The great educational exhibits had long been closed and only a few sections of the amusement 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 space-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!"

"Yeah," agreed the other. "I never worked so hard in all my life. But it's worth it. Are we going to set the Solar Guard back on its ear!"

Wallace laughed. "Not only that, but think of what the boss will say when we show up with it!"

"You know, Wallace," said Simms, a sly look on his face, "we could take it and use it ourselves—"

"Don't even think a thing like that!" snapped Wallace.

"Oh, of course not," said Simms hurriedly. "It doesn't pay to cross the boss. There's enough here for all of us."

"You know," mused Wallace, "there's only one thing I regret."

"What's that?" asked his partner.

"That I didn't get a chance to kick the space dust out of that punk, Cadet Manning!"

"Forget him," said Simms, waving his hand. "You'll meet him again someday. Besides, why think about him, when you've got the whole universe at your finger tips?"

"You're right. But someday I'm going to catch him and tear him apart!" snarled Wallace. "Come on. We've got to change over to atomic drive on this baby. I don't want to hang around here any longer than I have to."

"Yeah," said Simms. "Be pretty stupid if we're caught now!"

The two men climbed down into the power deck and began the job of refitting the freighter from chemical to atomic drive. Having already outfitted the vessel with atomic engines, it was a simple matter to change the exhaust, reset the feed lines, and emplace the protective lead baffles. In an hour the two spacemen were ready to blast off.

"There she is," said Simms, standing back to survey their work. "As fast as anything in space, except the Solar Guard cruisers on hyperdrive."

"O.K.," said Wallace. "Let's get out of here!"

Minutes later, in a jet car speeding along the main highway toward the exposition grounds, Captain Strong, Mike Hawks, and the three cadets of the Polaris saw a rocket ship blast off. They watched it disappear into the dark space above.

"That might be they," said Strong to Hawks. "I'd better alert the patrol ship near the space station and tell them to pick them up."

"That couldn't be Wallace and Simms, sir," said Astro.

"How do you know, Astro?" asked Strong.

"That was an atomic-powered ship. The wagon Wallace and Simms have is a chemical job. I know the sound of her jets almost as well as I do the Polaris."

Hawks looked at Strong.

"You can depend on Astro's opinion, Mike," said Strong. "He was born with a rocket wrench in his hand and cut his teeth on a reactor valve."

They soon reached the outskirts of the exposition grounds and were forced to slow down as they wound their way through the darkened streets. In the amusement section, the last of the whirlaway rides and games of chance had closed down and only the occasional roar of a caged animal in the interplanetary zoo disturbed the night.

Hawks drove the low, sleek jet car around the fair, taking a short cut through the outdoor mercuryball field and pulled up in front of the Polaris.

The five spacemen turned toward the concession site across the promenade and stopped, aghast.

"Gone!" exclaimed Strong. "Astro, you made a mistake! It was their ship we saw blasting off. It's too late to warn the space-station patrol. Wallace and Simms could be anywhere in space now!"

"But, sir," protested Astro, "I'm certain that an atomic-powered ship blasted off. And their old freighter was a chemical burner!"

"Well," said Hawks resignedly, "they're not here."

"Come on," said Strong, getting out of the jet car. "Let's take a look around."

Strong and Hawks hurried across the street to the empty lot and the three cadets followed.

"Take it easy, Astro," said Tom, when he saw the big Venusian gripping his fists in frustration. "Anyone could make a mistake."

"That's just it," said Astro. "I'm not mistaken! Those jokers must have changed over from chemical fuel to reactant drive!"

"But why?" asked Roger. "That would cost more than they could make in ten years of hauling passengers on joy rides!"

Astro whirled around and faced the two cadets. "I'm telling you the ship that blasted off from here was an atomic drive. I don't know any more than that, but I do know that!"

There was a sudden shout from Strong and the three boys hurried to the shack. The Solar Guard captain and the exposition commissioner were standing inside and playing the beam of an electric torch around the walls.

"Looks as though you were right about the atomic drive, Astro," said Strong. He flashed the light into one corner where a tangled jumble of lines lay on the floor. "That's feed-line gear for a chemical burner, and over there"—he played the light on some empty cartons—"is what's left of the crate's lead baffling it shipped in. They must have changed over to atomic drive recently."

Astro accepted the statement with a nod. It wasn't in the nature of the big cadet to boast. Now that the secret of the ship had been resolved, he turned, like the others, to the question of why?

"I think the best thing we can do," said Strong, "is to spread out and search the whole area. Might find something to indicate where they went." Commissioner Hawks nodded his head in agreement.

While Tom, Roger, and Astro searched outside, Strong and Hawks went through the drawers of the dusty desk standing in one corner.

"Nothing here but a record of the flights they made, bills for chemical fuel delivered, and the like," said Hawks at last. "They were losing money on the operation, too. Think they might have just gotten fed up and pulled out?"

Strong was rummaging around in one corner of the shack. "I'd go along with that, but for one thing, Mike," he said. "Take a look at this." He held up a small cloth bag. "There's dirt in the bottom of this bag. And there are about fifty more bags in that corner."

"Dirt!" exclaimed the commissioner.

"Yep," said Strong grimly. "So we found out who was dumping the dirt. But we still haven't found out why."

"Or where it came from," said Hawks.

Strong tossed the bag into the corner. "Well, I guess I'd better make a report to Commander Walters."

Hawks moved to the corner where the pile of chemical feed-line equipment lay on the floor. "Want to take a look at this stuff? Might be something important in it."

Strong thought a moment. "We can have the cadets do that. I want to get this report off to Walters right away, and issue an order to pick up Wallace and Simms."

"On what charges, Steve?" asked the commissioner. "I mean, what's wrong with what they've done?" The commissioner's question was based on one of the cardinal rules among all Solar Guard officers of authority. "Has the man committed any crime?"

Steve realized this and answered slowly. "They've changed over to reactor drive without a license or permission. That's a violation of the space code, section twenty-one, paragraph A. That is punishable by a suspension of space papers, and if the intention proved to be willful neglect of the code, a year on a penal asteroid. I think we can get them on that."

The captain stepped to the door and called the cadets.

"Find anything?" he asked, when they entered the shack.

"Nothing, sir," replied Tom. "Except more evidence that they changed over to atomic drive."

"That's enough" said Strong. "I'm going to send a report to Commander Walters. Is the teleceiver on the Polaris hooked up, Roger?"

"Yes, sir," replied Roger. "But Astro will have to start up the auxiliary generators to give you power."

"Very well, then," said Strong. "Corbett, you give Astro a hand on the power deck. And while we're gone, Manning, you go through that feed-line junk there in the corner and see if there's anything important in it!"

"Aye, aye, sir," replied Roger.

Strong and Hawks, followed by Tom and Astro, left the shack and hurried to the Polaris.