Can my employer force me to travel internationally?

So I've been working for a company for a few years now and suddenly we've been told to get passports and that they intend to send us to Russia, China, etc.

The thing is I don't have a passport, they don't exactly pay well enough for me to get 1 now, and we're expected to pay this out of pocket before the end of the month. I've told them that this is an issue financially and that, since I can't afford to travel and especially since I never expected to travel internationally in the next few years that I view this as a business expense.

They told me that this is my personal expense since it is my passport. My contract doesn't state anything about travelling outside the country. Could they fire me if I can't afford a passport? If anyone can help?

Edit: I know what this opportunity could bring, but I'm not joking if I say they don't pay enough to cover the cost of the passport. Even if they cover the cost and I pay it back, I will have to pay it back over 4+ months of nothing goes wrong to make sure I can cover my basic needs.