Carl Sagan and Isaac Asimov first met in the 1960s. “I visualized him as an elderly person (the stereotype of the astronomer at his telescope,)” Asimov recounted in his autobiography, “but what I found him to be was a twenty-seven-year-old, handsome young man; tall, dark, articulate, and absolutely incredibly intelligent.” The two went on to be good friends for more than 25 years as Asimov’s first impression was not only confirmed but amplified.

From the altogether fantastic Yours, Isaac Asimov: A Lifetime in Letters (public library), edited by Asimov’s younger brother Stanley, come a few short and infinitely delightful letters Asimov wrote to and about Sagan over the course of their friendship, brimming with equal parts good-natured humor and overwhelming respect.

There’s so much to love in this note Asimov sent to another friend on March 22, 1966:

Sagan has read half through my book on the universe and has caught one fundamental error so far. In my rendering of Eddington’s theories on stellar structure, I talked of radiation pressure. Apparently, I didn’t have to. Fortunately, it just means correcting a sentence here and there. But that’s what I need Sagan for. Anything he doesn’t catch isn’t there to be caught. If only he were a little faster about it. I said to him I realized he was awfully busy, too, but then I added with my particular brand of ingenuousness, “But then, what is your work compared to mine?” And he said, “You say it in such a way that I can take it as a joke. But you really mean it, don’t you?” So I made the best of it. I said, “Yes, I do.” A very smart fellow, that Sagan.

Jest aside, however, Asimov held profound admiration and respect for Sagan — but never revealed it in the raw, uncushioned by that same “particular brand of ingenuousness.” On December 13, 1973, he sent Sagan a short note of appreciation, with the appropriate twist of affable irreverence:

I have just finished The Cosmic Connection and loved every word of it. You are my idea of a good writer because you have an unmannered style, and when I read what you write, I hear you talking. One thing about the book made me nervous. It was entirely too obvious that you are smarter than I am. I hate that.

On June 15, 1985, Asimov sent another admiring note:

I just heard your talk on nuclear winter on Public Broadcasting. I am so proud of you, I almost burst with it. It was absolutely the sanest best speech I could imagine on the subject. It delighted me so much to find that I was on your side in every sentence of your talk.

But most heart-warming of all is this short limerick Asimov sent on the occasion of Carl Sagan’s marriage to Anne Druyan, one of the most epic love stories of modern history, in 1980:

Three loud cheers for Carl Sagan and Ann

Who today have become woman and man.

Be your lives bright as day

As the broad Milky Way

As the Big Bang with which all began.

One final note on the affectionate faux-rivalry between the two appears in a letter Asimov sent to another friend on March 15, 1986:

Half a year ago, Carl Sagan published Contact and that knocked half the sales off Robots and Empire. (These days, who can afford to buy two hard-covers?)

Yours, Isaac Asimov: A Lifetime in Letters is full of many more such gems from Asimov’s singular mind and heart. Complement it with Asimov on curiosity, risk-taking and the value of space exploration in this magnificent interview by the Muppets and Carl Sagan on the meaning of life.