New NASA Administrator Michael D. Griffin has decided to replace about 20 senior space agency officials by mid-August in the first stage of a broad agency shake-up. The departures include the two leaders of the human spaceflight program, which is making final preparations to fly the space shuttle for the first time in more than two years.

Senior NASA officials and congressional and aerospace industry sources said yesterday that Griffin wants to clear away entrenched bureaucracy, and build a less political and more scientifically oriented team to implement President Bush's plan to return humans to the moon by 2020 and eventually send them to Mars.

The moon-Mars initiative has put severe pressure on NASA's budget, forcing Griffin into a difficult balancing act -- trying to build quickly a next generation spaceship without crippling programs ranging from Earth observation satellites and aeronautics research to maintaining the Hubble telescope.

At the same time, the sources said, Griffin wants to restore NASA's glamour, reasserting the engineering and science leadership that has been eroding since the Apollo era. To this end, the sources said, he is willing to oust as many as 50 senior managers in a housecleaning rivaling the purge after the 1986 Challenger explosion.

"Some people make a lot of changes; some people make a few," said Ed Weiler, director of the Goddard Space Flight Center. "He's going to want people that are on his wavelength, and his wavelength is that he's an engineer and a scientist."

The sources said the targeted officials will be officially notified Monday, 60 days after Griffin took over as administrator and 60 days before the earliest date that federal regulations allow senior civil servants to be involuntarily reassigned.

Griffin, who will be in Europe next week to attend the Paris Air Show, had no immediate comment on the personnel moves. Griffin spokesman Dean Acosta said he could not discuss ongoing personnel matters, saying only that the process "is not unusual. When a new leader comes in, he wants to shape his team."

Sources familiar with the plans did not wish to be identified, either because they are not authorized to speak for NASA or did not want to discuss personnel matters before the designated officials are officially notified.

The sources said it was unclear how many of the officials, who include at least three associate administrators, NASA's highest-ranking career employees, would accept reassignment rather than resign or retire.

The sources confirmed that Associate Administrator for Space Operations William F. Readdy, a former astronaut, and retired Air Force Maj. Gen. Michael C. Kostelnik, Readdy's deputy for the shuttle and the international space station, will leave their jobs.

The two are supervising preparations for space shuttle Discovery's scheduled July 13 launch, the first time a shuttle will have flown since Columbia disintegrated during reentry Feb. 1, 2003. The sources said Readdy will stay in his post through the July mission but has not decided whether he will remain with NASA. He will be succeeded by William Gerstenmaier, who currently manages the international space station program, the sources said.

The sources also confirmed that Associate Administrator for Science Alphonso V. Diaz, who oversees the Mars rovers, the Cassini mission to Saturn and other exploration projects, will be replaced. His successor has not been chosen.

Only one of the officials, retired Navy Rear Adm. Craig E. Steidle, has confirmed his departure. The sources said Steidle, NASA's associate administrator for exploration systems, will take a job as vice president-international for the Aerospace Industries Association, a trade organization.

Griffin, 56, a blunt-spoken physicist and engineer, has made no secret of his enthusiasm for the moon-Mars initiative but clashed almost immediately with Steidle, a military procurement specialist hired in early 2004 to oversee development of the "crew exploration vehicle."

Steidle denied a month ago that he and Griffin had differences and told The Washington Post that he had no plans to abandon NASA in "the near term." Several sources at the time, however, said Steidle would quite likely leave, and other sources confirmed that he began discussing the Aerospace Industries job almost immediately.

The independent Web site reported last week that Steidle had resigned, and Steidle confirmed his departure in a Wednesday letter to exploration systems employees posted on the directorate Web site.

At the same time, rumors arose concerning the expected departures of other top officials as Griffin, a former NASA chief engineer and associate administrator for exploration, settled into his new job.

"He's wanted to be NASA administrator for a long time and has given a lot of thought to what has been done well or badly," one congressional source said. "Because of that, he is not going to take a year or two to get to know the organization."

Instead, the sources said, he expressed dismay that NASA over the past several years had put a lot of people in top management positions because of what one source described as "political connections or bureaucratic gamesmanship -- not merit."

Several sources spoke of a corps of younger scientists and engineers, including Griffin, who had been groomed in the 1970s and 1980s as NASA's next generation of leaders only to be shoved aside during the past 15 years. They said Griffin hopes to bring them back.

"The people around him will be quite outstanding," one source said. "The philosophy is that good people attract outstanding people. This is going to be a very high-intensity environment, and NASA needs experienced, outstanding people."

NASA Administrator Michael D. Griffin is to make the personnel changes by mid-August.Craig E. Steidle, NASA's associate administrator for exploration systems, confirmed he is leaving.