Dr. McDonald’s career in space physics spanned more than a half-century, starting with experimental research in the mid-1950s to measure the radiation above the Earth’s atmosphere. His mentor in this effort was James A. Van Allen, a University of Iowa physicist who became a leader in space research and for whom the radiation belts encircling the Earth were later named.
With the help of the Navy, he and Van Allen concocted “rockoons” — small rockets lifted to 70,000 feet by balloons that would then thrust up to 350,000 feet. The rockoons carried primitive equipment intended to learn more about cosmic rays, the high-energy particles trapped in Earth’s magnetic field.
“The rockoon program was great fun,” Dr. McDonald said in a 2008 lecture, “but it had its moments that established that we really were not rocket scientists.”
One rocket exploded aboard a military ship, severely injuring a Navy officer. “We got him to shore, he made a complete recovery, but I had to come down to Washington and face the Navy,” Dr. McDonald said. “I must say, in all of my years, I have never been dressed down quite as strongly as they did.”
In 1959, Dr. McDonald became one of the earliest scientists to join the new Goddard Space Flight Center in Greenbelt, and he continued cosmic ray research as head of the energetic particles branch for the next 11 years.
During that time, Dr. McDonald provided the conceptual framework on a series of small spacecraft known as the international monitoring platforms, or IMP. Built at Goddard, the IMPs sent scientific instruments to explore the region of space above Earth’s atmosphere, above and beyond the Van Allen radiation belts. Among the scientific data collected by the IMPs was how auroral lights in Earth’s polar regions are affected by solar activity.
From 1970 to 1982, Dr. McDonald was chief of Goddard’s lab of high energy astrophysics and helped design a satellite program with instruments that could study X-rays, gamma rays and cosmic rays — emissions from distant galaxies that showed “the most violent aspects of the universe,” said Joseph K. Alexander, a radio astronomer who became Dr. McDonald’s deputy as NASA’s chief scientist in the 1980s. “The laboratory enabled the initial steps of a new field looking into how the cosmos behaves.”
Also in the 1970s, Dr. McDonald was a leader on experiments flown on space probes that have gone to the remotest corners of the solar system. For the Pioneer 10 and 11 and Voyager 1 and 2 crafts, he helped design, build and use instruments to measure cosmic rays.
The Pioneer missions gave scientists the first comprehensive look of the energetic particles trapped in the magnetic fields of Jupiter and Saturn, according to NASA.
In recent years, Voyager 1 has been at the edge of what scientists believe is the end of our solar system and the start of interstellar space. Dr. McDonald continued to monitor information from the probe until shortly before his death, curious whether expectations would prove true of a “termination shock,” or shock wave, where the two regions collide.
“We’ve learned many new things about cosmic rays, new cosmic ray components, the change with radial distance, the change in solar cycle. So the data is constantly evolving over that period,” he said in 2003. “Half the fun is getting there.”
Frank Bethune McDonald was born in Columbus, Ga., on May 28, 1925. He was a 1948 graduate of Duke University in Durham, N.C. At the University of Minnesota, one of the early academic centers for space research, he received a master’s degree in physics in 1951 and a doctorate in physics in 1955.
His first wife, the former Virginia Ballew, from whom he was separated, died in 1977.
Survivors include his wife of 25 years, Irene Kelejian McDonald of University Park; three children from his first marriage, Kyle Jossi of Olney, Robert McDonald of Rockville and Douglas McDonald of Germantown; three stepchildren, David Kelejian of Catonsville, Md., Douglas Kelejian of Wilmington, N.C., and Melinda Kelejian of Reston; 12 grandchildren; and four great-grandchildren.
As chief NASA scientist from 1982 to 1987, Dr. McDonald was a principal adviser to the NASA administrator and other senior officials. In that role, he helped start a NASA partnership with historically black colleges to fund research projects by faculty members and graduate students.
He also played an active role launching NASA’s teacher-in-space program to connect schoolchildren with science and space exploration. Christa McAuliffe, the first teacher in space, died in the space shuttle Challenger disaster in 1986.
Dr. McDonald retired from NASA in 1989 as associate director and chief scientist at Goddard. During his career, he was project scientist on nine NASA missions and principal investigator on 15 space experiments. He wrote more than 300 scientific publications and was elected in 1986 to the National Academy of Sciences.
Dr. McDonald’s extracurricular interests were in gourmet cooking and fine wines. He once vacationed in southern France and was said to have been pleased to find a winery that hinted at his former employer. It was called Godard.