Shipstar (49 page)

Still: Who did this? Maybe the Bowl was first made for just living beneath constant sunshine. So at first the Builders may have basked in the glow of their smaller sun, developing and colonizing the Bowl with ambitions to have a huge surface area with room for immense natural expanses. But then the Bowl natives began dreaming of colonizing the galaxy. They hit on the jet idea, and already had the Knothole as an exit for it. Building the Mirror Zone took a while, but then the jet allowed them to voyage. It didn’t work as well as they thought, and demanded control, which they did by using large magnetic fields.

The system had virtues for space flight, too. Once in space, you’re in free fall; the Bowl mass is fairly large, but you exit on the outer hull at high velocity, so the faint attraction of the Bowl is no issue. Anyone can scoot around the solar system, and it’s cleared of all large masses. (The Bowl atmosphere serves to burn any meteorites that punch through the monolayer.)

The key idea is that a big fraction of the Bowl is mirrored, directing reflected sunlight onto a small spot on the star, the foot of the jet line. From this spot the enhanced sunlight excites a standing “flare” that makes a jet. This jet drives the star forward, pulling the Bowl with it through gravitation.

The jet passes through a Knothole at the “bottom” of the Bowl, out into space, as exhaust. Magnetic fields, entrained on the star surface, wrap around the outgoing jet plasma and confine it, so it does not flare out and paint the interior face of the Bowl—where a whole living ecology thrives, immensely larger than Earth’s area. So it’s a huge moving object, the largest we could envision, since we wanted to write a novel about something beyond Niven’s Ringworld.

For plausible stellar parameters, the jet can drive the system roughly a light-year in a few centuries. Slow but inexorable, with steering a delicate problem, the Bowl glides through the interstellar reaches. The star acts as a shield, stopping random iceteroids that may lie in the Bowl’s path. There is friction from the interstellar plasma and dust density acting against the huge solar magnetosphere of the star, essentially a sphere 100 astronomical units in radius.

So the jet can be managed to adjust acceleration, if needed. If the jet becomes unstable, the most plausible destructive mode is the kink—a snarling knot in the flow that moves outward. This could lash sideways and hammer the zones near the Knothole with virulent plasma, a dense solar wind. The first mode of defense, if the jet seems to be developing a kink, would be to turn the mirrors aside, not illuminating the jet foot. But that might not be enough to prevent a destructive kink. This has happened in the past, we decided, and lives in Bowl legend.

The reflecting zone of mirrors is defined by an inner angle,
Θ
, and the outer angle,
Ω
. Reflecting sunlight back onto the star, focused to a point, then generates a jet which blows off. This carries most of what would be the star’s solar wind, trapped in magnetic fields and heading straight along the system axis. The incoming reflected sunlight also heats the star, which struggles to find an equilibrium. The net opening angle,
Ω
minus
Θ
, then defines how much the star heats up. We set
Ω
=
30 degrees, and
Θ
=
5 degrees, so the mirrors subtend that 25-degree band in the Bowl. The Bowl rim can be 45 degrees, or larger.

The K2 star is now running in a warmer regime, heated by the mirrors, thus making its spectrum nearer that of Sol. This explains how the star can have a spectral class somewhat different from that predicted by its mass. It looks oddly colored, more yellow than its mass would indicate.

For that matter, that little sun used to be a little bigger. It’s been blowing off a jet for many millions of years. Still, it should last a long time. The Bowl could circle the galaxy itself several times.

III. B
OWL
D
ESIGN

As the book says, the Bowl star is

K2 STAR. SIMILAR TO EPSILON ERIDANI (K2 V). INTERMEDIATE IN SIZE BETWEEN RED M-TYPE MAIN-SEQUENCE STARS AND YELLOW G-TYPE MAIN-SEQUENCE STARS.

So its light is reddish and a tad less bright than Sol. There is a broad, cylindrical segment of the Bowl at its outer edge, the Great Plain. This is huge, roughly the scale of Ringworld, with centrifugal gravity Earth normal times 0.8, so humans can walk easily there. Beyond that is the bowl curve, a hemisphere that arcs inward toward the Knothole. On the hemisphere, the Wok, the centrifugal gravity varies with latitude, and is not perpendicular to the local ground. To make a level walking surface, the Bowl has to have many platforms that are parallel to the jet axis, so gravity points straight down.

The local apparent centrifugal gravity has two vector components:

A: Centrifugal gravity that is
perpendicular to the local level surface on the bowl,
vs angle
Ψ
(in radians). Here
Ψ
is measured away from the polar bowl axis—that is, the jet axis. The curve peaks at 90 degrees, where the Great Plain has a local g of 1 in this plot. (It’s 0.8 of Earth’s.)

B: Below shows the magnitude of centrifugal gravity that is parallel to the local level surface on the bowl, vs angle
Ψ
—thus, it’s the felt force pushing
away from the pole where the Knothole lies,
along the local level.

So the pushing-away force is largest at the mid-latitudes, then falls away because the total force is small at the poles. This component also vanishes on the Great Plain.

