"Stop your ship and be recognized!"

The rasping voice on the audioceiver was sharp. A command to be obeyed.

Tom turned away from the control board and looked at Strong who was already reaching for the ship's intercom.

"Full braking rocket thrust, Astro," he yelled into the microphone, "and make it quick or we'll all be blasted into protons!"

Tom and the captain gripped their chairs tightly as the ship bucked against the deceleration force of the powerful braking rockets. Gradually the freighter Dog Star slowed and came to a dead stop in space.

"Hey!" yelled Astro over the intercom from the power deck. "What's going on up there?"

"We've just entered the outer circle of defense on the prison asteroid, Astro," replied Strong. "We have to stop so they can sweep us with their radar and identify the ship."

"But I sent them a message in Solar Guard code that we were coming," interjected Roger who was listening from the radar bridge.

"They still have to make sure it's us," said Strong.

"Identify yourselves!" commanded the voice over the audioceiver again.

"This is space freighter Dog Star under temporary command of Captain Strong of the Solar Guard," answered Strong.

"What's your business here?" demanded the voice again.

"Interrogation of one of your prisoners. We have sent a coded message, under code Z for Zebra to your prison commandant, Major Alan Savage. If you'll check with him, you'll find everything in order," said Strong.

"Very well," replied the voice crisply, and then added, "Remain where you are. Do not move from your present position or attempt to send any messages. If you fail to comply with these conditions you will be blasted!"

"Very well," said Strong, "conditions are understood."

"Boy," chimed in Roger, as he climbed down the ladder from the radar bridge, "they sure don't want any company here."

"And for good reason," said Strong. "The most vicious criminals in the whole universe are confined here. Every one of them is capable of committing any crime in the solar code. And most of them have. The men here are the worst. They have refused psychotherapeutic readjustment to make them into new men."

"But I thought they had to go through it, sir?" said Tom.

"No," replied Strong. "Even criminals have certain rights in our society. They can either remain criminals and stay here, or be psychoadjusted and given new personalities. The ones that refuse are the ones on this Rock."

"You mean," gasped Roger, "that the men on this asteroid deliberately chose to remain criminals?"

"Yes, Manning," said Strong. "Rather than become healthy citizens of the system, they prefer to stay here and waste their lives in isolation with no hope of ever returning to society."

"Can they change their minds after they get here?" asked Tom.

"Any time. But when they get this far, they usually stay here. The men on Prison Rock didn't surrender easily. They are the toughest, most ruthless men in the universe."

"Attention! Freighter Dog Star! Attention!" the audioceiver rasped into life again. "You have been given temporary clearance. A space launch will ferry you to the asteroid. You are warned that any weapons discovered on your person, or acts that may be construed as providing aid and comfort to the inmates of this prison, will be considered treason against the Solar Alliance and you will be subject to immediate disciplinary action."

Tom and Roger glanced at each other, a worried look in their eyes. Strong just smiled. "Don't worry, boys. That little speech is read to every visitor to the asteroid."

"Just the same, sir," said Roger huskily, "I would prefer to remain aboard the Dog Star and give you, Tom, and Astro the pleasure of the visit."

Strong laughed. "They won't let you, Roger. They'll send up a crew of guards to search the ship. And the way these boys search makes a customs inspection look like a casual glance."

"Attention Dog Star!" A younger voice suddenly came in on the audioceiver. "This is Lieutenant Williams aboard the space launch. We are approaching your starboard catapult deck. Please open the air lock and take us aboard."

"They sure don't waste any time," commented Tom as he turned to the audioceiver. "Freighter Dog Star, Cadet Tom Corbett to Lieutenant Williams," he called, "the air lock is open and the catapult deck is ready to receive you." At the same time, the young cadet turned the valve that would open the outer air lock to the jet-boat deck.

