(去火星)

我不知道你在火星上的原因;但我很高兴你在那里，我希望我与你们同在。

"Whatever the reason you're on Mars is, I'm glad you're there, and I wish I was with you."

This one is very good speaking of  order of phrases.

PS

'不管'is often used in Chinese (both speaking and writing) which means'whatever''wherever''whoever',etc.

For example,

Whatever you say, I won't change my decision.

不管你怎么说，我都不会改变我的决定。

(去火星)

不管你为什么在火星上，

[ if you mean "I don't care why you're on the Mars", this is the word you should say, other wise: 不知道你为什么在火星上 for "I don't know why you're on the Mars." ]

我都很高兴你在那，

[ use "都" for whatever ]

我希望我是和你一起去的。

[You used "was" so "是" and "去的" for past time, ]

(去火星)

我不知道你在火星上的原因无论你在火星的原因是什么;但我很高兴你在那里，我希望我与你们同在。

"Whatever the reason you're on Mars is, I'm glad you're there, and I wish I was with you."