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Astro Bot can be acquired as early as Level 9. The only requirements seem to be that you have to breed two hybrids together. I've gotten some with Flower + Safety, Harvester + Flower, Magnet + Gravity, and Safety + Welder.
I used Magnet + Gravity Bot (I wasn't even going for Astro. I use this combo to to get XP and Robux fast). These are Science hybrid Bots that can be obtained at Level 16.
You don't need these two to get Astro though. I've gotten it with Harvester + Flower and Safety + Flower. I've even seen someone else get it with Flower + Flower. It seems any two hybrids can yield an Astro Bot. It's just really rare though.