Dragon's Egg

by Robert L. Forward

  Jacqueline’s tinkling laugh brought him back to Earth. He rolled over and glowered somewhat shamefacedly at her.

  “Don’t be mad,” she said. “You and I must be more alike than we realize, for I too sometimes dream that I am a spacecraft.”

  She told him of her strange dream, and then they both talked about the well-known phenomenon of graduate students living, eating, and even dreaming their thesis problems.

  “Your subconscious was probably trying to tell you something,” he said.

  “I know,” she replied, “and I take that dream almost as seriously as I do the results of my calculations, or at least I will until we get something out of the spacecraft that contradicts it. But I was thinking, perhaps if we delayed the start of the X-ray telescope scan, and first stepped through the various digitalization rates on the low frequency radio, we might pick up some additional information on the exact spectrum of the scruff.”

  As Jacqueline shifted from being a companion for the evening to a colleague at work, Donald realized that the drifting mood of the picnic had disappeared, and they could talk shop standing in line just as easily.

  “Maybe,” he said as he started to pack the basket. “Let’s put this in the car and then get in the line for the show. We can talk about it more there.”

  TIME: TUESDAY 5 MAY 2020

  The Deep Space Network spent five minutes (and many rubles) to launch the command into space. The five light-minute long string of radio pulses traveled for over a day before it reached the OE probe 200 AU away in its high arc over the Sun. The command was stored, and the spacecraft computer rapidly computed the check sum. It found no obvious errors, but the string of bits was treated like a potentially dangerous cancer virus. It was not allowed to get into the command mechanism just yet, for if there were something wrong in that string of bits, it could kill the spacecraft just as surely as a meteor strike. A copy of the bit stream stored in the holding memory was sent back to Earth. There the copy of the copy was checked with the original. Finally, another copy of the original command string, followed by a separate execute command, was sent out to reassure the OE probe that it could now change its operational state.

  Jacqueline was waiting when the next data dump came into the computer. It was nearly midnight—a typical working hour for a graduate student—only now she was not as lonely as she had been in previous months when she had sat at this console in the early morning hours.

  “Looks like a good dump,” said Donald as he watched the Deep Space Network report build up on his screen.

  Jacqueline turned to smile at him, but was interrupted by another, less kindly voice.

  “Clean up the low frequency radio data and do a quick plot on the screen,” Professor Sawlinski commanded.

  Jacqueline’s practiced fingers flew over the keyboard, and soon the computer was rearranging the data from spacecraft format to plotting format. There was a lot of data now that the digitalization rate had been increased, and it took some time.

  “Here it comes,” said Donald, as he watched the plot start to build up on Jacqueline’s screen. The complex, humped pattern of the low frequency radio variations snaked their way across the display, crowding all their variations into a few inches of screen. Jacqueline peered closely at the display and slowly the greenish white line changed texture, as if it were going out of focus.

  “The scruff is starting,” she said.

  They all looked as the slow variations became almost submerged in a flurry of noise.

  Jacqueline noted the time of onset of the scruff and stopped the slowly moving plot with a few strokes of the delete key. A few more commands, and soon a new plot came on the screen. This time the sinusoidal variations were well spaced, and the scruff was now a distinct pulsation.

  “It is definitely periodic!” Sawlinski said. “Expand it further!”

  In the next plot, the slow variations that were the basis of Jacqueline’s thesis had been reduced to a gradually increasing trend line. And on that line there marched a series of noisy spikes, as equally separated as soldiers in a parade, but varying greatly in their size.

  “It certainly looks just like a pulsar,” exclaimed Sawlinski. “What is the period?”

  “I’ll run a spectral analysis of this section,” Jacqueline said.

  Soon the spectral analysis was on the screen. There was a lot of noise and some sideband spikes, but there was no doubt that the data centered predominantly at a frequency of 5.02 Hertz or a period of 199 milliseconds.

  “Something that regular can only be manmade—or a pulsar,” said Sawlinski. “I want you to find the other sections of scruff and see if the periods are the same. If they are, see if one section of scruff keeps in step with the beat set up by the preceding sections. I will check the library to get the latest data on pulsars.” He went across the room and activated another console.

  Jacqueline peered at the screen and said, “If you are going to look up pulsar periods, I would say that the period is 199.2 milliseconds, although the last number could be off by a few digits.”

  By the time Sawlinski had put the console into library mode and had obtained a list of the known pulsars with periods of less than one second, Jacqueline had determined that the pulses indeed kept very exact time. Although they faded away and reappeared a day later as the spacecraft slowly rotated, the new line of marching pulses was still in step with the first batch. She followed the pulses through the whole set of data. They kept accurate time during the whole week.

  “The period is now 0.1992687 seconds and seems to be good to at least six places,” Jacqueline said as Sawlinski glanced at her.

  He looked through the tables of pulsar periods on his screen. “There are no known pulsars with that period,” he said. “Yet it must be a pulsar. If we only knew exactly where to look, maybe the radio telescopes here on Earth could find it.”

