Saturn Rukh (12 page)

“One hundred twenty kilometers…” intoned Seichi, reading off the length of tether that had been unreeled from the drum in
Sexdent
’s nose. Chastity did a quick mental calculation and her fingers lifted the joyball, increasing the speed of the penetrator. “One-thirty... one-forty…” The small crater was now a large crater that filled the holoviewport...

“… One hundred fifty kilometers ... one hundred fifty-five kilometers…” reported Seichi.

“Contact!” said Chastity as the penetrator struck the icy surface of Helene and the image from the penetrator nose camera turned black. Sliding her fingertip across the tension control icon, Chastity began braking the still-unreeling tether. As
Sexdent
started to pull on the tether, gravity returned. It wasn’t much, just four-tenths of a gee. The energy going into the electromagnetic brake was electrically shunted to resistance-heated radiator vanes that could be seen overhead out of the top of the viewports. The vanes glowed a deep red as they radiated away the energy that had been generated during the deceleration of
Sexdent.

For thirteen minutes they swung around Helene, their nose pointing constantly at the icy planetoid, while Chastity carefully let out the tether. The pull of the lengthening tether winding around the surface of the icy rock caused their ship to lose speed.

“Slowing down nicely,” said Rod. “Six hundred ninety meters per second ... six-eighty …”

“Tether length reaching maximum,” warned Seichi. “One hundred ninety kilometers ... one hundred ninety-five kilometers ...”

Just as Rod’s velocity countdown reached 640 meters per second, and Seichi’s length count reached 200 kilometers, they had swung 180 degrees around Helene and were now moving in the opposite direction.

Chastity watched her console carefully until the yellow line indicating the predicted trajectory for
Sexdent
rotated until it was parallel with the blue line for the optimum outgoing trajectory from Helene. When the two lines merged and became a single green line, she tapped a red icon on the screen marked “Tether Release.” The tether was cut at the penetrator and snaked its way along the gray icedust surface as they left Helene behind and started their fall inward above the E ring toward the growing orange globe of Saturn—which was now eighteen degrees across.

“Nice job,” said Rod approvingly as the ship returned to free fall. He looked at the image on his console screen. The yellow and blue lines weren’t exactly parallel. They started to diverge at the edge of his screen. He could have brought them into exact parallelism with a short burst from the vernier jet, but decided not to. The small error wouldn’t affect their arrival at the next moon significantly. He quickly wiped his screen so Chastity wouldn’t see it.

“Perfect alignment!” he announced. “Nothing for me to do.”

Chastity looked at her screen. It contained an identical image showing the small residual error she had left after her tether turn maneuver. She quickly wiped her screen so Rod wouldn’t see it. With the screen now blank, she turned to grin her thanks at him, violet eyes glittering with pride at her successful maneuver. If he wanted to act like a gentleman instead of showing her who was boss, she would be lady enough to let him.

That evening she received a congratulatory message from Art Dooley over her personal comm link.

“That was an amazing demonstration of how powerful tethers can be,” said Art. “For years, I have been trying to convince the venture capitalists I know to invest in space tethers for interorbit and interplanetary transportation. When meta was invented, however, all the early interest in space tethers went away, because meta-fueled rockets could do most space transportation jobs, and tethers were an untried and therefore risky technology. Although a space tether transportation system uses no fuel, and would cut space transportation costs way down, even below what we expect from using cheap Saturn meta, I could never get anybody interested. You just demonstrated that a good space tether can horse a two-hundred-ton payload around at a half-gee acceleration. The investors now believe that tethers are a ‘proven’ technology and are willing to put their money into it. Today I lined up enough of them to buy out this small business, Tethers Unlimited, that has the patents on the high-strength, failsafe Hoytether. We’re going to pump a lot of money in it, turning it from a small business to a big business. Having someone as famous as you in the company management would be a big asset in our marketing efforts. Would you like to be on the board of directors?”

Chastity replied that she was flattered to be asked, but she would have to reserve her decision until she and the rest of the crew had completed their mission. After all, once one had a billion dollars in the bank, there was no need to work anymore, unless it was fun, of course.

