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.