio demonstrations and rocket launching contests for children. There was also an exhibit of classic cars, a moon bounce and games. Tours offered especially for the open house included the Spacecraft Fabrication Facility, where satellite parts are constructed; the Spacecraft Test and Integration Facility, where participants viewed a giant centrifuge; and Building 28, home of the Go Explore! Exhibit Pod, where moon rocks from an Apollo mission were on display. And there was the demonstration by David Beverley, NASA Goddard's information technology administrator.

Playing his own electric bass guitar, keyboard synthesizer and computer equipment, Beverley showed visitors how technology is used to create music.

"As a child, I wanted to be an astronaut and I became a musician," said Beverley, a former rock guitarist. "Now, I'm working at NASA, and today I'm showing people how the same imaging technology that is used in space projects can be used to create music."

The day offered Goddard visitors access to the scientists and engineers who normally work in secret behind locked gates. The scientists readily demonstrated, for example, how thermal covering is used to protect the Hubble Space Telescope, and the effect vibrations have on space equipment.

Mark Hess, Goddard spokesman, said the event was held after the center's director determined that "we need to get the public back inside the gates." Such events, he said, help to explain not only what happens in the center's facilities, but also how science is applied in everyday life.

Noreen and Clark Scrandis of Clarksville, in Howard County, brought their children Lauren, 7, Rachel, 5, and Peter, 2, to allow the girls to find out more about recognizing stars in constellations.

Aaron Solomon, 9, of Greenbelt, a ham radio operator who went to the event with his brother Isaac, 7, and his father, Francis, spent several minutes working with Goddard engineer Marco Midon, trying to call someone on a ham radio operated by the Goddard Amateur Radio Club. After several minutes, the boy was treated to a response from a ham radio operator at the Indianapolis Motor Speedway for this week's Brickyard 400 NASCAR race.

The chance to educate is why Branch, who is also a deejay specializing in rap music, said he likes to talk to children at such events -- to help them understand that "there are rocket scientists who like rap music."

Mike Calabrese, left, shows how to view sunspots. The Goddard Space Flight Center's open house yesterday was the first since Sept. 11, 2001.Standing in front of a model of the Hubble Space Telescope, Kevin Hartnett explains some of the telescope's features to visitors.