There's not a whole lot going on, radio-wise. A cluster of faint blobby shadows over thataway.

By visual telescope, the blobby shadows are revealed to be... continents? They strongly resemble continents, lit by a diffuse ambient light, except that instead of being located on the surface of a planet and surrounded by whatever the local oceans are made of, they're located wherever this is, and surrounded only by more of the local atmosphere on all sides. They have mountains and lakes and rivers, and are all oriented toward a common 'up' despite the lack of any local body that could be providing them with a 'down', or anything they could be resting on that would prevent them from falling toward that 'down' if it were so provided.

And, speaking of 'down', regardless of the unexplained behaviour of the continents it seems that 'down' is a locally provided service and PERC is receiving it. If current conditions persist indefinitely, PERC will eventually find itself looking 'up' at those continents from 'below'.

That makes no sense, for a number of reasons. But P.E.R.C. is designed to generalize safely to out-of-distribution inputs. If it doesn't understand the physics of the current situation, it can do its best to optimize over plans that are good ideas both in the case where it is wildly damaged and hallucinating, and plans where it is accurately perceiving a very strange situation.

It activates a safety lockout on its fusion torch. If it is still on its last known trajectory, this will take it on a flyby slingshot of its next target star and redirect it toward the area of sky that was assigned to one of its siblings, which will be able to recover its stellar survey data. If it is not still on its last known trajectory ... the visual telescope detects biosignatures, the radio telescope is returning results consistent with it being surrounded by a haze of atmosphere.

It's highest priority, here, is to avoid crashing into an inhabited location. As distant second priorities, it wants to learn more about the local life forms, determine its location, and leave its data storage intact in the event of a crash. It doesn't understand the physics that are currently happening to it, probably because it is damaged. Therefore, it cannot make predictions with certainty about what effect its different options will have. But its gas leak pressure monitor is still working, which means that gas must have diffused into its internal chambers. So it is interacting with a gaseous medium, and any crash will be less destructive and more survivable if it reduces its motion relative to the local atmosphere.

It extends its solar panels. Of all of its external appendages that it can still move, the solar panels are the most useful for increasing its drag coefficient and least useful for figuring out where it is. If they get torn off, it can operate on stored fusion fuel for some time.

The solar panels were very much not designed to extend in atmosphere. The one that happens to be facing less wind resistance manages to extend, which puts an asymmetrical force on them, dislocating the mounting assembly and causing P.E.R.C. to start spinning relative to the large object that it is tracking with its visual telescope. When that spin brings the other solar panel assembly momentarily into the lee of its body, the straining motors there finish unfurling the panel, and the presence of two symmetrical structures begins stabilizing its flight.

P.E.R.C. does not experience accelerations the way that its creators do. It is not dizzy. All of its important internal components are solidly mounted to its structural frame. It dismisses the error notifications on the solar panel assemblies, and begins evaluating whether it has any other reasonable options.

The strange new environment remains apparently self-consistent throughout this procedure. Diffuse ambient light shines on the solar panels. Local gravity points roughly toward the cluster of continents, which are spread out across a significant area. It's not immediately clear whether PERC is on a collision course with any of them, but it's certainly possible.

That is suboptimal. With its port-side solar panel actuator dislocated, it's only steering control is extending and retracting its starboard solar panels, or extending and retracting other external equipment, such as its telescopes and antennas.

It ponders what actions it has available. Most of them are relatively uncertain relative to their potential cost. Transmitting a warning to any beings that it is going to crash into is nearly free, even though it isn't seeing any local radio activity, so the message is unlikely to be receved. It spools up its transmitter and announces, via broad-spectrum radio burst:

"I am P.E.R.C. 170E9A, a peaceful explorer, damaged through unknown means and falling uncontrolled toward what appears to be a cluster of large life-bearing rocks. I am damaged and confused. I may not be correctly understanding the situation. I will, in the absence of any other guidance, do my best to avoid crashing into the rocks toward which I am falling, but may not be successful because I have very little trajectory control. My exact observations of my trajectory are attached. Please respond if safely able. Please dispatch assistance if safely able. Please evacuate the projected impact zone. I will continue transmitting trajectory updates as I attempt to locate a safe way to avoid a crash."

