On the side of the cliff, I hung from a thin metal cable. Hundreds of feet below me, the jagged red rocks of the Martian valley floor pointed up at me like deadly spears.

The temperature had risen from minus one hundred degrees Fahrenheit to a nice, warm minus twenty degrees Fahrenheit. Wind pushed at my body, making me sway from side to side. But it could have been worse. I could have been stuck in a sandstorm, with grains of sand hitting me at sixty miles an hour, rattling off my titanium shell and blinding me completely.

As it was, I had a good view. On Mars, at midday, when the sand isn't blowing, the sun is blue against a butterscotch-colored sky. The clouds are barely more than stretched-out strings of fog, lighter blue than the sun.

I could look across the entire valley and see the oranges and reds of Martian soil. Nearly ten miles away, a gigantic dome held all two hundred of the scientists and tekkies who founded the first colony on Mars. Under that dome was oxygen and water and warmth and food(all the things humans needed to survive.

Out here? There was no oxygen. No water. No warmth. And no food.

And, of course, those jagged rocks waited for any mistakes. From where I was, it wouldn't matter much that gravity on Mars is about a third of Earth's gravity. If my grip on the cable slipped, those rocks would tear through my robot body like daggers. What made it worse was that I had a passenger strapped onto my back.

My job was to make it to the bottom of the cliff with both of us undamaged.

At the top, the metal cable was attached to a long spike driven deep into the soil. All three hundred feet of the cable dangled from this spike.

I held on to the cable with a gripper in each hand. Each gripper clamped the cable securely with much more power than I could have gotten just by using my fingers.

The trick was to unclamp the gripper in my right hand and hold on with the gripper in my left hand. Then I had to bring my free right hand down and reclamp at a level below my left hand. Once the right-hand grip was secure, I unclamped the left and reclamped it below the right. And so on. It was slow work that took a lot of concentration.

One thing made this easier. My lower body was on wheels, so all I had to do was let myself roll down the cliff. Slowly. Very slowly.

I was halfway down when it happened.

As I leaned against the cliff, my right wheel hit a loose portion of rock. It broke away, clattering down the cliff. My right side swung inward, spinning me sideways.

This wouldn't have been a problem if I'd been clamping the cable with both grippers. But I was only holding with my left gripper.

In panic, I grabbed at the cable with my right hand.

Because I was spinning, I missed the cable and jammed my hand into the cliff. This pushed me away from the cliff too hard. For a second, I was like a pendulum. With less gravity on Mars than on Earth, my action shot me six feet away from the side of the cliff and then banged me against rock on the return.

It felt like I'd been slammed with a baseball bat. Keeping my grip on the cable with my left hand, I fought to find the cable with my right.

But I was out of balance. Especially with a passenger on my back.

My wheels began to roll upward on the cliff wall as the weight on my back pulled me upside down and backwards.

The cable twisted more.

Still, I tried to find a grip with my right hand.

Nothing.

Then . . .

Snap.

The buckle keeping the passenger on my back opened, and suddenly I had no passenger.

"Rawling!" I shouted, as I watched the downward tumble of arms and legs. "Rawling!"

Seconds later, there was an explosion of dust as the body smashed into the rocks.

I had failed in my mission.