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Voidcrawler (Mycocelium Nocturnis) is a voracious invasive species, most similar to a fungus but much hardier and with some singularly unique properties:

Firstly it absorbs not only light, but also heat from its surroundings and, through processes unknown, both uses the energy to absorb and use nearby materials and stores it as electricity using some strange form of internalised capacitors. As a result Voidcrawler infestations are always dark and cold. The fungal bodies themselves have a near 0 albedo and stay just above 0 degrees Celsius.

Secondly voidcrawler infestations will produce house sized spores that, somewhere in the myriad whorls and twists of their thoroughly inexplicable cellular structures, house a naturally occurring reactionless engine. These spores will eventually, with a bit of a kickstart from their parent body, break away from their parent planet and parent star, accelerating out into the interstellar gulf; feeding on starlight and slowly using up their reserves of stored energy.

When a Voidcrawler spore lands on a planet (they can generally decelerate pretty handily against gravity so they're not impacting at obscene speeds) it almost always spells doom for any species that might live there, eventually rendering the host planet a cold, dark ball that dissolves over the aeons into ever more spores.

The question is this: Given that space is very, very big, and even we (with our telescopes and brains) find it very hard to spot planets around other stars, how do these unintelligent spores navigate to a new star, slow down, navigate to a new planet, and land?

You can assume that concerns about energy, acceleration and continued function in the interstellar void are taken care of.

These aren't your normal button mushrooms.

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  • $\begingroup$ "how do these unintelligent spores navigate to a new star, slow down, navigate to a new planet, and land?" shouldn't the second "navigate to a new planet" be removed? It seems redundant and a bit out of place to navigate to the planet after they already started slowing down to land. $\endgroup$ – Secespitus Aug 30 '17 at 8:08
  • $\begingroup$ before braving atmospheric entry, how do they survive the space radiation? (I'm willing to give you the benefit of doubt how they make to space.) $\endgroup$ – user6760 Aug 30 '17 at 8:18
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    $\begingroup$ 0 C or 0 K? 0 C can be pretty hot for general space vacuum... $\endgroup$ – L.Dutch Aug 30 '17 at 8:26
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    $\begingroup$ Go with the flow of galactic winds. Until at one point a certain planet pulls at them hard enough. $\endgroup$ – Totumus Maximus Aug 30 '17 at 15:17
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    $\begingroup$ "navigate' implies intelligence and intention in direction. Spores are generally agnostic as to where they go and are at the mercy of the medium into which they are released (air current, water current, etc...) $\endgroup$ – NKCampbell Aug 30 '17 at 21:05
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The very point of a spore is to be sprouted so cheaply that the parent organism can produce so many of them. They don't navigate, they don't consume nor process energy. Therefore they can lie dormant almost indefinitely. Depending on your definition, a spore is not even an actual life form, but only a potential one. They hedge against time with having no life functions and they hedge against size of space by their sheer numbers. Eventually, after aeons, one of the countless number of spores will land on something and sprout.

What you propose is not a spore, but more kind of larva. It's not an adult organism, but it's "alive", so it can process it's surroundings and respond. But they also barely navigate - the parent organism sends many of them, in all directions, most of them will die as well, but some will come close enough to a star to notice it, and then they'll move towards it's new prey.

I think the obvious issue is how to make them NOT come back to it's home star, but you can take care of that by being launched by the parent really fast and perhaps in some kind of egg form, so they develop only after millennia, when they'll be far enough to not get confused by home star.

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    $\begingroup$ Spores experience a fate worse than death — pre-life! Then, eventually, death. $\endgroup$ – can-ned_food Aug 30 '17 at 14:39
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    $\begingroup$ You raise an excellent point. It is more of a larval or nascent voidcrawler than a spore. $\endgroup$ – Joe Bloggs Aug 30 '17 at 14:58
  • $\begingroup$ Also to this: Once a planet is completely enveloped by the crawler, it starts to turn itself into a disk at the equator, stretching further and further out. Once it's big enough to have solar (!) escape velocity, the rim starts to produce the spores, flaying them away until the planet has completely been transformed into spores, leaving nothing but a small clump below escape-velocity behind $\endgroup$ – Hobbamok Apr 4 at 13:07
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I think you're probably stretching things a little with this, but you mention yourself you're already breaking some fairly major laws of physics, so what's a few more?

The mechanism required to do what you want basically already exists; tropism.
From Wiki:

A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus.

The two most common of which are phototropism (the response to light) and geotropism (the response to gravity).
So when your spore first launches itself into space it uses it's ridiculously sensitive phototropic response to align itself with a distant star and begin moving (if you want to stretch things a little further you could even possibly say it can tell which stars are nearer based on light output).
Then it basically settles down to hibernate and spends probably thousands of years travelling towards this distant target.

