In November, after the walnuts and grapes were in, Salvador and Julia Hernandez packed their four children in the car and headed south through California's Central Valley to the Mexican state of Michoacan, 1,700 miles away.

"It took 21/2 days," remembered Jose M. Hernandez, the youngest of the four. "My dad put cans of Campbell's soup on the engine manifold so they would heat up. Then we'd open them and eat the soup in the back seat."

On May 6, the National Aeronautics and Space Administration chose Jose Hernandez, the son of Mexican migrant farmworkers from Stockton, Calif., and Michoacan, as an astronaut candidate, destined, perhaps, to be the next human to set foot on the moon.

"I still remember as a kid the tail end of the Apollo missions, and the desire to become an astronaut never went away," Hernandez said in a recent interview at NASA headquarters. "I always aggressively sought jobs that would enhance my credentials."

His credentials are considerable. Hernandez, now 41, is a materials engineer at Houston's Johnson Space Center and an expert in X-rays, tomography, ultrasound, and other nondestructive means of medical and materials analysis. As part of the 11-member astronaut class of 2004, he will train as a mission specialist.

Before joining NASA in 2001, he worked for the Department of Energy, where he helped develop the inspection techniques and monitoring procedures used in the disposal of 15 tons of Russian enriched uranium.

And at the DOE's Lawrence Livermore National Laboratory in the 1990s, he and a colleague used experience gained working on the Reagan administration's X-ray laser defense initiative to develop the first full-field digital mammography system for detecting breast cancer in women.

Along the way, he also become president of the Society of Mexican-American Engineers and Scientists, an avid runner who has participated in the Marine Corps Marathon and the father of five children.

All this from a short, slightly built man with a mustache who did not speak English until he was 12: "It was probably because of the language barrier that I had a greater knack for numbers and math," he said.

Hernandez was born in French Camp, Calif., just outside Stockton, the family's final stop on its annual "California circuit," which began in February, when they arrived in California's Central Valley and headed for the strawberry fields near the city of Ontario.

"Then we'd move north to Salinas, for lettuce," Hernandez recalled. "And then to Stockton, for cherries, cucumbers, apricots, peaches, tomatoes. . . . We finished with grapes" in October, he said, then drove back to Michoacan for an extended Christmas vacation.

But "during all the stops, we went to school," Hernandez said. "We didn't work Monday through Friday, but always on the weekends. And late in the school year, when all the kids were looking forward to summer vacation, I was dreading it."

It was sometime during grammar school in the early 1970s when Hernandez's itinerant life began to change. He went to his teacher late in the harvest season to pick up enough homework to tide him over until the next circuit began, but the teacher insisted on talking with his parents.

They had some bright kids, who needed some stability, the teacher told them. " 'You should put down some roots' is the way it came out," Hernandez recalled. "After that we stayed with the same school district -- we adapted well."

And performed well. Hernandez's sister is an accountant and his two brothers are a mechanic and a DOE engineer. Salvador Hernandez eventually ran his own business driving a fertilizer truck and is semi-retired in Stockton with Julia.

"My mom never learned English," Hernandez said, which is the chief reason he speaks fluent Spanish. And with two Mexican-born parents, Hernandez enjoys dual citizenship, to the delight of Mexico, whose embassy feted him here after his selection as an astronaut.

Hernandez went to Stockton's University of the Pacific on a scholarship, graduating with a degree in electrical engineering in 1984, and then earned a master's at the University of California at Santa Barbara.

He had joined Lawrence Livermore as a work-study undergraduate and was rehired when he returned from UCSB. But even as a young Energy Department up-and-comer, his ultimate goal was to become an astronaut.

"I remember [in 1981] when NASA chose [Costa Rican-born] Franklin Chang-Diaz as an astronaut, opening the road to Latinos," Hernandez said. "I said to myself that I had no excuse now. I can't say they don't let Latinos in."

Ten years later, "I decided to get serious," he said. He discussed his dream with Adelita, his fiancee, and told her that "she had to be very supportive, or there would be no wedding." He need not have worried, he said, as Adelita has "been behind me all the way."

Hernandez reached the interview stage of the astronaut selection process twice before clearing the final hurdle this year. By that time he had been working for three years at NASA, where, among other projects, he has supervised development of an on-board capability to repair the space shuttle's thermal insulation.

But his astronaut class may be the first in a generation never to fly the shuttle, scheduled to be phased out by 2010 and replaced by a "crew exploration vehicle," the spacecraft expected to implement President Bush's plan to send humans back to the moon and eventually to Mars.

"I don't expect to reach Mars during my life as an astronaut," Diaz said. "But I'm hoping one of us [in the Class of 2004] will be the next person to set foot on the moon."

Jose M. Hernandez, a mission specialist candidate in NASA's 2004 astronaut class, traveled from Mexico to California as a child to work on several farms.