Then, silently, she reached into her pocket again. With some difficulty, she drew out a small silver pin. Its 1960s design was tacky: a shooting star soaring upward, a long, cometlike tail.

For you, Ben.

She dropped the pin into the little ditch, after the diamond marker. Then she kicked dust back into the hole, and scuffed over the surface.

The footprints Armstrong and Muldoon had left behind on the Moon’s surface were still there — would remain there for many millions of years, until micrometeorite erosion finally obliterated them. But it was different on Mars. The prints she was making today would last for many months, perhaps years; but eventually the wind would cover them over.

In a few years her footprints would be erased by the wind, the first little pit she’d dug all but untraceable.

“…Natalie?”

She hadn’t said anything, she realized.

She turned to Challenger. The human artifact was a squat, white-painted toy, diminished by the distance she had come; the sun made the sky glow behind it. She could still see the pearl-gray interior of the airlock, embedded at the center of the MEM, and above that she could make out the fat cylinder of the ascent stage, with its propellant tanks clustered like berries around a stalk.

There was a single set of footsteps, crisp in the duricrust, leading from Challenger to where she stood, beyond the circular splash of dust from the MEM’s landing rocket. They looked like the first steps on a beach after a receded tide; they were the only footsteps on the planet.

By God, she thought, we’re here. We came for all the wrong reasons, and by all the wrong methods, but we’re here, and that’s all that matters. And we’ve found soil, and sunlight, and air, and water.

She said: “I’m home.”

LOST MARS

In our world, Challenger was the name — not of a Mars lander — but of the shuttle orbiter which was destroyed in January 1986, killing its crew of seven. It was a disaster which brought the U.S. space program, in 1986, to a nadir, rather than the new zenith of a Mars landing.

But it might have been very different.

After the liftoff of Apollo 11 in July 1969, an exuberant Vice President Spiro Agnew proclaimed that the U.S. “should articulate a simple, ambitious, optimistic goal of a manned flight to Mars by the end of the century.” And NASA had strong, feasible plans to achieve that goal.

America has never again been so close to assembling the commitment to go to Mars.

What went wrong in 1969? Why did President Nixon decide against the Mars option?

And how would things have worked out, in an alternate universe in which Natalie York walked on Mars?