Once it reaches the solar system of this star it then uses a combination of geo and photo tropism to detect planets, it can recognise the reflected light and tell them apart from stars. When it gets close enough to a planet it can sense it's gravity and starts to move towards it, slowing down as the pull of gravity increases.

It may seek to oppose the pull of gravity to slow its descent further, but actually I imagine a hard and fast landing that spreads the organism's matter across a wide area may actually be desirable so it probably doesn't slow too much, just enough to survive.

There you go, fully automated movement; no intelligence required.

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    $\begingroup$ how does a free falling object sense gravity? $\endgroup$ – ths Aug 30 '17 at 11:05
  • $\begingroup$ @ths it doesnt have to sense it, it just flies towards the detected planet and away from it if its outer shell gets too hot or the small hairs on its outside detect strong winds. $\endgroup$ – C.Fe. Aug 30 '17 at 11:57
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    $\begingroup$ @ths By tidal force. $\endgroup$ – Agent_L Aug 30 '17 at 14:29
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    $\begingroup$ Like, ludicrously sensitive. $\endgroup$ – can-ned_food Aug 30 '17 at 14:35
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    $\begingroup$ @MikeNichols : or very large. Possibly fine tendrils... $\endgroup$ – Joe Bloggs Aug 30 '17 at 14:57
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One can characterize a planet's atmosphere, temperature and some characteristics of its surface by how it reflects light.

from http://www.as.utexas.edu/astronomy/education/fall08/scalo/secure/309l_sep25_plandet.pdf

[infrared spectra of Venus, mars, earth[1]

Your spores drifting in space would have plenty of time to stare in a given direction, characterizing the reflected infrared light coming from that area and looking for suitable planets. Probably some planets are better than others.

Slowing down and landing are part of the question but are not going to be issues because for one the OP states

/You can assume that concerns about energy, acceleration and continued function in the interstellar void are taken care of./

and for two they have reactionless drives, so they can scoot around, decelerate etc as they see fit.

An interesting question is intraspore competition. Spores coming from a given planetary source would see similar surroundings. If they all make for the nearest suitable planet, the first one there gets a considerable fitness advantage. Competition strategies would come into play - for example:

  • look for a more distant target and head for that, letting your siblings fight it out over the low hanging fruit.

  • break off sporelet decoys which radiate tantalizing frequencies, simulating a desirable planet in the distance.

  • physical sabotage of sibling spores (or more distantly related conspecific spores).

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Bacteria move through solutions by a somewhat random walk known as Chemotaxis, in which they are marginally more likely to travel towards the food/energy source than in any other direction. Could your spores do something similar with travelling towards the direction of light sources?

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  • $\begingroup$ Welcome to WorldBuilding Mithra! Interesting answer. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Aug 30 '17 at 16:14
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Though it is not clear exactly how far can we go with our imagination here, maybe this will work. Please forgive me any inaccuracies for I'm not an astrophysicist, really. Below is only the general idea.

Each star system is covered with something called Heliosphere. Basically, this is an empty space filled with solar wind with a specific radius (a tiny particles flying away from the star). When such particle leaves the heliosphere, its speed is reduced to minimal. Voidcrawler can have some kind of mechanism that enables 'scanning' for solar wind after certain period of time. For example, after half of its stored energy is consumed, we consider voidcrawler is already far away from its home system and ready to start tracking solar winds coming from other stars.

When first signs of star wind do appear, the organism can somehow follow the track of these particles backwards, searching for maximum-impact direction, if applicable. Anyway, it changes its course to inside the system and now we're ready to search for the planets.

That's the trickiest part - we can't just smell the gravity from planets, nor do they emit anything noticeable on a space level. I suggest navigating to ecliptic (flat plane in which all planets and stuff are located) and patrolling the space between entry point and the star back and forth until you got lucky and bump into an atmosphere of some celestial object that passes by. Though I'm afraid that the probability of such collision lies somewhere near zero... That's where the large numbers of voidcrawler may help.

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I envisage a low density spore, as big as a house, but of very little mass. It moves slowly but steadily.

In free space it naturally seeks out light sources (light = food) and moves towards them. How it does this is unclear; I would suggest using gravity waves, but how it would differentiate between the gravity of a solar system and that of a black hole (which I assume it would want to avoid) is unclear.

Once within a solar system, the solar wind acting upon the high volume low mass spore keeps it away from the sun, but where it can naturally be drawn, sooner or later, into the gravity field of a victim planet, while still feeding on light energy.

Having very little mass and high volume it enters the atmosphere slowly, floating down following gravity. They do not really need to decelerate.

Once landed it starts consuming material resources. Since it has no need of atmosphere, it consumes the natural atmosphere as well, converting that into electrical energy and reproducing itself.

