Tom glanced at the astral chronometer over the control board of the Polaris and sighed with relief. It was nine P.M. He turned to the intercom.

"Attention, please! Attention, please! The exhibit is now closing for the night. All visitors will kindly leave the ship immediately." He repeated the announcement again and turned to smile at the last lingering youngster ogling him before being yanked toward an exit by a tired and impatient mother.

The hatch to the radar bridge opened and Roger climbed down the ladder to flop wearily in the pilot's seat in front of the control panel.

"If one more scatterbrained female asks me how the astrogation prism works," groaned the blond cadet, "I'll give it to her and let her figure it out for herself!"

Astro joined them long enough to announce that he had made sandwiches and brewed hot chocolate. Tom and Roger followed him back to the galley.

Sipping the hot liquid, the three cadets looked at each other without speaking, each understanding what the other had been through. Even Astro, who normally would rather talk about his atomic engine than eat, confessed he was tired of explaining the functions of the reaction fuel force feed and the main valve of the cooling pumps.

"The worst of it is," sighed Astro, "they all pick on the same valve. What's so fascinating about one valve?"

Tom's job on the control deck was less tiring, since his was more of a command post, which demanded decisions, as conditions arose, rather than a fixed routine that could be explained. But even so, to be asked over and over what the astral chronometer was, how he could read time on Earth, Mars, Venus, Titan, Ganymede, and all the satellites at the same time was wearing on the toughest of young spirits.

Eager to forget the grueling day of questions and answers, the cadets turned their thoughts to the mysterious midnight activity that had been taking place around the spaceship concession during the last ten days.

"I just can't figure out what those guys are up to," said Roger, blowing on his hot chocolate. "We've watched those guys for over a week now and no one has even come near them with anything that could be smuggled."

"Could be a small package," suggested Astro, his mouth full of ham sandwich. "Somebody could take a ride and slip it to them."

"Hardly," said Tom. "Remember, that ship blasts off like she's loaded to the nose with cargo. And then she comes back like a feather. You can tell by the sound of her jets. So it wouldn't be anything small enough for someone to carry."

"Yeah, I guess you're right," agreed Astro.

"Well," said Tom finally, "I'm stumped. I think the only thing left to do is to decide if it's anything important enough to tell Captain Strong about. Working on the Polaris twelve hours a day and staying up all night to watch those two jokers has me all in."

Roger and Astro looked at each other and then silently nodded their agreement.

"O.K.," said Tom, "we'll go to the skipper's hotel in Venusport and tell him the whole thing. Let's see what he makes of it."

At that moment Captain Strong was in the office of Exposition Commissioner Mike Hawks trying to make sense out of a series of reports that had landed on the commissioner's desk. Hawks watched him carefully as he studied the papers.

"You say this is the ninth report you've received since the fair opened, Mike?" asked Strong finally.

Hawks nodded. He hadn't known whether to laugh off or seriously consider the nine space skippers' reports that the sky over the exposition site was dirty.

"Yes, Steve," he said. "That one came from the skipper of an express freighter. He blasted off this morning and ran through this so-called dirt. He thought it was just a freak of nature but reported it to be on the safe side."

"I don't suppose he took a sample of the stuff?"

"No. But I'm taking care of that," replied Hawks. "There's a rocket scout standing by right now. Want to come along?"

"Let me finish these reports first."

"Sure thing."

As Strong carefully checked each report, Commissioner Hawks rose and began to stride restlessly back and forth across the spacious office. He stopped in front of the window and stared out over the exposition grounds, watching the thousands of holiday visitors streaming in and out of the buildings, all unaware of the strange mystery in the sky above them. Hawks' attention was drawn to the giant solar beacon, a huge light that flashed straight out into space, changing color every second and sending out the message: "Quis separabit homo"—Who shall separate mankind?

This beacon that at the beginning of the exposition had reached into the black void of space like a clean bright ray was now cloudy and murky—the result of the puzzling "dirty sky."

"All right, Mike," Strong announced suddenly. "Let's go."

"Get anything more out of those reports?" asked Hawks, turning back to his desk.

"No," replied the Solar Guard officer. "They all tell the same story. Right after blast-off, the ships ran into a dirty sky."

"Sounds kind of crazy, doesn't it?"

"Crazy enough to check."

Hawks pressed a button on the desk intercom.

"Yes, sir?" replied a metallic voice.

"Have the rocket scout ready for flight in five minutes," Hawks ordered. He snapped off the intercom without waiting for a reply and turned to Strong. "Let's go, Steve."

The two veteran spacemen left the office without further comment and rode down in the vacuum elevator to the highway level. Soon they were speeding out to the spaceport in Hawks' special jet car.

At the blast-pitted field they were met by a young Solar Guard officer and an elderly man carrying a leather case, who were introduced as Lieutenant Claude and Professor Newton.

While Claude prepared the rocket scout for blast-off, Strong, Hawks, and Newton discussed the possibility of lava dust having risen to great heights from another side of the planet.

"While I'm reasonably sure," stated Newton, "that no volcano has erupted recently here on Venus, I can't be sure until I've examined samples of this so-called dirt."

"I'll have Lieutenant Claude contact the University of Venus," said Hawks. "Their seismographs would pick up surface activity."

Claude stuck his head out of the hatch and reported the ship ready for blast-off. Strong followed the professor and Hawks aboard and strapped himself into an acceleration chair. In a moment they were blasting through the misty atmosphere of Venus into the depths of space.

Fifteen minutes later, Hawks and Strong were standing on the hull of the ship in space suits, watching the professor take a sample of a dirty black cloud, so thick it was impossible to see more than three feet. Strong called to the professor through the spacephone.

"What do you make of it, sir?" he asked.

"I wouldn't want to give you a positive opinion without chemical tests," answered the professor, his voice echoing in Strong's fish-bowl helmet. "But I believe it's one of three things. One, the remains of a large asteroid that has broken up. Two, volcanic ash, either from Venus or from Jupiter. But if it came from Jupiter, I don't see how it could have drifted this far without being detected on radar."

Now, holding a flask full of the black cloud, the professor started back to the air lock.

"You said three possibilities, professor," said Strong.

"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 spacemen 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 space suits.

The hatch opened again and the two spacemen 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 Space 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 analyzed 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 space?"

"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!"