The year is 2076, and a crew of astronauts is preparing to return to Earth after spending two years in a laboratory on Mars. They're tired, hungry and ready to go home.

Meanwhile, the craft carrying their replacement crew from Earth has just entered Martian orbit.

For the next two hours, the two crews work methodically to make sure their missions are completed successfully.

The simulation of the futuristic voyage is taking place in an unassuming red brick building in Old Town Alexandria, home of the Challenger Learning Center of Greater Washington. For 17 years, hundreds of groups -- mostly schoolchildren -- have come through its doors for an introductory two-hour, hands-on course on space exploration and travel.

The center is part of the Challenger Center for Space Science Education, a nonprofit organization founded in 1986 by families of the astronauts lost in the Challenger shuttle tragedy that year. There are 47 learning centers nationwide.

"We're dedicated to continuing the educational spirit of the Challenger mission," said Howard Wahlberg, vice president of marketing and network development for the center. "The centers use the excitement of space exploration to create positive learning experiences."

Participants in the center's workshops are transformed into astronauts, engineers and scientists, and they work in a replica of an orbiting space station and a mission control room modeled after NASA's Johnson Space Center in Houston.

Students participate in one of four simulations -- "Rendezvous with a Comet," "Return to the Moon," "Voyage to Mars" and "Encounter Earth" -- and work as teams, testing test their decision-making, problem-solving and communication skills. Total cost for the two-hour session is $450 per group. While most classes at the center consist of students from Washington area schools, there are occasional adult classes. Wahlberg said some companies send employees to the center because skills that the scenarios require, such as cooperative decision-making and teamwork, are similar to those needed in the workplace.

On one recent afternoon, 15 Exxon Mobil employees visited the center for a "Voyage to Mars."

Lisa Kashur, a data coordinator for the company, said she brought the program to her employer's attention after visiting the center with her daughter Chelsea's sixth-grade class last year.

"I got a kick out of it myself, and I thought, 'Why can't we try this?' " said Kashur, 40. "Look around. Everyone is enjoying themselves."

That was true. The adults started self-consciously. But as the afternoon went on, they seemed to shed their armor and become more relaxed in their space lab coats. Instructors wear the dark blue NASA training suits that astronauts wear.

The center's mission control and space station look like something from the set of "Star Trek." Mission control has a bank of monitors on which data can be read. Footage from actual shuttle missions is shown. During this particular mission, for example, actual photos of Martian terrain -- canals and canyons -- is shown.

To enter the space station, travelers are "beamed up." They stand on shimmering discs while a fluorescent light flashes above them. Doors open to an encapsulated space station with several work stations. Participants are assigned to teams with different responsibilities, including communications, data, navigation, remote controls and life support.

The communications team, for example, is responsible for verbal communication between space station and mission control. The life support team tests water supplies and reads solar panels. Those working as remote control members get to operate a robotic arm encased in the space laboratory where they analyze data on mass, volume and chromatography, the method of separating a mixture of compounds.

"They mimic the research activities that go on in space," Wahlberg said.

Before they begin their work, however, students gather for an overview in a briefing room filled with folding chairs and pictures of real astronauts, including a framed photograph of the seven astronauts killed in the Columbia shuttle disaster on Feb. 1. George Kasunich, an education specialist who teaches many of the classes, said that, to his surprise, only one student has asked a question about the tragedy.

Although some class participants have said they think about the Columbia tragedy when they see the photograph, they come in "focused on their assignments," Kasunich said.

After an hour, the teams switch places. Those who were in the space station return to mission control, and their counterparts go to space.

"It does help us work as a team," Carol Rich, a data coordinator for Exxon Mobil, said after switching with her partner, "and communications is a big issue with your counterparts."

Colleague Kevan McCrae, an analyst, agreed: "It's a decision-making process, and it's similar to what we do in the workplace."

While adult classes are infrequent, the center is booked through the end of the year with students from area schools. Soon after the Columbia disaster, sixth-graders from Buzz Aldrin Elementary School in Reston visited for "Rendezvous With a Comet." Their school is named for the astronaut who in July 1969, with Neil Armstrong, was part of the first manned landing on the moon and followed Armstrong onto the surface.

The students asked a few questions at the beginning: What is a comet? What is a simulator? When did the center start?

But it was clear that they were eager to go into the space center where they could be space explorers for the afternoon. Their mission: to study Comet Encke.

Teacher Jeanne de La Vergne said she has been bringing classes to the Challenger Center for nine years. "We book six months in advance," she said. "We make a trip every year. The kids love it."

A bulletin board at the center is a testament to its popularity. It's covered with letters and drawings from children everywhere thanking the staff for the space adventure.

Unlike the adults, the children enter without trepidation. They take easily to the computers, visuals and playacting.

"Mission Control, this is the space station. I have a message for the medical team," 11-year-old Chin May said with convincing authority as he read from the script.

Next to him, partner Philip Anderson, 11, slipped into youth-speak. "Dude, where's the NAV?" he asked Chin.

The high point of the students' mission was discovery of a new comet. Their reward: They get to name it. Being politically savvy, they decided to name it after their teacher, Comet de La Vergne.

"I was thrilled," de La Vergne said. "I never had a comet named after me."

Afterward, the students seemed enthralled with space travel. "I only had one little boy say, 'I don't want to go up there,' " she said.

Kashur, the Exxon Mobil employee, said her colleagues were also happy with their "trip."

"We got to know and work with each other in a different atmosphere," Kashur said, "and you have an appreciation for the people who go up in space, that's for sure."

Ira Morrow of Buzz Aldrin Elementary School in Reston participates in a mission to locate a comet at the Challenger Learning Center in Alexandria.Lisa Kashur, a data coordinator for Exxon Mobil, works through a problem during a Mars mission with fellow employees, including Frank Kelly, left, and Kevan McCrae. At near right top, Prem Kumar, from left, Matthew Young, Kashur, Kelly and Flight Director George Kasunich celebrate a successful probe launch.Above, Flight Director Ashley Johnson leads a group through a mission at Alexandria's Challenger Learning Center, one of 47 in the nation. At left, Johnson instructs students to power down their computers in a simulated emergency.