Here is this sentence with some stylistic improvements (more idiomatic German; register of speech: casual but not sloppy oral communicaiton):
Ich habe zwar nicht viel Ahnung von Astronomie, aber: Was könnte die Menschheit je gegen so ein riesiges Objekt ausrichten, wenn es direkt auf die Erde zuraste?
This does perhaps not answer your question for better terms for "kleiner Mensch" (e.g. winziger Mensch). But I do not find the entire sentence very idiomatic. One thing is the picture that the sentence evokes: it presents us one single person trying to stop the asteroid. However, more realistically, this would be a project for not one single person but rather for (hopefully) the entire planet, or at least 2-3 of the larger countries who have the most advanced equipment. So I think it would be most natural to speak of die Menschheit. Now, the Menschheit ist a pretty large thing, or at least a numerous one. Speaking of a "tiny" Menschheit would be strange. If you anyway want to stress the insignificance of the Menschheit as compared to the power of the asteroid, you could perhaps say:
Ich bin zwar kein Experte für Astronomie, aber: Was könnte die Menschheit in ihrer ganzen Winzigkeit [or: Schwäche] je gegen so ein riesiges Objekt ausrichten, wenn es direkt auf die Erde zuraste?
Other stylistical notes:
"zusteuerte" is not a good choice here for what the asteroid does. The asteroid does not have the ability to redirect its trajectory, so there is no "steuern" there. Therefore I use rasen.
"überwältigend groß": somehow it does not feel right. One would not say überwältigend groß for an asteroid. I do not have a fool-proof explanation for this, but I think we use überwältigend rather for emotional effects, not for physical dimensions. So a good concert could be überwältigend, or the love of a woman, or whatever. For physically large things you simply can use riesig.
As this answer was more about the entire sentence, not specifically about "un petit humain de rien du tout": If you want solutions for this in German, please give some more expamples in French (from other areas than astronomy perhaps).