A Step Farther Out (23 page)

Although most of this was published several years ago, there has been little need for revision; surprising, given the progress of science.

At the 1978 meeting of the AAAS Dr. Joseph Weber reported that he still has events; he is reanalyzing his older data; and so far it is too early to determine whether the 12-hour periodicity really exists or was an artifact of the data analysis. Thus there's nothing new to report.

Since my spacedrive column was published I have corresponded with a consulting engineer who was invited to test a "spacedrive"; he wrote eagerly to tell me that hanging the fool thing on a pendulum and turning it on produced a very small, but very real, displacement from vertical—which is to say, there was a definite non-Newtonian force! That was exciting.

I was set to go up and view this marvel, but a few days ago I received a letter: my engineer correspondent had put a plastic bag over the apparatus; and the tiny "thrust" had vanished. Evidently there was some kind of standing wave effect. Sigh.

So that all the evidence on spacedrives is as described in the essay. I wish there were more, but there isn't.

As to Hawking's oracles: the last place I know of that he presented that paper was to a Vatican conference of Catholic scientists. I expect they found it as disconcerting as I did. And no one has refuted Hawking's equations.

"Halfway To Anywhere" was the first column I wrote for
Galaxy.
I haven't changed it, because (1) the numbers are still correct, and (2) it's interesting for its history: the column was written in November of 1973, and the tables were made up from slide rules and logs; archaic methods indeed! I'd never do that much work now, were I robbed of my calculators and computers.

The second chapter, "Those Pesky Belters and Their Torchships," was never printed. There were too many tables and too many numbers. A much watered-down version eventually appeared as the second column I'd ever done, and then-editor Ejler Jakobbsen gave me to understand that in future I'd not include
quite
so much detail. He may have been right; I haven't yet made up my mind. Certainly when Jim Baen took over the editor's job at
Galaxy
a lot more detail and many more equations crept back into the column.

Science writers have a problem: how much detail do we include, and how technical can we get? After all, our first purpose is to entertain; if we can't do that, there's no point in writing a column or article for the general public. On the other hand, there's buried in most of us a frustrated teacher: we want the readers to
understand
and even to be able to work these things out for themselves.

And many readers do want the gory details; especially in this era of cheap but excellent calculators and computers. Thus there's the temptation to include the equations, and a lot of tables and charts. At the same time, most adult readers did
not
grow up in the era of the calculator and are understandably a bit, uh, perhaps not afraid of, but uncomfortable with numbers and equations. Those who open a book and see a ton of equations are likely to put it back down again unless they're part of the small minority who like such things.

I've always tried to strike a balance: to write these columns so that you could follow the arguments without having to work the equations, but to give enough details to let those genuinely interested play the game themselves. I am told I've been reasonably successful at that.

Anyway,
this section presents "gory details"; but you really don't have to do the math to see what's being said, and hopefully to enjoy the speculations about space travel.

One of my rivals in the science-writing field usually begins his columns with a personal anecdote. Although I avoid slavish imitation, success is always worth copying. Anyway, the idea behind this column came from Robert Heinlein, and he ought to get credit for it.

Mr. Heinlein and I were discussing the perils of template stories: interconnected stories that together present a future history. As readers may have suspected, many future histories begin with stories that weren't necessarily intended to fit together when they were written. Robert Heinlein's box came with "The Man Who Sold the Moon." He wanted the first flight to the Moon to use a direct Earth-to-Moon craft, not one assembled in orbit; but the story had to follow "Blowups Happen" in the future history.

Unfortunately, in "Blowups Happen" a capability for orbiting large payloads had been developed. "Aha," I said. "I see your problem. If you can get a ship into orbit, you're halfway to the Moon."

"No," Bob said. "If you can get your ship into orbit, you're halfway to
anywhere."

He was very nearly right.

* * *

Space travel isn't a matter of distances, it's a question of velocities. Now most space systems designs begin with rough-cut estimates of present and near-term predicted technological capabilities; and one of the best measures used in design analysis is called "delta vee." This is engineer talk for a change in velocity, and comes from the general mathematical symbol for change, the Greek letter delta or Δ. Delta vee, written Δv, is the total velocity change a ship can make.

The nice part about delta v is that for rough analysis it doesn't matter how you expend your fuel. You can burn it all up at once, or make a whole series of velocity changes: the sum of delta v achieved will be the same. Moreover, the total delta v can be calculated from the Specific Impulse (a measure of efficiency) of the fuel used and the fraction of the total ship weight that's made up of fuel. No other numbers are needed, not even total ship's weight. Given the total delta v, you can determine what kind of missions the ship can perform.

The other nice feature is that delta-v requirements for any journey in the solar system can be calculated from well known parameters: mass of the sun, masses of the planets you're leaving and going to, and the distances of the planets from the sun. There are a lot of possible refinements, but rough estimates of delta v requirements for any minimum energy journey can be run off on a slide rule in no time.

The least costly method of long-distance space travel involves transfer orbits, sometimes called Hohmann orbits after the German architect Dr. Walter Hohmann, who first calculated the energy requirements to get from place to place in the solar system. Hohmann's book, THE ATTAINABILITY OF THE CELESTIAL BODIES, was published in the mid-thirties and was a very important book indeed, because it showed that space travel really was possible with chemical rocket fuels.

