Computer-Two, like the other three that chased each other's tails in orbit round the Earth, was much larger than it had to be. 

It might have been one-tenth its diameter and still contained all the volume it needed to store the accumulated and accumulating data to control all space flight. 

They needed the extra space, however, so that Joe and I could get inside, if we had to. And we had to. 

Computer-Two was perfectly capable of taking care of itself. Ordinarily, that is. It was redundant. It worked everything out three times in parallel and all three programs had to mesh perfectly; all three answers had to match. -If they did not, the answer was delayed for nano-seconds while Computer-Two checked itself, found the mal-functioning part and replaced it.

There was no sure way in which ordinary people would know how many times it caught itself. Perhaps never. Perhaps twice a day. Only Computer-Central could measure the time-delay induced by error and only Computer Central knew how many of the component spares had been used as replacements. And ComputerCentral never talked about it. The only good public image is perfection.

And it's been perfection. Until now, there was never any call for Joe and me.

We're the troubleshooters. We go up there when something really goes wrong; when Computer-Two or one of the others can't correct itself. It's never happened in the five years we've been on the job. It did happen now and again in the early days, but that was before our time.

We keep in practice. Don't get me wrong. There isn't a computer made that Joe and I can't diagnose. Show us the error and we'll show you the malfunction. Or Joe will, anyway. I'm not the kind who sings one's own praises. The record speaks for itself.

Anyway, this time, neither of us could make the diagnosis.

The first thing that happened was that Computer-Two lost internal pressure. That's not unprecedented and it's certainly not fatal. Computer-Two can work in a vacuum after all. An internal atmosphere was established in the old days when it was expected there would be a steady flow of repairmen fiddling with it. And its been kept up out of tradition. Who told you scientists aren't chained by tradition? In their spare time from being scientists, they're human, too.

From the rate of pressure loss, it was deduced that a gravel-sized meteoroid had hit ComputerTwo. Its exact radius, mass, and energy were reported by Computer-Two itself, using the rate of pressure loss, and a few other irregularities, as data.

The second thing that happened was the break was not sealed and the atmosphere was not regenerated. After that came errors and they called us in.

It made no sense. Joe let a look of pain cross his homely face and said, "There must be a dozen things out of whack."

Someone at Computer-Central said, "The hunk of gravel ricocheted very likely."

Joe said, "With that energy of entry, it would have passed right through the other side. No ricochets. Besides even with ricochets, I figure it would have had to take some very unlikely strikes."

"Well, then, what do we do?"

Joe looked uncomfortable. I think it was at this point he realized what was coming. He had made it sound peculiar enough to require the trouble-shooters on the spot-and Joe had never been up in space. If he had told me once that his chief reason for taking the job was because it meant he would never have to go up in space, he had told it to me 2` times, with x a pretty high number.

So I said it for him. I said, "We'll have to go up there."

Joe's only way out would have been to say he didn't think he could handle the job, and I watched his pride slowly come out ahead of his cowardice. Not by much, you understand-by a nose, let's say.

To those of you who haven't been on a spaceship in the last 15 years-and I suppose Joe can't be the only one-let me emphasize that initial acceleration is the only troublesome thing. You can't get away from it, of course.

After that it's nothing, unless you want to count possible boredom. You're just a spectator. The whole thing is automated and computerized. The old romantic days of space pilots are gone totally. I imagine they'll return briefly when our space settlements make the shift to the asteroid belt as they constantly threaten to do-but then only until additional computers are placed in orbit to set up the necessary additional capacity.

Joe held his breath through acceleration, or at least he seemed to. (I must admit I wasn't very comfortable myself. It was only my third trip. I've taken a couple of vacations on Settlement-Rho with my husband, but I'm not exactly a seasoned hand.) After that, he was relieved for a while, but only for a while. He got despondent.

"I hope this thing knows where it's going," he said, pettishly.

I extended my arms forward, palms up, and felt the rest of me sway backward a bit in the zero-gravity field. "You," I said, "are a com puter specialist. Don't you know it knows?" "Sure, but Computer-Two is off."

"We're not hooked into Computer-Two," I said. "There are three others. And even if only one were left functional, it could handle all the space flights undertaken on an average day."

"All four might go off. If Computer-Two is wrong, what's to stop the rest."

``Then we'll run this thing manually."

