Norfolk, Va.

MOST technological advances are actually just improvements. One thing builds on the next: from shoddy to serviceable, from helpful to amazing. First you had a carriage, then a car, and then an airplane; now you have a jet. You improve on what is there. Technological advances are like that.

Except for the one that involved landing on the Moon. When a human went and stood on the Moon and looked back at the Earth, that was a different kind of breakthrough. Nothing tangible changed when Neil Armstrong’s foot dug into the lunar dust and his eyes turned back at us. We didn’t get faster wheels or smaller machines or more effective medicine. But we changed, fundamentally. What had been unknown, was known. What had been unseen was seen. And our human horizon popped out 200,000 miles. Forever, we would see the Earth differently, because we had seen it from someplace truly foreign.

This is why Mars is important. When we get a human to Mars — in the next few decades, NASA has predicted — our horizon will expand 1,000 times farther, and it will never go back.

Watching the first images from the rover, Curiosity, which landed on Mars early this month, I was reminded of a short story by Ray Bradbury called “Mars Is Heaven!” In it, Mars is populated by aliens who fool visiting Earthlings into thinking they’re in a familiar environment before murdering them. It’s about how stupid nostalgia is, how it tricks us into wanting things that were never that great in the first place. What strikes me about the story is that, just over 60 years ago, someone could seriously write about aliens on Mars.