Should I hold the same writing standards for international students as I do for domestic students? I teach courses in physics, astronomy and planetary science.

Often, but certainly not always, international students (and even some domestic students, for whom English is not their primary language), write worse than their domestic peers who have grown up with English. 

For most assignments, I take up to a couple points off for negligence of spelling and grammar, depending on how bad their writing was. The point is to remind students that their writing should be professional. I never nit-pick about a typo here or there, but if there is a whole slew of them, I'll make a note and take off what would amount to be a few percentage points of the assignment's total score. 

I'd like to give grace to those for whom English is not their primary language. At the same time, I need to be fair and grade equally. As it stands, I grade equally for everyone, but I try to give more feedback to those who need it. 

Does anyone have any better ideas about how to do this?
There are two different types of value judgement that you are discussing here---communication goals and grading---and I think it's important to carefully separate them.


Communication Goals: I think that it is unfair to a person who isn't a native English speaker to not expect them to aim for the same goals of clear and lucid communication.  When giving feedback, don't brush over the language and grammar errors just because a person is not a native speaker.  They should have the same chance to learn and improve as anyone else.
Grading: Unless the class is about English grammar, making English quality a significant part of the grade is generally a bad way to evaluate a person's work.  If the language is bad enough that it interferes with your ability to understand the class-relevant content, then yes, that's a problem and should receive proportional demerits.  If the class-relevant content is clear, however, it's questionable to me whether any points should be taken off at all.


In short: grade very generously on language issues, but point out the mistakes so the student can continue to learn and improve.