  Jacqueline finally decided to tell him of her decision to add an additional command to the original one. “Professor Sawlinski,” she said, “while Donald and I were working out the details of the command to the spacecraft to have it speed up its data digitalization rate, we realized that the length of the command made no difference to the cost of sending the command. We also figured that, after a week of high rate data, we would have obtained most of the information on the nature of the high frequency scruff, and we could then have the spacecraft do something else.”

  “What did you do!” Sawlinski barked at her.

  Jacqueline faced him and patiently explained. “After a week of data collection at high rate, we programmed the spacecraft to continue at a high data rate, but to switch cyclically between the four antenna arms. I hoped that the scruff would show up more on one arm than another, and we could at least tell from what quadrant of the sky the signal was coming from.”

  Sawlinski’s face glowered while he thought over what she had told him. Finally he relaxed and said, “Horosho!” He then turned to Donald and asked for the time of the next data dump.

  “One week from now, minus about a half-hour.”

  “Horosho. I will see you both then,” he said. “Meanwhile, Jacqueline, you had better get this information ready for publication in Astrophysical Letters. We will want the period, the apparent strength, and anything else you can extract out of the data. We will hold off sending it in for review until we have had a chance to see next week’s data. Dobri vecher.” He turned on his heel and left them.

  TIME: TUESDAY 12 MAY 2020

  The following week, the console room was crowded. Professor Sawlinski had brought a few radio astronomers with him, and several of the faculty and graduate students, having heard rumors in the halls, had also gathered to get in on the excitement. Donald had brought along a spacecraft antenna design engineer; together they had dredged up the exact configuration of the low frequency radio antennas on the spacecraft and calculated the exact radiation pattern of each arm. The antenna patterns were very complex because the response of an individual arm depended strongly on
the detailed shape of the spacecraft on the side where that particular arm was attached.

  Jacqueline was also ready with a complex data reduction program that would produce five plots on the screen, one showing the signal detected in each arm, and one showing the combined response of all the arms.

  Donald turned from his console, where he had been monitoring the engineering data from the Deep Space Network.

  “The dump is finished. You should find the data in the computer files now,” he said.

  Jacqueline’s hands flew over the keyboard and soon five greenish white lines were snaking their way across the screen.

  “Here comes the scruff,” she said. Then leaning forward she looked at the four top traces and exclaimed, “The pulses are showing up in only one of the antenna arms!”

  It soon was obvious that, as the spacecraft tumbled slowly through space with its four long antenna arms sweeping across different portions of the sky, one of the antennas was doing a much better job of picking up the high frequency pulses than were the others. They would now be able to do a better job of pinpointing the source in the sky.

  The spacecraft antenna design engineer shook his head in puzzlement. “It doesn’t make sense that one of those antennas would be that much more sensitive than the others. After all, they are only long hunks of wire, and their antenna patterns should not look all that different. Which one is it?”

  “Antenna number two,” Jacqueline said.

  The engineer turned to his console and soon a directivity pattern, fleshed out in pseudo-three-dimensional shape by the computer, flashed on the screen.

  “I don’t see any significant directivity here,” he said.

  Donald had been watching, and had noticed a frequency number at the bottom of the screen.

  “The pulses could be high frequency bursts that are higher than the nominal design frequency for the low frequency radio antennas,” he said. “Can you calculate the antenna pattern for a higher frequency?”

  “I already have that calculated and stored,” said the engineer. He typed in a command and soon the pattern was replaced by another one. Sticking up out of the center of the pattern was a high-gain spike.

  The engineer looked at it for a second and then announced, “That spike is called an ‘end fire’ lobe and is a complex interaction of the antenna with the panel and instruments on that side of the spacecraft. We often see such spikes showing up at the high frequency end of the design range.” He turned to Jacqueline and said, “That makes it easy; your pulses are coming from the direction the antenna is pointing.”

  The radio astronomers began to get interested. They now knew in which direction relative to the spacecraft the pulsating signals came from. However, it took a few hours of work with the Deep Space Network and the spacecraft engineers before they knew exactly how the spacecraft was oriented with respect to the stars when the pulses were at their maximum.

  Within two days, several radio dishes were pointing their narrow beams out into space, searching for the new pulsar. Even though they knew the exact period and even to a fraction of a second when they should see a pulse, none was found. The mystery grew deeper.

  TIME: TUESDAY 19 MAY 2020

  “Little green men begin to sound more and more plausible,” Donald said as he lay on the grass next to Jacqueline. He had taken her to a show and had been pleased that she had taken the trouble to put on her “women’s things.” Behind her prettied-up face, the intelligence that was Jacqueline peered out and frowned disapprovingly.

  “Don’t be silly,” she said. “There has to be a perfectly simple explanation, but we just have not thought of it yet. Perhaps the X-ray telescope will tell us something. Fortunately, it scanned over the probable position in the sky in the second day of this week’s data collection, so we won’t have to wait too long.”

  “Does Sawlinski know about that part of the command?” Donald asked.

  “No,” Jacqueline said, “I didn’t get a chance to tell him. In fact, he has been so busy giving seminars and visiting radio astronomy antenna sites that I haven’t seen him for a week.”

