He still had vivid memories of that first interview, for the weightless microphone, wavering only slightly in the draught of air from the ventilators, had almost hypnotized him into incoherence. Yet no one would have guessed: his voice had its normal, professional smoothness.

They had been twenty million miles behind the comet, but swiftly overtaking it, when he had trapped Martens in the observatory and thrown that opening question at him.

“Dr Martens,” he began, “just what is Randall’s comet made of?”

“Quite a mixture,” the astronomer had answered, “and it’s changing all the time as we move away from the sun. But the tail’s mostly ammonia, methane, carbon dioxide, water vapour, cyanogen—”

“Cyanogen? Isn’t that a poison gas? What would happen if the Earth ran into it?”

“Not a thing. Though it looks so spectacular, by our normal standards a comet’s tail is a pretty good vacuum. A volume as big as Earth contains about as much gas as a matchbox full of air.”

“And yet this thin stuff puts on such a wonderful display!”

“So does the equally thin gas in an electric sign, and for the same reason. A comet’s tail glows because the sun bombards it with electrically charged particles. It’s a cosmic sky-sign; one day, I’m afraid, the advertising people will wake up to this, and find a way of writing slogans across the solar system.”

“That’s a depressing thought—though I suppose someone will claim it’s a triumph of applied science. But let’s leave the tail; how soon will we get into the heart of the comet—the nucleus, I believe you call it?”

“Since a stern chase always takes a long time, it will be another two weeks before we enter the nucleus. We’ll be ploughing deeper and deeper into the tail, taking a cross section through the comet as we catch up with it. But though the nucleus is still twenty million miles ahead, we’ve already learned a good deal about it. For one thing, it’s extremely small—less than fifty miles across. And even that’s not solid, but probably consists of thousands of smaller bodies, all milling round in a cloud.”