I can't see it being too much of a issue, I'm sure there are plenty of examples using similar elements and art styles. While they do look similar , I think you have changed it enough to quantify drawing inspiration from it , which is what most of art is :D

Found a couple of similar examples so you should be quite fine :)

https://www.google.com/imgres?imgurl=https%3A%2F%2Fc8.alamy.com%2Fcomp%2...

https://www.google.com/imgres?imgurl=https%3A%2F%2Fchrisdoescomics.files...

https://www.google.com/imgres?imgurl=https%3A%2F%2Fthumbs.dreamstime.com...

safe to use in your game? most likely yes.

safe to submit to OGA as an asset? it depends on how you created the image. i am not admin or the most knowledgeable about the legalities and definition of a derivative, so take what i say here as it is: my opinion.

derivative means it is derived from something else. did you trace the original spaceman head? did you resize the spaceman head and adjust the pixels to look good at low res? in other words, did you use the actual image of the original spaceman in the actual composition of the new spaceman. if yes, than it is a derivative work.

I would say, looking at the image, that the portrait is a little bit too close, but the rest is okay.

Usage in your own game is a bit different from submission on OGA. Since you're talking about the Summer Art Challenge, I'll assume your interest is predominantly about submtting on OGA:

^ Pretty much what they said; Should be fine, but it depends on how you made the more specific sprites, like that astronaught face. More on that below.

Using the same/similar color palette is fine. That won't make it a derivative. Palettes can't be copyrighted.

You'll want to avoid labelling your asset pack with the Manga title or author name. "The Crater by Osamu Tezuka Asset pack!" could invite Trademark issues and implies endorsement by the manga author that he has not given. Saying it's "inspired 'The Crater' manga" is fine, though.

How was your astronaut character and astronaut face created? Did you copy/scan the manga and then alter the image? Did you trace any part of the manga art to get the right shape and proportion? What parts of the manga, if any, did you use as a base or guide?

The Italian version would indicate the artist if it weren't Osamu Tezuka.

Using the artwork as an indirect reference is borderline derivative. Could go either way. I'd say best not to use it unless you're able to remake the asset without using any references.

It's an astronaut and the same colours but the helmets a different shape, the face is different. It doesn't look the same as the manga picture...change the shape of the cracks on the visor maybe for even less similarities.

Well, now I could probably redo it based on my own piece as reference... :)

Hahaha! Well, you should note that using a derivative of the original as a reference when making a new version would make the new version a derivative of the original as well. 

Otherwise I could just... trace the shape and layout of Chrono from Chrono Trigger to make my new character "Shrono". Shrono is a derivative of copyrighted content, so then I trace the shape and layout of Shrono to make my totally-original-not-copyright-infringing-character "Brono". Copyright-free, right!? </sarcasm> :P

Distinctive qualities help, but you can still end up with a derivative that looks nothing like the the original. It will still be a derivative if you used the original to copy it. It would be hard to prove it's infringing if it looks nothing alike, but it could still be trouble. Comparing similarity is not the only way to prove something was derived. I know a guy who made video blogs of himself kitbashing commercial game art until no one could tell what it came from. No one could prove it was a derivative by comparing the assets side-by-side, but someone still DMCA'd him and used his own video blog as evidence of copying.

Again, looking at the original while you create your own version is borderline. Usually it takes more than that to constitute "copying" in a legal sense. Your art is probably fine, but if you're not sure, I wouldn't risk it until you can redo the sprite without using another asset as a reference

Are differences enough make it not derivative? Is there a rule of thumb on how much different it should be?

Technically no. If "copying" (in a legal sense) occured, no amount of changes will fully innoculate it from risk of being a derivative. However, even some lawyers would say that if it is so different that you truly cannot tell that one came from the other, why would it matter? Then again, other lawyers have told me, "if the new work is nothing like the original, why did you need the original at all? Just create the new work without using existing works and eliminate the risk entirely."

How did your kitbashing story ended though? I'm sure it is derivative work, but that would probably fall under the fair use category, right?

Yes, it was deemed derivative. He argued it was Fair-Use, but it was only fair-use in featuring the assets in his video, so he didn't get a copyright strike against his channel, but it was not Fair-Use to use the derivatives in his game. The youtube videos were not infringing, but they were used as evidence that his game would be infringing. He still had to adhere to the DMCA for using the assets in his other projects.

In the same way, you could probably use derivative assets under the terms of Fair-Use, but using them in a game would mostl likey not qualify as Fair-Use. Similarly, submitting such assets to OGA would not be allowed because "Fair-Use" is not one of our accepted licenses.

Just can't not to write it. People's brain is the engine to create derivative objects and it can't do anything else. It can combine some things but everything produced by brain will be derivative of something... In a philosophical way everything produced by everyone is derivative at least for some value....

Just a question. If someone created a step by step tutorail and you followed each step? If the result isn't the same but similar..?

Depends if you were basing your work off of what they were doing or using the tutorial as a guide.

@DustDFG: ^Yep. What Umplix said. In fact, drummyfish and I consulted an attorney about exactly that scenario. 

But you are correct when you say "in a philosophical way". The difference between "derivation" and "inspiration" is philosophical. If you were inspired by some other work of art, and made your own, it is NOT a derivative. If you used some other work of art as a component or baseline for making your own, it IS a derivative. The line between inspired and derived is often fuzzy and only exists philosophically.

Unfortunately, there is legal precident surrounding that philosification, and believing in the "wrong" philosophy can still get you in trouble legally.

But if you got some "lego bricks" and method how to create a tower from them....

https://www.slynyrd.com/blog/2019/8/27/pixelblog-20-top-down-tiles

https://opengameart.org/content/pixel-art-top-down-tileset

Do you think the grass is derivative?

But I can recreate it just from my mind by looking on nothing without any references because I understood the method....

Tutorials give you bricks and say how to place them together...

But I can recreate it just from my mind by looking on nothing without any references because I understood the method....

if that's how you made them, then they aren't derivatives.

Ok... I expected another answer but I glad :)

Do you know where I can read about copyright laws and other related to it stuff?