Unfortunately, as Willy Ley noted in ROCKETS AND SPACE TRAVEL, Hohmann's book is nearly unreadable, combining Germanic scholarly thoroughness, unfamiliar subject matter, lots of mathematics, and a terribly complex style. Despite that, his work remains important and the transfer orbits he described are the only feasible methods of getting to other planets from Earth with chemical rockets.

In Hohmann orbits, the starting planet at GO and the target planet at the time of journey's end must be precisely opposite each other with the sun between. Naturally, then, the trip begins when the target planet hasn't yet got to opposition, and these journeys can start only at certain times. The ship departs on a trajectory that carries it into a highly elliptical orbit with one end of the ellipse just touching the orbit of the origin planet and the other touching the orbit of the target planet.

The delta v required for Hohmann trips to various places is shown in Figure 12. In every case it is assumed that the starting point is not on Earth, but in orbit around Earth. The numbers were calculated for me by Dan Alderson, who programs JPL's computers and is usually concerned with real spacecraft such as Pioneer and Mariner; they're quite accurate, given the model used. For those interested, we assume the planets have circular orbits and all lie in the same plane, and use conic section approximations.

The first important number is the fly-by delta-v requirement. This assumes you just want to get close to the target, and after that you don't care what happens to the ship. In the real world, fly-by probes can be useful afterwards: the Pioneer series Jupiter probes, for example, may round Jupiter in such a way that they use Jupiter's attraction to fling them on toward other planets, or out of the solar system altogether.

There was even a possibility of a Grand Tour, in which the spacecraft approached Jupiter, Saturn, and then either flew past both Uranus and Neptune, or went direct from Saturn to Pluto, each time using the delta v gained from a close approach to one planet to get to the next. Congress wouldn't fund the Grand Tour, and that opportunity is lost for our lifetimes because it takes a special configuration of planets for the Grand Tour; but as I write this they're preparing to launch Mariner 10, a probe that will use Venus as a slingshot to send it down to Mercury, and if all goes well Mariner will arrive at Mercury about the time this is published. (Mariner is scheduled to arrive at Venus on Feb. 5, 1974, and at Mercury on March 29, 1974.)

The Pioneer probes carry the famous gold plaque with a code showing the origin of the spacecraft and line drawings of human male and female, on the assumption that someday they may be picked up by beings in another star system. Since the probe will leave the solar system with a velocity of only a few kilometers per second, and must cross trillions of kilometers before there's any possibility of its being found, we don't have to worry much about the aliens using it to track us back to Earth and conquer us; by that time, if interstellar travel is possible, we'll have it.

It happens that I was present when that plaque—called "The Praque" by the TRW technicians who built Pioneer—was invented. NASA held a big press briefing at TRW, a dog and pony show for science reporters; the NASA, JPL, and TRW scientists concerned with Pioneer described the experiments aboard, and one happened to mention that Pioneer would definitely leave the solar system forever.

One of the reporters present was Eric Burgess, who with Arthur Clarke founded the British Interplanetary Society back in the 40's. Eric became very thoughtful, and later that afternoon spoke to Carl Sagan of Cornell and some of the others in charge of Pioneer, pointing out what a unique opportunity this was to send a message to anyone "out there." It might take a long time to arrive, but at least it was going. The idea caught on, and within a week the plaque was designed and installed.

__________

 
Values for Sun are very close approach and circular orbit at surface. Value for Earth is marginal delta v needed to escape Earth's gravitational effect. Asteroid capture values are large because the asteroids have essentially no mass, and thus do not aid appreciably in an attempt to catch up with them after arriving at their orbital distance.

__________

Then, of course, came the complaints about the "dirty pictures" of nude men and women, but that's another story.

Figure 12 shows in addition to fly-by delta-v requirements, the delta v you'd need to get into some kind of orbit around the planet: the bare minimum for capture, and a circular orbit from which you could land or observe closely. You can see the numbers come out at reasonable values, except when you're trying to get very close to the sun. One important number is the Sun escape velocity. If you have that much delta v capability, you can get to other stars: anywhere, for practical purposes. It is important to note, though, that Figure 12 assumes you don't start from Earth, but from
orbit around Earth.

Since you need 7.6 km/sec delta v to get into Earth orbit in the first place, Bob Heinlein's top of the head remark was very close to correct. Earth orbit is halfway to anywhere.*

In other words, the first step's the hard one. If you can get into Earth orbit, you can get most anywhere else. Unfortunately, the disintegrating totem poles we now use to get into orbit are just too cumbersome and expensive to make space flight routine. Worse, they use up nearly all their total delta v getting into orbit—and the rocket is thrown away, hundreds of millions of bucks into the drink.

The upcoming shuttle reusable ship will help and is sorely needed, but there's a system even better than that. The concept I'm about to describe can use old rocket boosters over and over again; in fact, the rocket motor never leaves the ground. Only payload goes up.

This magic feat is performed by lasers. The basic design of the system comes from A. N. Pirri and R. F. Weiss of Avco Everett research laboratories. What they propose is an enormous ground-based laser installation consuming about 3,000 megawatts. In practice, there would probably be a number of smaller lasers feeding into mirrors, and the mirrors would then concentrate the beam onto one single launching mirror about a meter in diameter. This ground station zaps the spacecraft; the ships themselves carry no rocket motors, but instead have a chamber underneath into which the laser beam is directed.