"What kind o' contraption is that in there?" Chow asked, peering through the pane intently.

"A reentry device for spacecraft," Tom replied. "It's called a Duratherm Wing, or Durathermor for short. 'Dura' because it's made of my lighter-than-air foam plastic, Durabuoy, and 'therm' because it uses heat from air friction."

"What's it suppose t' do?"

"Get the spacecraft safely down to the ground from orbit. The Durathermor will serve as a heat shield to keep the craft from burning up in the atmosphere, and also enable it to be steered down to a precise landing, like an airplane."

"Can't you do that already?" Chow queried.

"Sure, in my repelatron spaceship. But rockets and space capsules are different. Most of them can only reenter the atmosphere like a bullet and it's not easy to maneuver a bullet."

Tom explained that his Durathermor could be fitted to the nose of shuttle-ferry rockets making repeated trips to and from space stations. Even more important, it could be used to rescue disabled spacecraft, stranded in orbit.

"How in thunder would you do that, boss?"

"Well, a rescue ship would go into orbit beside the disabled craft. Then an astronaut would get out, lug the Durathermor across the void, and attach it to the stranded spaceship."

Chow scratched his double chin. "Be purty big fer one man to handle, wouldn't it?"

Tom chuckled. "The wing wouldn't be opened up yet. The whole apparatus would be neatly stowed in a compact, fairly lightweight package."

He explained that before firing a retro-rocket to plunge the ship into the atmosphere, the astronaut would trigger a tiny cartridge of highly volatile liquid. This liquid would vaporize, causing a fabric sheath to balloon out in the shape of the wing. Then Durabuoy would foam out of a tank and fill the sheath.

"I guess you know how hot a spacecraft gets during reentry," Tom went on.

"Brand my stew kettle, I sure do! The whole ship glows like red-hot stove lid."

"Much hotter, actually. Air friction can shoot the temperature above 20,000 degrees Fahrenheit. But my Duratherm Wing will stay fairly cool."

The secret of the cooling, Tom explained, lay in a myriad of tiny semiconductor heat cells -- somewhat like radio transistors -- embedded in the fabric sheath. These would convey much of the heat into electricity, which would then be stored in batteries for use later in the flight.

Chow went back to his galley and brought a lunch cart, then stayed to watch as the wind tunnel test continued. He saw Tom and Arv turning small control knobs. These caused the nose and wing tips to curl up and down, or the twin tails to curve slightly from side to side.

"How do you make them thingamabobs curl thataway, Tom?" the cooked asked, puzzled.

"Well, the Durabuoy is allowed to absorb just enough heat to become slightly pliable. But the actual bending is done by fine bimetal strips that are woven right into the fabric."

Chow squinted. "What kind o' metal?"

"Bimetal -- actually two separate kinds of metal bonded together. The storage batteries are used to heat them by electrical resistance. One metal expands more from the heat than the other, so the strip curls slightly. It's the same principle that moves the needle of your oven thermometer. And when al the strips curl, they exert enough force to flex the wing surface."

"Wal, fry me fer an oyster! An' that's how you aim to steer an' fly the whole contraption?"

"Right." Tom added that his Durathermor also had a special emergency crash-landing device, which was activated somewhat like the wing.

Ö

As he pressed a button, the cylindrical package on the Firefly's nose opened. The huge, glistening fabric sheath slowly inflated to its delta-wing shape and the twin tail booms streamed aft.

"Now the Durabuoy foam is gushing into the sheath and hardening," Tom radioed. "As you can see on your monitor screens, the wing encloses and shields the whole forward end of our craft. But during reentry a small portion of the wing will purposely be allowed to burn away, to provide pilot visibility during the landing maneuvers."

"Exactly how does it work?" one of the board members asked.

Tom explained that for aircraft use, the device would be mounted in a small pod under the nose of the plane. Using the easel chart in the conference room, he sketched out its operation.

"The pilot would trigger the device as soon as he knew that the plane was about to crash," Tom said. "This would fire a breaking rocket to slow the craft's fall."

"The same blast would eject a fabric sheath with a foreword, slightly whirling motion. Part of the rocket's exhaust would be used to balloon out the sheath. Then plastic foam -- the same lighter-than-air Durabuoy used in my Duratherm Wing -- would gush out and solidify inside the sheath."

Tom illustrated the result on the chart pad -- a huge, mushroom-shaped Durabuoy crash shield foreword of the plane's nose.

"With its lighter-than-air buoyancy, this would act as both a balloon and a parachute and further help to slow the plane's fall," he went on. "At the actual moment of crash, the cellular structure of the foam mushroom would shatter and absorb most of the shock."

The spaceship was moored again, and Tom's atomic earth blaster was hauled outside to a suitable drilling site. A launching rig was set up and anchored to the rock with explosive bolts. Then the torpedo-shaped blaster was positioned nose down in the rig.

By this time Petronius' brief night had fallen, so Tom called a halt. The astronauts returned to the Challenger to eat and nap. As Chow served a hearty meal, Tom explained the drilling operation.

"You see, with the planetoid's low gravity, the blaster would tend to rebound out of the drill hole instead of grinding up the rock effectively. So I've added the magnetic clamps to hold it tight to the walls of the shaft."

"Then how can the blaster move along and keep drilling?" a crewman asked.

