Round-Trip Laser-Pushed Lightsail Propulsion System
[J. Spacecraft, Vol. 21, No. 2, pp. 187-195 (1984)]
"We want to get to Barnard as fast as possible, so we will use more laser power to push the sail while at the same time keeping it below its melting point. This will require boosting the power of the laser generators by a factor of thirty. The lightsail will accelerate at thirty percent of Earth gravity and reach half the speed of light in less than two years. At this speed, the time dilation factor is thirteen percent. We could continue to accelerate to higher coast velocities, but the radiation generated from hitting dust grains and hydrogen atoms starts to become hazardous. As before, as the lightsail approaches the target star, the inner rendezvous stage separates from the decelerator stage, the laser light from the solar system strikes the decelerator stage and reflects back on the rendezvous stage, bringing it to a halt in the target star system. The crew can then use the rendezvous stage as a solar sail for transport within the target star system. When the crew is ready to go home, they separate out the return stage from the remainder of the rendezvous stage, which now becomes the accelerator stage. The laser light from the solar system reflects off the accelerator stage onto the return stage, and it again accelerates at thirty percent of Earth gravity to half the speed of light. One final burst of laser light will then bring them safely to rest in the solar system. Assuming a coast velocity of half the speed of light, the roundtrip travel time to Barnard is about twenty-seven years. We can cut that some by continuing to accelerate to higher coast velocities and adding more radiation shielding."
"Twenty-seven years!" exclaimed Congressman Farquar from the end of the subcommittee table. "Isn't there a better and faster method of getting there?"
"Yes. Antimatter propulsion," replied Ross. "What is desired is a spacecraft that can accelerate at one to three Earth gravities in order to reach speeds closer to the speed of light more quickly, can provide rapid transport around the target star system once it arrives there, can land on and take off from all but the largest planets in that system, and can head for home the instant the crew is ready to do so. An antimatter rocket provides that capability—if we can build it."
"Are we going to be able to afford it?" asked Congresswoman Polk at the other end of the subcommittee table. "I was under the impression that antimatter cost billions of dollars a gram."
"It used to," replied the administrator. "And if that were still true, I wouldn't be suggesting its use. But some recent experiments in particle physics have discovered a Higgs-mediated charge-exchange resonance in the near collision of protons with electrons. The resonance was so narrow that it was missed up until now. But being very narrow, it is also very strong. If a beam of polarized electrons at just the right speed is shot at a beam of polarized protons at just the right speed, and the two polarizations are set at just the right angle, then the protons and the electrons exchange charges. The electron becomes a positron or antielectron, while the proton becomes an antiproton. The two types of antimatter are easily separated from the normal matter in the beams, decelerated to a stop, put together to make antihydrogen, and the antihydrogen stored without loss in electromagnetic traps until you are ready to use it—in our case for space propulsion. Amazingly enough, essentially no energy is required, since you can get back most of the particle beam energy during the deceleration process. My engineers inform me that within a few years, we could have antimatter factories producing literally tons of antimatter a year—more than enough for an interstellar mission, if we can build an engine robust enough to utilize the antimatter at its full potential. Let me show you an idealized picture of an antimatter powered interstellar rocket."
The holoscreen went through its "audience attention focusing" routine, and the focusing ball faded away and was replaced by a long, thin spacecraft that reached from nearly floor to ceiling in the corner of the hearing room. It looked like a fire-arrow, with glowing red feathers on its tail, and a purplish-blue flame coming out of its rear.
"This is pretty much what an antimatter rocket would look like. The antimatter engine is at the rear, emitting a high temperature plasma made of energetic particles moving at almost the speed of light. To keep the engine from melting, it must be cooled, and the liquid-drop radiators for the cooling are the three bright red triangular 'tail-feather' structures at the rear. In front of the radiators is a heavy tungsten radiation shield that blocks most of the gamma rays being emitted by the engine. In front of that are the storage tanks for the propellant that will be heated by the antimatter, and in front of that are the storage bottles for the antimatter itself. Way at the front, at the end of a very long boom, is the crew compartment, far away from the gamma rays emitted by the engine during its operation, and shielded by the mass of the tungsten shield and the mass of the propellant. The critical part in the entire design is not the container that holds the antimatter, but the engine that uses the antimatter. We're going to have a tough time making it. Let me show you roughly what it must do."
The three-dimensional image of the antimatter rocket disappeared from the holoscreen, and a black-and-white drawing showed up on the flatview screen to the left.
"This is an idealized schematic of what happens inside an antimatter rocket engine. The antihydrogen is injected into the reaction chamber along with a stream of regular hydrogen. The positrons in the antihydrogen immediately annihilate with the electrons in the hydrogen, producing weak gamma rays and releasing the antiprotons which then immediately annihilate with the protons from the hydrogen. Unlike the annihilation of electrons and positrons, which always produces gamma rays, the annihilation of protons with antiprotons always produces between three and seven subatomic particles called pions. They are roughly equally divided into three types, uncharged, positively charged and negatively charged The uncharged pions almost instantly produce highly energetic gamma rays, which is why antimatter engines are dangerous to be near when they are operating and need shielding.
