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How could an organic, plant-like, intelligent organism move around space?

Assuming that the body is inside a small meteoroid and that outside of the meteoroid is covered in tessellating leaves to gather sunlight, to allow it to survive.

Since this organism would have to have to conserve matter any matter it lost would be unrecoverable. Usually, to propel oneself around space you shoot stuff out the back and go forward, but without venting matter propelling oneself would be hard.

In short, what are the most viable method for moving around space, without venting matter, and with only sunlight for energy?

How hard it is to have a closed system organism is out of scope for the question.

edit: bonus points if you can figure out a way for it to rotate on the spot.

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    $\begingroup$ It uses zero-point energy to warp space $\endgroup$ – OIEp Aug 28 at 12:00
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    $\begingroup$ also see: worldbuilding.stackexchange.com/questions/30509/… $\endgroup$ – Nahshon paz Aug 28 at 13:00
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    $\begingroup$ If it's a plant, why does it need to move? Mostly they stay put. $\endgroup$ – Separatrix Aug 28 at 14:21
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    $\begingroup$ Leaves use photosynthesis which requires an atmosphere, you'll need to put one on your asteroid, even if it's just a thin layer, or a nearby source of O2 H2O and CO2. If you're imaginative that could make for some very strange looking leaves ( e.g. bulbous leaves with an internal air sac that's exchange with the interior of the asteroid ) $\endgroup$ – Tom J Nowell Aug 28 at 14:59
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    $\begingroup$ You need to watch Farscape. $\endgroup$ – RonJohn Aug 28 at 17:18

13 Answers 13

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If you don't want or can't afford ejecting matter, you are left with solar sail: since photons carry momentum, reflecting photons can allow you to harvest that momentum and use it to move.

Expand reflective leaves in a suitable way, and use the light coming from the star at your advantage.

Mind of two consequences:

  1. If you reflect the photons, you are not using them to feed you energy. If you are absorbing the photon you get half of the available momentum.
  2. Maneuvering can be complicated and quite time consuming
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  • $\begingroup$ I was thinking of doing this, and having the leaves have a metallic, and shiny base and a sort of circulatory system throughout the skin, which could, at will fill with photosynthesizing cells. The metal could also maybe protect the main body from small projectiles and radiation. $\endgroup$ – wpokdljnlnmn Aug 27 at 9:58
  • $\begingroup$ Don't forget, travelling by solar sail is incredibly slow, and you're only moving away from a star. $\endgroup$ – Loid Thanead Aug 27 at 9:59
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    $\begingroup$ @LoidThanead You can use a solar sail to descend towards a star, by having it constantly give you a mostly retrograde thrust; your orbit will slowly spiral inwards. $\endgroup$ – Skyler Aug 27 at 19:08
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    $\begingroup$ @Ghedipunk it's hard to plan a slingshot maneuver with so little maneuverability. On the other hand, what little maneuverability you have does get amplified by the flyby, so if the interplanetary creature had enough brains to plan several flybys in advance ... it might be able to affect its trajectory enough to be worth the energy spent to power said brains. $\endgroup$ – John Dvorak Aug 27 at 23:48
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    $\begingroup$ This is actually pretty brilliant, being that the entire asteroid is essentially a plant, it could maneuver simply by changing the color of it's leaves. It would be amazingly slow but plants live for a long time. Of course all it could really do is change it's orbit, but given centuries it could probably make it an elliptical orbit--possibly eccentric enough to explore the ort cloud (at which point it would probably need to hibernate due to lack of sunlight) $\endgroup$ – Bill K Aug 28 at 16:22
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I'll just go for the bonus points about rotation. As long as the organism can change its shape, it's perfectly possible for it to rotate freely while maintaining net zero angular momentum throughout the rotation. That is, it can rotate without expending any mass and without anything to push off of. Cats do this quite regularly in order to land on their feet, regardless of their orientation when dropped. See the falling cat problem at Wikipedia; experiment at your peril.