"It operates in pulses," Tom replied. "The current to the electromagnets flows only while grinding is going on. When the ground-up rock is spewed out of the shaft, the current shuts off. The heat reaction drives the blaster forward until it presses against solid rock. Then the pressure switches the magnets on again."

The blaster was bracketed slowly out of the drill shaft. While most of the crewman remained at their posts, Tom took off in the Challenger. He maneuvered the huge ship aloft so as to aim the microwave beam from its telesampler gun down into the drill hole. When the beam was reflected, it would carry back particles of the substance at the base of the shaft."

Tom swallowed hard, realizing that countless lives might depend on his efforts. "Yes, I still think we can bring the planetoid down to a controlled landing, Dad. But I'm not sure I can give you all the answers right now."

Frowning, the young inventor paces back and forth near the conference table. "It's a cinch we'll need more than one method to do the job," he mused aloud. "The Durathermor technique will just give us a start on the problem."

Mr Swift looked doubtful. "You certainly won't be able to impart much aerodynamic lift to a mass as big as Petronius."

"Not with the kind of wing I used before," Tom agreed. "But a parasol-mounted delta wing would at least give us some control. And a Durathermor drogue chute, combined with braking rockets, could partly slow its fall."

Tom sat down and grabbed a pencil and paper. "Look, Dad! We'd have to time our operations to the precise second, but here's how we might do it. The chute and rockets would slam on the brakes, but only temporarily. Now, if we set up powerful repelatrons, we could spear the planetoid with our repulsion beams--before it could build up velocity again."

"Hey!" Bud exclaimed. "Why not use repelatrons right now and kick Petronius back into a stable orbit?"

Tom shook his head. "Too late for that, I'm afraid, in view of what Dad and Dr. Volenti have told us about the effects of the Van Allen belt."

"How come?"

"Think how many megatons of H-bombs it took to move Petronius in the first place. To get enough effect, we'd have to keep pouring power with our repelatrons for a long period of time."

"So keep pouring!" Bud urged.

"No used. It would be like putting water into a sieve. Petronius was on a three-hour orbit; but the orbital time is shorter now than the planetoid's sinking. Much of the time it's passing through the Van Allen belt. That would drain off its orbital energy as fast as we could feed it in. Right, Dad?"

The ender scientists nodded. "If our calculations are correct, that answer is hopeless."

Bud scratched his head. "Okay, then how about using H -bombs?"

"Too risky at Petronius' present altitude," Mr. Swift replied. "The effects of radioactive fallout would be impossible to predict."

"Back to you then, genius boy," Bud said, turning to Tom. "Tell us the rest of your plan."

"Well, once we braked the planetoid inside the atmosphere, repelatron beams could keep its rate of fall from accelerating too dangerously."

"Even so, there'd still be a terrific seismic shock from its landing," Dr. Volenti warned.

"True," Tom conceded. "But we could do a switch on the Durathermor crash-landing technique. I mean, instead of using a crash shield to protect the falling object, we put a shield on the ground itself."

Volenti and Mr. Swift bent over the table eagerly. "How do you mean, son?"

Tom penciled a quick sketch. "We build up a mountain of Durabuoy foam beforehand at the exact spot of landing. Of course much of the rocky crust of Petronius would burn away during entry. When the sapphire cure hits, the foam mountain will shatter and melt. The impact energy will be turned into heat an spread out safely over a large area of the desert."

Mr. Swift and Dr. Volenti exclaimed their approval of Tom's plan.

"Son, this gives us a real hope of staving off disaster!"

"I see no reason why the plan won't work." The astronomer chimed in, "providing our calculation s are sufficiently precise."

"Does the Defense Department know of the danger yet, Dad?" Tom asked.

"Yes, we notified Washington early today, but we'd better call again and explain your plan."

"How long before Petronius hits?"

"Ten days by our estimate," Mr. Swift replied.

Tom gave a worried whistle. "Then we'd better get cooking fast!"

The young inventor hurried to his laboratory and plunged frantically into work. He decided that four super-powerful repelatrons would be required for the planetoid landing. These and the Durabuoy mountain posed no technical problems. "Hank Sterling and Arv can handle them," Tom decided. He called the two into his laboratory and explained the requirements.

"With round-the-clock shifts, we can have the repelatrons ready in a week or so," Hank promised. "Where's the landing to take place?"

"The Great Salt Lank Desert in Utah."

"After Hank and Arv had left, Tom turned his attention to the Durathermor equipment and electronic gear needed for the planetoid operation. Scarcely pausing to nibble lunch from a tray, he worked on through the day. It was almost midnight when Tom finally left his drawing board and went to bed.

Ö

Two odd-looking helicopters were skimming back and forth, each squirting out a ribbon of Durabuoy, like toothpaste from a tube. Tom told Chow this was to build up a huge flat-topped mountain of the white plastic foam. The young inventor had used similar craft to lay an aerial road above the Ngombian jungle in Africa, as related in Tom Swift and his Repelatron Skyway.

Ö

Soon the Challenger was zooming aloft. Tom steered it into orbit alongside the tiny, hurtling planetoid, then landed gently. Two holes were bored through the rocky crust--one at the stern end of Petronius, the other in the side facing away form earth. The Durathermor drums were lowered into the holes and firmly anchored against the sapphire core. Finally a thrust chamber was drilled into the planetoid's nose and filled with rocket fuel.