~ * ~

A day later, they began to catch up with Calypso, the leading Trojan companion of Tethys.

“Hmmm…” said Rod as he set up his pilot board for the maneuver. “There must be something wrong somewhere, Jeeves. It says here that Calypso is sixty-five degrees ahead of Tethys. I thought that Trojan moons were always at sixty degrees ahead or behind their primary.”

“The L4 and L5 Trojan points, which indicate the minimum of the gravitational well, are at exactly sixty degrees,” replied Jeeves. “But if a small moon at that point is perturbed—which happens often in a moon system as complex as that of Saturn— then the small moon moves in an orbit around the minimum point. The orbit is not circular or elliptic, however, it is tadpole shaped.”

“Tadpole shaped?” said Rod.

Jeeves changed the image on Rod’s console screen from that of a telescopic view of Calypso to a plan drawing, showing the large moon Tethys and its leading and trailing Trojan moons. One was Telesto, a roundish potato-shaped moon about twenty-two kilometers in diameter. The other was Calypso, an elongated yam-shaped moon that was thirty kilometers by twenty-four kilometers by sixteen kilometers. Superimposed on the images were lines indicating the L4 and L5 points, with contours around them that were circular on the side toward the primary moon, and cusped on the side away. The circular portion looked like the head of a tadpole while the pointed cusped portion looked like the tail.

“As you can see,” said Jeeves, using a blinking arrow to point out the moonlet on the screen, “Calypso is almost at the cusp point in its orbit around the leading L4 point, so it is sixty-five degrees ahead of Tethys.”

“Now I see what you mean,” said Rod. “The orbit does have the shape of a tadpole. What a weird shape. How far do those tadpole orbits stretch? Can the cusp point for the Calypso orbit go all the way around to the trailing Trojan point?”

“If Saturn were perfectly round, there were no other moons, and Calypso were given just the right perturbation, the cusp, or tail, of the tadpole orbit would reach all around to the point in the orbit opposite to the primary moon, where it would meet the tail of the maximal orbit from the trailing Trojan point. A tiny perturbation there could cause a switch.”

“But in real life, switches don’t happen,” said Rod. “The perturbations of the other moons will kick it somewhere else long before it gets to the halfway point.”

“Actually,” replied Jeeves, “there is an example of such switching in the Saturn system.”

“Really?” said Rod, intrigued. “Show me.” His screen changed.

“There are two small inner moons of Saturn, Epimetheus and Janus, that are almost the same size, and share almost the same orbit. Janus, approximately two hundred kilometers in diameter, is about four times as massive as Epimetheus, which is about one hundred twenty kilometers in diameter. Since neither can be considered the ‘primary,’ there aren’t the usual Trojan points with their tadpole orbits. Instead, both move in horseshoe-shaped orbits that lie on either side of the nominal circular orbit for their joint angular momentum. Janus, being heavier, moves in a small crescent that only covers a few tens of degrees of the circle, while Epimetheus moves in a large horseshoe orbit that stretches from plus thirty degrees all the way around to minus thirty degrees.”

Rod looked at the diagram on his screen. “Say ... Epimetheus is significantly outside, the nominal orbit.”

“That is correct,” said Jeeves. “The nominal orbit for the combined system is 151,432 kilometers. Janus is presently orbiting at 151,422 kilometers—ten kilometers inside the nominal orbit—while Epimetheus is orbiting at 151,472 kilometers—40 kilometers outside. In a few years, the distances will be reversed. Epimetheus will be 40 kilometers inside, while Janus will be 10 kilometers outside.”

“Curiouser and curiouser...” muttered Rod. “But right now we have Calypso to catch.” He reset the pilot console to the telescopic image that he had started with, while Chastity settled in at the science console, setting up the screen icons for tether control. After some careful calibrations and consultation with Jeeves, Rod gave a short burst from the vernier rockets to adjust their incoming trajectory. The burst of noise and the slight acceleration brought most of the rest of the crew out of their habitats to see the action.