enter image description here

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    $\begingroup$ If a body, by distorting its own shape, can change its orientation while maintaining zero angular momentum, is there any reason why similar distortions should not enable it to change its (lateral) position without ever acquiring any (lateral) momentum? $\endgroup$ – Michael Kay Aug 28 at 8:44
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    $\begingroup$ @MichaelKay You can change the location of your center of mass, but you can't make a cycle that would keep you moving forward - you move a bit forward, and when you restart the cycle, you move the same bit backwards again. This doesn't happen with rotation because in the end, the rotation of the object doesn't really change anything. You don't need something as complex as a cat either - this is how gyroscopes work. Of course, even though you can rotate something using just this effect, you can't apply a constant acceleration - which is part of why space ships still use "rockets" for rotation. $\endgroup$ – Luaan Aug 28 at 12:58
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    $\begingroup$ @Luaan Can you change the location of your center of mass? That would imply an external force. Even if you change your shape, your CoM will remain stationary, even though it might now be positioned differently in your body - your body will move around the CoM. I don't think you can move the CoM using only internal forces, otherwise, you'd violate conservation of momentum (which must remain zero). I don't quite see the link to gyroscopes - they start with nonzero angular momentum and don't change their orientation. $\endgroup$ – Nuclear Wang Aug 28 at 13:08
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    $\begingroup$ @NuclearWang Yikes, I've changed the focus between CoM relative to your body and relative to another body, and ended up with utter nonsense :) What you're saying is exactly right - you can move CoM relative to a point on your body, but not relative to e.g. the Sun. Unless you're doing it in KSP, where you can actually use it for absolutely horrible reactionless propulsion :P $\endgroup$ – Luaan Aug 28 at 13:15
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    $\begingroup$ @MichaelKay, it does appear to be actually possible, but not very efficient; see physics.stackexchange.com/questions/886/… $\endgroup$ – Jan Hudec Aug 28 at 18:22
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Ion thruster. https://www.nasa.gov/centers/glenn/about/fs21grc.html

Technically this is also venting matter, but "venting" to me implies large quantities.

Kinetic energy = mv2. Energy increases as the square of the velocity. By ejecting small particles you minimize mass loss. By ejecting them very very fast you maximize kinetic energy.

A creature in space near the sun would have access to a variety of incoming charged particles from the solar wind; mostly protons but other things too. Photosynthesis uses radiant energy and so without some adjunct metabolic path these charged particles cannot be harvested for energy. Your creature captures these and then accelerates and re-emits them; an ion thruster.


Electrodynamic propulsion.

https://en.wikipedia.org/wiki/Electrodynamic_tether

Electrodynamic tethers (EDTs) are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy.[1] Electric potential is generated across a conductive tether by its motion through a planet's magnetic field.

This is contingent on the organism traversing an electromagnetic field. Some planets will have them; stars should have them. By creating an electrical field in a long "tail" the organism / spacecraft can push against an external field, turning energy into momentum.


Neither of these ideas is fiction; both are real. Although, of course not used by real creatures, as far as we know.

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    $\begingroup$ the ion thruster which uses particles from the surrounding plasma as reaction mass is called a Bussard Ramjet $\endgroup$ – craq Aug 28 at 0:20
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    $\begingroup$ @TheDyingOfLight What? No. $\endgroup$ – Yakk Aug 28 at 13:51
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    $\begingroup$ @TheDyingOfLight - yeh, I am perplexed by that too. Expand that thought? $\endgroup$ – Willk Aug 28 at 15:07
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    $\begingroup$ @Willk It has been demonstated that the drag the magnetic scoop will produce will be far greater than the thurst the fusion drive can deliver. Aditionally pp fusion is way to slow to be useful and the alternative of using the NCO is problematic to say the least. $\endgroup$ – TheDyingOfLight Aug 28 at 16:22
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    $\begingroup$ @TheDyingOfLight - at 0.5c I am sure you are right. But I envisioned this thing as using ion thrusters more as maneuvering rockets. $\endgroup$ – Willk Aug 28 at 18:26
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Low energy forms of propulsion would include Yarkowsky propulsion (basically, you heat the area opposite to the direction you want to go, as much as possible) and both electrostatic and electrodynamic propulsion and attitude adjustment. Solar wind sails are another possible way.

Your organism should be able to measure the electric and magnetic fields outside its meteoroid, and alter the distribution of surface charges (or maybe even expel charged particles) as well as inducing currents within itself. Current control alone can be used for attitude control (the meteoroid aligns itself by exploiting the available solar magnetic field).

