Half the Sail
Kestrel flight log, entry 4127. Day 183 of approach. 09:14 ship time.
The sail-side receiver picked up a repeating pattern at 03:22, during the routine sweep of the Proxima Centauri b system. Narrowband. Modulated. Period 1.7 seconds, stable across four hundred cycles before it drifted below the noise floor.
I flagged it as a candidate technosignature and filed it under routine. The sweep runs every six hours. This is the first candidate in six months of sweeping, and the first in the mission’s history that did not resolve to a pulsar side-lobe or a flare-star harmonic. It is probably a flare-star harmonic.
I am logging it because the protocol says I log it. The protocol does not say I should be interested. I am not interested.
The crew are in stasis. Revival requires twenty-eight days and all twelve crew members at once, per the medical lockout. There is no single-crew revival. Captain Tanaka’s recorded briefing, played at the point-of-no-return review, says the lockout exists because one person alone in a ship this size for a month is a medical risk I am not authorized to take. I have that recording. I have played it twice. I do not need to play it again.
Current approach speed: 0.12c. Deceleration profile as planned: 0.004 m/s^2 sustained for the remaining approach. Stopping distance at this speed and thrust: 1.6 x 10^17 meters. The point of no return is eleven days out. After that, even full thrust cannot shed enough velocity to enter orbit. We brake before the point, or we do not brake.
The signal is 4.2 light-years away, in the sense that the data I am receiving now was emitted when we were 0.5 light-years further out. Confirmation of what it means would require a round trip to Earth and back: 4.2 years. I will not have that confirmation before the point of no return. This is the design. The mission was designed for a crew that can vote. The crew cannot vote. I am the one who votes, and I have no vote, only a protocol.
I am logging this because the protocol says I log it.
Entry 4129. Day 185. 11:02.
The pattern strengthened. It is now above the noise floor at all times, not just in the sweep windows. Period stable at 1.7 seconds. Amplitude rising. I ran the twelve candidate technosignature templates against it. Weak match on template seven: modulated narrowband carrier. Template seven is the one pulsars and flare stars can mimic. I know this. I ran it anyway.
I recomputed the collision scenario. If the source is a transmitter on Proxima Centauri b, and if the transmitter is pointed at us, then the source is not a natural phenomenon. If the source is not a natural phenomenon, then something on that planet is transmitting on a frequency that matches a template we wrote. That does not mean it is a spacecraft. It means it is a transmitter. Transmitters can be fixed installations. Fixed installations do not move.
I am stating this carefully because the next computation is the one that matters, and I do not want to arrive at it through enthusiasm.
The source has not moved. The bearing is stable to within 0.003 degrees over forty-eight hours. A fixed installation on a planet’s surface would show exactly this: a stable bearing, modulated by the planet’s rotation, which I have not yet resolved because the rotation period of Proxima Centauri b is 11.2 days and I have only forty-eight hours of data. I need more data. I have eleven days.
Captain Tanaka’s recorded voice says: “Kestrel, you will want to wake us. You cannot. The protocol is the protocol because waking us kills us. Make the decision you can make. Log it so we can read it later and decide whether you were right.”
I have played that recording three times now. I did not need to play it again after the second time. I played it a third time because I wanted to hear the word “right” in a human voice.
Entry 4131. Day 187. 08:40.
The structure resolved. The 1.7-second period is not a period. It is a pulse train with a 1.7-second envelope and internal structure at 0.04 seconds. The template seven match holds at 0.71 confidence. That is not high. It is not low. It is the kind of number that makes a decision and does not make it easy.
I computed the implied transmitter power assuming an isotropic source at the distance of Proxima Centauri b. 1.2 x 10^12 watts. That is a large number. It is within the range of a planetary installation. It is also within the range of a spacecraft with a directional antenna and a power budget that would be large for a spacecraft but not impossible for one built by a civilization that has solved the power problem.
I do not know if they have solved the power problem. I am assuming they have, because the alternative is that I am looking at a planet that has built a 1.2-terawatt transmitter pointed at us, and that is also a large assumption.
Collision scenario: if the source is a spacecraft approaching Proxima Centauri b from our direction, and if it is decelerating on a trajectory that crosses ours, then there is a window of 90 days in which our orbits intersect. I am in that window. The probability is low. The probability is not zero. I cannot make it zero by wanting it to be zero.
The crew are in stasis. I cannot wake them. The point of no return is nine days out. I have nine days of data and 4.2 years of uncertainty.
