Thor Heyerdahl, the Norwegian scientist and explorer who challenged accepted views of the spread of civilization and whose account of his “Kon-Tiki” voyage across the Pacific on a balsa wood raft stirred the imaginations of generations of readers, died Thursday.

Heyerdahl, 87, died April 18 after slipping into a coma two days earlier at a hospital near his home in the coastal town of Colla Michari on the Italian Riviera.

His career as a researcher and explorer spanned more than 60 years, and he remained active until shortly before his death. He was, in fact, working on preparations for a research project in Samoa that would have involved faculty and student researchers at the University of Maine when he became ill around Easter.

Heyerdahl underwent surgery for cancer last summer and recently had decided to forgo further medical treatment, according to his son, Thor Heyerdahl Jr. He had left Oslo earlier this year to return to the Italian home that he bought and restored in the 1950s.

“Thor was the most honest and loyal person I’ve ever met,” said Daniel Sandweiss, professor of anthropology and quaternary and climate studies at the University of Maine. He worked with Heyerdahl in Peru in the 1980s. “He would always tell you what he thought, and he would go to the mat for his friends.”

Many in the Bangor area will remember Heyerdahl’s speech at the Maine Center for the Arts in 1997 where more than a thousand people sat spellbound for two hours listening to him unravel tales of adventure and exploration.

“People turned out from every part of Maine. People came who had never set foot on the campus before,” said George Jacobson, director of UM’s Institute of Quaternary and Climate Studies. “He was magnetic because of who he was and what he had done. He had captured all of our imaginations as young people.”

Jacobson remembered Heyerdahl as a charismatic person whose personality filled the room.

UM President Peter Hoff, in a statement, said, “We are fortunate to have been touched by his warm spirit and dedication to science.” Hoff noted that Heyerdahl received an honorary degree from UM in 1998. In addition to making his 1997 Maine Center for the Arts speech, Heyerdahl spoke at the May 1998 commencement.

Heyerdahl’s connection with Maine stems from a chance meeting in a Peruvian hotel lobby in 1988 with Sandweiss, then a graduate student. The two worked together for three years and maintained a friendship over the years. Years later, with Sandweiss at UM, Heyerdahl was named a distinguished research associate at UM’s Institute of Quaternary and Climate Studies.

That title was more than honorary, and Heyerdahl remained involved in the work of the institute until his death.

Heyerdahl enjoyed his relationship with the university, in part because he had been at odds with academic institutions throughout his career, Sandweiss said.

“I think he was very pleased to be associated with us because the work we were doing was the kind of research he believed to be important,” he said. “And I think he was pleased to be associated with us as scientists. Of course, it meant a great deal to us to have someone of his stature associated with the institute.”

Last fall, Heyerdahl approached the university and Sandweiss about participating in a project he was planning to research step pyramids in Samoa. Sandweiss met with Heyerdahl in February to discuss the project.

“He really wanted the university to be involved in this, and we would have been,” Sandweiss said.

A storyteller who enjoyed relating amusing tales about his life, Heyerdahl also loved to laugh “a full laugh, with his head thrown back, his mouth open and his face turning red,” Sandweiss said.

His fame came from his exploits on the Kon-Tiki voyage as a result of his early training as a scientist, although some discounted his research because he lacked advanced academic degrees. But his ideas on the spread of civilization around the world broke ground in that field and are being studied by a new crop of researchers looking into early cultural exchanges.

“Thor was as good a scientist as anyone I’ve ever met,” Sandweiss said. “His ideas were new and went very much against what established researchers were thinking about people, especially people in the Pacific. But he not only had the ideas, he tested them.”

Heyerdahl was born in Larvik in 1914 and enrolled in the University of Oslo, where he studied zoology and geography. At 22, he traveled to Polynesia to study the animals and vegetation in the Marquesas Islands. During that time Heyerdahl developed the theory that Polynesia had been populated by ancient travelers who arrived there on balsa rafts from South America.

That idea ran contrary to the prevailing theories, which held that the first inhabitants of Polynesia had come from Asia.

After World War II, during which Heyerdahl served in the Free Norwegian Forces, he continued to pursue the idea of westward travel and, in the face of rejection from established scientists, set out to prove it could have happened. He built the raft Kon-Tiki out of balsa, a material that would have been available to the people of South America thousands of years ago, and, with a crew of five, traveled the 4,300 miles from Peru to Polynesia.

The expedition proved that such voyages were possible and set Heyerdahl on a lifelong path of archaeological exploration and research into questions of long-distance migration and cross-cultural contacts in ancient times.

“The Kon-Tiki expeditions opened my eyes to what the ocean really is,” Heyerdahl wrote in the foreword to the 35th edition of the book recounting the expedition. “It is a conveyor and not an isolator. The ocean has been man’s highway from the days he built the first buoyant ships, long before he tamed the horse, invented wheels and cut roads through the virgin jungles.”

He continued his research in the Pacific with expeditions in the Galapagos Islands and at Easter Island, and later attempted two voyages across the Atlantic Ocean from Morocco to Barbados in replicas of ancient reed boats, which he called the Ra Expeditions. His research continued with projects that took him to the Maldives in the Indian Ocean and back to Easter Island, before heading to Tucume, Peru. In 1994, he moved to the Canary Islands to investigate a group of pyramids there. There he helped to found the Foundation for Exploration and Research on Cultural Origins, which awards annual research grants for scholars to explore the interactions between ancient peoples.

He wrote several books after “Kon-Tiki,” including “Aku-Aku,” about his work on Easter Island, and “The Ra Expeditions.” His last book, “The Hunt for Odin,” was published in 2001 in Norwegian.

Heyerdahl was considered by some a visionary whose ideas and methods set an example for a generation of researchers who came after, and as a man whose intellectual and physical exploration continued until his death.

“The age of exploration is not over,” he told a Maine audience in 1997. “With our advanced technology, we are on the threshold of great discoveries. But one thing is for sure – we’re entirely mistaken if we think that to make those great new discoveries we have to go to other planets. From my experience, I can tell you that we do not know nearly enough about the origins of ancient cultures. There is nothing in the solar system that could give us the surprises that are left on this planet.”