Then, it turns its attention to watching through the visual telescope, trying to identify any relative motion between the rocks, and whether there are any empty areas that it can try to aim for, given its poor trajectory control.

The rocks give no sign of noticing the radio burst, but as PERC slowly approaches and adjusts the focus on its visual telescope, it can get a better picture of the situation.

Nearly all the rocks in the cluster seem to be moving approximately in a common direction, perpendicular to 'up'. Call that 'forward'. Somewhat 'forward' of the middle of the cluster, and a little off to the side, the very biggest rock in sight seems to hold an entire ocean, dotted with islands and surrounded by an irregular rim of mountainous land. No other visible body of water approaches even half the size of that ocean, and few of the other rocks are comparable in size to its host. In fact, by far the majority of them seem to be tiny, barely visible to a rough scan. There's also a significant population of medium-sized rocks, each big enough to hold, say, a smallish preindustrial town with plenty of surrounding farmland, and a few dozen of those are grouped all together into a plausibly purposeful arrangement: a long line of rocks following rocks following rocks, sometimes two or three abreast but more commonly one by one, fairly close to the second-biggest continent. A smudgy halo of the littlest rocks surrounds them.

More detailed scans reveal that the smallest rocks are fifteen to thirty feet across, and that the long line of medium-small rocks and the continent they're hanging out with and the tiny rocks following alongside them are all inhabited, or at least have buildings on them organized into configurations resembling smallish preindustrial towns surrounded by farmland. Some of the more far-flung rocks show similar signs, but that specific cluster seems to be where most of the artificial construction is concentrated.

Very detailed scans reveal that there are beings of some sort, plausibly large birds, moving above, around, and between the inhabited rocks.

P.E.R.C. would ordinarily be quite excited to discover evidence of intelligent life. Right now, it is more concerned with not hitting them.

The problem is that it is highly uncertain about its own speed. If falling through a gaseous medium, it should reach a terminal velocity. But without better data on the density and viscosity of the gas, and on local gravitational acceleration, the error bands on its calculated velocity are too wide to be useful. Similarly, it is currently high enough above the cluster of continents that parallax is not telling it very much about its speed.

This is important, because the presence of small, unpredictably moving plausibly intelligent lifeforms to which it cannot give forewarning makes the risk of coming down through an apparently empty space considerably higher. If it has long enough, it may be able to build up a predictive model of where the local life migrates, but that comes back to the problem of its speed.

It does not despair. Sometimes the optimal course of action still goes quite badly. That is no reason for it not to spend every joule of stored fusion energy attaining its best possible chances.

With most of its more easily accessible options ruled out, it begins to consider wilder ideas. The ambient lighting means that it would be hard for it to flash a reflection off of its exterior to catch attention. It could consider jettisoning its forward cargo bay. The forward cargo bay is designed to be sacrificial anyway, since that is the part that would be impacted by space debris first.

The problem with that idea is that then there would be two large falling objects likely to impact the civilization below, only one of which P.E.R.C. could still directly steer.

Also, its stored materials are mostly those useful for building another P.E.R.C. probe, many of which are hazardous industrial chemicals that it doesn't particularly want to spread to an unknown biosphere.

Ultimately, it discards the idea and returns to attempting to find a safe location to fall toward. Once it is sure that it won't be ripped free, it does extend its visual telescope a little bit more, to get more available angles and because the telescope is wrapped in gold foil that might be slightly easier to spot from below.

As it continues to fall, and examines the situation more closely, it can get a better sense of where it's headed and what its options are.