Five minutes later, the ship was swarming with tight-lipped enlisted Solar Guardsmen, who spoke to Strong and the cadets with cool courtesy. These were men who signed up for two years as guards on the Rock after competing with thousands of other enlisted men. A guard on the Rock was paid triple wages for the two-year isolation. But more than anything else the right to wear the bright white patch with a paralo-ray gun in the center denoting their service as guards on the Rock was prestige envied even by commissioned officers of the Solar Guard.

After what Tom thought to be the most thorough search he had ever seen was over, Lieutenant Williams reported to the control deck where Strong and the cadets had been politely but firmly detained. He informed them that they were now ready to blast off to the Rock, adding that a more detailed search of the area between the ship's outer and inner hulls would be conducted after they had gone.

"You mean," said Tom, amazed, "that you actually search the four inches between the two hulls? What in the universe could we possibly hide in there?"

"I don't know, Corbett," replied Williams. "We've never found anything there." He turned to Strong and smiled. "But there's always a first time, isn't there, sir?"

"Yes, of course," agreed Strong. "You do a thorough job, Williams. Very good indeed!"

"Thank you, sir," said Williams. "You know, we've heard about you and the Polaris unit here on the Rock." He turned to Tom, Roger, and Astro. "We have a stereo of that mercuryball game you played at the Academy when you were Earthworms."

"What?" cried Tom. "You mean that game was recorded?"

"It sure was," said Williams. "But we've seen it at least fifty times."

"Well, blast my jets!" said Astro in amazement.

The game was one that the cadets had played when they first entered the Academy. It had done much to unify the boys into a fighting team.

An enlisted sergeant suddenly appeared, snapping to attention in front of Lieutenant Williams. "Ready to blast off, sir," he said.

"Very well," said Williams, then turned to Strong and the cadets. "Follow me, please."

In a few moments the space launch was blasting away from the freighter and heading for a tiny planetoid in the distance. As they drew near, Strong and the cadets peered out of the ports to get a view of the prison, but were disappointed when Williams ordered the ports covered.

He smiled apologetically at Strong and explained, "All approaches are secret, sir. We can't allow anyone to see where our defenses are located."

"You fellows certainly believe in keeping prisoners in and visitors out!" commented Strong.

"Anyone interested in coming to the Rock, sir," said Williams, "is under natural suspicion."

The three cadets gulped, duly impressed with the severity of the prison routine.

Soon they felt the unmistakable jar and bump of the small space vessel touching the surface of the planetoid. The jets cut out suddenly and Williams stood up.

"Please follow me. Do not speak to anyone, and do not stop walking. Keep your hands in front of you and maintain a distance of ten feet between you and the man in front of you."

He walked through the open hatch where a hard-faced enlisted guardsman stood rigidly, holding a paralo-ray gun at the ready.

With a quick nod to the cadets Strong followed Williams through the hatch. At ten-foot intervals they followed him out of the hatch, with Tom bringing up the rear and the enlisted guardsman behind him.

As Tom stepped out onto the surface of the asteroid he wasn't quite sure what he expected to see, but he certainly wasn't ready for the sight that greeted his eyes.

As far as he could see, there was grass, spotted with small one-story buildings. To the left was a single towering structure built of Titan crystal and on top of it was the largest atomic blaster he had ever seen. He turned to ask the guardsman about the gun but was motioned ahead with a curt, "No questions. Keep walking."

Tom continued to walk. He noticed that they were heading for the tower. As he drew nearer, he could see men walking around a narrow catwalk at the top. They all carried paralo-ray rifles with miniature grids mounted on the barrel. Inside the rifle was a tiny radar direction finder. It was a simple but effective control against escaping prisoners. Each of the inmates of the Rock wore small metal disks welded to a thin chain around their waists. The disk was sensitive to radar impulses, and with no more effort than snapping a thumb catch on the rifle, the guard could locate and paralyze the nearest disk-wearing inmate.

Tom was so full of questions it was necessary for the guard to warn him again, only this time in sharper tones.

Entering the tower, they were scrutinized and cleared by an electronic beam that passed through their bodies and indicated any metal they might carry. Once through this last barrier, they were escorted to a slidestairs, where Williams left them.