Occasionally, she turned about to look at Sol. Even ten centuries out she could still distinguish Jupiter and Saturn. These were auspicious observations.
Finally it was time for the mid-course correction. She had spent the preceding century refining her alignment and her navigational observations. The burn was to be only one hundred meters per second, so accurate had been her perihelion impulse. Nevertheless, without that correction she would miss the Centauran system entirely. When the second arrived and her alignment was perfect, Ilse lit her tiny rocket—and discovered that she could obtain at most only three quarters of the rated thrust. She had to make two burns before she was satisfied with the new course.
For the next fifty years, Ilse studied the problem. She tested the rocket’s electrical system hundreds of times, and even fired the rocket in microsecond bursts. She never discovered how the centuries had robbed her, but extrapolating from her observations, Ilse realized that by the time she entered the Centauran system, she would have only a thousand meters per second left in her rocket—less than half its designed capability. Even so it was possible that, without further complications, she would be able to survey the planets of both stars in the system.
But before she finished her study of the propulsion problem, Ilse discovered another breakdown—the most serious she was to face:
She had forgotten her mission. Over the centuries the pattern of magnetic fields on her programmed memory had slowly disappeared—the least used programs going first. When Ilse recalled those programs to discover how her reduced maneuverability affected the mission, she discovered that she no longer had any record of her ultimate purpose. The memories ended with badly faded programs for biochemical reconnaissance
and planetary entry, and Ilse guessed that there was something crucial left to do after a successful landing on a suitable planet.
Ilse was a patient sort—especially in her cruise configuration—and she didn’t worry about her ultimate purpose, so far away in the future. But she did do her best to preserve what programs were left. She played each program into her own memory and then back to the programmed memory. If the process were repeated every seventy years, she found that she could keep the programmed memories from fading. On the other hand, she had no way of knowing how many errors this endless repetition was introducing. For this reason she had each of her subminds perform the process separately, and she frequently checked the ballistic and astronomical programs by doing problems with them.
Ilse went further: she studied her own body for clues as to its purpose. Much of her body was filled with a substance she must keep within a few degrees of absolute zero. Several leads disappeared into this mass. Except for her thermometers, however, she had no feeling in this part of her body. Now she raised the temperature in this section a few thousandths of a degree, a change well within design specifications, but large enough for her to sense. Comparing her observations and the section’s mass with her chemical analysis programs, Ilse concluded that the mysterious area was a relatively homogeneous body of frozen water, doped with various impurities. It was interesting information, but no matter how she compared it with her memories she could not see any significance to it.
Ilse floated on—and on. The period of time between the midcourse maneuver and the next important event on her schedule was longer than Man’s experience with agriculture had been on Earth.
As the centuries passed, the two closely set stars that were her destination became brighter until, a thousand years from Alpha Centauri, she decided to begin her search for planets in the system. Ilse turned her telescope on the brighter of the two stars … call it Able. She was still thirty-five thousand times as far from Able—and the smaller star … call it Baker—as Earth is from Sol. Even to her sharp eye, Able didn’t show as a disk but rather as a diffraction pattern: a round blob of light—many times larger than the star’s true disk—surrounded by a ring of light. The faint gleam of any planets would be lost in that diffraction pattern. For five years Ilse watched the pattern, analyzed it with one of her most subtle programs. Occasionally she slid occulting plates into the telescope and studied the resulting, distorted, pattern. After five years she had found suggestive anomalies in the diffraction pattern, but no definite signs of planets.
No matter. Patient Ilse turned her telescope a tiny fraction of a degree,
and during the next five years she watched Baker. Then she switched back to Able. Fifteen times Ilse repeated this cycle. While she watched, Baker completed two revolutions about Able, and the stars’ maximum mutual separation increased to nearly a tenth of a degree. Finally Ilse was certain: she had discovered a planet orbiting Baker, and perhaps another orbiting Able. Most likely they were both gas giants. No matter: she knew that any small, inner planets would still be lost in the glare of Able and Baker.
There remained less than nine hundred years before she coasted through the Centauran system.
Ilse persisted in her observations. Eventually she could see the gas giants as tiny spots of light—not merely as statistical correlations in her carefully collected diffraction data. Four hundred years out, she decided that the remaining anomalies in Able’s diffraction pattern must be another planet, this one at about the same distance from Able as Earth is from Sol. Fifteen years later she made a similar discovery for Baker.
If she were to investigate both of these planets she would have to plan very carefully. According to her design specifications, she had scarcely the maneuvering capability left to investigate one system. But Ilse’s navigation system had survived the centuries better than expected, and she estimated that a survey of both planets might still be possible.
Three hundred and fifty years out, Ilse made a relatively large course correction, better than two hundred meters per second. This change was essentially a matter of pacing: It would delay her arrival by four months. Thus she would pass near the planet she wished to investigate and, if no landing were attempted, her path would be precisely bent by Able’s gravitational field and she would be cast into Baker’s planetary system.
Now Ilse had less than eight hundred meters per second left in her rocket—less than one percent of her velocity relative to Able and Baker. If she could be at the right place at the right time, that would be enough, but otherwise …
ILSE PLOTTED THE ORBITS OF THE BODIES SHE HAD DETECTED MORE AND MORE accurately. Eventually she discovered several more planets: a total of three for Able, and four for Baker. But only her two prime candidates—call them Able II and Baker II—were at the proper distance from their suns.
Eighteen months out, Ilse sighted moons around Able II. This was good news. Now she could accurately determine the planet’s mass, and so refine her course even more. Ilse was now less than fifty astronomical units from Able, and eighty from Baker. She had no trouble making spectroscopic observations of the planets. Her prime candidates had plenty of oxygen in their atmospheres—though the farther one,
Baker II, seemed deficient in water vapor. On the other hand, Able II had complex carbon compounds in its atmosphere, and its net color was blue green. According to Ilse’s damaged memory, these last were desirable features.