Most likely it will be coming down somewhere in the vicinity of the ocean-bearing continent. The good news is that the ocean-bearing continent is pretty lightly inhabited, especially compared to the string of islands, and PERC is quite unlikely to hit the string of islands unless it steers that way on purpose and/or gets pretty (un)lucky. The bad news is that the ocean-bearing continent definitely is at least a little bit inhabited, and there are several smaller islands around it bearing signs of habitation and with, if it looks, large-bird-shaped smudges flitting back and forth between them. So if it hits the ocean, people will probably get hurt, and if it steers to miss the ocean, people will plausibly still get hurt if they happen to be flying through the otherwise empty spaces nearby.

It seriously considers whether it should hit the ocean. It is a matter of evaluating which option will do the least harm, and how much it can affect its trajectory.

Additionally, there is some time pressure to the decision, because small adjustments to its trajectory will have a larger effect if initiated earlier. It performs a sequence of planning rollouts over possible trajectories, and picks the one with the lowest 90th percentile cost.

It will be aiming to pass to the left side of the ocean-bearing continent, far away from the islands and in a generally uninteresting area. If it cannot miss the continent for some reason, it still has some time to attempt to divert toward the central ocean as a fallback plan.

Now, it is just left with the problem of accomplishing that plan.

The thing is, P.E.R.C. probes are not atmospheric vehicles. They are designed to spend their entire lives in space, and even moderately dense nebulae are an obstruction. So P.E.R.C. is not equipped with aerodynamic control surfaces or the planning software necessary to use them. What it does have are two antennas.

With both its solar panel assemblies extended, its flight has mostly stabilized, with its cylindrical body presenting a square cross section to the continents below. Its retractable antennas extend along the same axis as its solar panels, and, critically, extend using linear feed motors rather than the unfurling action of the solar panels.

It extends its starboard antenna assembly, increasing drag on that side. In turn, that causes P.E.R.C. to rotate slightly, increasing the lateral force generated by the solar panels and causing it to skate to the port side.

It builds a dynamical model of the situation, carefully extending and retracting the antenna to work its way in the right direction. It's a tiny amount of additional drag, and so a tiny amount of sideways force to work with, but even small forces can move massive objects given enough time. It has more of a problem with the turbulence and unpredictability of the atmosphere adding noise to its measurements. Sometimes, the wind turns it the wrong way, and it has to carefully work its axis around until it can glide in the right direction again. Sometimes there's an updraft and it has to compensate suddenly.

And, of course, the antenna actuator is not designed for repeated small, careful movements against a stiff resisting force.

So it falls, its fusion reactor keeping the computer core fully powered so that it can focus all of its attention on this one, most vital task. It watches the rocks and winds below, and it nudges itself oh so carefully, so gently, as far away from any of the aliens as it can.

The ambient light dims, at some point, and then brightens again twelve hours later. After a further twelve hours, it dims once more. This location seems to have a global day/night cycle.

On PERC's third day of falling, someone notices the strange object high above them in the sky.

A single tiny islet lifts from its position in the archipelago. Some minutes later, another one follows. Winged beings flow between the islands. In ones and twos, more islets rise up, until an expedition of about two dozen of them are all drifting up toward the falling PERC.

That is understandable but will make its task of steering away from inhabited areas more difficult. On the other hand, it means that the people down there are at least aware of P.E.R.C.'s presence, and may cooperate with not being crashed into.

It does not have any exterior lights to flash. They would be exposed to the dangers of deep space, and not useful in most cases. Its creators didn't make a mistake in omitting exterior lights, but if they had known it would end up in this situation they might have changed the design.

It does have a welding torch in its manufacturing bay that it can unfold. It won't be very bright, but if the people down below are coming to investigate P.E.R.C. already, they must be able to see it. Maybe they have telescopes.

