Throughout this sequence our imaginary observer is struck above all by the earth's isolation.
Sunlight, starlight and cosmic rays, and occasionally some interplanetary debris, arrive
at the earth's surface, but in all those eons nothing save a little hydrogen and helium leaves
the planet. And then, less than 20 years ago, the planet suddenly begins, like a dandelion gone to seed,
to fire tiny capsules throughout the inner solar system.
First they go into orbit round the earth and then to the plant's lifeless natural satellite, the moon.
Six tiny capsules, larger than the rest, set down on the moon and from each two small organisms emerge,
briefly explore their immediate surroundings and then sprint back to the earth,
having tentatively extended a toe into the cosmic ocean.

- Carl Sagan, "The Solar System"

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