“It was a tremendous sensation,” he remembered. “Some crazy people thought it was a propaganda story, a hoax.”

He still ardently hoped that humans would continue to explore the solar system. We talked about Mars and sending humans there, something he had written about for many years. “I should say that we could send a manned flight to Mars in 10 years if there was the incentive, but certainly in 20 years,” he said.

I asked him about terraforming Mars, changing the Red Planet so that it would be more like Earth. He wrote a book about the process in the 1990s, trying to use software on his computer to model how Mars would change with terraforming. I asked him if his ideas had changed since then.

“Start terraforming Mars by remote-control systems,” he said. “It'll be a joint process, humans and machines.” Then he added mischievously, “I hope the machines don't get annoyed with us!”

Clarke turned 90 on 16 December 2007. The government of Sri Lanka organized a birthday celebration for him. A few days before his birthday, when asked how he felt, he replied with characteristic humor, “Well, I actually don't feel a day older than 89. Of course, some things remind me that I am indeed qualified as a senior citizen. As Bob Hope once said, you know you are getting old when the candles cost more than the cake.”

“In my time, I have been very fortunate to see many of my dreams come true,” he noted. “Growing up in the 1920s and 1930s, I never expected to see so much happen in the span of a few decades.”

Clarke sat next to Walter Cronkite on the historic day when Neil Armstrong first walked on the moon. I asked Clarke about his memories of that day.

“I have no specific memories,” he said. “I have the audiotapes and videotapes. My polio episode has wiped out so many of those memories.”

Since the 1980s, Clarke had been afflicted with what is known as postpolio syndrome (PPS), which is characterized by muscle fatigue, joint pain, and some memory lapses. It is a consequence of the polio episode he had in 1959 (from a vaccination). He had to use a wheelchair for years. Yet at the time I met him, he was still keeping a pretty full schedule and answering e-mail quickly, with the help of his secretary. While I was waiting for Clarke to catch his breath, I asked his assistant, Gunawardene, to describe how Clarke worked with e-mail.

“Sir Arthur's day has shrunk in terms of waking hours because of his postpolio condition and because he is not as strong as he used to be,” Gunawardene said. “He would wake up around 8:00, have a leisurely breakfast, and then come to his desk between 9:30 and 10:00 in the morning. Then he would look at the overnight e-mail and decide which he would answer immediately and which could wait a couple of days.”

I asked Clarke if he remembered his interaction with Stanley Kubrick on the screenplay of 2001: A Space Odyssey(1968), which brought him immense fame.

“There weren't any real fights,” he quipped. Clarke spent a few weeks holed up in an apartment on Manhattan's Upper West Side finishing the screenplay, based on his 1948 short story “The Sentinel.”

We came back to the present. He said that in many ways, being confined to a wheelchair had left his mind free to roam the cosmos. He was spending a lot of time thinking.

“My main interest is astronomy and the discovery of extraterrestrial life,” he said. “I'm sure the ETs are all over the place. I am surprised and disappointed they haven't come here already—assuming they haven't. Maybe they are waiting for the right moment to come. And I hope they are not hungry!”