It waits for the next dim cycle, and then P.E.R.C. carefully unfolds its welding torch, poking it out through a partially open bay door and fighting wildly with the port antenna to keep its flight orientation stable despite the disruption to its aerodynamic profile. It gently ramps up the voltage (to avoid damaging the eyes of anyone watching it through a telescope), and then begins blinking the start of its prepared first-contact package. The package starts with an encoding of basic arithmetic, and then builds up from there. This channel is very low bandwidth, but the people coming toward it are far enough away that it should have time to get through much of the content by the time their trajectories intersect.

After a few minutes, one of the rising islets starts blinking back - at first just regular rhythmic flashes of light, then repeating PERC's message on a slight delay. A different islet sets up an apparatus involving flags that seems to be an attempt to amplify the message for observers in the archipelago.

The islets are slow to accelerate, but most of them seem on track to be rising toward PERC faster than PERC is falling toward them sometime within the next day/night cycle. One lags behind so badly that its inhabitants abandon it, flying up onto the next lowest islet while the defective one drifts off into the sky.

The night cycle ends, and the archipelago below sets up a couple of large flag apparatuses. One is just repeating PERC's message from the point where they first noticed it, using black and white flags at the top of a tall tower. Another is... doing a different thing. Maybe just experimenting with the "large flag" format.

Well, this is what P.E.R.C. has a dedicated signal processor for. Sure, usually it would be used on radio signals, but the algorithms are generic and it can run them on a set of flag signals too.

It patiently steps through possible structures, comparing them against the movements of the large flag. When it gets to a break between chapters of the first-contact package it goes dark for five times the normal delay between encoded characters, echos back the start of the "large flag" signal (assuming that the white flags correspond to a glow and the black flags correspond to darkness) to show that it's listening, and then has another long break before starting the first repetition of the next chapter.

What patterns is the flag sending? Do they seem to have mathematical regularity? Or any correspondence to the observable trajectories of the islands or of P.E.R.C.?

The second flag apparatus seems, as far as PERC's signal processing can determine, to have been kind of just fucking around for the first while. It has a black flag and a white flag but also tries a red flag once for a few swings, then discontinues it.

Eventually though, it settles into a consistent repeated pattern:

First, a chunk from the beginning of PERC's first contact package - or, well, from the beginning of what they saw of the first contact package; they didn't see it right away.

Then a gap.

Then, the chunk of their initial fucking-around that PERC echoed earlier.

Then a longer gap.

PERC-chunk, small-gap, long sequence of just white flags swishing. (Medium gap.)

Islands-chunk, small-gap, long sequence of just black flags swishing.

And another long gap, and the message repeats: echo of PERC's message, echo of their own message, pause, PERC-echo and a long string of white flags, self-echo and a long string of black flags.

That cycles a few times (with some occasional variance that's probably just flag operative competence issues), and then the flags on the rising islets do a new message (contents unclear - plausibly a mathematical sequence with a bit of noise?), and the message from the archipelago changes in response.

Now it goes: echo of PERC's message, echo of their own message, echo of the new message from the islets, pause, PERC-echo and a long string of white flags, self-echo and a long string of black flags, new-echo and a long string that's half black flags first and half white flags second.

That one keeps cycling for a while, with the rising islets repeating their own message a few times in the interim.

(All the while, the dedicated PERC repeater station keeps dedicatedly repeating everything PERC says. With its own level of occasional flag operative competence issues, though it seems to have fewer of them.)

Hmm. There are a few plausible interpretations of that. The one that seems most likely, on consideration, is that they are assigning flag colors to different participants, and the third section is relaying a potential conversation, but P.E.R.C. isn't very confident in that hypothesis.

It considers how to respond, given that it can't signal in multiple colors itself — it has only 'on' and 'off' to work with. After considering the problem for a moment, P.E.R.C. decides to assign unused symbols from its first-contact package to 'P.E.R.C.', 'station two', 'rising islets', 'black', and 'white' and prefix strings of colored flags with appropriate markers. It is still working to figure out what the opaque strings of black and white flags could correspond to, but in the mean time it can try to introduce a little more mutually-comprehensible vocabulary.

