“Look, Don,” I asked him, “have you ever seen anything like that?”

“No,” he answered after a careful examination. “What holds it together? It hasn’t changed its shape for the last two minutes.”

“That’s what puzzles me. Whatever it is, it should have started to break up by now, with all that disturbance going on around it. But it seems as stable as ever.”

“How big would you say it is?”

I switched on the calibration grid and took a quick reading.

“It’s about five hundred miles long, and half that in width.”

“Is this the largest picture you can get?”

“I’m afraid so. We’ll have to wait until it’s closer before we can see what makes it tick.”

Don gave a nervous little laugh.

“This is crazy,” he said, “but do you know something? I feel as if I’m looking at an amoeba under a microscope.”

I did not answer; for, with what I can only describe as a sensation of intellectual vertigo, exactly the same thought had entered my mind.

We forgot about the rest of the cloud, but luckily the automatic cameras kept up their work, and no important observations were lost. From now on we had eyes only for that sharp-edged lens of gas that was growing minute by minute as it raced toward us. When it was no farther away than is the moon from Earth, it began to show the first signs of its internal structure, revealing a curious mottled appearance that was never quite the same on two successive sweeps of the scanner.

By now, half the Observatory staff had joined us in the radar room, yet there was complete silence as the oncoming enigma grew swiftly across the screen. It was coming straight toward us; in a few minutes it would hit Mercury somewhere in the center of the daylight side, and that would be the end of it—whatever it was. From the moment we obtained our first really detailed view until the screen became blank again could not have been more than five minutes; for every one of us, that five minutes will haunt us all our lives.