After he had completed his Ph.D., he was offered a job at EG&G Corporation Energy Measurements in Goleta, California, to work on crystal growth. EG&G was a defense contractor of the United States government and dealt with sensitive information and science. In 1975, this required Van den Berg to become a naturalized U.S. citizen.
During his work at EG&G, Van den Berg asks NASA for permission to conduct crystal growth experiments in space, which NASA approved. Van den Berg and his colleagues designed the EG&G vapor growth crystal system experiment apparatus for a Space Shuttle flight. NASA decided that it would be easier to train a crystal growth scientist to become an astronaut, then it would be the other way around and so they asked EG&G and Van den Berg to compile a list of eight people who would qualify to perform the science experiments in space and to become a Payload Specialist. Van den Berg and his chief, Dr. Harold A. Lamonds could only come up with seven names. Lamonds subsequently proposed adding him to the list, joking with Van den Berg that due to his age, huge glasses and little strength, he would probably be dropped during the first selection round but at least they would have eight names. The first selection round however, consisted of a selection based on science qualifications in the field in question, which Van den Berg easily passed. The final four were tested on physical and mental qualifications which he also passed, while two of the others failed due to possible heart issues. He was now part of the final two, and NASA always trains two astronauts, a prime and a back-up. In 1983 he started to train as an astronaut and six months before the launch he was told that he would be the prime astronaut, much to his own surprise. When he went into space he was 53 years old, making him one of the oldest rookie astronauts.
Van den Berg was assigned as Payload Specialist on STS-51B Challenger (April 29–May 6 1985). STS-51B, the Spacelab-3 mission, was launched from the Kennedy Space Center, Florida, and returned to land at Edwards Air Force Base, California. It was the first operational Spacelab mission. The seven-man crew aboard Challenger conducted investigations into crystal growth, drop dynamics leading to containerless material processing, atmospheric trace gas spectroscopy, solar and planetary atmospheric simulation, cosmic rays, and laboratory-animal and human medical monitoring. By the end of the mission, Van den Berg had traveled over 2.9 million miles in 110 Earth orbits, and logged over 168 hours in space.
Van den Berg has over twenty years of research and management experience in the preparation of crystalline materials — in particular, the growth of single crystals of chemical compounds, and the investigation of associated defect chemistry and electronic properties. where he is responsible for the operation of a crystal-growing facility, which produces various kinds of crystal by vapor transport methods. Van den Berg is a co-investigator on the Spacelab-3 mission Vapor Crystal Growth System (VCGS) experiment. In that capacity, he is responsible for the crystal growth aspects of the VCGS experiment. He has intimate knowledge of VCGS and Fluid Experiment System (FES) hardware, and has participated in all major design and science reviews of those systems. He has broad experience in crystal growth and characterization, including vapor transport, solution, and melt growth techniques. He is an international Authority on vapor growth techniques with an emphasis on mercuric iodide crystals and its application in the nuclear industry as gamma ray detectors.