Saturn Rukh, page 9
Rod and Chastity helped Seichi with the engine checkout, activating the magnoshield from inside, while Seichi reached up into the nozzle bell with a magnetometer to check the field configuration. The two also examined the parachute, balloon, and shrouds that would bring them to a halt in the clouds and float them there during their six-month stay on the giant planet.
“That’s the trouble,” said Rod with a resigned shake of his head as he and Chastity looked at the neatly laid-out shrouds on top of fold after fold of tough fabric. “The only way to really check out a parachute is to jump with it.”
~ * ~
Finally, they were ready. At dinner that night, Rod announced, “Mission Control says they are set up and ready to have us climb down the rings. As soon as Rhea is in the right position, we’ll do a burn at Titan and drop down to our first ‘rung’ on the ladder.”
Sandra, puzzled, asked Rod a question. “What I don’t understand is how come if Sexdent is able to climb up out of Saturn’s gravity well using its rockets, why can’t it use its rockets to climb down?”
“ ‘Cause of the extra mass we have to carry down,” said Rod. “If all we had to carry was the fifty-ton main capsule, the hundred and twenty tons of fuel in the tanks of the main capsule would do the job. But we also have to take down the thirty-five-ton meta factory, the five-ton heat shield, and the ten tons of balloon and shrouds—another fifty tons. We’d ran out of fuel trying to slow down that load—and burn up. So we’ve got to take it in steps—using every trick in the book—to climb part of the way down the gravity well using the moons and rings—while saving our fuel for that last big step down into the atmosphere.”
~ * ~
When the orbits were just right, Rod drifted Sexdent over to Titan and let it fall in toward the smoggy moon on an essentially parabolic orbit. At the point of their closest approach Rod fired the rockets for about half a minute at one gee.
“We’re on our way,” he reported. “We’ll be at Rhea in five days.”
“Let’s have a party!” suggested Pete, but then he noticed that everyone had a concerned look on their faces. They were committed now ... to a very risky mission ... a mission that might cost them their lives.... He dropped the idea.
~ * ~
As they approached Rhea, Sandra got some good images of the bright, crater-covered moon.
“Just a well-worked ball of ice,” reported Sandra, looking critically at the images she had collected. “Not much different from the bright side of Iapetus.”
Rod brought them to a halt at Rhea with a half-minute burn at three gees.
“We’ll have to wait here until the orbit timing of all the moons we’re going to use is right,” he said.
“Here we are... right outside the E ring,” said Sandra, looking at the fine line across the sky that indicated the plane of the rings, “but I can’t get any pictures—wrong angle. Have to make do with Saturn.”
Soon she and Dan were back at their task of imaging features on the rapidly rotating face of Saturn. The giant planet now covered thirteen degrees of the sky—almost filling the holoviewports.
~ * ~
“Mission Control says the time for the next drop down is early tomorrow,” announced Rod a few days later.
“What’s the next stop?” asked Pete. “Dione? That’s the next big moon inward.”
“Not quite,” said Rod. “We’re going to leave sixty degrees early so we drop down on Dione’s Trojan companion, Helene, instead. It’s just the right size for Chass to try a tether whip maneuver. Save us some fuel.”
The next day, at their closest approach to Rhea, Rod fired the rockets for only ten seconds. But that was enough to drop them downward again. Their trajectory skimmed only a few hundred kilometers above the E ring and Sandra got excellent pictures. For the next two days, they watched as Saturn loomed ever larger in their viewports.
“The white spot is catching up with Brown Spot One again,” reported Dan from the biviewer. “You ought to get a shot of it.”
“As soon as we stop, I’ll take pictures of Saturn, but right now I’d better concentrate on the E ring,” replied Sandra.
“We’re not going to stop,” said Chastity, who was controlling the orientation of Sexdent for the two scientists. “With a tether whip, you leave as fast as you came.”
“Oh!” said Sandra. “Then you’d better turn us around so I can shoot Brown Spot now.”
~ * ~
Sexdent soon caught up with Helene. It was a free-floating potato-shaped megamountain about thirty-five kilometers in diameter. Rod, Chastity, and Seichi were at their stations, while the other three looked over their shoulders from their handholds on the ladder that ran through the center of the ship. The acceleration levels expected during this tether whip maneuver weren’t large, only half a gee, so they didn’t have to strap down on acceleration couches as they did for the rocket bums.
“We’ll be catching up with her at eight hundred meters per second,” reported Rod as the elliptical orbit of Sexdent started to intersect the circular orbit of Helene. “It should be a nice easy practice run for you. I’m ready with rockets in case you miss.”
Chastity’s long-nailed fingers flew over the keyboard to bring up a seldom-used menu on her screen. She touched the icon that flipped back the conical nosecone of Sexdent, exposing a metapropelled penetrator inside, its sharp tungsten-carbide tip pointing outward. Some hours ago, Jeeves had used its mechbot, Mouser, to move the penetrator from its storage rack on the wall to the launcher, and hook the flame-resistant metal-braid “tail” of the penetrator to the end of the two-hundred-kilometer tether stored on the reel.
Rod, working in concert with Chastity, used the attitude jets to direct the nose of Sexdent toward the still-distant moonlet.
“Launch,” said Chastity quietly as the penetrator took off using a low-temperature exhaust of warm helium gas. After the penetrator was well clear of Sexdent the exhaust flame brightened into a reddish-purple plasma. The penetrator accelerated toward its target, dragging the tether behind it—the multiline structure automatically dilating to its extended shape as it was pulled from the reel.
Chastity had a plan view of the action on her console screen and a view from the penetrator’s nose camera on the holoviewport. Her manicured right hand was in the controller’s box under the console, directing the flight of the penetrator as her eyes alternated between the holoviewport and the console screen.
“We need to get a little closer, Rod,” she said, tapping the image on the console touchscreen with the tip of a long glittering-gold fingernail. “I want a closest distance of a hundred and sixty.”
Rod tweaked the joyball in the controller’s box under his console. The vernier jets on Sexdent gave a short burst, the projected path line of the spacecraft shifted slightly, and the number indicating the distance between the trajectory of Sexdent and the trajectory of the asteroid at their point of closest approach changed from 210 to 160. He kept a close eye on that number and his hand ready around the joyball, in case the number started to change, but he couldn’t help glancing out the viewport in front of him as they closed in on the slowly tumbling grayish-white mountain—growing larger each second.
The tumbling gray icerock was much larger in the holoviewport in front of Chastity. Knowing that Rod would keep Sexdent on course, she concentrated on the image in front of her, looking for the best place to put the penetrator. If it hit a hard surface, it might not penetrate enough and would be pulled out as tension was applied. If it was placed in a field of rocks, the rocks might cut the tether as it tilted over from the vertical during their turn. The best place was the middle of a small crater, as fresh as possible. She spotted one—and with minuscule adjustments to the joyball in her controller, the cross-hairs on the holoviewport shifted to the center of the crater.
“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.
