crystal, it soared above the cluster of buildings that surrounded the grassy quadrangle and dominated Space Academy like a translucent giant.

The cadets stepped off the slidewalk as it glided past the Tower building and ran up the broad marble stair. At the huge main portal, Tom stopped and looked back over the Academy grounds. All around him lay the evidence of mankind's progress. It was the year 2353, when Earthman had long since colonized the inner planets, Mars and Venus, and the three large satellites, Moon of Earth, Ganymede of Jupiter, and Titan of Saturn. It was the age of space travel; of the Solar Alliance, a unified society of billions of people who lived in peace with one another, though sprawled throughout the universe; and the Solar Guard, the might of the Solar Alliance and the defender of interplanetary peace. All these things Tom saw as he stood in the wide portal of the Tower Building, flanked by Astro and Roger.

Turning into the Tower, the three cadets went directly to the office of their unit commander. The training program at Space Academy consisted of three cadets to a unit, with a Solar Guard officer as their teacher and instructor. Steve Strong, captain in the Solar Guard, had been their cadet instructor since the unit had been formed and he now smiled a welcome as the cadets snapped to attention in front of his desk.

"Polaris unit reporting as ordered, sir," said Tom, handing over the audiogram order he had received the day before.

"Thank you, Corbett," said Strong, taking the paper. "At ease."

The three boys relaxed and broke into wide grins as Strong rounded his desk and shook hands with each of them.

"Glad to have you back, boys," he said. "Did you enjoy your leave?"

"And how, sir," replied Tom.

"Tom's mother showed us a whale of a good time," chimed in Roger.

"And how she can cook!" Astro licked his lips involuntarily.

"Well, I hope you had a good rest—" said Strong, but was suddenly interrupted by the sound of a small bell. Behind his desk a small teleceiver screen glowed into life to reveal the stern face of Commander Walters, the commander of Space Academy.

Strong turned to the teleceiver and called, "Yes, Commander Walters?"

"Did the Polaris unit arrive yet, Steve?" asked the commander.

"Yes, sir," replied Strong. "They're here in my office now, sir."

"Good," said the commander with a smile. "I just received a report the exposition will open sooner than expected. I suggest you brief the cadets and raise ship as soon as possible."

"Very well, sir," answered Strong. The screen darkened and he turned back to the cadets. "Looks like you got back just in time."

"What's up, sir?" asked Tom.

Strong returned to his chair and sat down. "I suppose you've all heard about the Solar Exposition that opens on Venus next week?"

Tom's eyes lit up. "Have we! That's all the stereos and visunews and teleceivers have been yacking about for weeks now."

"Well," said Strong with a smile, "we're going!"

The three cadets couldn't restrain themselves and burst out in a happy shout. Then Roger calmed down enough to comment, "Sounds more like another vacation than an assignment, sir."

"Hardly, Manning," replied Strong. "You see, every industry, society, organization, and governmental agency is setting up exhibits at the exposition to show the people what's taking place in their part of the solar system. There'll also be an amusement section." Strong chuckled. "I've seen pictures of some of the tricks and rides they've developed to entertain the younger generation. Believe me, I'd rather take full acceleration on a rocket ship than ride on any of them."

"But what will we do, sir?" asked Tom.

"Our job is very simple. We're to take the Polaris to the exposition and land on the fairgrounds. When the fair opens, we show all the visitors who are interested, everything about her."

"You mean we're going to be"—Roger swallowed—"guides?"

"That's right, Manning," said Strong. "You three will guide all visitors through the Polaris."

"How long will we be there, sir?" asked Tom.

"A month or so, I guess. The Polaris will be the first Academy exhibit. When you leave, another unit will replace you with their ship and do the same thing."

"But—but—" stammered Astro, "what will we say to them? The visitors, I mean?"

"Just answer all their questions, Astro. Also, make up a little speech about the functions of your particular station."

Strong looked at his watch and rose to his feet. "It's getting late. Check the Polaris over and stand by to raise ship in an hour."

"Yes, sir," said Tom.

The cadets came to attention, preparing to leave.

"One thing more! Don't get the idea that this is going to be a space lark," said Strong. "It's very important for the people of the Solar Alliance to know what kind of work we're doing here at the Academy. And you three have been selected as representatives of the entire Cadet Corps. So see that you conduct yourselves accordingly. All right, dismissed!"

The three cadets saluted sharply and filed out of the room, their skipper's final words ringing in their ears.

Fifteen minutes later, having packed the necessary gear for the extended trip, the Polaris unit rode the slidewalk through the grassy quadrangle and the cluster of Academy buildings, out toward the spaceport. In the distance they could see the rocket cruiser Polaris, poised on the launching ramp, her long silhouette outlined sharply against the blue sky. Resting on her four stabilizer fins, her nose pointed toward the stars, the ship looked like a giant projectile poised and ready to blast its target.

"Look at her!" exclaimed Astro. "If she isn't the most beautiful ship in the universe, I'll eat my hat."

"Don't see how you could," drawled Roger, "after the way you put away Mrs. Corbett's pies!"

Tom laughed. "I'll tell you one thing, Roger," he said, pointing to the ship, "I feel like that baby is as much my home as Mom's and Dad's house back in New Chicago."

"All right, all right," said Roger. "Since we're all getting sloppy, I have to admit that I'm glad to see that old thrust bucket too!"

Presently the three cadets were scrambling into the mighty spaceship, and they went right to work, preparing for blast-off.

Quickly, with sure hands, each began a systematic check of his station. On the power deck Astro, a former enlisted Solar Guardsman who had been admitted to the Cadet Corps because of his engineering genius, stripped to the waist and started working on the ship's massive atomic engines. A heavy rocketman's belt of tools slung around his waist, he crawled through the heart of the ship, adjusting a valve here, turning a screw there, seeing that the reactant feeders were clean and clear to the rocket firing chambers. And last of all he made sure the great rocket firing chambers were secure and the heavy sheets of lead baffling in place to protect him from deadly radioactivity.

On the radar bridge in the nose of the ship, Roger removed the delicate astrogation prism from its housing and cleaned it with a soft cloth. Replacing it carefully, he turned to the radar scanner, checking the intricate wiring system and making sure that the range finders were in good working order. He then turned his attention to the intercom.

"Radar bridge to control deck," he called. "Checking the intercom, Tom."

Immediately below, on the control deck, Tom turned away from the control panel. "All clear here, Roger. Check with Astro."

"All clear on the power deck!" The big Venusian's voice boomed over the loud-speaker. The intercom could be heard all over the ship unless the many speakers were turned off individually.

Tom turned his attention back to the great control panel, and one by one tested the banks of dials, gauges, and indicators that controlled the rocket cruiser. Tom Corbett had wanted to be a