In nominating Rear Admiral Richard Truly to take over as chief of the National Aeronautics and Space Administration, President Bush has picked one of the most qualified people possible for the job - and the first astronaut to ever head the space agency.

Truly will replace James C. Fletcher, who retired as NASA administrator last month after more than two years on the job. Fletcher, former president of the University of Utah, also headed NASA from 1971 to 1977 and was called back to rebuild the agency after the Challenger disaster in 1986.Truly was part of that rebuilding project. A former astronaut who had flown two shuttle missions, he was recalled from the Navy in 1986 to become NASA's associate administrator for space flight and headed the recovery team and the redesign of the shuttle's booster rockets.

The 51-year-old Truly has a reputation as a cool and competent pilot, engineer and administrator who never loses his composure. Despite his years as a hot-shot carrier pilot, he is known as a careful astronaut who doesn't take chances. He has had 17 years of experience with NASA.

In his new position, Truly will be in charge of more than shuttle flights. He will direct the development of NASA's space station, oversee a variety of scientific and technical space experiments, and plan for possible landings on the moon and Mars in the 21st century. He will also have to convince Congress to provide the money for all these things.

Truly already is on the job preparing for a flight of the shuttle Atlantis later this month. Atlantis will launch a radar-mapping device known as Magellan, which will hurtle toward Venus and go into orbit around that cloud-covered planet in mid-1990. Magellan will spend eight months mapping the surface of Venus.

While dealing with Congress may turn out to be his most frustrating job, Truly has some things going for him. He is extremely popular and well-respected on Capitol Hill. The post as NASA chief requires Senate approval, but no difficulty is expected in gaining confirmation. To accommodate Truly, Congress already has agreed to waive a requirement that the head of NASA be a civilian.

The space agency still appears to be in very good hands.