I'm imagining roughly human-sized creatures which have 'sails' that they use to absorb solar energy and steer themselves around the solar system. I'm trying to think of a feasible way such a being could land on a world, and whether or not they could achieve escape velocity again.

I think in my head I imagine them as mostly made up of energy or something.


Thank you all very much for your responses. Like I said in comments, I'm new to this, so wasn't even sure what to think about, or ask for.

The world in which this would be taking place is more fantasy than sci-fi, but I had started to consider more traditionally sci-fi elements (creatures from space being mistaken as something divine), and had wondered if I could make it scientifically plausible. The conclusion I've come to is that this will only work the way I intend if I go the 'handwaving magical' route, as several of you have suggested, and I'm absolutely fine with that. Thanks again!

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    $\begingroup$ Quite related : How could my solar-powered fairies exist in outer space? $\endgroup$ Commented Nov 11, 2023 at 3:38
  • $\begingroup$ For future reference: (a) you're asking at least two questions (land & take off) and we only allow one per post. (b) You need to provide details. There's almost nothing to work with here. Are the sails large enough to work as parachutes? Are they strong enough to handle atmospheric braking? What's their mass? (c) When you ask questions, remember to say, "I want to do X and have Y to work with, what can I do to achieve Z?" Asking "could this happen?" is lazy and risks question closure (it's your world, it's always possible...). $\endgroup$
    – JBH
    Commented Nov 11, 2023 at 5:20
  • $\begingroup$ Please read through our tour and the following two Help Center pages (help center and help center) to better understand the pros and cons of this site. Stack exchange like specific, well-defined problems. Vague is bad. $\endgroup$
    – JBH
    Commented Nov 11, 2023 at 5:21
  • $\begingroup$ Apologies, I'm totally new to all this. I will take the time to edit my post with more detail above tomorrow and try and make it worthwhile. (It's very late where I am right now) $\endgroup$ Commented Nov 11, 2023 at 5:55
  • $\begingroup$ It's not obvious that such creatures could function in space in the first place. Their sails wouldn't be long-term useful. Yes, they could use them to steer, but only as they continually move farther and farther away from the sun. There's no way to get back. $\endgroup$ Commented Nov 14, 2023 at 18:41

2 Answers 2


If the sails are also human-scale...

Not a Chance

A solar sail's force per square meter is 9.08 micro-newtons per square metre at Earth's orbit. Even assuming you could get sunlight through the atmosphere normal to the surface (tricky), you would need a square sail a kilometre on a side to lift a single kg against Earth's gravity.

If a solar sail is all your creatures have, if they ever enter a gravity well, they're stuck.


If they've evolved to live in microgravity in a vacuum, then the experience of a warm planetary atmosphere would be a little like putting a human in a centrifuge, spinning them up to bone-crushing levels and then boiling them.

Even the action of entry into Earth's atmosphere would probably completely disrupt them, as they are unlikely to experience significant acceleration forces under normal circumstances and hitting even the quite diffuse upper atmosphere of Earth at orbital speeds will likely tear them apart even if it didn't cook them.

On the assumption they they can survive immersion in an atmosphere for some possibly magical reason, they'd need to be able to construct themselves a (re)-entry shield in order to land on a world with an atmosphere. This doesn't have to be particularly substantial, but it will require some kind of industrial process... just plain old rock or ice will usually not work, and break up in the upper atmosphere. Something like a paracone might be as minimalist as you can get, so they need to be able to make and shape metal foil, somehow.

I think in my head I imagine them as mostly made up of energy or something.

If they're basically magical, you'll be OK. Don't try to handwave them as being made of plasma or something, because that couldn't reasonably exist in Earth's atmosphere. Honestly, I'd try not to go into any detail if it could be avoided, because details are where you can be wrong, and just glossing over stuff is something most readers/players/consumers are basically happy to do so long as your setting is otherwise coherent.

sunlight and gas for energy and propulsion

With a bit of handwaving, and not looking too closely at the details, you might be able to have them use solar-powered jet engines of one kind or another, so long as their "wings" were quite large and they were very lightweight indeed, which covers flight. Actually developing enough velocity to escape into orbit seems unlikely, because you need a lot of energy to get up there, the sun isn't that intense at Earth's distance, and the atmosphere provides a lot of drag. Even ultra-efficient and suspiciously high-thrust ion engines are unlikely to help here. Once fallen, your particular angels seem likely to be staying here until someone gives them a lift back out again.

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    $\begingroup$ It's worth considering that current (experimental) solar sails can hold up to handling and assembly in Earth's surface environment because they're built here, but that very fact limits their performance severely. Drexler's proposal for a high performance solar sail involved 30-100 nm thick aluminum panels. This kind of structure isn't even going to survive on Earth's surface, let alone usefully function. (Short of magic, as you mentioned.) $\endgroup$ Commented Nov 13, 2023 at 2:27

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