"The third," replied the professor, "could be--"

The professor was interrupted by Lieutenant Claude calling over the intercom.

"Just received a report from the University of Venus, sir!" said the young officer. "There's been no volcanic activity on Venus in the last ten years serious enough to create such a cloud."

Strong waited for the professor's reaction, but the elderly man was already entering the air lock. Before Strong and Hawks could catch up to him, the air-lock hatch slammed closed.

"Hey," exclaimed Strong, "what does he think he's doing?"

"Don't worry about it, Steve," replied Hawks. "He probably forgot we were out here with him, he's so concerned about this dirt. We'll just have to wait until he's out of the air lock."

The Solar Guard officer nodded, then looked around him at the thick black cloud that enveloped the ship. "Well," he said, "one of the professor's theories has been knocked out."

"Yes," replied Hawks. "Which means this stuff is either the remains of a large asteroid or--"

"The third possibility," finished Strong, "which the professor never explained."

Suddenly the air-lock hatch opened again and the two s.p.a.cemen stepped inside. Closing the hatch behind them, they waited until the pressure was built up again to equal that of the ship, and then they removed their helmets and s.p.a.ce suits.

[Ill.u.s.tration: _The hatch opened again and the two s.p.a.cemen entered the air lock_]

Leaving the air lock and walking down the companionway, Hawks suddenly caught Strong by the arm.

"Have you considered the possibility of this cloud being radioactive, Steve?" he asked.

Strong nodded slowly. "That's all I've been thinking about since I first heard about it, Mike. I think I'd better report this to Commander Walters at s.p.a.ce Academy."

"Wait, Steve," said Hawks. "If you do that, Walters might close the exposition. Wait until you get a definite opinion from Professor Newton."

Strong considered a moment. "I guess a few more minutes won't make a lot of difference," he said finally. He realized how important the exposition was to his old friend. But at the same time, he knew what would happen if a radioactive cloud suddenly settled on the city of Venusport without warning. "Come on. Let's see what the professor has to say about this stuff."

They found the professor on the control deck bending over a microscope, studying samples taken from the flask. He peered intently into the eyepiece, wrote something on a pad, and then began searching through the pages of a reference book on chemicals of the solar system.

Lieutenant Claude stepped up to Hawks and saluted sharply. "Power deck reports they've got a clogged line, sir. It's in the gas exhaust."

Strong and Hawks looked at each other, and then Hawks turned to the young officer. "Send a couple of men outside to clear it."

"Aye, aye, sir," said Claude, and then hesitated. "Shall the men wear lead suits against possible radioactivity, sir?"

Before Hawks could answer, Newton turned to face the three men. The professor was smiling. "No need to take that precaution, Lieutenant. I never did tell you my third opinion, did I, Captain Strong?"

"Why, no, you didn't, sir," said Strong.

The professor held up a sheet of paper. "Here's your answer. Nothing but plain old Venusport topsoil. Pure dirt!"

"What?" exclaimed Hawks hastily, reaching for the paper.

"Well, blast me for a Martian mouse," muttered Strong under his breath.

"But how?"

Newton held up his hand. "Don't ask me how it got here. That isn't my line of work. All I know is that, without a doubt, the black cloud is nothing more than dirt. Plain ordinary dirt! And it comes from the area in and around Venusport. As a matter of fact, certain particles I a.n.a.lyzed lead me to believe it came from the exposition site!"

Hawks looked at Newton dumbfounded. "By the craters of Luna, man, we're a thousand miles over the exposition!"

The professor was stubborn. "I can't tell you how it got here, Commissioner Hawks. But I do know it's Venusian dirt. And that's final!"

Hawks stared at the elderly man for a second, still bewildered. Then he suddenly smiled and turned to Claude. "As soon as that exhaust is cleared, blast off for Venusport, Lieutenant. I'm going to find out who dirtied up the sky!"

Two hours later, when Captain Strong returned to his hotel in Venusport with Mike Hawks, he was surprised to see the three cadets of the _Polaris_ crew slumped, sleepy-eyed, on a couch in the lobby.

"What are you doing here, boys?" he asked.

The three cadets came to attention and were wide awake immediately. Tom quickly related their suspicions of Wallace and Simms.

"And we've watched them every night, sir," Tom concluded. "I don't know what it is, but something certainly is going on in that shack they use for an office."

"Yes, sir," agreed Astro, "and no one is going to fool me about a rocket ship. I know when they blast off loaded and return light."

Strong turned to Hawks who said quietly, "Wallace and Simms are the only ones in this whole area that blast off regularly without a customs search."

"You mean," stammered Strong, "Wallace and Simms are dumping"--he could hardly say the word--"_dirt_ in s.p.a.ce?"

"They have a ship. The cadets say the ship blasts off loaded and returns light. And we've got the sky full of dirt. Venusian dirt!"

"But why?"

"I suggest we go out to the exposition grounds right now and ask them!"

said Hawks coldly. "And believe me, they'd better have some rocket-blasting good answers!"

CHAPTER 5

The great educational exhibits had long been closed and only a few sections of the amus.e.m.e.nt park of the big exposition remained open. The giant solar beacon, its brilliant colors changing every second, maintained a solemn solitary watch over the exhibition buildings, while here and there groups of fair visitors wandered wearily back to their hotels.

There was a sudden flurry of activity at the s.p.a.ce-ride concession. Gus Wallace and Luther Simms tumbled out of the shack and raced into their ship. Once inside the ancient craft, they secured the hatch and turned toward each other smiling broadly. Wallace stuck out his hand.

"Put 'er there, Simms. We did it!"

The two men shook hands heartily.

"By the craters of Luna," said Simms, "I thought we'd never make it! And if we did, that it wouldn't be there!"

"But it was, Simms! It was! And now we've got it!"