"You'll do it, I suppose? You know how-I think not?"

"So they'll talk me in."

"For the love of Eniac," he groaned.

There was no problem, actually. We moved out to Computer-Two as smooth as vacuum and less than two days after take-off, we were placed into a parking orbit not ten meters behind it.

What was not so smooth was that, about 20 hours out, we got the news from Earth that Computer-Three was losing internal pressure. Whatever had hit Computer-Two was going to get the rest, and when all four were out, space flight would grind to a halt. It could be reorganized on a manual basis, surely, but that would take months at a minimum, possibly years, and there would be serious economic dislocation on Earth. Worse yet, several thousand people now out in space would surely die.

It wouldn't bear thinking of and neither Joe nor I talked about it, but it didn't make Joe's disposition sweeter and, let's face it, it didn't make me any happier.

Earth hung over 200,000 kilometers below us, but Joe wasn't bothered by that. He was concentrating on his tether and checking the cartridge in his reaction-gun. He wanted to make sure he could get to Computer-Two and back again.

You'd been surprised-if you've never tried it-how you can get your space-legs if you absolutely have to. I wouldn't say there was nothing to it and we did waste half the fuel we used, but we finally reached Computer-Two. We hardly made any bump at all, when we struck Computer-Two. (You hear it, of course, even in vacuum, because the vibration travels through the metalloid fabric of your spacesuits-but there was hardly any bump, just a whisper.)

Of course, our contact and the addition of our momentum, altered the orbit of ComputerTwo slightly, but tiny expenditures of fuel compensated for that and we didn't have to worry about it. Computer-Two took care of it, for nothing had gone wrong with it, as far as we could tell, that affected any of its external workings.

We went over the outside first, naturally. The chances were pretty overwhelming that a small piece of gravel had whizzed through ComputerTwo and left an unmistakable hole. Two of them in all probability; one going in and one coming out.

The chances of that happening are one in two million on any given day-even money that it will happen at least once in six thousand years. It's not likely, but it can, you know. The chances are one in not more than ten billion that, on any one day, it will be struck by a meteoroid large enough to demolish it.

I didn't mention that because Joe might realize that we were exposed to similar odds ourselves. In fact, any given strike on us would do far more damage to our soft and tender bodies than to the stoical and much-enduring machinery of the computer, and I didn't want Joe more nervous than he was.

The thing is, though, it wasn't a meteoroid.

"What's this?" said Joe, finally.

It was a small cylinder stuck to the outer wall of Computer-Two, the first abnormality we had found in its outward appearance. It was about half a centimeter in diameter and perhaps six centimeters long. Just about cigarette-size for any of you who've been caught up in the antique fad of smoking.

We brought out our small flashlights.

I said, "That's not one of the external components."

"It sure isn't," muttered Joe.

There was a faint spiral marking running round the cylinder from one end to the other. Nothing else. For the rest, it was clearly metal, but of an odd, grainy texture-at least to the eye.

Joe said, "It's not tight."

He touched it gently with a fat and gauntleted finger and it gave. Where it had made contact with the surface of Computer-Two it lifted, and our flashes shone down on a visible gap.

"There's the reason gas pressure inside declined to zero," I said. 

Joe grunted. He pushed a little harder and the cylinder popped away and began to drift. We managed to snare it after a little trouble. Left behind was a perfectly round hole in the skin of Computer-Two, half a centimeter across. 

Joe said, "This thing, whatever it is, isn't much more than foil."

It gave easily under his fingers, thin but springy. A little extra pressure and it dented. He put it inside his pouch, which he snapped shut and said, "Go over the outside and see if there are any other items like that on it. I'll go inside." 

It didn't take me very long. Then I went in. "It's clean," I said. "That's the only thing there is. The only hole." 

"One is enough," said Joe, gloomily. He looked at the smooth aluminum of the wall and, in the light of the flash, the perfect circle of black was beautifully evident. 

It wasn't difficult to place a seal over the hole. It was a little more difficult to reconstitute the atmosphere. Computer-Two's reserve gas-forming supplies were low and the controls required manual adjustment. The solar generator was limping but we managed to get the lights on. 

Eventually, we removed our gauntlets and helmet, but Joe carefully placed the gauntlets inside his helmet and secured them both to one of his suit-loops. 

"I want these handy if the air-pressure begins to drop," he said, sourly.