The mission profile, Dana soon saw, was close to the classic minimum-energy Hohmann transfer profile he’d sketched out to Jim, that day in the shop at the back of his house in Hampton.

The chemical mode had some advantages. The development program would be comparatively cheap, since the hardware would be based on incremental upgrades of Saturn technology, for example the use of an enhanced Saturn second stage to serve as an orbital injection booster.

But the nuclear camp from Marshall, led by Udet and Conlig, didn’t find it hard to pick holes in the case. Compared to the NERVA profile, twice as much mass would have to be hurled into Earth orbit, for a mission twice the length. Chemical technology couldn’t manage much better than that. Not without imagination, anyway, Dana thought; not if you stick to direct transfer…

Dana knew that most of the points raised in the discussion were a repeat of the sterile arguments which had plagued NASA for some months.

At the end of the question session Seger didn’t call for any applause.

Lunch turned out to be steak and chicken served buffet style. The debate continued during the meal, with delegates making points by jabbing bits of steak or fried potato at each other.

Dana spotted the sleek, handsome figure of Wernher von Braun himself. He was talking to an astronaut: Joe Muldoon, a moonwalker, tall, erect, his thinning, gray-blond hair clipped to military neatness.

Few people spoke to the obscure little man from Langley with his peculiar presentation. Venus swing-by modes? What the hell is that about? That suited Dana. He left the lunch early and returned to his seat in the hall; he didn’t much like steak anyway.

The conference looked at two more options, before Dana’s pitch. Both of those were more ambitious, technically, than either the main chemical or nuclear options reviewed earlier; Dana suspected they had been explored just to make sure nothing obvious was missed before the primary mode was selected.

A representative of McDonnell presented a so-called nuclear electric option, together with representatives of NASA and ARPA, the government’s Advanced Research Projects Agency. Plasma — a charged gas — would be accelerated electrodynamically out of a rocket nozzle. A plasma rocket’s thrust was tiny, but would last for months; plasma rockets would move spaceflight techniques away, at last, from the antique Jules Verne kick-and-coast model. The technology was unproven, but there had been some trials; an electric rocket had been operated at high altitude as long ago as 1964.

The McDonnell man flashed up a conceptual design for a manned nuclear-electric ship. It was a staggering arrangement, like a three-armed windmill. Two of the arms — each fifty yards long — contained reactors, and the third the habitable section. The rockets were mounted at the hub of the rotor, and the whole thing was designed to spin about the hub to provide artificial gravity. It would be, Dana thought, like a great metal snowflake, spinning toward Mars. It was a terrific concept, and utterly impractical.

Next up was a project manager from General Dynamics. He got to his feet with a broad grin gleaming from out of a California tan. “I’ve got to tell you,” he told the audience deadpan, “that I can beat you NERVA folks hands-down. With two million pounds in Earth orbit I can get to Mars and back in just 250 days — not much more than half your time — and taking no less than twenty guys. Gentlemen, I give you Project Putt-Putt.”

The idea was to throw one-kiloton nuclear bombs out of the back of the spacecraft — thirty devices every second — and set them off, a thousand feet behind the ship. The shocks would be absorbed through water-cooled springs, and the ship would be driven forward. “Like setting off firecrackers behind a tin can. Am I right?”

The concept seemed ridiculous, but General Dynamics had done some preliminary studies, called “Project Orion,” in the early 1960s, and the presenter was able to show photographs of a small flight-test model which had used high explosives to hurl itself a few hundred feet into the air.

The technical problems were all around the high temperature flux on the rocket’s back end structure, which would have to radiate away excess heat between explosions. And of course the system had one major drawback, the General Dynamics man said, and that was the radioactive exhaust. But that hadn’t seemed such an obstacle back in 1960, when the first Orion studies had been initiated. Then, it was thought that the unscrupulous Soviets might use this quick-and-dirty method to short-cut to space, so we had to look at it, too.

The General Dynamics man joshed and wisecracked his way through his talk. When he sat down he got the biggest hand of the day.

Dana felt himself shrink into his seat. How the hell do I follow that?

When he got to the podium Dana shuffled with his notes and foils, trying to avoid looking out over the sea of sleek suits before him. There was a spotlight on him; it seemed to impale him. It was already four-thirty, and after the General Dynamics pitch the delegates had lost concentration; they were still laughing, talking.

Dana began to read from his notes. “Manned Mars stopover missions of duration twelve to twenty-four months are characterized by Earth return velocities of up to seventy thousand feet per second, over the cycle of mission opportunities. A promising mode for reducing Earth entry velocities to forty to fifty thousand feet per second, without increasing spacecraft gross weight, is the swing-by through the gravitational field of Venus. Studies indicate that this technique can be applied to all Mars mission opportunities, and in one-third of them, the propulsion requirements actually can be reduced below minimum direct-mode requirements…”

There was a ripple of reaction in the audience, a restless shifting. Dana plowed on. He felt sweat start over his brow, around his collar.

He hurried through the idea of gravity assist. He tried to emphasize the history and intellectual weight of the idea, showing that his own computations had built on the work of others. “The concept within NASA of using a Venus swing-by to reach Mars dates back to Hollister and Sohn, working independently, who published in 1963 and 1964. This was further elaborated by Sohn, and by Deerwester, who presented exhaustive results graphically in a format compatible with the direct flight curves in the NASA Planetary Flight Handbook…”

It was a little like a game of interplanetary pool, he said. A spacecraft would dive in so close to a planet that its path would be altered by that world’s gravitational field. In the swing-by — the bounce off the planet — the spacecraft would extract energy from the planet’s revolution around the sun, and so speed up; in exchange, the planet’s year would be minutely changed.

In practical terms, bouncing off a planet’s gravity well was like enjoying the benefit of an additional rocket stage at no extra cost, if your navigation was good enough.

“We have already studied the Mariner Mercury mission, which would have swung by Venus en route to Mercury. A direct journey would have been possible, using, for example, a Titan IIIC booster; but the gravity assist would have allowed the use of the cheaper Atlas-Centaur launch system…”

“Yeah,” a voice called from the audience, “but Mariner Mercury got canned. And there were no men on it anyhow!”

Laughter.

Dana pressed on, brushing the sweat from his eyes. There were two ways Venus could be used to get to Mars, he said. The spacecraft could swing by Venus outbound, and use Venus’s gravity to accelerate it toward Mars. Or Venus could be used to decelerate the craft, on its way back to Earth.

“First estimates show a mass in Earth orbit of two million pounds would be required for a mission duration of 640 days.” Same weight as nuclear; two-thirds the trip time of chemical. “Thus a mission profile close to optimal is delivered, without the need for ambitious new technologies, and hence significantly reduced development costs compared to other candidate modes…”