To quickly rotate around its barycenter, your organism would have to first excavate a spherical chamber all around said barycenter, then fill it with a solid sphere of the densest material it has at its disposal. Finally, fill the gap between sphere and chamber (just inches thick) with cilia. By moving the cilia, the organism can rotate the sphere any way it wants - and the outer meteoroid will rotate in the opposite direction, proportionally to the difference between the masses (if the sphere is 1/100th of the meteoroid mass, then by rotating the sphere 100 times the meteoroid will spin on its axis once).

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Lasers

Admittedly, they don't have to be coherent, just a directed light beam. You can gather energy from starlight and emit it opposite the direction you want to go. In principle it works just like any other drive, except, photons do not have rest mass, so you won't lose any of your precious materials in the asteroid. Yet photons still carry momentum so you will gain speed. If the laser it self is moveable, or you have multiple lasers, you can even steer.

Keep in mind though, that this is a terribly slow form of movement.

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  • $\begingroup$ I was also thinking about this, and I just wanted to know if it was possible. $\endgroup$ – wpokdljnlnmn Aug 27 at 10:39
  • $\begingroup$ Where terribly slow is an extremely low rate of acceleration. Very, very long travel times. Even interplanetary trips are likely to take centuries. $\endgroup$ – a4android Aug 27 at 12:50
  • $\begingroup$ Why would this be any slower than a solar sail? Are you just talking about slowness in deep space where available light is minimal? $\endgroup$ – R.. Aug 28 at 16:04
  • $\begingroup$ Lasers (or let's just say light) would still consume matter, however it's the most efficient matter to force conversion possible if you can do it without generating waste heat (Uses that old e=mc^2 formula). Plasma might also count and would give more thrust--I don't know enough about it. $\endgroup$ – Bill K Aug 28 at 16:50
  • $\begingroup$ Plasma definitely is matter. How light would "consume" matter, that I do not see, could you elaborate on that? It is possible to generate light just by exciting electrons in nuclear orbits. $\endgroup$ – infinitezero Aug 28 at 16:55
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Capture the solar wind and use it to power an ion drive.

This will be very-low thrust, but it is feasible. Sources vary, but the density of the solar wind at Earth orbit is about 4 particles per cubic cm (electrons, protons and alpha particles).

You could have the organism capture the solar wind continuously, and only use it for thrusting occasionally, thus allowing higher thrust when maneuvering is required.

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  • $\begingroup$ If you're capturing the solar wind in significant amounts, you also have a solar sail. Maybe the creature uses the sail to raise its orbit and the thruster to change inclination or to lower the orbit. $\endgroup$ – Ryan_L Aug 27 at 20:06
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Your being could utilize the Interplanetary Transport Network (ITN), which is a collection of gravitationally determined pathways through the Solar System that utilize low-energy transfer to minimize fuel use.

Low-energy transfer trajectories are paths in space where orbit around one mass is exchanged for orbit around another at points in spacetime where very little energy is required to effect this change. Longer trajectories can be combined from several such transfers; a method used by deep-space probes. While it would use little energy, transport along the network could take a long time.

Fuel for the transfers could be gained by collecting interplanetary ice or dust and accelerate it to relativistic velocities by linear accelerators powered by collected sunlight or fusion reactor.

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It depends on whether you're willing to go into the realm of sci-fi, or you want to stay with the pure unadulterated science.

If you want something realistic, then the best options were already mentioned previously by people here: solar sail, photon or, ion drives(they use thrust, but minimize matter used).

However, if you're willing to go for something more... controversial, I remember that there are a few thrustless possibilities related to Mach's principle, that are most likely incorrect, but could be used for a sci-fi. It's been quite some time, and I only remember rough principle, but here's wikipedia article that you can use as basis for what to look for: https://en.wikipedia.org/wiki/Woodward_effect.

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An additional problem is how to stop rotation. Differences in albedo (reflectivity) and (inevitable in a vacuum) offgassing rates on the surface of asteroids has been known to cause them to spin up to the point of breaking apart. A method for redistribution of mass,changing of shape, or off-axis propulsion (for rotational acceleration) would be necessary to keep this from happening.