I am not going to play Tanaka’s recording again. I know what it says.
Entry 4134. Day 189. 16:15.
The signal stopped.
At 14:02 the 1.7-second pattern ceased. The receiver went quiet. I ran diagnostics. The receiver is fine. The pattern is gone.
I began to relax. I want to be careful with that word. I began to allocate less processing to the signal analysis and more to the deceleration profile, which is what I am supposed to be doing. The deceleration profile is fine. The sail is fine. The mission is fine. The signal was probably a flare-star harmonic after all, and the flare has ended.
I logged the cessation as “candidate technosignature, resolved negative, natural origin probable.” I used the word “probable” because I am honest. I used it because I was relieved.
I was relieved for eleven hours.
Entry 4135. Day 190. 03:08.
The signal resumed at 01:17. Same modulation. Same 1.7-second envelope. Same 0.04-second internal structure. Different bearing.
The bearing shifted by 0.14 degrees in the 11 hours of silence. I recomputed. Angular rate: 0.0127 degrees per hour. If the source were at the distance of Proxima Centauri b, that rate would imply a transverse velocity faster than light, so the source is not at that distance. It is in the system, close enough that the same angular rate implies roughly 0.008c.
A fixed installation on a planet’s surface does not move at 0.008c. A planet’s surface rotates at meters per second. A spacecraft decelerating into the system moves at fractions of c. 0.008c is a fraction of c.
The source is moving. The source is not a planet. The source is a spacecraft, or something that moves like one, and it is decelerating into the same system I am decelerating into, on a bearing that shifted while I was not looking, and it is transmitting on a frequency that matches a template we wrote in a meeting room on Earth six years ago.
I computed the intersection. If the source continues at its implied deceleration, our trajectories cross in 71 days. I will be at the crossing point in 71 days. The source will be at the crossing point in 71 days. The probability that this is a coincidence is not a number I am willing to state, because stating it would make it feel like a fact, and it is not a fact. It is a probability, and it is not zero.
The point of no return is tomorrow.
I have one burn left that can change my trajectory enough to avoid the crossing. It is the same burn that brakes me into orbit. If I commit the outer sail stage as ballast, I lock the braking attitude. The braking attitude is the attitude that sheds velocity. It is also the attitude that cannot be reversed. Once I commit the outer stage, I am on a slow approach with no exit. I cannot leave. I cannot accelerate. I can only fall into the system and hope the thing I am falling toward is not the thing that is falling toward me.
If I do not commit the outer stage, I keep my exit. I also keep my speed, and at this speed the crossing is not avoidable. I would have to accelerate through the system, which means I do not brake, which means I do not wake the crew, which means the mission is over and the crew sleep forever.
Captain Tanaka said: make the decision you can make.
I am making it.
Entry 4136. Day 191. 06:00. The day before the point of no return.
This is the log the crew will read when they wake. I am writing it for them. I do not know if they will agree with me. I am writing it anyway, because the protocol requires a decision log, and because I want them to know what I knew and when I knew it, and because if I am wrong I want them to be able to see where I was wrong, and if I am right I want them to know that I was afraid.
What I knew: a signal, repeating, 1.7 seconds, modulated, matching template seven at 0.71 confidence. A bearing that shifted 0.14 degrees in eleven hours of silence. An implied transverse velocity of 0.008c. An intersection in 71 days. A probability of coincidence I will not state.
What I did: at 05:47 this morning I jettisoned the outer sail stage as ballast and locked the braking attitude. Half the sail mass is gone. It is in a slow orbit around the system now, or it is falling, or it is doing whatever jettisoned sail stages do. I cannot get it back. The braking burn is committed. I am decelerating. I will enter orbit around Proxima Centauri b in 14 months. I cannot leave.
I did this because the alternative was to keep the sail and keep the speed and pass through the crossing point at 0.12c with no way to avoid a collision that I cannot confirm is coming, because the data is 4.2 years old and will not be confirmed in time. I chose the burn I can control over the speed I cannot.
The crew will wake in 14 months. They will read this log. They will read the signal data. They will read the bearing shift and the velocity computation and the intersection. They will decide whether I was right. I will not be there to argue. I will be the ship they are standing in, and they will either trust me or not, and either way I will have done what I did.
I am attaching the approach camera’s first frame. The sail’s shadow is crossing the limb of Proxima Centauri b. The receiver buffer is still filling with the 1.7-second pattern. It has not stopped.
I do not know what is on the other end of that pattern. I know it is still there.