was going out. Of course it can't be," he added hastily. "Not possibly! But there is an extraordinary number of sunspots. Maybe they'll clear. But meanwhile the amount of heat reaching us is dropping. As far as I know there's no parallel to it. Night temperatures are thirty degrees lower than they should be. Not only here, either, but at all the robot weather stations that have been spotted around the planet. They average forty below zero minimum, instead of ten. And—there is that terrific lot of sunspots...."

He looked hopefully at Massy. Massy frowned. Sunspots are things about which nothing can be done. Yet the habitability of a borderline planet, anyhow, can very well depend on them. An infinitesimal change in sun heat can make a serious change in any planet's temperature. In the books, the ancient mother planet Earth was said to have entered glacial periods through a drop of only three degrees in the planet-wide temperature, and to have been tropic almost to its poles from a rise of only six. It had been guessed that glacial periods in the planet where humanity began had been caused by coincidences of sunspot maxima.

This planet was already glacial to its equator. There was a genuinely abnormal number of sunspots on Lani, its sun. Sunspots could account for worsening conditions here, perhaps. That message from the inner planet could be bad, thought Massy, if the solar constant drops and stays down a while. But aloud he said:

"There couldn't be a really significant permanent change. Not quickly, anyhow. Lani's a Sol-type star, and they aren't variables, though of course any dynamic system like a sun will have cyclic modifications of one sort or another. But they usually cancel out."

He sounded encouraging, even to himself. But there was a stirring behind him. Riki Herndon had come silently into her brother's office. She looked pale. She put papers down on her brother's desk.

"But," she said evenly, "while cycles sometimes cancel, sometimes they enhance each other. They heterodyne. That's what's happening."

Massy scrambled to his feet, flushing. Herndon said sharply:

"What? Where'd you get that stuff, Riki?"

She nodded at the sheaf of papers she'd just laid down.

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