Magnetic Nozzle
"The other two thirds of the pions, which are moving at ninety-four percent of the speed of light, contain two-thirds of the annihilation energy, are charged, and can be contained and directed by a magnetic field—if it is strong enough. These charged particles will travel a spiral path that is twenty-one meters long before they decay. During that travel, they not only push against the magnetic field of the engine, providing rocket thrust, they also will heat up the excess hydrogen atoms that were not annihilated, and that hot plasma will also provide additional thrust. The charged pions decay into other charged subatomic particles called muons, which also will be trapped and directed by the magnetic field, providing further push to the magnetic field and additional heating of the hydrogen gas. These muons travel over a mile before they decay into electrons and positrons. By this time they will have left the engine, having used up most of their energy pushing the rocket. It is easy to make an antimatter rocket engine that is good enough for missions in the solar system—you just accept poor performance out of the engine and put a little more antimatter in the fuel tanks. But for fast interstellar missions, we are going to have to get nearly all the energy out of those charged pions and muons before they leave the engine nozzle, and to do that we are going to need superstrong magnetic fields. We can make superstrong magnetic fields with the new 'room temperature' superconductors, but although they are called 'room temperature', they really are not. The superconducting coils require cooling to dry ice temperatures, and that is next to impossible to obtain in a coil built into an antimatter engine that is emitting high energy gamma rays. What we need is not a 'room temperature' superconductor, or even a 'high temperature' superconductor, but a superconductor that can carry high currents with no loss when it is red hot."
"What are the chances of that?" asked Chairman Ootah.
"Not good," admitted Ross. "In the next year or so I will have my materials scientists working on coming up with a true 'high temperature' superconductor. If they are successfu
l, then we can design and build an antimatter rocket that will provide fast, convenient transport back and forth to Barnard. If they are not successful, then we will be stuck with the slower round-trip laser lightsail system."
"We all hope you will be successful," said Chairman Ootah. "That concludes our testimony for today. The subcommittee is adjourned until the next call for a meeting."
TEACHERS
"DOESN'T it remind you of Christmas?" Jinjur said as they set up a picnic on the beach around the old Council Rock. Time and tides had washed away all signs of the beach camp that they had deserted four years earlier, but the shape of the shoreline was still welcoming and familiar. The whole colony had come down to meet the second drop of supplies from Prometheus, and the thought of new and better tools had everyone in a good mood.
"I know what I am looking forward to most," said Cinnamon wryly.
"Don't forget, the birth control pills will take several days to reach full effectiveness," John warned.
The medical staff and the computers on board the lightsail craft had decided that the best way to control the population on Eden would be with oral contraceptives for the men. For the last five years, the men and women on the planet had had to avoid actual intercourse, or risk pregnancy. After decades of indulging freely on board Prometheus, the restriction had seemed severe. Indeed there were several small children to attest to their lack of restraint. But while some of the children had not been planned, all were wanted and loved, raised as they were with the cooperation and help of the whole community.
The six-year-old firstborns were working intently down by the edge of the surf constructing an elaborate sand castle. Adam, insisting that as the oldest, he was in charge, was ordering the others around, and thanks to the support of his "brothers" Dirk and Everett, the children fell in line with his grand plans. Freeman, Lavender, and their little brother Justin were slowly canvassing the beach looking for shells to decorate the structure. Eve and her two little sisters, Rebecca and Sarah, hauled sand from Dirk's moat to Adam and Everett's tower, and Shannon carefully added flourishes and tiny details.
The adults had learned long ago that, usually, the best thing for their bright and opinionated progeny was to leave them alone, and so they were sitting comfortably in the spot that had once been the center of camp. David and Nels were setting out the festive food that the men had made for this special occasion, and Arielle was, as usual, eating it.
"Don't eat too much, Arielle," John cautioned. "You are setting yourself up for heartburn." The petite blonde was at the end of her second pregnancy—the flouwen had informed them it was a boy—and the former beauty queen never seemed to stop eating.
"Ha! I have stomach like horse! You only want more food for yourself." Arielle snickered, her mouth full of fruit.
"Don't worry about it," said David to the others. "I've grown used to my woman's appetite. I brought along plenty."
"But Nels had us work so hard getting each recipe just right, arranging everything so fancy ..." John muttered. "And no one gets to see how beautiful it is, if you eat it before it's even put on the table."
"Relax, John," Nels said. "I think you've been living alone too long. People eating the food with pleasure is the best compliment a cook can have. I like watching people eat." He looked toward Cinnamon, nursing their fourth daughter. The baby's downy blond hair looked golden against Cinnamon's tan breast.
"That's right," said John. He was used to the kidding he received as the only bachelor. "I'm getting old and crotchety and set in my ways. Humph. I like my ways. Good ways." John had offered to live with Jinjur and his children after Shirley had died, but she had declined. Carmen rejected his offer, too. Maybe ... he thought for the millionth time, maybe I should have asked instead of offered.
