Dr. A'Hearn uses a plastic bottle to describe the rotation of the Comet Hartley 2. (Reed Saxon/AP)

Michael A’Hearn, the chief architect and director of an intentional space collision of a comet and spacecraft to advance the eternal human search for the secrets of the solar system, died May 29 at his home in University Park, Md. He was 76.

The cause was pancreatic cancer, said his wife, Maxine A’Hearn.

An astronomer and professor emeritus at the University of Maryland, Dr. A’Hearn was the chief scientist for the NASA mission known as Deep Impact, in which an impactor spacecraft was launched directly into the path of a speeding comet.

“Nothing like it had ever been attempted before,” Jim Green, NASA’s director of planetary science, said in an interview. What Dr. A’Hearn was proposing, Green said, was to “blow a hole” in the comet to see what was inside.

At 1:52 a.m. on July 4, 2005, the 23,000 mph collision occurred 83 million miles from Earth. It made headlines and led news broadcasts around the world. To achieve the symbolic Fourth of July date, the spacecraft had been launched from Cape Canaveral, Fla., on Jan. 12, 2005.

Dr. A'Hearn at a blackboard. (Courtesy of University of Maryland)

The machinery consisted of two parts: a “flyby” craft equipped with sophisticated recording equipment, and a detachable “impactor,” an 820-pound, refrigerator-size craft for the comet to strike. The comet, known as Tempel 1, was about 9 miles long and 3.7 miles wide and moved around the Sun in a 5½ -year elliptical orbit between Jupiter and Mars.

There was a blinding shower of light and a giant plume of gas and icy debris when the comet and the impactor spacecraft collided with the force of what was said to have been the equivalent of 4½ tons of dynamite.

Scientists had long believed that comets contain remnants of matter left over from the formation of the solar system. They wanted to look inside Tempel 1 to see if an interior view might yield more information.

From the Tempel 1 collision and later flyby observations of other comets, the scientific community gained “a complete rethinking of our understanding of the formation of comets and of how they work,” Dr. A’Hearn said in a 2013 announcement of the formal end of the Deep Impact mission.

“These small, icy remnants of the formation of our solar system are much more varied, both one from another and even from one part to another of a single comet, than we ever had anticipated,” he said.

Michael Francis A’Hearn was born in Wilmington, Del., on Nov. 17, 1940, and grew up in Boston. He graduated from Boston College and then received a doctorate in astronomy from the University of Wisconsin in 1966. That year he joined the faculty at the University of Maryland. In 2000, he was named a distinguished university professor. He took emeritus status in 2011 but had continued doing research at the university.

Survivors include his wife, Maxine Ramold A’Hearn, whom he married in 1963, of University Park; three sons, Brian J. A’Hearn of Oxford, England, Kevin P. A’Hearn of Vienna, Va., and Patrick N. A’Hearn of Seattle; and five grandchildren.

Dr. A'Hearn speaks to the media with an image, behind him, of the collision of the Deep Impact impactor and the Tempel 1 comet. (Brendan McDermid/European Pressphoto AGency)

Outside the lab and classroom, Dr. A’Hearn was an avid sailor. He told friends that if he had not become an astronomer, he would most liked to have been a ship’s captain. He once combined his vocation and avocation by teaching a course on navigation by the stars. He favored casual dress, and it was sometimes said that for him, formal attire meant shorts and a Hawaiian shirt.

On Monday, he was honored posthumously with NASA’s Exceptional Public Service Medal.

After the collision with Tempel 1, the Deep Impact flyby craft continued moving through space. In November 2010 and again in January 2012 it passed close enough to other astronomical bodies to record valuable scientific data.

Scientists at NASA and at the University of Maryland remained in touch with the craft and tracked its movements. But in August 2013, they lost contact, never to regain it. They formally ended the mission a month later.

As for the comet Tempel 1, it continues on its elliptical orbit as if little or nothing ever happened.