"A mass-conversion ship has plenty of power to do just that. At one hundred per cent efficiency, it would use up about one per cent of her mass as energy and another one per cent as reaction mass. That's what the Star Rover is going to do when it is finished."

One of the younger kids was waving his hand. "Mister Chief Engineer?"

"Yes, son?"

"Suppose it goes on a few weeks longer and passes the speed of light?"

Mr. Ortega shook his head. "It can't."

"Why not, sir?"

"Eh, how far have you gone in mathematics, sonny?"

"Just through grammer school calculus," the kid answered.

'Tm afraid there is no use in trying to explain it, then. Just take it from me that the big brains are sure it can't be done."

I had worried about that very point more than once. Why can't you go faster than light? I know all that old double-talk about how the Einstein equations show that a speed faster than light is a meaningless quantity, like the weight of a song or the color of a sound, because it involves the square root of minus one—but all of that is just theory and if the course we had in history of science means anything at all, it means that scientists change their theories about as often as a snake changes his skin. I stuck up my hand.

"Okay," he says. "You with the cowlick. Speak up."

"Mr. Ortega, admitting that you can't pass the speed of light, what would happen if the Star Rover got up close to the speed of light—and then the Captain suddenly stepped the drive up to about six g and held it there?"

"Why, it would— No, let's put it this way—" He broke off and grinned; it made him look real young. "See here, kid, don't ask me questions like that. I'm an engineer with hairy ears, not a mathematical physicist." He looked thoughtful and added, "Truthfully, I don't know what would happen, but I would sure give a pretty to find out. Maybe we would find out what the square root of minus one looks like— from the inside."

He went on briskly, "Let's go on about the Mayflower. You probably know that when the original Star Rover failed to come back, the Mayflower was designed to be the Star Rover II, but the design was obsolete before they ever started putting her together. So they shifted the name over to the new intersteller ship, the Star Rover III, renamed this one the Mayflower and grabbed her for the colonial service.

"You kids should consider how lucky you are. Up to now, emigrants to Ganymede have had to spend two years and nine months in space, just to get there. You're making it in two months."

"Couldn't we go faster?" somebody wanted to know.

"We could," he told us. "But we don't need to and it runs up the astrogation and control difficulties. In these new ships the power plant has gotten 'way ahead of the instrumentation. Be patient; your grandchildren will make the trip in a week, blasting at one g all the way. There'll be so many ships they'll have to have traffic cops and maybe we can come close to shipping out as many people as there are extras bom each year.

"Enough about that," he went on. "Who here can tell me what 'E equals M C squared' means?"

I could have answered but I had already spoken up once and it doesn't do to get a reputation for apple polishing. Finally one of the older kids said, "It means that mass can be converted into energy."

"Right!" Mr. Ortega agreed. "The first real demonstration of that was the atom bomb they set off 'way back in 1945 at Alamogordo, New Mexico. That was a special case; they still didn't know how to control it; all they could do was to make one whale of a big bang. Then came the uranium power plants, but that still didn't amount to much because it was a very special case and only a microscopic percentage of the mass was converted into energy. It wasn't until Kilgore's energy transformation equations—don't worry about them; you'll study them when you are older if you are interested—it wasn't until Kilgore showed how it could be done that we had any idea of how to do what Dr. Einstein's energy-mass equation said, clear back in 1905.

"And we still didn't know how to control it. If we were going to turn mass into energy, we needed more mass with which to surround the reaction, a very special sort of mass that would not turn into energy when we didn't want it to and would hold the reaction where we wanted it. Ordinary metal wouldn't do; one might as well use soft butter.

"But the Kilgore equations showed how to do that, too, when they were read correctly. Now has anyone here any notion of how much energy you get when you convert a chunk of mass into raw energy?"

Nobody knew. "It's all in that one equation," he said, "good old Doc Einstein's 'E equals M C squared.' It comes out that one gram of mass gives nine times ten to the twentieth power ergs." He wrote it down for us: 1 gm. = 9 x l0²⁰ ergs.

"Doesn't look like much, does it?" he said. "Now try it this way:" He wrote down

900,000,000,000,000,000,000 ergs.

"Read it off. Nine hundred thousand million billion ergs. It still doesn't mean much, does it? Figures like that are impossible to comprehend. The nuclear physicists keep a barrel of zeroes around handy the way a carpenter does a keg of nails.

"I'll try once more," he went on. "A pound of mass, any old mass, say a pound of feathers, when converted into energy equals fifteen billion horsepower-hours. Does that give anyone a notion of why the Mayflower was assembled out in an orbit and will never ever land anywhere?"

"Too hot," somebody said.

"'Too hot' is an understatement. If the Mayftower had blasted off from Mojave space port the whole Los Angeles Borough of the City of Southern California would have been reduced to a puddle of lava and people would have been killed by radiation and heat from Bay City to Baja California. And that will give you an idea of why the shielding runs right through the ship between here and the power plant, with no way at all to get at the torch."

We had the misfortune to have Noisy Edwards along, simply because he was from the same bunk room. Now he spoke up and said, "Suppose you have to make a repair?"

"There is nothing to go wrong," explained Mr. Ortega. "The power plant has no moving parts of any sort"

Noisy wasn't satisfied. "But suppose something did go wrong, how would you fix it if you can't get at it?"

Noisy has an irritating manner at best; Mr. Ortega sounded a little impatient when he answered. "Believe me, son, even if you could get at it, you wouldn't want to. No indeed!"

"Humph!" said Noisy. "All I've got to say is, if there isn't any way to make a repair when a repair is needed, what's the use in sending engineer officers along?"

You could have heard a pin drop. Mr. Ortega turned red, but all he said was, "Why, to answer foolish questions from youngsters like yourself, I suppose." He turned to the rest of us. "Any more questions?"

Naturally nobody wanted to ask any then. He added, "I think that's enough for one session. School's out."

I told Dad about it later. He looked grim and said, "I'm afraid Chief Engineer Ortega didn't tell you the whole truth."

"Huh?"

"In the first place there is plenty for him to do in taking care of theauxiliary machinery on this side of the shield. But it is possible to get at the torch, if necessary."

"Huh? How?"

"There are certain adjustments which could conceivably have to be made in extreme emergency. In which case it would be Mr. Ortega's proud privilege to climb into a space suit, go outside and back aft, and make them."

"You mean——"

"Imean that the assistant chief engineer would succeed to the position of chief a few minutes later. Chief engineers are very carefully chosen, Bill, and not just for their technical knowledge."