“Looks like a kid’s first snowball,” said Pete, looking at the rapidly approaching moon out the viewport over Chastity’s shoulder as she prepared to fire her penetrator. “After this tether whip, I presume we’ll be going to Enceladus next?”

“Nope,” said Rod. “Enceladus is the wrong size—too large for a tether whip and too small for a gravity whip. Its diameter is five hundred kilometers, while our tether is only two hundred kilometers. If Chastity stuck one end of the tether into Enceladus and started to make a one-hundred-and-eighty-degree turn around it, we’d smack into the surface after only going ninety degrees. But even though it’s five hundred kilometers in diameter, its escape velocity is less than two hundred meters a second—not enough to help a perigee burn significantly. So instead of burning something like a whole klec’s worth of fuel to stop and start at Enceladus, we’re going to bypass it. I’m going to add a burn to Chastity’s tether whip at Calypso. That’ll drop down past the orbit of Enceladus to the orbit of Mimas. But even then we can’t use Mimas directly. Again it’s the wrong size. Its diameter is only two hundred kilometers, but that’s still too large for a tether whip. We’re going to use a big boulder Sandra found tadpoling around its leading Trojan point.”

They quickly caught up with Calypso.

“I’m going to do this one at one gee,” said Chastity, “so make sure you hold on tight.” She fired the penetrator and unreeled only fifty kilometers of tether before striking Calypso at its “waistline.” This time Chastity hardly used the brake at all and the acceleration level quickly climbed to one gee as they started their rapid turn around the moonlet on the end of their short tether. Pete found his legs buckling under the unaccustomed acceleration and sat down on the grating. Chastity noticed the motion out of the comer of her eye.

“Only four minutes of this, Pete,” said Chastity. “Then you’ll be back in free fall.”

“Don’t forget my minute,” Rod reminded her.

“Right... five minutes, Pete.”

A few minutes later, Chastity turned to Rod. “Almost there,” she warned.

“Ready,” said Rod, his hand on the joyball in his controller.

“Cut!” announced Chastity as they completed their 180-degree turn around Calypso. The tether came free, and as the acceleration from the centrifugal force of the tether from the nose dropped off, Rod replaced it with the acceleration force from the rocket engines at the rear. For a full minute, the engines roared at one gee. The rockets stopped and they were back floating in free fall, freely falling even farther down into Saturn’s immense gravity well.

~ * ~

A day and a half later, they reached the orbit of Mimas at the inner portion of the E ring and headed for the kilometer-sized moonlet at Mimas’s trailing Trojan point.

“We’re coming in much faster this time—one-and-a-half klecs,” Chastity warned the crew that morning at breakfast. “So the gee level will be higher. You don’t need to be strapped in acceleration couches—it’ll only be one-and-a-third gees— but you’d better sit down.”

After an eight-minute tether whip and a twelve-second rocket bum, they were on their way down again.

“What’s our next stop?” asked Pete, as he floated upward off the grating in the free fall that followed the maneuver.

“We’re going all the way over the A ring into the Huygens Gap,” said Chastity, pointing to her screen. “There’s a relatively clear region in the Cassini Division between the A and B rings. Sandra found out why—there’s a pair of moonlets keeping it swept clean. We’re going to use one of them. We’ll be there in eight hours.”

~ * ~

Six hours later, Chastity appeared again on the control deck. Rod, Dan, and Sandra were busy taking one shot after another as they zoomed just above the A ring.

“This next tether whip is going to be at high gees,” she announced. “We’re going to need acceleration couches.”

“Lessee ...” said Rod, switching gears. “Do I need to make a burn on this one?”

“No fuel needed for this one,” replied Chastity. “We can do it all by braking.”

~ * ~

The penetrator struck the Huygens Gap moonlet with the tether length at 140 kilometers. Chastity, lying on her back in the acceleration couch, used her controller to increase the braking on the tether while letting it pay out slowly. The acceleration level reached 2.6 gees, while the radiator fins sticking out of the sides of the capsule turned from red hot to white hot to blue hot as the kinetic energy taken out of the spacecraft was converted into heat and dissipated into space. For almost six minutes the crew endured the high acceleration, then suddenly it was all over, and they were back in free fall again.