But there Dr. Vorhulst began to rein in the class’s speculation. “Friction,” he said succinctly. “Don’t forget friction. Remember what reentry did to a lot of the early spacecraft. If you used a Lofstrom loop, you’d need to accelerate your capsule to that seven miles a second of escape velocity that I was talking about the other day before you let go of it, and then the air friction would burn it right up.”
He paused, eyes roving over his class, expression as good-natured as always but with a faint twinkle that made Ranjit expect some sort of surprise was coming. “So,” the teacher said sociably, “have any of you junior-grade astronauts figured out what kind of rocket drive your ship is going to have yet?”
Ranjit hadn’t thought of anything beyond the usual fuel-plus-oxidizer. He kept his mouth shut, though, because he knew, from the fact that the professor had raised the question in the first place, that Dr. Vorhulst had something else in mind.
So did his seatmate, but he responded in a different way. His hand went right up. “You’re not talking about a chemical rocket, are you, Dr. Vorhulst? So, then, what do you think? Maybe a nuclear-powered one?”
“Good guess,” the teacher said, “but, no, I don’t think a nuclear-powered rocket would be your best bet, at least not the kind of nuclear power you mean. Oh, there are designs around for rockets that were driven by exploding atom bombs, one after another. We can talk about them if you like, but for getting from LEO to Mars I think there are two much better possibilities. Both of them are tailor-made to be used with some kind of space elevator to boost them into low earth orbit because they’re both way too feeble to lift anything off Earth’s surface and into space. One is the solar sail. The other is the electric rocket.”
Ten minutes later Dr. Vorhulst had given short and convincing reasons for avoiding nuclear explosions as rocket propellants—the need for heavy shielding to protect astronauts from the deadly radiation, and anyway who wants to shoot a few hundred atomic bombs off into space? Solar sails had a lot to recommend them, he conceded, but they were dreadfully slow and not very maneuverable. However, the electric rocket, while also pretty slow to accelerate, required no fuel storage and produced no undesired by-products. Where did the electricity come from? Perhaps from an onboard nuclear power plant, Vorhulst conceded, but just as easily from solar power—that is, from solar power in space, where there were no nights or cloudy weather, so that the sun always shone. “And what do you do with that electricity? You use it to ionize some working fluid—a gas like xenon, for instance—and the gas fires itself out of your rocket nozzles at very high velocity, and off you go.”
He paused for a breath. “All right,” he admitted, “an electric rocket would not accelerate very fast.” But it would keep right on accelerating as long as you liked, and the longer the acceleration, the greater its velocity would be. You could accelerate until you were halfway there. Then you could turn around and decelerate until you arrived. Did anyone see what that implied?
Vorhulst gave them a few moments to figure it out, but no one did. “It means,” he told them, “that the farther your trip, the faster the speed that you’ll attain. You wouldn’t want to use an electric rocket to go to the moon. Short trip; you don’t really have time to get going very fast. For Mars, though, it’s optimal. And for the outer planets, say Uranus or Neptune, why, that trip doesn’t take much longer than to Mars! And if you’re really going to go a far piece, say to the Oort cloud, you build up so much velocity with all that acceleration that that enormous journey in fact becomes feasible!”
Then he stopped and grinned. “Well,” he said, “I don’t want to oversell you on the electric rocket, because it has one serious fault. That is, we don’t have any.” He overrode the faint groans of disappointment. “Oh, it’s legitimate in principle, all right. But nobody has ever built one because if you have to start your flight from Earth’s surface, they won’t work. They need something to lift them into low earth orbit first, and then they can strut their stuff. Something like an Artsutanov space elevator, and, as you know, we just don’t have one of those around anywhere.”
He gave them a rueful smile. “Oh, one day we will,” he promised. “Then we’ll have electric rockets by the zillion, and I’d be willing to bet that more than one of you will be riding to all sorts of weird and wonderful places. But not yet, because at the present time they don’t exist.”
Which, when you stopped to think about it, was true enough, at least for the little volume of space near Earth, though it wouldn’t be for long.
Actually, somewhat farther away, there were 154 of those electric rockets that were already taking direct aim for Earth, and the individuals aboard them didn’t think they were unusual at all.
These individuals were the One Point Fives, and they (or their ancestors) had been traveling from star to star in spacecraft just like these for many, many generations. Always on much the same errand, too. The fact of the matter was that the One Point Fives had a unique place among the subordinate sapient species of the galaxy.
Basically they were the Grand Galactics’ hit men.
To a casual observer the One Point Fives might not have seemed to be good candidates for that sort of employment. Stripped of their shields and prostheses the average One Point Five wasn’t much bigger than a terrestrial cat. That casual observer would not be likely, however, to see a One Point Five in that stripped-down condition. A One Point Five’s indispensable protective devices massed just about half as much as his body itself (hence the name One Point Five), and every last bit of these devices was vitally necessary. Some of the devices guarded the fragile organic being inside against radiation—from the ionizing spillover from their nuclear power plants or from the residues of their many long-ago nuclear wars. Or even against the lethally high ultraviolet rays that came from their star and were no longer warded off by their planet’s ozone layer because their earlier activities had resulted in their planet’s no longer having one. Some of their chemical processors removed poisons from the air they breathed or the food and water they ingested. Some merely kept them from going insane from the unbearable din that suffused every part of their world (that took blanketing sound absorbers backed up with frequency nullifiers). Other processors toned down the maddening flashes and flares that accompanied their industry.
There were a few isolated spots on their planet where a One Point Five could strip naked and survive. Those places were the breeding rooms and the birthing rooms, as well as a scattering of spots where medical and surgical procedures were performed. There weren’t many of those. Because there was so much to guard against, neutralize, or prevent on that ravaged world, such places were not only scarce but expensive.
That being so, one might wonder why a species as technologically savvy as the One Point Fives didn’t just go ahead and build themselves a fleet of spaceships and proceed to start a new life on some unspoiled planet somewhere else in space.
Actually, the One Point Fives had done that… once.
The project had not been a success, however. Oh, the ships had got invented and built, all right, and a benign enough planet had been located. But the Grand Galactics had stepped in. After that happened, it had been so little of a success that, though many thousands of years had passed, the One Point Fives had never considered trying it again.
By and large the school year had been a disappointment, but the summer began well for Ranjit Subramanian. Take his grades, for instance. When they were posted, he was not surprised at the gentleman’s C he got in philosophy (his grade in psychology didn’t matter, because he’d dropped it out of boredom) and not particularly surprised, either, though pleased, by his A in astronomy. But the A in statistics had been a total mystery. Ranjit could only conjecture that it was the result of the advanced reading he had picked up for himself when he couldn’t stand