Local Gravity versus Angle from the Jet Axis

The Builders designed it this way so that some of the lands are hard to walk upon in the direction of the Knothole. This discourages others from simply traveling to the Knothole by a slog across the entire Bowl; it takes a lot of work, working against a slanted local “gravity”—especially near the Mirror Zones, which are in the mid-latitudes. Remember also that you must pump fluids around, since local forces drive rivers to flow and either they return through clouds and rain, or you must pump them when the weather doesn’t perform well. There’s a tendency for fluids to wind up in the lower gravity regions, too.

We did other such calculations, and many such didn’t get into the final book. But they lurked in our minds. This may be an example of Ernest Hemingway’s dictum that the more you know about a story’s background, the more you can then leave out, and the detail will still make the story stronger because of the confident way you write it.

This odd centrifugal gravity also presents the Builders with a big stress problem. Holding together this whirling, forward-driving system demands nuclear-force levels of strength.

The atmosphere is quite deep, more than two hundred kilometers. This soaks up solar wind and cosmic rays. Also, the pressure is higher than Earth normal by about 50 percent, depending on location in the Bowl. It is also a reservoir to absorb the occasional big, unintended hit to the ecology.
Compress Earth’s entire atmosphere down to the density of water, and it would only be thirty feet deep. Everything we’re dumping into our air goes into just thirty feet of water.
The Bowl has much more, over a hundred yards deep in equivalent water. Too much carbon dioxide? It gets more diluted.

Sideways “Gravity” versus Angle from the Jet Axis

This deeper atmosphere explains why in low-grav areas, surprisingly large things can fly—big aliens and even humans. We humans Earthside enjoy a partial pressure of 0.21 bars of oxygen, and we can do quite nicely in a two-bar atmosphere of almost pure oxygen (but be careful about fire). The Bowl has a bit less than we like: 0.18 bar, but the higher pressure compensates. This depresses fire risk, someone figures out later.

Starting out, we wrote a background history of where the Builders came from, which we didn’t insert into the novel. It lays out a version of that distant history that isn’t necessarily what we ended up implying and partially describing:

Long before 65 million years ago, there were dinosaurs who maintained internal temperatures through feathers, in a largely warmer world. But they ventured out with rockets into a solar system chilly and hostile. Still, they needed metals and did not want to destroy their biosphere with the pollutants from smelting, fast energy use, excess carbon dioxide, and the like. So the Bird Folk split into two factions:

•    the Gobacks who wanted to return to simple habits compatible with the world they once knew, using only minimal technology, and

•    the Forwards, who wanted to re-create around the Minor Star (which became the Bowl’s) a fresh paradise that fulfilled the warm, comfortable paradise the Folk had once known. That could send the Forwards out into the galaxy that beckoned, full of living worlds ripe for the spread of the evolving Folk and all they stood for.

Some of the Forwards were impatient to see what worlds lay millennia away. Many had themselves put in stasis to await a planetary rendezvous. Some faiths arose, hoping to commune somehow with the Godminds whose SETI signals told of great feats of engineering … but these turned out to be funeral pyre signals, of greatness departed long before. By that time, Earth was far behind the Bowl and shrouded in nostalgic legend.

So came the Separation, with the warmth-loving Forwards leaving and the Gobacks remaining on Earth. There they returned to the free life available in the ancestral lands. They kept their numbers low and gradually came to dislike the technologies they had inherited from the Forwards and the earlier civilizations. They reverted to a quiet, calm, agricultural culture. And they prospered, until a bright, flaring tail appeared in their skies.…

After all, by then, the dinosaurs didn’t have a space program.

—April 2013

B
OOKS
BY
G
REGORY
B
ENFORD
AND
L
ARRY
N
IVEN

Bowl of Heaven

TOR
BOOKS
BY
GREGORY
BENFORD

Jupiter Project

The Stars in Shroud

Shiva Descending

Artifact

In Alien Flesh

Far Futures

Beyond Human

TOR
BOOKS
BY
LARRY
NIVEN

N-Space

Destiny’s Road

Rainbow Mars

Scatterbrain

Ringworld’s Children

The Draco Tavern

Stars and Gods

Playgrounds of the Mind

A
BOUT THE
A
UTHORS

GREGORY BENFORD is a professor of physics at the University of California, Irvine, and lives in Irvine. He is a winner of the United Nations Medal for Literature, and the Nebula Award for his classic novel
Timescape
. Visit him at
www.gregorybenford.com
.

LARRY NIVEN is the author of the Ringworld series and many other science fiction masterpieces. His
Beowulf’s Children,
coauthored with Jerry Pournelle and Steven Barnes, was a
New York Times
bestseller. He lives in Chatsworth, California. Visit him at
www.larryniven.net
.

This is a work of fiction. All of the characters, organizations, and events portrayed in this novel are either products of the authors’ imaginations or are used fictitiously.

SHIPSTAR

Copyright © 2014 by Gregory Benford and Larry Niven

All rights reserved.

Cover photograph of Galaxy 4499 by NASA, ESA, A. Aloisi (STSCI/ESA), and The Hubble Heritage (STSCI/AURA)—ESA/Hubble Collaboration

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