Just as the midday eclipse was half over, the children called out, "There it is! There it is!", and a shooting star flared across the sky. A moment later, there was the distinctive "boom" as the aeroshells shock wave reached them.
"Hurray! Hurray!" The children were all dancing on the sand at the edge of the water. The adults knew that it would take a while for the flouwen to find the crawler and bring it to them, but the sense of excitement continued to grow.
"Hello!" Little Red finally called from the surf. "Little Pink is bringing the crawler!" The flouwen had decided to let the youngling do the honors now that it had gained enough bulk to hold the crawler. Little White and Little Purple were escorting the youngling, and Little Red had gone on ahead to tell the humans they had found the package from the skies.
Nels, Richard, and Jinjur headed into the water and together they wrestled the heavy metal box onto the sand. "Crawler" was a misnomer, as the motor that ran the treads had been removed to allow more mass for precious cargo. The outer hatch was carefully removed, and each item was unpacked gently as they enjoyed the touch of civilization sent to them by their friends aloft. The time they had spent aboard Prometheus seemed as distant as a dream now. Except for the occasional conversation over the comm-links, the Edenites had little contact with their past. Even Josephine, the computer in the ship trapped below the lagoon, was visited only in extreme emergences ... just a few of the children had made the risky underwater trip.
Most of the cargo space was filled with vitamin pills. Data fed to the Prometheus from the flouwen concerning the nutritional content of the local foodstuffs had led to a weekly supplement about the size of bird shot, but with years between drops and the growing population, the vitamins were precious indeed.
Also included in this drop were "Teachers" for the children; small handheld computers filled with educational software. Each of the almost indestructible, light-powered Teachers had been specifically programmed for each child. Shannon's, for instance, had an animated dragon to explain each lesson, and the explanations themselves were be tailored to appeal to her joy of art and drawing. Eve, who had inherited her mother's musical sense, would find that her computer would use songs to teach her spelling, math, and the periodic table, while Freeman's curiosity would be rewarded with more and more complicated explanations as he requested them. The firstborn took the teachers reverently amid the oohs and ahs of their siblings, and each vowed to work diligently. The parents, more realistically, had agreed to rotate the chore of supervising the children's learning so that none would return to Earth with an incomplete education.
For the adults there were spare communicators and battery packs, as the older batteries slowly lost their ability to recharge fully. There were tools to replace those that had been lost, and new multipurpose tools that Caroline had fabricated to order in the automated machine shop on Prometheus. There was also an updated picture of the crew up above. They were getting older and grayer, and Sam was missing, having died ten months ago.
Each treasure was lovingly bundled up for safe transport back to the village, where it would be used and valued. Technology that they had once taken for granted now seemed as magical as gifts from the gods, and no tool would be lost or mistreated even by a child. Heartened that their everyday chores would be made a little lighter by these aids sent down by their family overhead, the party on the beach lasted far into the afternoon. Then, as Barnard slipped into the ocean, the weary but happy humans straggled back to the homes they had carved out for themselves on this alien world.
FOLLOW ON
THE THREE astronauts met in person for the first time on the bridge of the antimatter-powered rescue ship Succor. Their mission was to get help to the explorers stranded on Zuni as quickly as possible. Their high-speed space-craft would be followed within the next few years by a larger exploration and development fleet that would include contingents of scientists and engineers, along with their accompanying flocks of robots. While the three astronauts had all conversed at length over the solnet in the process of helping to design and program the space-craft, they had been too busy getting ready to spend the next thirty years away from home to get together physically before.
Laura Brooks was the las
t to arrvive. She had taken a well-deserved lunar-day-long vacation at her home in Mare Imbrium on Luna before selling off the house and taking the daily Lunavator capsule between Imbrium and the space station Goddard. The station robots brought her out in a Goddard flitter to the kilometers-long interstellar spacecraft, which was floating in space a safe distance away from the station. First making sure that she was securely tethered, the robots led her through a temporary shielded passway they had erected across the short gap between the small flitter and the airlock entrance to the human quarters portion of Succor. The airlock cycled and she pulled her off her helmet. Her long blond hair was pinned into a crown on her head, exposing the implant set into the top of her spine. She had been upgraded for this mission, and the soft down of her back hairline was still stubble from the preoperative shaving. Laura patted her hair and licked her lips nervously. It had been weeks since she had even talked to another human being face-to-face, and these men would be her sole living companions for more than six years.
She stepped from the lock, and with an expert push with her foot, she floated into the Succor's central deck. Two men floated before her in the room, looking as nervous as she felt. Laura smiled and nodded a greeting.
"Laura Brooks, logging on," she said.
The shorter and younger of the two men formally returned the nod. He gestured to the slender black man standing beside him. "May I present Orson Pratchette of Valkyrie, Mars?" Orson bowed at the introduction, somehow managing to do the full Japanese business-bow in free-fall without looking ridiculous. He was bald, and his slender frame and smooth dark skin called attention to the beauty of his well-shaped head. The shorter man continued. "And I am Beauregard Darlington Winthrop the Sixth," he said. Then his face broke into a broad friendly grin. "Just call me Win. Welcome aboard."
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