The centuries had shrunk to decades, then to years, and finally to days. Ilse was within the orbit of Able’s gas giant. Ten million kilometers ahead her target swept along a nearly circular path about its sun, Able. Twenty-seven astronomical units beyond Able gleamed Baker.
But Ilse kept her attention on that target, Able II. Now she could make out its gross continental outlines. She selected a landing site, and performed a two hundred meter per second burn. If she chose to land, she would come down in a greenish, beclouded area.
Twelve hours to contact. Ilse checked each of her subminds one last time. She deleted all malfunctioning circuits, and reassembled herself as a single mind out of what remained. Over the centuries, one third of all her electrical components had failed, so that besides her lost memories, she was not nearly as bright as she had been when launched. Nevertheless, with her subminds combined she was much cleverer than she had been during the cruise. She needed this greater alertness, because in the hours and minutes preceding her encounter with Able II, she would do more analysis and make more decisions than ever before.
One hour to contact. Ilse was within the orbit of her target’s outer moon. Ahead loomed the tentative destination, a blue and white crescent two degrees across. Her landing area was around the planet’s horizon. No matter. The important task for these last moments was a biochemical survey—at least that’s what her surviving programs told her. She scanned the crescent, looking for traces of green through the clouds. She found a large island in a Pacific-sized ocean, and began the exquisitely complex analysis necessary to determine the orientation of amino acids. Every fifth second, she took one second to re-estimate the atmospheric densities. The problems seemed even more complicated than her training exercises back in Earth orbit.
Five minutes to contact. She was less than forty thousand kilometers out, and the planet’s hazy limb filled her sky. In the next ten seconds she must decide whether or not to land on Able II. Her ten-thousand-year mission was at stake here. For once Ilse landed, she knew that she would never fly again. Without the immense booster that had pushed her out along this journey, she was nothing but a brain and an entry shield and a chunk of frozen water. If she decided to bypass Able II, she must now use a large portion of her remaining propellants to accelerate at right angles to her trajectory. This would cause her to miss the upper edge of the planet’s atmosphere, and she would go hurtling out of Able’s planetary system. Thirteen months later she would arrive in the vicinity
of Baker, perhaps with enough left in her rocket to guide herself into Baker II’s atmosphere. But, if that planet should be inhospitable, there would be no turning back: she would have to land there, or else coast on into interstellar darkness.
Ilse weighed the matter for three seconds and concluded that Able II satisfied every criterion she could recall, while Baker II seemed a bit too yellow, a bit too dry.
Ilse turned ninety degrees and jettisoned the small rocket that had given her so much trouble. At the same time she ejected the telescope which had served her so well. She floated indivisible, a white biconvex disk, twelve meters in diameter, fifteen tons in mass.
She turned ninety degrees more to look directly back along her trajectory. There was not much to see now that she had lost her scope, but she recognized the point of light that was Earth’s sun and wondered again what had been on all those programs that she had forgotten.
Five seconds. Ilse closed her eye and waited.
Contact began as a barely perceptible acceleration. In less than two seconds that acceleration built to two hundred and fifty gravities. This was beyond Ilse’s experience, but she was built to take it: her body contained no moving parts and—except for her fusion reactor—no empty spaces. The really difficult thing was to keep her body from turning edgewise and burning up. Though she didn’t know it, Ilse was repeating—on a grand scale—the landing technique that men had used so long ago. But Ilse had to dissipate more than eight hundred times the kinetic energy of any returning Apollo capsule. Her maneuver was correspondingly more dangerous, but since her designers could not equip her with a rocket powerful enough to decelerate her, it was the only option.
Now Ilse used her wits and every dyne in her tiny electric thrusters to arc herself about Able II at the proper attitude and altitude. The acceleration rose steadily toward five hundred gravities, or almost five kilometers per second in velocity lost every second. Beyond that Ilse knew that she would lose consciousness. Just centimeters away from her body the air glowed at fifty thousand degrees. The fireball that surrounded her lit the ocean seventy kilometers below as with daylight.
Four hundred and fifty gravities. She felt a cryostat shatter, and one branch of her brain short through. Still Ilse worked patiently and blindly to keep her body properly oriented. If she had calculated correctly, there were less than five seconds to go now.
She came within sixty kilometers of the surface, then rose steadily back into space. But now her velocity was only seven kilometers per second. The acceleration fell to a mere fifteen gravities, then to zero.
She coasted back through a long ellipse to plunge, almost gently, into the depths of Able II’s atmosphere.
At twenty thousand meters altitude, Ilse opened her eye and scanned the world below. Her lens had been cracked, and several of her gestalt programs damaged, but she saw green and knew her navigation hadn’t been too bad.
It would have been a triumphant moment if only she could have remembered what she was supposed to do
after
she landed.
At ten thousand meters, Ilse popped her paraglider from the hull behind her eye. The tough plastic blossomed out above her, and her fall became a shallow glide. Ilse saw that she was flying over a prairie spotted here and there by forest. It was near sunset and the long shadows cast by trees and hills made it easy for her to gauge the topography.
Two thousand meters. With a glide ratio of one to four, she couldn’t expect to fly more than another eight kilometers. Ilse looked ahead, saw a tiny forest, and a stream glinting through the trees. Then she saw a glade just inside the forest, and some vagrant memory told her this was an appropriate spot. She pulled in the paraglider’s forward lines and slid more steeply downwards. As she passed three or four meters over the trees surrounding the glade, Ilse pulled in the rear lines, stalled her glider, and fell into the deep, moist grass. Her dun and green paraglider collapsed over her charred body so that she might be mistaken for a large black boulder covered with vegetation.