The graduate program looks for a Bachelor's degree with a strong undergraduate academic and research background, typically in physics, astrophysics, or space science, or similarly at the Master's level. For international students, English proficiency is required, although this can be satisfied in multiple ways (TOEFL or equivalent, interview, documentation of attending an English language school).
Faculty look for students who will be able to be happy, healthy, and productive in their chosen research area. The precise qualities otherwise vary by group, as some groups place emphasis on academic and mathematical performance, while others look for computational, hardware, or engineering skills. Applicants are encouraged to provide in their personal statement an area (or areas) of interest, as then the applicant's skill set can be evaluated against what faculty in those areas value most.