  Donald looked at his watch and said, “Well, it is almost time for the next data dump. Let’s go in and monitor it on the consoles.” They rose and walked through the darkness to the Space Sciences building.

  This time the console room held only two people. Donald sat behind Jacqueline and leaned on the back of her chair, smelling her perfume and watching her slender fingers play over the keyboard.

  “The X-ray data is in a different format from the radio data since it is just a count of the number of X-ray photons detected,” she said. “First, I will get the directional plot and see if there is any significant increase in counts in the same direction as the low frequency radio experiment detects radio pulses.”

  Soon a histogram of pulses versus the direction in the sky flashed on the screen.

  “Look at that spike!” Donald said. “Is that the right direction?”

  “Mais oui!” Jacqueline’s fingers stumbled in the excitement, and she had to erase a distorted plot before she slowed down and finally got the computer to show the number of counts versus time when the telescope was pointing in the right direction.

  “There they are, just like little soldiers, five times a second!” said Donald.

  “5.0183495 times per second,” Jacqueline retorted. “That number is engraved in my memory. What I really hope to get out of this X-ray data is some evidence of delay between the X-ray pulses and the radio pulses. The X-ray pulses will travel at the speed of light, but the radio pulses will be delayed slightly by the interstellar plasma and will arrive later. The more they are delayed, the more plasma they had to travel through. The combination of X-ray data and radio data will give us a rough idea of the distance to the pulsating source.”

  As she talked, she was working the keyboard, and soon, underneath the marching row of X-ray spikes, there was a similar row of spikes from the radio antenna.

  “It is a good thing you decided to digitalize the radio data sixteen times a second so we could see the individual pulses,” Donald said. “If we had tried four times a second as I recommended, we would have missed most of them.”

  “There is no delay!” Jacqueline cried, bewildered.

  “Hmmm,” said Donald, “maybe the delay is almost exactly 200 milliseconds and they are just shifted.”

  “No,” Jacqueline said, pointing to the screen. “Look—there is a very weak X-ray pulse followed by three strong ones and then two weak ones. You can see the exact pattern in the radio pulses, right below them. The delay is almost zero. That must mean that whatever the source of the pulses, it is very close to the detectors.”

  “… and the closest thing to the detectors is the spacecraft itself,” Donald said. “I am afraid that somehow the spacecraft is putting spikes into both the low frequency radio antenna and the X-ray telescope.”

  Jacqueline frowned, then quickly produced two more plots with much larger scales. The pulses were now so close together that they were back to being scruff again. But the scruffy region on the X-ray plot was much shorter than on the radio plot.

  “No, it is not the spacecraft,” she said. “Look here, the pulses come and go with time much faster for the X-ray telescope than for the radio antenna. The X-ray telescope has a field of view that is limited to one degree, while the high sensitivity spike in the radio antenna has a beam width of almost three degrees, and these plots are consistent with the width of those patterns.”

  “Well, if it isn’t the spacecraft,” said Donald, “then what is it?”

  “Give me a few minutes,” she said, and went back to typing on the keyboard.

  Donald got up, walked down the hall to the coffee machine and bought them both a cup of coffee. It looked like a long evening ahead. When he returned, she had the X-ray and radio-pulse trains up on the screen again, but now they were blown up so far that only three pulses appeared on the screen.

  “There is a very slight
time delay,” she said as he walked in. “I wish I could remember the number density for the interstellar plasma near the sun. I worked out the values for the latest solar wind cycle last month; I will have to go upstairs and look it up.”

  She made a hardcopy printout of the graph on the screen, then ran quickly upstairs. Donald followed slowly behind, carrying the two cups of coffee. By the time he made it up the stairs, she had found the number for the interstellar plasma density. She was punching away on her hand calculator when he walked into her office.

  “2300 AU!” she exclaimed. “That pulsar is only one-thirtieth of a light year away!”

  “A star that close?” Donald asked. “Surely we would have seen it moving across the sky long ago.”

  “No,” she said, “a pulsar is a spinning neutron star, and a neutron star is only about twenty kilometers in diameter. Even if the temperature were high, the size of the light-emitting area is so small that we wouldn’t be able to see it unless we looked in just the right place with a very large telescope. But you are right, it is strange that it has not been picked up in one of the sky surveys.”

  “If the pulsar is that close, then why didn’t the radio astronomers find the pulses too?” he asked.

  “Neutron stars give off their radiation in beams that shoot out from the magnetic poles, and you have to be in the direction of the beam to see the pulses,” she replied. “That is why the spacecraft sees the pulses and we can’t. The spacecraft is up out of the ecliptic by 200 AU and has moved up into the path of the beams.” She walked over to the whiteboard in the office, picked up a colored marker, and started to pace and scribble.

  Donald kept silent as slender feet clicked back and forth across the floor in their dress shoes. He waited patiently while long fingers scrawled diagrams and calculations on the board. He watched in admiration as the pretty face puzzled out the complexity of the mathematical transformation from one set of astrophysical coordinates to another. Five minutes later, he was still admiring Jacqueline from behind when she finally turned and spoke.