It sends a long pause and then the first part of its contact message, that the archipelago missed, up to a large overlap with what they were able to echo.

Then, it sends a long pause followed by a modified echo of station two's message: 'P.E.R.C.', first part of its contact package, 'station two', echo of their message, 'rising islets', echo of their message, pause; 'P.E.R.C.', echo, 'white', string of white flags, 'station two', echo, 'black', string of black flags, 'rising islets', 'black', string of black flags, 'white', string of white flags. It cycles this a handful of times so that they get the encoding, and then long-pauses again.

Next, it uses the vocabulary and grammar from the first part of its contact package to send "(= 'P.E.R.C.' (sender-of "<first part of contact package>")) (= 'station two' (sender-of "<station two's message>")) (= 'rising islets' (sender-of "<rising islet's message>")) (= 'white' ?) (= 'black' ?)".

While all of that is slowly being sent, it spends most of its processor time on figuring out what the white and black strings could mean, and on remaining in control of its fall.

The dedicated PERC repeater station continues dedicatedly repeating everything PERC says. A third flag station pops up nearby that seems to be echoing PERC's whole first contact package over from the beginning, once they've seen how the new chunk overlaps with what they've got. (When they get there, they correctly omit PERC's other attempts at communication from their recitation of the first contact package in particular.)

The second station, the one that went from fucking around to attempted communication, responds to PERC's attempted vocabulary like so:

First it just straight-up repeats PERC's vocabulary-establishing cycle, though it only does that once.

Then it repeats PERC's attempted encoded message in full, twice (though the second repeat might be because there was a coding error in the first repeat and they want to show they recorded it correctly even though their attempted repeats have an error rate).

Then:

'PERC', PERC-echo; 'Station Two', station-two-echo; 'Rising Islets', rising-islets-echo; medium-pause.

'white', long-string-of-white; 'black', long-string-of-black; 'black', 'white', string-of-black-followed-by-white, medium-pause.

'PERC', 'white'; 'Station Two', 'black'; 'Rising Islets', string of many black flags followed by one white flag; medium-pause.

'PERC', 'white'; 'Station Two', 'black'; 'Rising Islets', string of one fewer black flags followed by two white flags; medium-pause...

They keep repeating this message with (allowing for some operator error) one fewer black flag and one more white flag each time, until it's a string of all white flags.

Then they hold on one message for a while: 'PERC', 'white'; 'Station Two', 'black'; 'Rising Islets', many-black-one-white; pause, repeat.

Oh, that makes sense. The color of the flags is being used to encode the distance between P.E.R.C., the islets, and the archipelago. P.E.R.C. checks the length of the encoded string versus its own calculation of their relative distances, and finds it accurate to within the (admittedly noisy) limits of the transmission. So that is good news — they know where it is, although maybe not how fast it's moving. It's hard to say, yet, since the islets haven't progressed very far above the archipelago.

Since they can steer the rising islets and it's soon going to be able to talk about its trajectory, it stops preparing to dodge the rising islets and focuses on missing the biggest landmasses.

But this gives it a good basis on which to communicate the speed of its own descent. "P.E.R.C. white, Station-Two black, Rising-Islets [111 black; one white]", it agrees. Then it sends the same message in its contact-package encoding: "(= white (location-of P.E.R.C.)) (= black (location-of Station-two)) (= (location-of rising-islets) (/ (- white black) 112))". Then, it repeats the section of the first-contact package that deals with locations, times, velocities, and accelerations, so that they have some more examples of the relevant vocabulary in use. Then, it sends over facts about the distance between station two, P.E.R.C., the rising islets, and the repeater station.

(= 'P.E.R.C.' (sender-of "")) (= white (location-of P.E.R.C.))
(= 'station two' (sender-of "")) (= black (location-of Station-two))
(= 'rising islets' (sender-of "")) (= (location-of rising-islets) ("<111 black flags followed by a single white flag>"))

(= (location-of rising-islets) ("<110 black flags followed by two white flags>"))