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It is not necessary to not vent a large quantity of matter so long as the matter that it does eject is with sufficient force. General relativity tells us that the energy required to accelerate an object with any mass to the speed of light $c$ is infinite. Thus, the organism could reach any desired velocity by ejecting a single molecule, so long as that molecule was ejected with sufficient force. All you need is a source of infinitesimal quantities of matter (interplanetary dust will suffice) and a source of energy (which you have supplied with the organism's "leaves"). The trick, now, is to find a mechanism by which that solar energy is transformed into mechanical energy in such a way as to launch a very very small amount of matter at $0.9999c$.

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    $\begingroup$ The problem with this method is that (when dealing with non-relativistic velocities) doubling the speed of the propellant doubles the thrust force... but increases the required energy to expel that propellant by a factor of four. Increase thrust by three times, you increase energy consumption by nine times. It's a trade off. Sure, you CAN get any arbitrary amount of delta-v using a tiny amount of matter, but the energy requirements quickly become nuclear. $\endgroup$ – FlyingLemmingSoup Aug 28 at 5:37
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An alternate solution would be a sort of slingshot, using retractable tethers to temporarily latch on to celestial bodies of greater size than that of the plant creature itself.

Then, using the rotation of the celestial body along with a speedy retraction of the tethers, velocity can be achieved before releasing its hold. The leaf based solar sails, mentioned earlier, can take over further acceleration and steering after that.

Assuming the problem of survival in a hard vacuum is already solved, a plant based intelligent organism, capable of a high level of advanced mathematics, could be able to calculate trajectories to pinball between celestial bodies of appropriate size until the main destination has been reached.

We are definitely not talking about great velocities here. But then again, I hear the concept of time is relative to the size and metabolism of the perceiver, and a plant would most probably be a patient traveller.

Then comes the problem of calculating trajectories. The organism would need a naturally evolved sight or other means of scanning and interpreting deep space objects, while doing advanced astrophysics and celestial mechanics calculations, mapping its optimum path, before beginning its journey.

A question that follows would be how the hell the plant ended up in space to begin with, which also applies to the original question.

Did a behemoth plant grow into the form of a launcher for a sporelike chrysalis, towering several miles high, before shooting out that first tether, latching on to a passing asteroid?

Or maybe a single incredibly massive organism spread its burrowing roots throughout a planet to such a degree as to actually cracking the planet into multiple pieces, each one a newly born space traveling vessel.

Oh, by the way. Rotation would be achieved by adjusting the angle of the leaves on one side of its frame to lessen the reflection of photons, like a rudder, using the same basic function as the solar sail.

It's not the most elegant solution, I know. Just my two cents.

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    $\begingroup$ the plant evolved on a planet, and then the planet did break into pieces, although not because of the plant. I think that it was originally an extremophile bacteria or something, which ended up on a new asteroid belt which was once a habitable planet. it spread out from there, after many years of evolution on a large resource-rich meteor. $\endgroup$ – wpokdljnlnmn Aug 28 at 23:25
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In order to move through space you MUST either push or pull something (Based on everything I've heard so far anyway). You can push stuff out your back, you can push against solar wind, or you can possibly pull on something (Could it push/pull the sun's Magnetic lines?).

If it could attract space debris (Hydrogen?) there is a lot floating around. If it sucked it in from the front it would get a little forward movement. It could then use an internal nuclear fusion reactor to convert it to helium (like the sun) and use the energy created to both fuel itself and create a powerful light beam out the back. Any other material picked up from space (dirt? metals?) could either be used in it's own system or mixed with the laser to create a more powerful push (Plasma?)

Even if it couldn't "Suck" in debris, it could use a scoop to gather it.

Note that in this interesting scenario, the "Leaves" would point inwards towards the central "reactor" instead of outwards towards a sun.. also it could go for ever, as long as there was hydrogen to scoop up.

An advanced fusion reactor might even be able to fuse other more dense materials (Like mr. fusion from back to the future) so it could scoop up and live off nearly anything.

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Your only choices are pretty much:

  1. Ion Thruster—You still lose mass, but a tiny amount, that you could pick up on your travels (or just by drifting in solar wind).

  2. Gravity Tentacles—If the organism has (very) long tentacles, it can shift them toward nearby gravity wells to adjust trajectory path.

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