In the process it is destroying the structural integrity of the planet, which leaves it vulnerable to natural forces breaking it apart - in the process helping to spread more spores.

-- Edited to cater for new spores --

New spores, as bred from a contaminated planet and released, would be characteristically of lower mass but offer a higher surface area to catch the solar wind to take them beyond the gravitation of the solar system where, over time, they mature gaining mass and the ability to seek out a new host planet.

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  • $\begingroup$ If new spores are released by the planet breaking down, they will remain trapped on home star orbit, along with rest of the debris. That doesn't help escaping star's gravity. $\endgroup$ – Agent_L Aug 30 '17 at 14:30
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    $\begingroup$ @Agent_L - that is a very good point, and I have to confess that I hadn't thought that through, although the OP had more or less already got that covered: These spores will eventually, with a bit of a kickstart from their parent body, break away from their parent planet and parent star, accelerating out into the interstellar gulf; feeding on starlight and slowly using up their reserves of stored energy. - how they actually do that, though, is unclear to me. $\endgroup$ – Lee Leon Aug 30 '17 at 14:37
  • $\begingroup$ Note that gravity waves caused by movement of planets and their star are comparable to the electromagnetic waves caused by the movements of planet and star-sized magnets, i.e. tiny. $\endgroup$ – wizzwizz4 Aug 31 '17 at 14:53
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Expanding on @Mithra's answer: The head toward the light.

The spores launch from the surface into orbit as they become ready. In orbit they start preparing for the trip storing energy. And use parallax over a year to pick a star by estimating distance and favoring closer ones.

They boost toward the star. While they are still close to a star so have plenty energy they accelerate quickly, but reduce power to the drive as they leave, dwindling to off as they pass the point of getting enough energy to maintain themselves. Now they drift and hibernate living off their stores. Powering the engine only if they drift too far from their target.

Eventually (tens of thousands? millions? of years later) the arrive close enough to a star to receive enough energy to wake them up. They move to their Goldilocks zone and circularize their orbit. There is plenty of energy to maneuver and metabolize so now they look for planets to spawn on. Interesting planets should be easily visible, Venus and Mars are among the brightest things in our sky, and they move relative to the background stars.

Matching orbits with a reaction-less drive and nearly limitless energy is trivial even if it picked a 'polar' or counter rotating orbit on entering the system.

All you need is a stable view point and long exposure photo-receptors.

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Another aproach to this question could be to create a spore-cluster formation similar to the Stark Industries missile demonstrated in the beginning of Iron Man 1.

A huge clumb of spores is sent in a direction, then, when the chance of colliding with a planet is unlikely, the spore spreads, covering the holes in between the larger clumbs. Then, when these again become unlikely to hit, it spreads again.

Returning to the Stark Industries missile, think about how unlikely a single missile hitting a target in that area is, but by splitting the way it does, the chance of hitting the target (planet) is signifigantly larger.

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Given that interstellar travel requires tremendous speeds that are very hard to alter, it may be sufficient to exhibit chemotaxis towards blue-shifting stars - those that are already moving in the direction of the spore.

Then once body is close enough to get a significant amount of solar power, one can use tidal phototaxis with blue-shifted object upweighted.

Of course one would assume long dormant period after the spore launches, so one could assume that it matures in interstellar space in extremely low temparature using a stored energy.

Of course it may be advantageous to differentiate close stars and distant galaxies by the spectra.

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  • $\begingroup$ how can a spore determine blue shifting stars? $\endgroup$ – L.Dutch Aug 30 '17 at 18:37
  • $\begingroup$ @L.Dutch If it can detect the energy from a star, then it could prefer those with a shorter wavelength over those with a longer wavelength. $\endgroup$ – Luke Aug 31 '17 at 1:18
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    $\begingroup$ @Luke, blue shift and blue spectrum are two different things. The OP talks about blue shifted. $\endgroup$ – L.Dutch Aug 31 '17 at 4:12
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    $\begingroup$ @Luke, to discriminate between a truly blue star (i.e. Bellatrix) and a blue shifted star, one has to analyze its emission spectrum, check for known spectral lines and see if they match the known wavelength. If these differs, the star is said to be either blue or red shifted. Now, I hope you agree with me that having a spectrometer AND reference spectral lines is hard for a spore. $\endgroup$ – L.Dutch Aug 31 '17 at 5:31
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    $\begingroup$ @L.Dutch I think the whole concept as presented goes beyond a 'spore'. I'm assuming that since an organism has evolved to organic intergalactic travel by consuming whole planets and launching spores outside of its solar system, it wouldn't be a bigger stretch to assume its perception has evolved more around the astronomical than ours. $\endgroup$ – Luke Aug 31 '17 at 5:53

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