In the words of Carl Sagan

Margaret Hamilton, who led the team that wrote the code that put humans on the moon, has died at 90 🕊️

It's just a bad model, we claim, the straight line, dedicated exploration of every star in the galaxy." Source: Carl Sagan, interview. The paper: W. I. Newman & C. Sagan, "Galactic civilizations: Population dynamics and interstellar diffusion," Icarus 46 (1981), 293–327.

And when you do the diffusion physics with the appropriate diffusivity, that is, the time to random walk, there are many cases in which the time for an advanced civilization to fully explore the galaxy in the sense of visiting every star system is considerably longer than the age of the galaxy.

Interstellar space flight is going to be hard, you're going to go slow, you're going to go to the nearest star systems first, you're going to explore those stars. It is not a straight line but a diffusion question.

What are we talking about? That they're sending out 400 billion spacecraft, all at once, simultaneously, to every star in the galaxy? Not at all.

William Newman and I published a paper on this very point, in which we point out: Imagine there is a civilization that has capable interstellar spacecraft and now they start exploring.

The galaxy is 10 billion years old, they should be here. And if you say you can't travel at the speed of light, take a tenth of the speed of light, a hundredth of the speed of light, still much less than the age of the galaxy.

Carl Sagan 🧵 "The Fermi paradox essentially says that if there's extraterrestrial high technology intelligence anywhere they should have been here because if they travel at the speed of light, the galaxy is 100,000 light-years across, it takes you 100,000 years to cross the galaxy.

Carl Sagan summarizes the 2,000+ year history of books in 2 minutes: Video Credit: Cosmos Studios

A humbling reminder from Carl Sagan: Even if you read 1 book a week for your entire life, you'd only scratch the surface of human knowledge.

Even in darkness, we glow. In this image of Earth taken by the Artemis II crew, we can see the electric lights of human activity. In the lower right, sunlight illuminates the limb of the planet.

The Moon is around 250,000 miles from Earth. It's so far that you could fit the other seven planets into the space between Earth and the Moon.

“Knowing a great deal is not the same as being smart; intelligence is not information alone but also judgement, the manner in which information is coordinated and used.” — Carl Sagan

‘Earthrise’ was captured by astronaut William Anders during the Apollo 8 mission, while orbiting the Moon with Frank Borman and Jim Lovell. Anders later reflected, “We came all this way to explore the Moon, and the most important thing is that we discovered the Earth.”

Think of the rivers of blood spilled by all those generals and emperors so that in glory and in triumph they could become the 'Momentary' masters of a 'Fraction' of a 'Dot' — Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space

“We are one planet. One of the great revelations of the age of space exploration is the image of the earth finite and lonely, somehow vulnerable, bearing the entire human species through the oceans of space and time.” — Carl Sagan

“Houston, we’ve had a problem.” Apollo 13 launched on April 11, 1970, with Jim Lovell, Jack Swigert and Fred Haise bound for the Moon. An oxygen tank explosion crippled the spacecraft on April 13, and the mission was forced into an emergency return trajectory.

“We are an impossibility in an impossible universe.” ― Ray Bradbury

I've already got the prize. The prize is the pleasure of finding the thing out, the kick in the discovery, the observation that other people use it—those are the real things; the honors are unreal to me. I don't believe in honors. — Richard Feynman